Homebuilders Guide to 
Earthquake-Resistant 
Design and Construction 

FEMA 232 - June 2006 

Prepared by the 
Building Seismic Safety Council 
for the 
Federal Emergency Management Agency 
of the Department of Homeland Security 

National Institute of Building Sciences 
Washington, D.C. 


NOTICE: Any opinions, findings, conclusions, or recommendations expressed in this publication do not 
necessarily reflect the views of the Federal Emergency Management Agency. Additionally, neither 
FEMA nor any of its employees make any warranty, expressed or implied, nor assume any legal liability 
or responsibility for the accuracy, completeness, or usefulness of any information, product, or process 
included in this publication. 

The opinions expressed herein regarding the requirements of the International Residential Code do not 
necessarily reflect the official opinion of the International Code Council. The building official in a 
jurisdiction has the authority to render interpretation of the code. 

This report was prepared under Contract EMW-1998-CO-0419 between the Federal Emergency 
Management Agency and the National Institute of Building Sciences. 

For further information on the Building Seismic Safety Council, see the Councils website -- 
www.bssconline.org -- or contact the Building Seismic Safety Council, 1090 Vermont, Avenue, N.W., 
Suite 700, Washington, D.C. 20005; phone 202-289-7800; fax 202-289-1092; e-mail bssc@nibs.org. 

In Memoriam 

Edwin G. Zacher 1920-2005 

The Federal Emergency Management Agency and 
the Building Seismic Safety Council dedicate this guide to 
Edwin G. Zacher in recognition of his significant contributions 
to the profession of structural engineering and the nations 
codes and standards development efforts. The residential built 
environment and all those who work to create it and use it have 
benefited tremendously from his work. 


PREFACE


The Federal Emergency Management Agency (FEMA), which is part of the Department of 
Homeland Security, works to reduce the ever-increasing cost that disasters inflict on the nation. 
Preventing losses before they occur by designing and constructing buildings and their 
components to withstand anticipated forces from various hazards is one of the key components 
of mitigation and is one of the most effective ways of reducing the cost of future disasters. 

The National Earthquake Hazards Reduction Program (NEHRP) is the federal program 
established to address the nation's earthquake threat. NEHRP seeks to resolve two basic issues: 
how will earthquakes affect us and how do we best apply our resources to reduce their impact on 
our nation. The program was established by Congress under the Earthquake Hazards Reduction 
Act of 1977 (Public Law 95-124) and was the result of years of examination of the earthquake 
hazard and possible mitigation measures. Under the NEHRP, FEMA is responsible for 
supporting program implementation activities, including the development, publication, and 
dissemination of technical design and construction guidance documents. 

Generally, there has not been much technical guidance addressing residential buildings unless 
they are located in areas of high seismicity or exceed a certain size or height. This is because 
most residential buildings were thought to perform fairly well in earthquakes due to their low 
mass and simple construction. While buildings may not normally experience catastrophic 
collapse, they can still suffer significant amounts of damage, rendering them uninhabitable. This 
is especially true when construction techniques are less than adequate. What is particularly 
important from FEMA's point of view is that, given the sheer number of this type of building, 
even minor damage represents a significant loss potential and temporary housing demand that 
will need to be addressed after an earthquake by all levels of government. 

After the San Fernando earthquake in 1971, a study of residential buildings and the damage they 
suffered was conducted by a team of experts under funding from the Department of Housing and 
Urban Development (HUD) and the National Science Foundation (NSF). HUD utilized these 
data to develop a non-engineering document entitled Home Builder's Guide to Earthquake 
Design. This manual, originally published in 1980, provided easy-to-follow information to the 
average homebuilder on steps for reducing potential earthquake damage. In July 1992 it was 
reprinted as a joint FEMA-HUD document, also known as FEMA 232. The manual was 
subsequently updated for FEMA and the revised publication, Home Builders Guide to Seismic 
Resistant Construction (FEMA 232), was published in August 1998. 

Since that time, there have been several significant changes that needed to be incorporated into 
this document to keep it current. The first and most important change was the completion of the 
FEMA-funded Consortium of Universities for Research in Earthquake Engineering (CUREE)-
Caltech Woodframe Project. This project was funded using FEMA Hazard Mitigation Grant 
Program funds available after the Northridge earthquake and was designed to address the 
unexpected amount of damage suffered by wood frame residential structures. Similar to the 
successful FEMA/SAC Steel Moment Frame Buildings Project, this effort combined academic 
research and testing of wood frame buildings and components with the development of 


engineering-based design guidance for future construction. The project yielded some interesting 
findings that needed to be captured in a guidance document. 

A second change was the development and publication of the 2000 International Residential 
Code (IRC) by the International Code Council. This model residential building code replaced 
the Council of American Building Officials (CABO) One- and Two-Family Dwelling Code, 
which did not adequately address earthquake loads. The IRC reflects on the NEHRP 
Recommended Provisions and is intended to adequately address the earthquake hazard. 

This publication presents seismic design and construction guidance for one- and two-family 
houses in a manner that can be utilized by homebuilders, knowledgeable homeowners, and other 
non-engineers. It incorporates and references the prescriptive provisions of the 2003 
International Residential Code as well as the results of the FEMA-funded CUREE-Caltech 
Woodframe Project. The manual includes prescriptive building detail plans based on state-ofthe-art earthquake-resistant design for use by homebuilders and others in the construction of a 
non-engineered residential structure. Further, the manual also uses the results of recent loss 
investigations as well as current research and analysis results to identify a number of specific 
above-code measures for improved earthquake performance along with their associated costs. A 
typical modern house is used to illustrate the application and benefits of above-code measures. 
This manual replaces the Home Builders Guide to Seismic Resistant Construction (FEMA 232) 
published by FEMA in August 1998 as well as earlier FEMA and HUD versions. 

Finally, FEMA wishes to express its deepest gratitude for the significant efforts of primary 
authors J. Daniel Dolan, Kelly Cobeen, and James Russell; the members of the Project Team; the 
many reviewers and workshop attendees; and the BSSC Board of Direction and staff. Their 
dedication and hard work made this document possible. 

Department of Homeland Security/Federal Emergency Management Agency 


ACKNOWLEDGEMENTS 


The Building Seismic Safety Council is grateful to all those involved in developing this updated 
guide to earthquake-resistant home design and construction. As chairman of the BSSC Board of 
Direction, it is my pleasure to express appreciation to the members of the team developing this 
guide: 

J. Daniel Dolan, PhD, PE, Professor, Washington State University, Wood Materials and 
Engineering Laboratory, Pullman (team leader) 

Kelly Cobeen, Structural Engineer, Cobeen and Associates Structural Engineering, 

Lafayette, California 

James E. Russell, Building Codes Consultant, Concord, California 

Special thanks also go to Gerald Jones, retired building official, and Douglas Smits, the chief 
building official of Charleston, South Carolina, who served with me and the writers on the 
committee overseeing this project. 

The BSSC also is grateful to those who participated in a workshop discussion (Appendix F) of an 
early draft of this guide and willingly shared their experiences with respect to earthquake-
resistant home building. Special thanks go to those individuals who reviewed and commented 
on subsequent drafts of this document; their input has made this a much more useful guide. 

I also wish to thank the members of the BSSC Board of Direction who have recognized the 
importance of this effort and provided sage advice throughout the project. Special thanks also 
are due to the BSSC staff who worked untiringly behind the scenes to coordinate the project 
resulting in this report. 

Finally the BSSC is grateful to Michael Mahoney, FEMA project officer, for his insight and 
ongoing support, to the International Code Council for their work in reviewing and disseminating 
this guide, and to the National Association of Home Builders for their assistance in 
disseminating this guide. 

Jim. W. Sealy 
Chairman, BSSC Board of Direction 


TABLE OF CONTENTS 


Preface.......................................................................................................................................... iii 
Acknowledgements ........................................................................................................................v 
Executive Summary ......................................................................................................................1 
Chapter 1 Introduction..................................................................................................................3 
1.1 
Background.........................................................................................................................3 
1.2 
Above-code Recommendations...........................................................................................4 
1.3 
The International Residential Code.....................................................................................5 
1.4 
IRC Seismic Design Categories...........................................................................................5 
1.5 
Building Response.............................................................................................................10 
1.6 
Important Concepts Concerning Homesite........................................................................12 
1.7 
Model House Used for Guide Analysis.............................................................................14 
1.8 
Other Hazards ....................................................................................................................17 
Chapter 2 Earthquake-Resistance Requirements.....................................................................21 
2.1 
IRC General Earthquake Limitations.................................................................................21 
2.2 
Load Path...........................................................................................................................22 
2.3 
House Configuration Irregularities....................................................................................29 
Chapter 3 Foundations and Foundation Walls.........................................................................49 
3.1 
General Foundation Requirements ....................................................................................49 
3.2 
Concrete Foundations ........................................................................................................53 
3.3 
Masonry Foundations ........................................................................................................56 
3.4 
Footing Width....................................................................................................................57 
3.5 
Special Soil Conditions .....................................................................................................58 
3.6 
Foundation Resistance to Sliding from Lateral Loads ......................................................59 
3.7 
Special Considerations for Cut and Fill Sites....................................................................61 
3.8 
Foundation Walls...............................................................................................................62 
3.9 
Foundation Wall Thickness, Height, and Required Reinforcing.......................................63 
3.10 
Wood-Framed Wall Bottom Plate and Foundation Sill Plate Anchorage .......................64 
3.11 
Required Location for Anchor Bolts along Exterior Walls.............................................65 
3.12 
Required Anchorage along Interior Braced Walls...........................................................65 
3.13 
Anchoring of Interior Braced Walls in SDCs D1 and D2.................................................67 
Chapter 4 Floor Construction.....................................................................................................69 
4.1 
General Floor Construction Requirements ........................................................................69 
4.2 
Wood-Framed Floor Systems............................................................................................70 
4.3 
Cantilevered Floors............................................................................................................71 
4.4 
Requirements for Blocking................................................................................................73 
4.5 
Connection of Floor Joists to Wall Top Plate or Foundation Sill Plate Below .................75 
4.6 
Floor Sheathing..................................................................................................................77 

4.7 
Lateral Capacity Issues for Wood Framed Floors Using Wood Structural Panels............77 
4.8 
Concrete Slab-on-Grade Floors .........................................................................................81 
Chapter 5 Walls...........................................................................................................................83
5.1 
Wood Light-Frame Construction.......................................................................................83 
5.2 
Stone and Masonry Veneer..............................................................................................103 
5.3 
Cold-formed Steel Houses...............................................................................................107
5.4 
Masonry Wall Houses .....................................................................................................110 
5.5 
Insulating Concrete Form (ICF) Wall Houses.................................................................115 
Chapter 6 Roof-Ceiling Systems...............................................................................................121
6.1 
General Roof-Ceiling Requirements ...............................................................................121 
6.2 
Special Framing Considerations......................................................................................122 
6.3 
Blocking and Lateral Load Paths for Roof Systems........................................................124 
6.4 
Connection of Ceiling Joists and Rafters to Walls Below...............................................126 
6.5 
Roof Sheathing ................................................................................................................126 
6.6 
Lateral Capacity Issues for Wood Framed Roofs............................................................128 
6.7 
Quality Control ................................................................................................................129 
Chapter 7 Chimneys, Fireplaces, Balconies, and Decks.........................................................131
7.1 
Chimneys and Fireplaces.................................................................................................131 
7.2 
Balconies and Decks........................................................................................................137 
Chapter 8 Anchorage of Home Contents.................................................................................141 
8.1 
General............................................................................................................................141 
8.2 
Water Heater Anchorage .................................................................................................141
8.3 
Securing Other Items .......................................................................................................144 
Chapter 9 Existing Houses ........................................................................................................151
9.1 
Additions and Alterations................................................................................................151 
9.2 
Earthquake Upgrade Measures........................................................................................154 
APPENDICES 
Appendix A Analysis of Model House Used in This Guide....................................................167 
A1 Model House....................................................................................................................167 
A2 Analysis Using Standard Engineering Design Methods..................................................171 
A3 Analysis Using Nonlinear Methods.................................................................................171 
A4 Analysis Results...............................................................................................................174 
Appendix B Earthquake Provisions Checklist for Builders and Designers .........................179 
Appendix C Earthquake Provisions Checklist for Designers and Plan Checkers...............187 
Appendix D Significant Changes for the 2006 International Residential Code ....................193 

D1 Revised Seismic Design Maps.........................................................................................193 
D2 Addition of Seismic Design Category D0........................................................................193 
D3 Change in Applicability of Irregular Building Requirements .........................................199 
D4 Clarification and Addition of Requirement for Masonry Veneer....................................199 
Appendix E References and Additional Resources.................................................................201 
Appendix F Homebuilders Guide Project Participants ........................................................207 
LIST OF FIGURES 
Figure 1-1 Seismic Design Categories......................................................................................... 6-9 
Figure 1-2 Forces induced in a house due to earthquake ground motion ......................................10 
Figure 1-3 Concept of actual vs. code seismic forces....................................................................11 
Figure 1-4 Concept of ductility......................................................................................................11 
Figure 1-5 Definition of drift .........................................................................................................12 
Figure 1-6 A house damaged due to fault movement....................................................................13 
Figure 1-7 Houses damaged due to a landslide at the site .............................................................13 
Figure 1-8 Elevation of model house used as an example for this guide ......................................14 
Figure 1-9 Model house floor plan ................................................................................................15 
Figure 2-1 Chain illustrating load path concept.............................................................................22 
Figure 2-2 Lateral loads induced in a building due to wind or earthquakes..................................23 
Figure 2-3 Loading and deflection of roof, ceiling, and floor systems..........................................24 
Figure 2-4 Loading and deflection of bracing wall systems..........................................................25 
Figure 2-5 Load transfer between components in a building ........................................................26 
Figure 2-6 Horizontal load path connections and deformations....................................................27 
Figure 2-7 Overturning load path connections and deformations .................................................28 
Figure 2-8 Depiction of ideal building configuration for earthquake resistance...........................30 
Figure 2-9 Irregular building configuration with open front .........................................................31 
Figure 2-10 Rotational response and resistance to torsion ............................................................31 
Figure 2-11 Concentration of loads and possible damage and failure due to irregular plan .........32 
Figure 2-12 Roof-level damage to building with a T-shaped plan ................................................33 
Figure 2-13 Deformation pattern due to soft-story behavior.........................................................34 
Figure 2-14 Change in deformation patterns associated with soft-story irregularity ....................34 
Figure 2-15 House experiencing soft- and weak-story behavior ..................................................35 
Figure 2-16 Apartment building experiencing soft- and weak-story behavior..............................35 
Figure 2-17 Detailing required for stepped foundation walls........................................................36 
Figure 3-1 Perimeter foundation with separately placed footing and stem wall ...........................49 
Figure 3-2 Sliding action resisted by foundation...........................................................................50 
Figure 3-3 Overturning action resisted by foundation...................................................................51 
Figure 3-4 Interior braced wall and floor framing.........................................................................52 
Figure 3-5 Foundation requirements for interior braced wall line on slab-on-grade construction52 
Figure 3-6 Recommended minimum reinforcement for concrete footings and stem walls...........54 

Figure 3-7 Above-code horizontal reinforcing lap at corners and intersections............................54 
Figure 3-8 Above-code use of vertical dowels to connect a slab-on-grade to a separately poured 
footing...........................................................................................................................................55 
Figure 3-9 Inverted-T footing dimensions.....................................................................................58
Figure 3-10 Lateral resistance provided by foundation .................................................................59 
Figure 3-11 Above-code stepped foundation reinforcing detail....................................................60 
Figure 3-12 Foundation supported on rock and fill .......................................................................61 
Figure 3-13 Example of damage caused in building on cut and fill site........................................62 
Figure 3-14 Interior braced wall on slab-on-grade ........................................................................66 
Figure 4-1 An unblocked floor diaphragm ....................................................................................69 
Figure 4-2 Slab-on-grade and perimeter footing transfer loads into soil.......................................70 
Figure 4-3 Cantilevered floor restrictions......................................................................................71 
Figure 4-4 Cantilevered joist at braced wall above .......................................................................73 
Figure 4-5 Interior bearing line......................................................................................................74 
Figure 4-6 Blocking below interior braced wall............................................................................74 
Figure 4-7 Blocking floor joists spaced apart for piping in floor ..................................................75 
Figure 4-8 Toe nail configuration requirements ............................................................................76 
Figure 4-9 Blocked diaphragm configuration................................................................................79 
Figure 4-10 Diaphragm loads on long and short sides ..................................................................79 
Figure 4-11 Reinforcing straps at large diaphragm openings........................................................80 
Figure 5-1 Sliding, overturning, and racking action resisted by walls and foundation .................84 
Figure 5-5 Detailing differences for three options when using wood structural panel sheathed 
Figure 5-9 Illustration of concept of equal numbers of fasteners in line for symmetric nailing 
Figure 5-15 Symmetric layout of walls to distribute loads uniformly and thereby prevent 
Figure 5-2 Exploded view illustrating load paths..........................................................................85 
Figure 5-3 Wall action for resisting lateral loads...........................................................................86
Figure 5-4 Exploded view of typical residential wall segment......................................................86 
walls .............................................................................................................................................87 
Figure 5-6a Plan view of crawl space for model house in SDC C ................................................92 
Figure 5-6b Plan view of first floor for model house in SDC C....................................................93 
Figure 5-6c Plan view of second floor for model house in SDC C ...............................................94 
Figure 5-7a Plan view of crawl space for model house in SDC D2 ..............................................95 
Figure 5-7b Plan view of first floor for model house in SDC D2 ..................................................96 
Figure 5-7c Plan view of second floor for model house in SDC D2 .............................................97 
Figure 5-8 Properly and overdriven nails ......................................................................................98 
schedule.......................................................................................................................................100 
Figure 5-10 Sheathing detail for extending the sheathing over the band joist.............................102 
Figure 5-11 Applications of masonry and stone veneer ..............................................................103 
Figure 5-12 Stone veneer damage during the earthquake in Northridge, California...................104 
Figure 5-13 Cold-formed steel house under construction............................................................107 
Figure 5-14 House constructed with masonry walls....................................................................111 
torsion .........................................................................................................................................114 
Figure 5-16 Insulated concrete form house under construction...................................................116 
Figure 5-17 Illustration of cross-grain bending of wood ledger..................................................118 

Figure 6-1 Typical light-frame roof-ceiling system.....................................................................121 
Figure 6-2 Ridge board for 12:12 pitch roof................................................................................123 
Figure 6-3 Gable end wall or gable truss bracing........................................................................124 
Figure 6-4 Blocking at rafters to exterior wall.............................................................................125
Figure 6-5 Typical roof diaphragm sheathing nailing when wood structural panels are used ....127 
Figure 7-1 Chimney damage........................................................................................................131 
Figure 7-2 Chimney damage in Northridge earthquake ..............................................................132 
Figure 7-3 Locations for earthquake anchorage of masonry chimney at exterior house wall.....133 
Figure 7-4 Chimney section showing earthquake anchorage .....................................................134 
Figure 7-5 Anchorage detail for framing parallel to exterior wall...............................................134 
Figure 7-6 Anchorage detail for framing perpendicular to exterior wall.....................................135 
Figure 7-7 Factory-built flue and light-frame enclosure..............................................................136 
Figure 7-8 Collapsed factory-built chimney, light-frame enclosure, and deck ...........................136 
Figure 7-9 Decks and balconies in residential contruction..........................................................138 
Figure 7-10 Hold-down device providing positive connection of deck framing to house framing........139 
Figures 8-1a & 8-1b Securing a water heater with wall bracing .................................................142 
Figures 8-2a & 8-2b Securing a water heater with corner bracing..............................................143 
Figures 8-3 a-f Reproductions of pages from FEMA 74 on securing household goods...... 145-150 
Figure 9-1 Alterations to existing house......................................................................................152 
Figure 9-3 Cape Mendocino earthquake damage to cripple wall house with slab-on-grade 
Figure 9-10 House located on a hillside site damaged during Northridge, California, 
Figure 9-2 Horizontal addition and vertical addition...................................................................153 
additions......................................................................................................................................154 
Figure 9-4 Common anchorage configurations ...........................................................................156 
Figure 9-5a House with collapsed cripple walls..........................................................................157 
Figure 9-5b A house with severe damage due to cripple wall collapse.......................................157 
Figure 9-6 Cripple wall bracing...................................................................................................158 
Figure 9-7 Weak and soft story bracing.......................................................................................159 
Figure 9-8 Common open-front occurrences in one- and two-family detached houses..............160 
Figure 9-9 Detailing of narrow bracing wall piers at open fronts................................................161 
earthquake...................................................................................................................................162 
Figure 9-11 Anchorage of floor framing to the uphill foundation...............................................163 
Figure 9-12 Split-level ties for floor framing ..............................................................................164
Figure 9-13 Anchorage of concrete or masonry walls to floor, roof, or ceiling framing ............165 
Figure A1 Slab-on-grade base .....................................................................................................168 
Figure A2 Level cripple wall base...............................................................................................168 
Figure A3 Hillside base ...............................................................................................................168 
Figure A4 Basement ....................................................................................................................168 
Figure A5 Analysis model ...........................................................................................................172 
Figure A6 Hyseteric parameters for model..................................................................................172 

Figure D1 Seismic Design Categories, Site Class D ........................................................... 194-198 
LIST OF TABLES
Table 2-1 Load Path Connections for Horizontal Sliding ...................................................... 38-40 
Table 2-2 Load Path Connections for Overturning................................................................. 41-43 
Table 2-3 IRC House Configuration Irregularities ................................................................. 44-47 
Table 3-1 Summary of 2003 IRC Continuous Foundation and Anchor 
Bolt Requirements for Braced Wall Lines in One- and Two-family Houses................................68 
Table 3-2 Summary of 2003 IRC Continuous Foundation and Anchor 
Bolt Requirements for Bearing Walls in One- and Two-family Houses.......................................68 
Table 5-1 Braced Wall Panel Construction Methods (Materials) Recognized by the IRC .........88 
Table 5-2 IRC Sheathing Requirements for Seismic Design Categories C, D1, and D2 ..............90 
Table A-1 Example Wall Bracing per 2003 IRC, Slab-on-grade Base Condition......................170 
Table A-2 Hysteretic Parameters Used for Nonlinear Dynamic Model.....................................173 
Table A-3 Selected Results for IRC Bracing Provisions, Slab-on-grade Base Condition..........175 
Table A-4 Selected Results for Above-Code Measures, Slab-on-grade Base Condition...........176 

EXECUTIVE SUMMARY 


This guide provides information on current best practices for earthquake-resistant house 
design and construction for use by builders, designers, code enforcement personnel, and 
potential homeowners. It incorporates lessons learned from the 1989 Loma Prieta and 
1994 Northridge earthquakes as well as knowledge gained from the FEMA-funded 
CUREE-Caltech Woodframe Project. It also introduces and explains the effects of 
earthquake loads on one- and two-family detached houses and identifies the requirements 
of the 2003 International Residential Code (IRC) intended to resist these loads. The 
stated purpose of the IRC is to provide: 

. . . minimum requirements to safeguard the public safety, health, and general welfare, 
through affordability, structural strength, means of egress facilities, stability, sanitation, 
light and ventilation, energy conservation and safety to life and property from fire and 
other hazards attributed to the built environment. 

Because the building code requirements are minimums, a house and its contents still may 
be damaged in an earthquake even if it was designed and built to comply with the code. 
Research has shown, however, that earthquake damage to a house can be reduced for a 
relatively small increase in construction cost. This guide identifies above-code 
techniques for improving earthquake performance and presents an estimate of their cost. 
Note that the information presented in this guide is not intended to replace the IRC or any 
applicable state or local building code, and the reader is urged to consult with the local 
building department before applying any of the guidance presented in this document. 

The information presented in this guide applies only to one- and two-family detached 
houses constructed using the nonengineered prescriptive construction provisions of the 
IRC. Applicable IRC limits on building configuration and construction are described. 

A typical model house is used to illustrate the concepts discussed and to identify 
approximate deflections under earthquake loading, which permits performance to be 
compared for various building configurations using the minimum code requirements and 
the above-code techniques. The above-code recommendations are based on an analysis 
of the model house as well as comparative tests performed by various researchers and the 
lessons learned from investigation of residential building performance in past 
earthquakes. A nonlinear time-history analysis was performed for the model building 
using the SAWS computer program developed as part of the CUREE-Caltech 
Woodframe Project (Folz and Filiatrault, 2002). Details of the analysis are presented in 
Appendix A. 

Additional appendices feature checklists for builders, designers, and plan checkers; 
explain significant differences between the 2003 and 2006 editions of the IRC; and 
present a list of reference materials. 


Chapter 1 
INTRODUCTION 


1.1 BACKGROUND 

After the 1971 earthquake in San Fernando, California, a study of residential buildings and the 
types of damage they suffered was conducted by a team of experts with funding from the 
Department of Housing and Urban Development (HUD) and the National Science Foundation 
(NSF). Subsequently, HUD utilized the results of that study to develop a nonengineering 
guidance document entitled Home Builder's Guide to Earthquake Design. This manual was 
originally published in June 1980 to provide homebuilders with easy-to-follow guidance for 
reducing potential earthquake damage. 

In July 1992, the Federal Emergency Management Agency (FEMA) reissued the HUD manual 
with some updated material as a joint FEMA/HUD publication identified as FEMA 232. By the 
mid-1990s, it was apparent that this publication was in need of updating, especially to take into 
account some of the early findings from the 1994 Northridge earthquake. This update was 
prepared for FEMA by SOHA Engineers and was published by FEMA in August 1998 as the 
Home Builders Guide to Seismic Resistant Construction, again as FEMA 232. 

Since that time, significant events have occurred warranting another updating of the guide. First 
was completion of the FEMA-funded CUREE-Caltech Woodframe Project. This project, funded 
under the FEMA Hazard Mitigation Grant Program, addressed the unexpected damage suffered 
by woodframe residential structures during the Northridge earthquake. Project testing of 
complete woodframe buildings and individual components that resist or transmit earthquake 
loads yielded some interesting findings that needed to be captured in a guidance document. 

Another significant event occurred in 1994 when the International Code Council was established 
to develop a single set of comprehensive and coordinated national model construction codes. 
Prior to this time, the three organizations that founded the ICC  the Building Officials and Code 
Administrators International, Inc. (BOCAI), the International Conference of Building Officials 
(ICBO), and Southern Building Code Congress International, Inc. (SBCCI)  each published a 
set of model building codes that generally were used in distinct regions of the nation. The initial 
editions of the ICCs International Building Code (IBC) and International Residential Code 
(IRC) were published in 2000 and updates were issued in 2003 and 2006. The IBC replaced 
BOCAIs National Building Code, ICBOs Uniform Building Code, and SBCCIs Standard 
Building Code. For prescriptive residential construction, the IRC replaced the One- and Two-
Family Dwelling Code of the Council of American Building Officials (CABO). (Note that the 
IBC contains prescriptive and engineering provisions for light-frame wood construction. In 
certain cases, the IBC prescriptive provisions are different from those in the IRC. The National 
Fire Protection Associations NFPA 5000 Building Construction and Safety Code does not 
contain provisions for prescriptive residential construction.) 


In order to address these and other changes, FEMA initiated a complete rewrite of the 1998 
document; this guide, which retains the FEMA 232 designation, is the result. One- and two-
family detached houses of light-frame wood construction are addressed; however, the discussion 
is relevant to other materials of construction likely to be used for houses including light-frame 
cold-formed steel and insulated concrete form. Explained in this guide are: 

 
The basic principles of earthquake-resistant design, 
 
The specific prescriptive seismic provisions of the 2003 International Residential Code, 
 
The results of recent research and analysis, and 
 
Measures exceeding code requirements that are expected to reduce the amount of 
damage from an earthquake (see Section 1.2 below). 


The guide also includes limited guidance on applying the principles of earthquake resistance to 
house additions and alterations and on anchoring typical house furnishings and equipment such 
as hot water heaters. Appendices describe the analyses performed in developing this guide, 
present checklists for builders, designers, and plan checkers; explain significant differences 
between the 2003 and 2006 editions of the IRC; and identify reference materials and participants 
in the project resulting in this guide. 

1.2 ABOVE-CODE RECOMMENDATIONS 

The above-code recommendations included in this guide describe details that, when incorporated 
into a house, can be expected to result in improved performance above that expected from a 
house designed and constructed following the minimum requirements of the IRC. The above-
code techniques reduce the deformations of the house during an earthquake and therefore reduce 
the amount of damage. Above-code recommendations are printed in boldface type in this guide 
and appear, with associated discussion, in boxes. While the above-code recommendations are 
expected to improve the performance of a house in an earthquake and thereby reduce damage, 
many will involve some added costs. The costs associated with above-code recommendations 
presented in this guide are based on an estimate prepared by a homebuilder in the Seattle area 
and are cited as a percentage of the basic framing cost for the model house analyzed during 
development of this guide. Presenting the cost increase for the various above-code 
recommendations in these terms permits homebuilders in any part of the nation to easily 
determine what the associated added cost will be in his or her area, thus allowing builders and 
potential homeowners to make reasonable cost-benefit decisions regarding implementation of the 
recommendations. 


1.3 THE INTERNATIONAL RESIDENTIAL CODE 

As already indicated, this guide focuses on the ICCs 2003 International Residential Code, 
which provides a comprehensive collection of requirements for prescriptive (nonengineered) 
residential construction. The IRCs stated purpose is to provide: 

. . . minimum requirements to safeguard the public safety, health, and general welfare, through 
affordability, structural strength, means of egress facilities, stability, sanitation, light and 
ventilation, energy conservation and safety to life and property from fire and other hazards 
attributed to the built environment. 

The IRC addresses other natural hazards in addition to earthquakes (up to limits described in the 
IRC scoping provisions). When considering above-code recommendations, construction details 
intended to reduce the risk from one hazard may be slightly different from those needed to resist 
another. Thus, care should be taken to consider all natural hazards that present a risk to a 
specific site and to formulate an appropriate mitigation strategy in accordance with the 
jurisdictions building code. Additional guidance is provided in Section 1.8 of this guide. 

1.4 IRC SEISMIC DESIGN CATEGORIES 

The IRC designates the level of potential seismic hazard for dwellings by assigning a house to a 
Seismic Design Category (SDC) based on its location. The IRC SDCs are A, B, C, D1, D2 and E, 
with A representing the lowest level of seismic risk applicable to residential construction and E, 
the highest. All residential buildings (detached houses and townhouses) in regions with SDC 
designations A and B, the lowest levels of seismic risk, are exempt from the seismic 
requirements of the IRC. SDC E regions have such a high level of seismic risk that, with a few 
exceptions, houses in these regions fall outside the scope of the IRC and must be designed using 
engineering principles following the International Building Code or NFPA 5000. 

Whether or not required by the IRC and across all SDCs from A to E, many of the recommendations in this guide will improve the resistance of a dwelling to seismic forces, wind forces, and 
possibly the effects of other natural hazards. The discussion and examples presented in this 
guide focus on houses located in SDCs C, D1, and D2. 

All U.S. model building codes provide maps identifying the seismic hazard. The 2003 IRC 
seismic design maps (IRC Figure R301.1(2) shown in Figure 1-1 designate the Seismic Design 
Categories for the nation and U.S. territories. It is a simplified version of the maps referenced by 
the IBC and NFPA 5000 for all building types. The legend correlates the Seismic Design 
Category with the acceleration expected at a location in terms of gravity (g). A value of 100% g 
is equal to the vertical acceleration effects of gravity on Earth. More detailed information can be 
found for a particular building site using a CD-ROM available with the building codes and from 
FEMA; the CD allows the user to input the latitude and longitude coordinates of the site or the 
zip code. Zip codes should be used with caution because they may not reflect the highest 
possible SDC in the area covered by the postal zip code. Similar information is available on a 

U.S. Geological Survey (USGS) website  http://earthquake.usgs.gov/research/hazmaps (click on 
seismic design values for buildings). 


Figure 1-1 Seismic Design Categories  Site Class D. 



Figure 1-1 Seismic Design Categories  Site Class D (continued). 



FEMA 232, Homebuilders Guide 

Figure 1-1 Seismic Design Categories  Site Class D (continued). 

Chapter 1, Introduction 

Figure 1-1 Seismic Design Categories  Site Class D (continued). 

When using the seismic maps associated with the building codes, the reader should be aware that 
local soil conditions have a major impact on the seismic hazard for each particular site. The IRC 
map incorporates an assumed Site Class of D, which is stiff alluvial soil. If the soil conditions 
are different from stiff soil (e.g., bedrock or the soft alluvial soils often found in valleys), the 
local building department may have established special seismic requirements for these locations. 
Check with the local building department to determine if any such special regulations apply to a 
particular building site. 

1.5 BUILDING RESPONSE 

The series of drawings in Figure 1-2 
illustrates how a house responds to 
earthquake ground motion. Before 
the earthquake occurs, the house is 
stationary, resisting only the vertical 
gravity loads associated with the 
weight of the house and its 
occupants and contents. When the 
ground starts to move during an 
earthquake, the foundation of the 
house moves sideways but the roof 
and upper stories try to remain 
stationary due to the inertia of the 
house. By the time the roof starts to 
move in the direction of the 
foundation, the foundation is already 
moving back towards its initial 
position and the roof and foundation 
are moving in opposite directions. 
This cycle repeats until the 
earthquake ground motion stops and 
the building movements slow and 
then stop. If the shaking has been 
severe enough, the house may be 
damaged and have a residual tilt or 
displacement. 

Figure 1-2 Forces induced in a 
house due to earthquake ground 
motion. 


To perform adequately in an earthquake, a house must have enough strength to resist the forces 
generated without failure. Actual earthquakes can generate forces considerably higher than those 
used for code-prescribed design (Figure 1-3). Nevertheless, design for the lower code forces 
generally has prevented life loss and therefore satisfies the purpose of the code. Remember that 


the primary goal of the building code is to prevent loss of life; the prevention of damage is only 
incidental. It is for this reason that this guide presents above-code recommendations that 
describe techniques intended to improve the performance of a house during an earthquake and 
result in less damage. 

Figure 1-3 Concept of 
actual vs. code seismic 
forces. 


The reasons one- and two-family houses tend to perform adequately in earthquakes even when 
designed to the minimum code forces are because houses often are stronger than recognized in 
code-level design and because they often are constructed with ductile earthquake-resisting 
systems. In residential construction, the finish materials and nonstructural partitions often add 
significantly to the strength provided by required bracing materials. Houses constructed from 
ductile earthquake-resisting systems generally will perform well during an earthquake because 
they can deform without breaking. An example of ductility is given in Figure 1-4. The ductile 
metal spoon simply bends while the brittle plastic spoon breaks. 

Figure 1-4 Concept of ductility. 


Stiffness also is important. The stiffer the house, the less it will move or deflect during an 
earthquake, which will reduce the amount of damage to finish materials and, therefore, repair 
costs. Stiffness is measured in buildings in terms of drift (horizontal deflection) and is usually 


discussed as the horizontal deflection in a particular story in terms of either the amount of drift in 
inches or the percent story drift, which is the deflection in inches divided by the floor to floor 
story heights in inches (Figure 1-5). Unfortunately, increased stiffness also generally results in 
increased seismic forces; therefore, the above-code recommendations made in this guide 
simultaneously increase strength and stiffness. 

Figure 1-5 Definition of drift. 


One way to understand how the strength and stiffness of a house affect its earthquake 
performance is to remember that the forces exerted by earthquake ground motion are resisted by 
a houses strength while the drift (deflection) is resisted by the houses stiffness. An analogy 
that can be used for this is a fishing pole. The amount the pole bends is dependent on how stiff 
the pole is; whether the pole breaks or not is dependent on how strong the pole is. A strong 
house will not fall down, but a house with relatively little stiffness will sustain considerable 
damage. 

1.6 IMPORTANT CONCEPTS CONCERNING HOMESITE 

Site characteristics also affect how a house will perform during an earthquake: 

 
Certain types of soil amplify earthquake ground motions; 
 
Some types of soil slide, liquefy, and/or settle as a result of earthquake ground motions, 
all of which will result in loss of vertical support for the house; and 
 
Fault rupture on a house site can result in both horizontal and vertical offsets of the 
supporting ground. 


It is most desirable to build on sites with stable, solid geologic formations. Deep and unbroken 
rock formations, referred to as bedrock, generally will minimize earthquake damage whereas 
deep soft sedimentary soils will result in the maximum seismic forces and displacements being 
transferred to the house. 

Sites located over known faults and in landslide-prone sites warrant additional attention. No 
matter how well designed, a house cannot accommodate earthquake ground motions at a site 
directly on top of an earthquake fault where the ground on either side of the fault moves in 
opposite directions. Check with the local building department to determine where known faults 


are located. Houses should not be built within 50 feet of a known fault and, even at that distance, 
damage will still be significant. A house damaged by direct fault movement is shown in Figure 
1-6. 

Figure 1-6 A house damaged due to fault movement. The fault passed under 
this house near Wright's Station, on the Southern Pacific Railroad, in Santa 
Cruz County, California. The house was severed by fault movement during the 
earthquake. Photo credit: R.L. Humphrey, U.S. Geological Survey. 

Sites where landslides are likely to occur also should be avoided. An example of damage due to 
a hillside collapsing in a landslide is shown in Figure 1-7. Engage a geotechnical or geological 
engineer to inspect the site and/or check with the local building department to minimize the risk 
of building on a landslide-prone site. 

Figure 1-7 Houses damaged due to a landslide at the site. 
Photo Courtesy National Information Service for Earthquake Engineering, 
University of California, Berkeley




1.7 MODEL HOUSE USED FOR GUIDE ANALYSIS 

A typical model house is used in this guide to illustrate various concepts and techniques (Figure 
1-8). An analysis of the model house identified approximate deflections under seismic loading, 
which permitted performance to be compared for various configurations using the minimum 
code requirements and the above-code techniques. 

Figure 1-8 Elevations of the model house used as an example for this guide. 


The floor plan of the model house is shown in Figure 1-9. IRC-conforming seismic bracing 
configurations were developed for SDCs C, D1, and D2. Gypsum wallboard was used to meet 
IRC seismic bracing requirements where possible because the wallboard would already be 
provided for wall finish. Where IRC bracing requirements could not be met with gypsum 
wallboard, wood structural panel (oriented strand board or OSB) wall bracing was added. 
Modifications to the basic floor plan included changing the foundation from a slab-on-grade to a 
crawl space (cripple wall or hillside condition) to illustrate the differences resulting from these 
changes. 


Figure 1-9 Model house floor plan. 



After the basic model house construction was analyzed, the following above-code construction 
techniques were evaluated: 

 
A completely sheathed shear wall system in which the exterior walls were fully sheathed 
(around door and window openings) with OSB and hold-down devices were provided at 
building corners, 
 
A shear wall system with hold-down devices provided at each end of each OSB bracing 
wall, and 
 
An oversize sheathing panel system with each OSB sheet lapping vertically onto and 
edge-nailed to the rim joist at each floor level. 


Approximate comparisons of the performance of the basic configuration of the model house with 
the above-code configurations were made using a two-part process. First, each house 
configuration was analyzed using a computer model to determine maximum drift or deformation 
during earthquake loading. 

Second, in order to translate the analytical results into an approximation of house performance, 
three ranges of peak transient wall drift and assumed approximate descriptions of performance 
were developed. The choice of drift ranges and performance descriptions are based on 
component and full-building test results and the judgment of those participating in the 
development of this guide. 

The approximate performance categories and corresponding story drift ranges are: 

 
Minor damage potential  Less than or equal to 0.5% story drift 
The house is assumed to suffer minor nonstructural damage such as 
cracking of plaster or gypsum wallboard and hopefully would be greentagged (occupancy not limited) by inspectors after an earthquake, 
which would permit immediate occupancy. Some repairs should still be 
anticipated. 


 
Moderate damage potential  Above 0.5% to 1.5% story drift 
The house is assumed to suffer moderate damage including possible 
significant damage to materials and associated structural damage, but the 
building is assumed to have some reserve capacity in terms of strength and 
displacement capacity. The house hopefully would be green-tagged or, 
more likely, yellow-tagged (limited occupancy) by inspectors after an 
earthquake and may or may not be habitable. Significant repairs should be 
anticipated. 





 
Significant damage potential  Greater than 1.5% story drift 


The house is assumed to have significant structural and nonstructural 
damage that could result in its being red-tagged (occupancy prohibited) 
by inspectors after an earthquake. Significant repairs to most components 
of the building should be anticipated, and it may be more economical to 
replace the house rather than repair it. 

Comparisons of performance based on this approach are discussed in a number of sections in this 
guide. See Appendix A for a detailed description of the analysis. 

1.8 OTHER HAZARDS 

Although this guide has been written primarily to address the earthquake risk to a house, 
homebuilders and homeowners need to be aware of many other natural hazards. These hazards 
can also affect how a house should be designed and constructed as well where it should be sited 
and even whether the site should be used at all. 

1.8.1 Wind 

Since wind acts on a house by imparting horizontal loads similar to those of an earthquake, the 
seismic design and construction criteria contained in the IRC and further detailed in this guide 
will provide a significant level of protection to a houses structural system against wind loads. 
However, wind loads also act on the homes cladding, walls, and roof coverings, and the wind 
provisions of the IRC should be followed carefully (due to the differences in the load path for 
wind versus earthquake loads). While the IRC wind provisions are adequate, additional guidance 
is available and should be considered. This guidance includes the American Forest and Paper 
Association (AF&PA) Wood Frame Construction Manual and the Southern Building Code 
Congress International (SBCCI) Standard for Hurricane-Resistant Residential Construction 
(SSTD 10). 

1.8.2 Flooding 

Care should be taken to ensure that the home site is not in a flood hazard area. If there is any 
doubt in this regard, the local building code enforcement or floodplain management office should 
be consulted. As part of the National Flood Insurance Program (NFIP), FEMA publishes local 
flood hazard maps that delineate the extent of the flood hazard, using the Base Flood Elevation 
or BFE, which is flood elevation that has a 1 percent chance of occurring in any given year. As 
part of its agreement to participate in the NFIP in order to make federally-backed flood insurance 
available to its residents, a community adopts and enforces a series of flood design and 
construction requirements. Generally, these requirements mean that a home located in a flood 
hazard area must have its lowest floor and utilities elevated to or above the BFE. 


FEMA has available a series of publications on protecting homes from flood damage. Some of 
these publications are: 

 
Design Guidelines for Flood Damage Reduction (FEMA 15) 
 
Elevated Residential Structures (FEMA 54) 
 
Above the Flood: Elevating Your Flood-prone House (FEMA 347) 


For flood protection of existing houses, the reader is referred to Homeowner's Guide to 
Retrofitting: Six Ways to Protect Your House from Flooding (FEMA 312). All of the above 
publications are available from the FEMA Publications Center at 1-800-480-2520. Additional 
information is also available online at www.fema.gov. 

For guidance on the flood resistant construction provisions of the IRC, the reader is referred to 
the document Reducing Flood Losses Through the International Codes: Meeting the 
Requirements of the National Flood Insurance Program. This publication is available online at 
www.fema.gov as well as for a modest fee from the International Code Council 
(www.iccsafe.org) at (800) 786-4452. 

Above-code Recommendations: If a house is to be located in a flood hazard area 
and must be elevated to the BFE, it is recommended that the lowest floor of the 
house actually be elevated 2 or 3 feet above the BFE. This will result in a 
significantly lower insurance premium for the homeowner and will provide a 
significantly higher level of protection for a minimal additional cost. 

If a house is to be located just outside of a flood hazard area and is not required to 
meet a communitys flood regulations, it is recommended that the home still meet 
the communitys flood ordinances, such as not using a basement foundation, to 
provide an additional level of protection. Further, the homeowner is still 
encouraged to purchase flood insurance. Even if a home is just outside of a flood 
hazard area, it can still be subject to flooding as higher floods than the design flood can 
and have occurred. As a further measure of protection, flood insurance is available 
outside of the flood hazard area, usually for a very reasonable premium. 

1.8.3 Coastal Hazards 

Homesites along the coast are subject to two different hazards from hurricanes and other coastal 
storms such as Northeasters  high winds and flooding from storm surge. Storm surge is 
caused by the storms low pressure and winds pushing the water onto land. In the recent 
Hurricane Katrina, storm surge heights of almost 30 feet were measured along the coast of 
Mississippi. Coastal flood hazard maps similar to the flood maps described above are prepared 
by FEMA; however, the coastal flood hazard area is divided into two zones, the coastal high 
hazard zone or V-Zone which has significant wave heights and the regular flood hazard zone 
or A-Zone which does not. 


Coastal homes must be designed and built to withstand both wind and storm surge loads. This 
requires elevating the house above mapped storm surge elevations and ensuring that the house 
can resist the loads associated with hurricane force winds. Further, since storm surge waters can 
flow with considerable velocity which generates substantial hydrodynamic loads on the area 
below the elevated floor, this area must be built to allow water to flow through without placing 
additional loads on the structure. For this reason, any walls below the lowest floor must be made 
of breakaway construction. 

Note also that the coastline is a dynamic and ever-changing environment and is often subject to 
erosion. In choosing a home site, consider whether the property is subject to erosion, which 
could easily undermine and destroy the house in a relatively short period of time. Erosion maps 
showing the rate of change of the shoreline may be available locally. 

Above-code Recommendations: For additional protection of coastal homes, the 
following is recommended: 

 
Houses should be elevated 2 or 3 feet above the storm surge flood elevation 
provided on local maps. 
 
The area below the lowest floor should be kept open and free of as many 
obstructions as possible. 
 
Houses located in A-Zones but still near the coast (also known as Coastal A-
Zones) may still be subject to waves and storm surge and should be 
constructed to V-Zone criteria. 
 
Houses should be designed to resist damage from wind and debris. This 
includes limiting the size and amount of glazing and actually installing the 
storm shutters before an event. 
 
Building sites should be investigated for erosion potential prior to purchase 
and sites subject to erosion should be avoided. Houses built where erosion is 
possible should be located as far from the shoreline as possible. 
 
Natural features such a sand dunes and vegetation should be used as much as 
possible to protect houses from coastal forces. 


All of the above recommendations will provide better protection than currently offered by 
the IRC with little additional cost. 

The reader is encouraged to consult the FEMA Home Builders Guide to Coastal Construction: 
Technical Fact Sheet Series (FEMA 499). These fact sheets contain design and construction 
suggestions based on lessons learned in FEMAs investigations of past hurricanes and how they 
impacted residential construction. This document is available from the FEMA Publications 


Center as well as online at www.fema.gov. For more detailed engineering design information, 
the reader may wish to consult the Coastal Construction Manual: Principles and Practices of 
Planning, Siting, Designing, Constructing, and Maintaining Residential Buildings in Coastal 
Areas (FEMA 55). This is a large document (500 pages) and is available from the FEMA 
Publications Center in paper or on a CD-ROM. 

1.8.4 Tornado 

Building codes generally do not contain tornado provisions primarily because the probability of a 
specific house being hit by a tornado powerful enough to do damage are sufficiently remote that 
it is not thought to be economically justifiable to design for one. Therefore, further guidance on 
resisting the loads from a tornado does not exist. If protection of a homes occupants is a 
concern, the reader is referred to Taking Shelter from the Storm: Building a Safe Room Inside 
Your House (FEMA 320). This document is available from the FEMA Publications Center as 
well as online at www.fema.gov. 


Chapter 2 
EARTHQUAKE-RESISTANCE REQUIREMENTS 


This chapter explains the International Residential Codes (IRCs) general earthquake-resistance 
requirements as well as specific IRC requirements concerning load path and house configuration 
irregularities. One- and two-family detached houses of wood light-frame construction are 
addressed; however, the discussion is relevant to other materials of construction likely to be used 
for detached houses including light-frame cold-formed steel. 

2.1 IRC GENERAL EARTHQUAKE LIMITATIONS 

The variety of configurations used for houses is very wide and they are constructed of an equally 
wide variety of materials. IRC Section R301.2.2 imposes some limits on configuration and 
materials of construction for one- and two-family detached houses in Seismic Design Categories 
(SDCs) D1 and D2. These IRC limitations reflect the desire to provide equal earthquake 
performance for houses designed using the prescriptive IRC provisions and for those with an 
engineered design. Application of the prescriptive IRC requirements to houses that do not 
comply with the limitations can be expected to result in inadequate performance. 

IRC Section R301.2.2 earthquake limitations do not apply to one- and two-family detached 
houses in SDCs A, B, and C; however, townhouses in SDC C are required to comply. In 
general, design using the IRC is prohibited in SDC E, and the designer is referred to the 
engineered design requirements of the International Building Code (IBC) or other adopted code. 
There are, however, two methods by which houses designated as SDC E in IRC Figure 
R301.2(2) may be designed in accordance with IRC provisions: (1) if site soil conditions are 
known, IRC Section R301.2.2.1.1 permits determination of the Seismic Design Category in 
accordance with the IBC, which may result in a reduced SDC, and (2) if the restrictions of IRC 
Section R301.2.2.1.2 are met, houses located in SDC E may be reclassified to SDC D2 and 
designed using the IRC provisions. IRC Section R301.2.2.1.2 requirements dictate a regular 
house configuration. 

The limitations imposed by IRC Section R301.2.2 are as follows: 

 
Weight Limitations  For houses in SDCs D1 and D2, IRC Section R301.2.2.2.1 specifies 
maximum weights for the floor, roof-ceiling, and wall assemblies or systems. Because 
earthquake loads are proportional to the weight of the house, an upper bound on assembly 
weight provides an upper bound on earthquake loads. The specified maximum assembly 
weights relate directly to the weights considered in developing the IRC earthquake 
bracing provisions. The effect of the maximum weights is the exclusion of heavier finish 
materials when using the IRC provisions. Where heavier finish materials are to be used, 
an engineered design must be provided. 



 
House Configuration Limitations  For houses in SDCs D1 and D2, IRC Section 
R301.2.2.2.2 places limits on house configuration irregularities. These limits are 
discussed in detail in Section 2.3 of this guide. 
 
House System Limitations  Another scope limitation for houses in SDCs D1 and D2 is 
given in the combined requirements of IRC Sections R301.2.2.3 and R301.2.2.4. These 
sections provide limits for number of stories based on building system and limits for 
anchored stone and masonry veneer and masonry and concrete wall construction. Some 
of these requirements are addressed in Chapter 5 of this guide. 
 
Story Height Limitation  IRC Section R301.3 provides a scope limitation that is not 
related solely to earthquake loads but rather applies in all SDCs. This section limits story 
height by limiting the wall clear height and the height of the floor assembly. This limits 
both the lateral earthquake and wind loads and the resulting overturning loads. 


The IRC requires design in accordance with accepted engineering practice when the general 
earthquake limitations discussed above are not met (weight limitations, house configuration 
limitations, building system limitations, and story height limitations). Engineered design is 
addressed in Section R301.1.3. This section permits design to be limited to just the elements that 
do not conform to the IRC limitations. Increased assembly weight and story height will globally 
increase seismic and wind loads, generally making engineered design of the entire house 
necessary. Design of portions of the house is particularly applicable when an irregularity such as 
a cantilever, setback, or open front occurs. The extent of design is left to the judgment of the 
designer and building code official. When multiple irregularities occur, engineered design of the 
entire house may become necessary in order to provide adequate performance. The IRC requires 
that engineering design methods be used but does not specify whether this must be done by a 
registered design professional. State or local law governs who can perform the design; the reader 
is advised to check with the local building department for requirements. 

2.2 LOAD PATH 

For a house to remain stable, a load applied at any point on the 
structure must have a path allowing load transfer through each 
building part down to the building foundation and supporting soils. 
The term load path is used to describe this transfer of load through 
the building systems (floors, roof-ceilings, bracing walls). 

Basic Concept  To understand the concept of a load path, a house 
can be represented by the chain shown in Figure 2-1. The chain is 
pulled at the top and the load is transferred from one link to the next 
until it is transferred to the ground. If any link is weak or missing, 
the chain will not adequately transfer the load to the ground and 
failure will result. 

Figure 2-1 Chain 
illustrating the load path 
concept. 



Likewise, houses must have complete and adequate load paths to successfully transfer 
earthquake loads and other imposed loads to the supporting soils. 

Load Path for Earthquake and Wind Loads  The example house in Figure 2-2 will be used to 
discuss load path. The arrows provide a simplified depiction of earthquake or wind loads 
pushing horizontally on the house. Although wind and earthquake loads can occur in any 
horizontal direction, design procedures generally apply the loads in each of the two principal 
building directions (i.e., longitudinal and transverse), one at a time, and this discussion of 
loading will utilize that convention. 

Internally, the house has to convey loads from the upper portions of the structure to the 
foundation. For the example house, this is accomplished by transferring the loads through: 

 
The roof-ceiling system and its connections to the second-story bracing wall system, 
 
The second-story bracing wall system and its connections to the floor-ceiling system, 
 
The floor-ceiling system and its connections to the first-floor bracing wall system, and 
 
The first-story bracing wall system and its connections to the foundation, and 
 
The foundation to the supporting soil. 


The following discussion focuses primarily on the connections between the various building 
systems; the systems themselves are addressed only briefly here but are discussed in detail in 
later chapters of this guide. 

Figure 2-2 Lateral loads induced 
in a building due to wind or 
earthquakes. 


Roof-ceiling and Floor Systems  In the example house, the roof-ceiling system will resist 
horizontal earthquake loads proportional to the weight of the roof, ceiling, and top half of the 
second-story walls. The series of arrows at the right of Figure 2-3a depicts this load. The roof-
ceiling system deflects horizontally under the load and transfers the load to the supporting walls 
at both ends. The single arrows at the roof-ceiling system ends depict the reaction loads to the 
supporting walls. Within the roof-ceiling system, the load is carried primarily by the roof 
sheathing and its fastening (see Chapter 6 of this guide for discussion of roof-ceiling systems). 


Figure 2-3 Loading and deflection of roof-ceiling and floor systems. 


Similarly, the floor system will resist horizontal earthquake loads proportional to its weight and 
the weight of walls above and below. As shown in Figure 2-4b, it will deflect and transfer load 
to the supporting walls in much the same way as the roof-ceiling system. Again, the loading is 
carried by the floor sheathing and its fastening (see Chapter 4 of this guide for discussion of floor 
systems). 

Bracing Wall Systems  The roof-ceiling reaction load is transferred into the second-story 
bracing wall system as depicted by the arrow at the top of the wall in Figure 2-4a. The wall 
deflects under this load and transmits the load to the wall base and through the floor system to 
the first-story wall. Resistance to the wall load is provided by the wall sheathing and its 
fastening. 


Figure 2-4 Loading and deflection of bracing wall systems. 


The first-story bracing wall system resists loads from both the second-story wall and the second-
story floor system as depicted by the arrow at the top of the wall in Figure 2-4b. The wall 
deflects under this load and transmits the load to the wall base and the foundation. Again, 
resistance to the wall load is provided by sheathing and its fastening. Figure 2-5 provides an 
exploded view of the example house that illustrates the combination of roof-ceiling, floor, and 
wall systems and their connection to the foundation below. 


Figure 2-5 Load transfer between components in a building. 


Requirements for Connections Between Systems  As previously noted, a complete load path for 
earthquake loads requires not only adequate roof-ceiling, floor, and bracing wall systems but also 
adequate connection between these systems. Connections between systems must resist two 
primary types of loads: horizontal sliding loads and overturning loads. 

Load Path Connection for Horizontal Sliding  Figure 2-6 depicts the end wall at the left side of 
the house illustrated in Figures 2-2 through 2-5 and provides a detailed illustration of one 
possible path for horizontal loads from the roof assembly to the foundation. The left-hand 
portion of the figure shows a section through the end wall in which each of the links in the 
load path is given a number, H1 through H11, corresponding to a connection or mechanism used 
to transfer the loads. The right-hand side of the figure shows an elevation of the same wall and 
illustrates the deformation that will occur if adequate connection is not made. Table 2-1 at the 
end of this chapter provides a detailed summary of the load path connections for the Figure 2-6 
wall. 


Figure 2-6 Horizontal load path connections and deformations. 


Load Path Connection for Overturning  Because the horizontal loads are applied high on the 
house and resisted at the foundation, overturning loads develop in the bracing walls. Figure 2-7 
illustrates one possible load path for overturning loads. The left-hand side of the figure shows a 
wall elevation in which each of the links in the overturning load path is given a number, OV1 
to OV8, corresponding to locations with uplift or downward loads due to overturning. The right-
hand side of the diagram shows an elevation of the same wall that illustrates overturning 
deformations that will occur with earthquake loading from the left to the right. Uplift or tension 
occurs at one end of a wall simultaneously with downward force or compression at the other end. 

The IRC only specifies connections (hold-down straps or brackets) to resist overturning loads for 
a limited number of bracing alternatives. The connectors used to resist horizontal loads in most 
IRC designs will be required to resist overturning loads as well as horizontal loads. This is a 
major difference between prescriptive design and engineered design in which resistance to 
overturning loads must be explicitly provided. The IRC requires use of hold-down devices in 
Section R602.10.6 for alternative braced wall panels; in Section R602.10.11, Exception 2, for 
SDCs D1 and D2 when braced wall panels are not located at the end of braced wall lines; and in 
Section R703.7 when stone or masonry veneer is used. Table 2-2 at the end of this chapter 
provides a detailed summary of the load path connections for the Figure 2-7 wall. 


Figure 2-7 graphically depicts use of straps to resist overturning loads; however, overturning can 
be resisted by connections employing such other devices as bolts, nails, or hold-down brackets. 
Because different device types may deform differently under load, it is preferable to use the same 
type of device for an entire story level. Variations in connector type from story to story are 
acceptable. When considering overturning in an engineered design, it is customary to include the 
effect of dead load (the actual weight of the house) in reducing uplift and overturning loads; 
however, this level of calculation detail is beyond the scope of the IRC provisions. Hold-downs 
should be provided wherever they are required by the IRC or recommended by this guide, 
irrespective of dead load. 

Figure 2-7 Overturning load path connections and deformations. 


System and Connection Strength and Stiffness Requirements  The roof-ceiling, floor, and 
bracing wall systems are the basic members resisting earthquake loads. Adequate earthquake 
performance of a house relies on: 

 
Adequate strength of roof-ceiling, floor, and bracing wall systems, 
 
Adequate stiffness of roof-ceiling, floor, and bracing wall systems to limit deformation, 
 
Adequate connection between systems to provide a functional load path, and 
 
Adequate connection to the foundation. 


For most houses, it is generally anticipated that bracing wall system behavior will have more 
influence on the behavior of the house than the roof-ceiling or floor systems. Further, it is 
anticipated that wall behavior at lower stories will generally be more critical than at upper stories 
due to larger earthquake loads. 

Accumulation of Loads in Systems and Connections  Wind and earthquake loads increase or 
accumulate towards the bottom of the house. This is particularly applicable to loads in the 
bracing wall systems and their connections for horizontal loads and uplift and downward loads 


due to wall overturning. For example, overturning connections must be sized to resist all of the 
loads generated above the connection location. In a two-story house, the second-floor uplift 
connection, such as OV1 in Figure 2-7, will need to resist loads from the second story. The first-
story uplift connection, such as OV5 in Figure 2-7, will have to resist the uplift loads from the 
second story plus the additional uplift from the first story. It can generally be expected that OV5 
will need to resist a load two to three times that resisted by OV1. Downward loads at the 
opposite ends of walls and horizontal loads accumulate similarly. Although this accumulation of 
load often is missed in design for wind and earthquake loads, it is important and should be 
explicitly considered when connections are being selected. 

Above-code Recommendation: For overturning loads, the use of hold-down 
connectors (brackets, straps, etc.) is recommended as an above-code measure in order 
to improve load transfer and thereby decrease damage. For most of the basic IRC wall 
bracing types, the provisions rely on building weight and the fasteners resisting horizontal 
loads to also resist overturning. Wall component testing, however, indicates that this 
method is not as reliable in resisting overturning as the use of positive anchorage devices. 

2.3 HOUSE CONFIGURATION IRREGULARITIES 

A houses configuration (shape) significantly affects its response to wind and earthquake loads. 
This section discusses the concepts and the IRC provisions for irregular house configurations. 

Ideal Earthquake-Resistant House Configuration  For earthquake resistance, the ideal house 
would have: 

 
A simple rectangular shape, 
 
Bracing walls distributed uniformly and symmetrically through the house, 
 
No large concentrations of weight, 
 
Bracing walls at upper stories located immediately above walls in stories below, 
 
Wall bracing lengths that increase in lower story levels compared to the story above, 
 
No split-levels or other floor level offsets. 


A version of this ideal house is shown in Figure 2-8. This ideal configuration results in loads 
and deformations being uniformly distributed throughout the house, which permits resisting 
elements to contribute equally to earthquake resistance. With good distribution of bracing walls, 
earthquake loads can be resisted very close to where they are generated (by house weight), which 
reduces the need for transfer of earthquake loads through floor and roof systems to other portions 
of the house. This helps reduce the poor performance that often results when such transfers are 
required. 


Figure 2-8 Ideal building 
configuration for earthquake 
resistance. 


Deviations from Ideal Configuration  Deviations from the ideal configuration are called 
configuration irregularities. As houses deviate from the ideal configuration, loads and 
deformations are concentrated, which causes localized damage that can result in premature local 
or even complete failure of the house. While the ideal configuration is attractive from the 
standpoint of earthquake resistance, houses with irregularities are much more popular and 
common than those without. Large open great rooms and walls of nothing but windows are 
examples of typical irregular configurations. 

House Irregularity Concepts  House irregularities often are divided into two types: plan 
irregularities and vertical irregularities. 

Plan irregularities concentrate earthquake load and deformation in a particular area of a house. A 
common cause is a center of mass (building weight) at a location different from the center of the 
resisting elements (bracing walls). This can be due to non-uniform mass distribution, nonuniform 
distribution of bracing walls, or an irregular house plan. Two common examples are a 
house with one exterior wall completely filled with windows with no wall bracing provided 
(Figure 2-9) and a house with a large masonry chimney at one end. Houses with plan 
irregularities generally experience rotation in addition to the expected horizontal deformation. 
House rotation, as illustrated in Figure 2-10, magnifies the displacements, resulting in increased 
damage. 


Figure 2-9 Irregular building configuration with open front (no second-story bracing walls 
supporting roof on right-hand portion). 


Figure 2-10 Rotational response and resistance to torsion. 



Other common plan irregularities occur in T- and L-shaped houses that concentrate loads at the 
corners where the different wings of the house connect. Figure 2-11 illustrates the concentration 
of loads in an L-shaped house. The noted location of load concentration is where damage would 
be anticipated. Adequate interconnection of the house wings is required for good performance. 
Figure 2-12 illustrates damage due to inadequate interconnections. Without additional 
consideration, poor performance and additional damage or failure would be expected for 
structures with such plan irregularities. 


Figure 2-11 Concentration of loads and possible damage and failure due to irregular plan. 


Figure 2-12 Roof-level damage to building with a T-shaped plan. The damage at the roof 


corner (right-hand side) is due to stress concentrations where the wings of the building connect. 

Vertical Irregularities  Vertical irregularities concentrate damage in one story of a multistory 
house. This occurs when the stiffness or strength of any one story is significantly lower than that 
of adjacent stories. When the stiffness of a story is significantly lower, the deformations 
associated with the earthquake loads tend to be concentrated at that soft story as illustrated in 
Figure 2-13. The configuration on the left (normal) provides a uniform stiffness in each story, 
while the configuration in the middle illustrates a soft story with the deflections being 
concentrated in that story. If the displacements get large enough, they can cause complete failure 
of the soft story as illustrated in the configuration on the right. 

The CUREE-Caltech Woodframe Project found that many two-story light-frame residential 
houses exhibit soft-story behavior to some extent because the first stories feature relatively large 
window and door openings and fewer partitions (less bracing) than the second stories where the 
bedrooms and bathrooms are located. Soft first-story behavior also was observed in the analysis 
of the model house used for this guide. Figure 2-14 provides an exaggerated illustration of 
deformation concentrated in the first story of the model house. Cripple walls around the 
perimeter of a crawlspace also can result in soft-story behavior. A weak story can cause damage 
or failure to be concentrated in that story if earthquake loading approaches the story strength. 


drift 

drift 
normal soft story 
Figure 2-13 Deformation pattern due to soft-story behavior. 
Deformation is concentrated in the first story. 


overstress 

Figure 2-14 Change in deformation patterns associated with soft-story irregularity. 


Soft and weak stories, although technically different, can occur at the same time. Combined 
soft- and weak-story house irregularities have been the primary cause of story failure or collapse 
and earthquake fatalities in wood light-frame houses in the United States. To date, story failure 
has only been observed in houses that would not meet the current IRC bracing requirements or 
would fall outside the scope of the IRC. Figure 2-15 illustrates soft- and weak-story behavior in 


a single-family house. Figure 2-16 illustrates the loss of a soft and weak story in an apartment 
building. 

Figure 2-15 House experiencing soft- and 
weak-story behavior in the Cape Mendocino 
earthquake. Deformed shape of garage door 
opening shows approximately 6 inches of 
horizontal movement in the first story. 


Figure 2-16 Apartment building 
experiencing soft- and weak-
story behavior in the Northridge 
earthquake. The two balcony 
rails with little separation at the 
center of the photo illustrate the 
collapse of the lowest story. 


IRC Approach to House Configuration Irregularities  The IRC incorporates two approaches to 
limitation of irregularities. The first and more explicit approach is found in IRC Section 
R301.2.2.2.2, which directly limits a series of irregular house configurations. Specific 
exceptions allow inclusion of less significant irregularities within IRC prescriptive designs. For 
one- and two-family detached houses, these provisions are applicable in SDCs D1 and D2,. The 
IRC limitations on irregularities are derived from those required for engineered buildings by the 
IBC and NFPA 5000. Table 2-3 at the end of this chapter provides detailed discussion of the IRC 
Section R301.2.2.2.2 limits. Additional discussion may be found in the commentary to the 2003 
NEHRP Recommended Provisions. 


The second and less obvious approach is the IRC requirement for distribution of wall bracing. 
Along with braced wall lines at exterior walls, interior braced wall lines must be added so that 
the spacing between wall lines does not exceed 35 feet per IRC Section R602.10.1.1 (adjustable 
to 50 feet by an exception) or 25 feet in SDC D1 or D2 per Section R602.10.11. Maximum 
spacing between braced walls in a braced wall line is also regulated. Houses perform better if 
the walls are distributed throughout, rather than concentrated in limited portions of the structure. 
This allows the earthquake load to be resisted local to the area where it is developed. Higher 
earthquake loads develop when loading must be transmitted to bracing walls in another portion 
of the house. Good distribution of bracing walls helps to mitigate the adverse effects of many 
irregularities. 

Stepped Cripple Walls  IRC Section R602.11.3 and Figure R602.11.3 provide specific detailing 
to reduce the effect of a concentration of load due to stepped concrete or masonry foundation 
walls. As illustrated in Figure 2-17, a direct tie to the tallest foundation segment provides 
uniform stiffness along the wall line. 

Figure 2-17 Detailing required for stepped foundation walls (ICC, 2003). 



Above-code Recommendations: The concentration of damage as a result of irregular 
building configuration is equally applicable to earthquake loading in all SDCs and to wind 
loading. To date, the IRC has limited application to areas of high seismic risk. Use of the 
irregularity limitations in SDCs A, B, and C and for wind loading will contribute to 
better building performance and is recommended as an above-code measure. 

Experimental and analytical results show that typical residential buildings are prone to soft-
story irregularities. Stiffening and strengthening of soft first stories is recommended as 
an above-code measure to reduce deformation and resulting damage. Approaches to 
increasing first-story strength and stiffness include: increasing the length of wood structural 
panel sheathing; decreasing center-to-center spacing on sheathing nailing (although spacing 
of less than 4 inches is not recommended due to the possibility of splitting the framing and 
because other weak links could develop); using perforated shear wall construction (also called 
continuously sheathed wall); using segmented shear wall construction (also called wood 
structural panel sheathed with overturning anchors); or providing wood structural panel 
sheathing on both faces of bracing walls. See Chapter 6 of this guide for detailed descriptions 
of these measures. 

Another common location of soft or weak stories is cripple walls. This is particularly true 
where perimeter cripple walls have no inside face sheathing and under-floor basement areas 
have few or no interior walls. Although the IRC has increased bracing wall lengths beyond 
those required by earlier codes, further increasing length and providing interior braced wall 
lines will help to limit deformation and damage. Additional length of wood structural 
panel sheathing at the building perimeter and interior is recommended as an above-code 
measure to limit cripple wall deformation. 


Table 2-1 Load Path Connections for Horizontal Sliding 


Table 2-2 Load Path Connections for Overturning 
 

Table 2-3 IRC House Configuration Irregularities.  


Chapter 3 
FOUNDATIONS AND FOUNDATION WALLS 


This chapter discusses foundations and foundation walls constructed using the two most common 
foundation materials  concrete and masonry. Although the IRC permits the use of treated wood 
for foundations and foundation walls and insulating concrete form (ICF) for foundation walls, 
this guide does not cover those materials other than to inform the reader that wood foundations in 
Seismic Design Categories (SDCs) D1 and D2 require engineering design and many of the 
reinforcement requirements for concrete also apply to ICF (see Chapter 4 of the IRC for more 
information on the use of these materials). The subject of frost protection of foundations also is 
not discussed in this guide but, where required by the code (see IRC Section R403.1.4.1) or local 
regulations, foundations must either extend below the frost line or be protected from frost using 
approved methods. 

3.1 GENERAL FOUNDATION REQUIREMENTS 

Foundations are the interface between a house and the supporting soils. Many issues must be 
considered when selecting a foundation system including site topography, soils conditions, 
retaining requirements, loading from the house above, frost depth, and termite and decay 
exposure. Foundations primarily provide support for vertical gravity loads from the weight of a 
house and its contents, but they also provide resistance to horizontal sliding resulting from 
earthquake ground motions and must resist vertical loads at the ends of braced walls. 
Regardless of Seismic Design Category, all houses require a continuous foundation extending at 
least 12 inches below undisturbed soil along all exterior walls as shown in Figure 3-1. 

Figure 3-1 Perimeter foundation with separately placed footing and stem wall. 



When earthquake ground motion occurs, the resulting ground movements, velocities, and 
accelerations are imparted to the foundation and, in turn, transferred to a house or other building. 
How well the house performs during an earthquake depends on the foundation being able to 
provide: 

 
Continued vertical support, 
 
Friction and passive bearing at the soil-to-foundation interface to minimize movement 
and damage, 
 
Anchorage at the foundation-to-house interface to minimize movement and damage, and 
 
Strength and stiffness sufficient to resist both horizontal loads and vertical loads resulting 
from racking and overturning of bracing walls within the house. 


The foundation of the house must resist the sliding and overturning actions associated with an 
earthquake. These two actions are illustrated in Figures 3-2 and 3-3. The soil surrounding a 
foundation can resist sliding using a combination of friction along the bottom and bearing along 
the sides of the foundation; therefore, a wider and deeper foundation provides greater friction 
and greater bearing resistance than a shallow and narrow foundation. The whole overturning 
action illustrated in Figure 3-3 is resisted at the foundation in two ways. The portion of the 
foundation being pushed downward will bear against the soil below so a wider footing will 
provide more surface area to resist that downward load. At the uplift end of the foundation, the 
weight of any soil located above a footing helps to resist the loads trying to pull the foundation 
out of the ground; therefore, a deep inverted T-shaped foundation will provide greater resistance 
to uplift than a shallow footing or than a foundation having a shape that avoids having any soil 
above the top of the footing. 


Figure 3-2 Sliding action resisted by foundation. 


Figure 3-3 Overturning action resisted by foundation. 


The IRC discusses foundations (footings and stem walls) and foundation walls separately and 
contains requirements for those elements based on the materials used for their construction. This 
guide chapter is similarly organized and addresses foundations placed monolithically with a slab 
on grade and foundations having a combination footing and stem wall as illustrated in Figure 
R403.1(1) of the IRC. Foundation systems such as pilings, drilled piers, and grade beams require 
the involvement of a licensed design professional and are not discussed in this guide. 

IRC foundation wall provisions have evolved from similar provisions in the CABO One- and 
Two-Family Dwelling Code and Standard Building Code. Neither the IRC nor the other building 
codes give definitive guidance on when to use the foundation wall provisions of IRC Section 
R404 rather than the footing stem wall provisions of IRC Section R403. The 2000 IRC 
Commentary notes that the foundation wall provisions are primarily for masonry and concrete 
basement walls. The provisions of IRC Section R404 become mandatory for wall heights of 5 
feet and greater and for walls retaining unbalance fill of 4 feet or greater. For wall heights and 
unbalanced fill heights less than this, there are few practical differences between the foundation 
and foundation wall provisions. 

IRC Section R403.1.2 contains a general rule applicable to buildings located in SDCs D1 and D2 
that requires interior braced wall lines to be supported on a continuous foundation when the 
spacing between parallel exterior wall lines exceeds 50 feet. However, IRC Section R602.10.9 
contains a slightly more restrictive requirement. For a two-story house in SDC D2, a continuous 
foundation is required below all interior braced walls, even when the distance between exterior 
walls does not exceed 50 feet, unless three additional conditions can be met. Those conditions 
are: 

 
The distance between braced wall lines does not exceed twice the building width 
measured parallel to the interior braced wall line; 
 
In houses having either a crawl space or basement, cripple walls cannot exceed 4 feet in 
height; and 



 
In houses having a crawl space or basement, first-floor interior braced walls are 
supported on double joists, beams, or blocking as shown in Figure 3-4. 


Figure 3-4 Interior braced wall 
on floor framing. 


Above-code Recommendation: The main consequence of not providing a continuous 
foundation below a first-story interior braced wall line is that the floor must be strong enough 
to transfer earthquake loads from that interior location to a parallel perimeter foundation. 
This transfer of lateral loads can be accomplished using a wood-framed and -sheathed floor 
system, but this solution will definitely impart additional stresses in the floor and into cripple 
walls at the perimeter that would not occur if a foundation was provided below the interior 
braced wall line. Vertical loads due to the overturning loads in the braced wall segments also 
must be transferred to the perimeter foundation through bending action. While the doubling 
of the floor joist is an improvement, supporting these walls directly on a continuous 
foundation is recommended to achieve above-code performance. 

For slab-on-grade construction in SDCs D1 and D2, when the conditions described above require 
an interior braced wall line to have a foundation, the foundation depth along that interior wall 
must be at least 18 inches below the top of the slab as shown in Figure 3-5. 

Figure 3-5 Foundation requirements for interior braced wall line on slab-on-grade construction. 



3.2 CONCRETE FOUNDATIONS 

Regardless of Seismic Design Category, the minimum specified concrete strength for 
foundations (and foundation walls) is 2,500 pounds per square inch (psi) with higher strength 
necessary when a foundation is exposed to the weather and the house is located in a moderate or 
severe weathering probability area as shown in IRC Figure R301.2(3). Specifying 2,500 psi 
refers to a measure of the concretes compressive strength. To enable a concrete foundation to 
resist all of the possible loads to which it may be exposed, compressive strength needs to be 
complimented with tension capacity. Since concrete is unable to resist tension stresses without 
cracking, steel reinforcing bars are added to resist tension. Reinforcing is particularly valuable 
when resisting cyclic earthquake loads because, within the span of a few seconds, the loads may 
start by causing compression and then reverse to cause tension in the same location. 

IRC Section R403.1.3 specifies minimum reinforcement of concrete footings located in SDCs D1 
and D2. Separate subsections within IRC Section R403.1.3 address reinforcing of foundations 
consisting of a footing and a stem wall and reinforcing of the footing along the perimeter of a 
slab-on-grade. IRC Section R403.1.3 also contains an exception that allows omitting the 
longitudinal reinforcing in concrete footings for houses that are three stories or less in height and 
constructed with stud walls, regardless of the Seismic Design Category. 

Above-code Recommendation: To obtain above-code performance in SDC C, it is 
recommended that the minimum foundation reinforcing requirements for SDCs D1 and 
D2 be used. This added reinforcing will provide better footing performance whether it is 
resisting earthquake loads or loads induced by differential soil settlement, expansive soils, or 
frost heave. 

Typically, the bottom portion of a concrete footing must have one horizontal No. 4 reinforcing 
bar located 3 inches up from the bottom of the concrete (clear from the soil along the bottom of 
the footing). When a foundation consists of both a footing (horizontal foundation segment) and 
short stem wall (vertical foundation segment), two No. 4 continuous horizontal reinforcing bars 
are required  one in the bottom of the footing and one near the top of the stem wall as shown in 
Figure 3-6. 


Figure 3-6 Recommended minimum reinforcement for concrete footings and stem walls. 


Above-code Recommendations: When horizontal reinforcing bars are used in foundations, 
they need to be continuous to perform their intended function. The IRC does not provide any 
specific guidance on reinforcing continuity; therefore, the following above-code 
recommendation is derived from the basic standard for concrete construction (ACI 318-02, 
2003). Where two or more pieces of reinforcing steel are used to provide continuous 
horizontal reinforcing, the ends of the bars should be lapped to provide continuity. The 
minimum recommended lap for No. 4 bars is 24 inches and for No. 5 bars is 30 inches. 
As shown in Figure 3-7, horizontal bars terminating at corners of perimeter foundations 
and where an interior foundation intersects a perimeter foundation should have a 
standard 90-degree hook of 8 inches for No. 4 bars or 10 inches for No. 5 bars. 

Figure 3-7 Above-code horizontal reinforcing lap at corners and intersections. 


In SDCs D1 and D2, No. 4 vertical reinforcing is required at 48-inch maximum spacing if a 
construction joint (also called a cold joint) occurs between the footing and the stem wall. This 
often is referred to as a two-pour foundation because the concrete for the footing and the 
concrete for the wall are poured at separate times. These vertical bars (also called dowels) 
must extend a minimum of 14 inches into the stem wall and be hooked at the bottom into the 
footing as shown in Figure 3-6. These dowels provide a very important connection because, 
without them, earthquake loads can cause sliding to occur along the joint between the two 
separate concrete placements. Sliding along a similar construction joint between a slab-on-grade 
and a separately poured footing below the slab edge occurred during the 1994 Northridge 
earthquake and caused severe damage in houses located in Simi Valley, California. 

When a house has a concrete slab-on-grade with a thickened edge forming its perimeter 
foundation (also called a turned-down slab edge), one No. 4 horizontal reinforcing bar is 
required in the top and bottom of this footing as shown in Figure 3-8. The exception to this is 
when the slab and footing are poured at the same time; in this case, a single No. 5 bar or two No. 
4 bars located in the middle third of the combined slab and footing depth may be used. For slab-
on-grade construction in SDCs D1 and D2, interior bearing walls and interior braced walls 
required to have a continuous foundation must have the concrete slab thickened to 18 inches to 
form a foundation as shown in Figure 3-5. 

Above-code Recommendation: Although the IRC is basically silent on how to reinforce 
these interior thickened slab foundations, it is recommended that they should be 
reinforced as described above for the foundation along the slab perimeter. Horizontal 
reinforcing in slab-on-grade foundations should be continuous as described earlier. 


Figure 3-8 Above-code use of vertical dowels to connect a slab-on-grade to a 
separately poured footing. 


Above-code Recommendation: When a slab-on-grade is poured separately from the footing 
below, the IRC does not specify any vertical reinforcing across this joint. When this 
condition occurs, however, the possibility of earthquake loads causing sliding along that joint 
is very real. The basic provisions of ACI 318-02, Sec. 11.7.9, for concrete require that all 
construction joints be provided with a mechanism for transferring loads through the joint. 

Therefore, it is recommended that the same amount of vertical reinforcing required for 
a separately poured stem wall and footing condition (No. 4 at 48 inches on center) be 
installed to connect a slab to a separately poured footing as shown in Figure 3-8. In 
addition the surface of the construction joint should be cleaned to remove loose debris 
prior to placing the concrete slab. 

Above-code Recommendation: Anchor bolts connecting a wood sill plate to a foundation 
also must be installed in the foundation or stem wall. Although it is possible to install 
anchor bolts during the placement of the foundation concrete before it has hardened 
(also called wet setting), this is not recommended. Anchor bolts should be installed 
and secured so that they will not move prior to placing the concrete. The reason is that 
a wet setting can create a void in the concrete adjacent to the bolt. Anchor bolts, whether 
they have heads or hooks on the embedded end, will either make a hole or carve a slot as 
they are pushed into the wet concrete. That hole or slot forms a void over the entire length 
of the bolts embedment. This void prevents the bolt from completely bearing against the 
surrounding concrete and could result in movement of the bolt when subjected to 
earthquake loads. Additional anchor bolt requirements are provided at the end of this 
chapter. 

3.3 MASONRY FOUNDATIONS 

Masonry foundation requirements are generally more dependent upon the Seismic Design 
Category of the site than concrete foundation requirements. Solid clay masonry or fully grouted 
concrete masonry and rubble stone masonry may be used for foundations in SDCs A, B, and C; 
however, rubble stone masonry is not allowed in SDCs D1 and D2 due to its relatively low 
strength and stability. 

Above-code Recommendation: Because of their relatively low strength and stability, 
rubble stone foundations are not recommended for use in SDC C as an above-code 
measure. 


Above-code Recommendation: Like concrete, a masonry foundation with a stem wall must 
have at least one No. 4 horizontal bar in the bottom of the footing and one No. 4 bar near the 
top of the stem wall. This horizontal reinforcing needs to be continuous just as it does in 
concrete construction. Therefore, the above-code recommendations for lap splices are 24 
inches for No. 4 bars and 30 inches for No. 5 bars with hook extensions at corners as 
shown in Figure 3-7. 

In SDCs D1 and D2, masonry stem walls also must have vertical reinforcing and must be grouted 
as required by IRC Sections 606.11.3 and 606.12. (See Chapter 6 of this guide for a more 
detailed discussion of grouting in masonry wall construction.) For a masonry stem wall 
supported on a concrete footing, the vertical reinforcing must be one No. 4 bar at 48-inches 
maximum spacing, extending into the footing with a standard hook at the bottom, similar to what 
is shown in Figure 3-6. This is necessary to prevent sliding along the bottom mortar joint 
between the concrete footing and the masonry stem wall. 

Above-code Recommendation: In order to prevent sliding along the bottom mortar 
joint between the concrete footing and the masonry stem wall, it is recommended that 
masonry stem walls in SDC C also use the vertical reinforcing required in SDCs D1 and 
D2. 

3.4 FOOTING WIDTH 

Footing width is not dependent upon Seismic Design Category but instead is solely based on 
vertical load considerations. The minimum width for a concrete or masonry footing is dependent 
on the load bearing capacity of the soil measured in pounds per square foot (psf) and the number 
of stories and the weight of the wall it supports. For instance, when brick veneer is installed, this 
added weight requires a wider footing. 

The minimum soil bearing capacity considered by the IRC is 1,500 psf. In this case, the 
minimum footing width is 12 inches but it can increase to as much as 32 inches for a three-story 
house with brick veneer as shown in Figure 3-9. 

Bearing capacity is determined based on the soil classification determined for the site. Soil 
classifications and corresponding bearing capacities are listed in IRC Table R401.4.1 at the 
beginning of IRC Chapter 4. The soil classification system is described in IRC Table R405.1. 
There are four distinct groups of soils that comprise a total of 15 separate soil classifications 
ranging from well graded gravels as the best and peat as the worst. Accurate determination of 
soil bearing capacity requires correct classification of the soil at the building site and may require 
the expertise of a soils engineer or geologist. However, most building departments have 
determined the soil classification for most of the sites within their jurisdiction. Accurate 
determination of the correct site soil classification is important not just for determining the 
minimum footing width but also for determining the minimum reinforcing required for concrete 
or masonry foundation walls. 


When a footing is constructed with a width at the bottom that is greater than the thickness of its 
stem wall or a width greater than the thickness required for a foundation wall (e.g., an inverted-T 
shape), the minimum thickness of the footing is 6 inches. IRC Section R403.1.1 also specifies 
both a minimum and maximum width for the footing portion of this inverted-T foundation. As a 
minimum, the projection of the vertical face of the footing must be at least 2 inches beyond each 
vertical face of the stem wall or foundation wall. The maximum projection may not exceed the 
thickness of the footing. These dimensional requirements for an inverted-T foundation are 
illustrated in Figure 3-9. These minimum and maximum projection limits of the footing beyond 
the stem wall are consistent with ACI 318 design of plain concrete footings. Based on those 
requirements, an L-shaped footing should not be permitted unless its dimensions and reinforcing 
are designed to account for the eccentricity of the vertical load on the footing. 

Figure 3-9 Inverted -T footing dimensions. 


3.5 SPECIAL SOIL CONDITIONS 

On some sites, such as those in marshy areas or bogs, the bearing capacity of the soil may be 
known or suspected to be less than 1,500 psf. In such a case, the IRC requires a soil 
investigation and report to determine the actual bearing capacity of the soil and to define an 
appropriate footing width and depth. Soil testing also is necessary when it is likely that existing 
soil has shifting, expansive, compressive, or other unknown characteristics. Local officials often 
are aware of such conditions in locations where previous buildings have been constructed and 
may have maps identifying such areas and/or special rules that apply to foundation construction 
in such areas. When existing soil data are not available, such as on previously undeveloped sites, 
it is always prudent to obtain tests to validate bearing capacity and to determine if expansive 
soils are present. 

When expansive soils that exhibit large changes in volume (usually in response to changes in 
moisture content) are encountered, foundations must be designed in accordance with IBC 


Chapter 18 because the prescriptive requirements of the IRC assume no such special conditions 
are present. Methods to address expansive soil include foundation designs to resist the stresses 
caused by the soil volume changes that are likely to occur, isolation of the foundation, removal 
of the expansive soil, or stabilization of the soil by chemical, dewatering, presaturation, or other 
methods. Failure to identify and adequately compensate for differential movements caused by 
expansive soils can result in excessive stresses on the foundation causing cracking of even 
reinforced concrete or masonry foundations. Foundation movement induced by expansive soil 
also can result in differential movement of the houses wood framed walls that can crack brick, 
gypsum wallboard, and stucco finishes. Movement of the walls can create stresses that loosen 
the nailed connections of wall sheathing used to provide lateral bracing. These effects can, in 
turn, weaken a buildings earthquake resistance so it is very important to address expansive or 
other special soil conditions to limit differential foundation movement. 

3.6 FOUNDATION RESISTANCE TO SLIDING FROM LATERAL LOADS 

Foundations are the final link in the load path within a building to transfer the earthquake loads 
to the ground. At the foundation level, the combined lateral loads from the entire building are 
attempting to push the building laterally. To resist movement, the foundation pushes against the 
soil that surrounds it. Consequently, footing width and depth are factors that determine the 
resistance that can be provided by the foundation to earthquake ground motions. This is because 
footing width determines the horizontal surface area of the bottom of a foundation in contact 
with the ground and the depth determines the vertical surface area bearing against the soil on 
either side. These surface areas provide resistance to sliding through a combination of friction 
and bearing against the soil as shown in Figure 3-10. Therefore, a wider or deeper foundation 
will be capable of resisting greater lateral loads than a narrower or shallower foundation. 
Similarly, the sliding resistance of a slab-on-grade house will be greater than that of one having 
only a perimeter foundation due to the added frictional surface area provided by the underside of 
the slab. 

Figure 3-10 Lateral resistance provided by foundation. 



Above-code Recommendation: For houses with the first floor located above a crawl space 
rather than a basement, IRC Section 408.5 allows finished grade under the floor to be located 
at the bottom of the footing except where the groundwater table is high, surface drainage is 
poor, or the area is prone to flooding. However, when the finished grade is located at the 
bottom of the footing, the vertical face of the footing on that side does not bear against soil. 

Therefore, as an above-code measure, in SDCs D1 and D2, it is recommended that crawl 
space perimeter footings have their entire depth below the finished grade of the crawl 
space. Embedment along both vertical faces of the footing provides additional bearing 
surface area to resist sliding loads perpendicular to the exterior walls. 

Above-code Recommendation: To aid in providing sliding resistance, the bottom of 
footings should be level. The IRC does allow the bottom of footings to be sloped at a rate of 
not more than 1 foot vertical in 10 feet horizontal. When this maximum slope along the 
bottom of the footing cannot be met, the bottom must be stepped. Similarly, the top of 
foundations must be level but can also be stepped. As an above-code measure when steps 
are used, the horizontal reinforcing should be bent to extend through the steps as shown 
in Figure 3-11. 

Figure 3-11 Above-code stepped foundation reinforcing detail. 



Above-code Recommendation: Special consideration also must be given to sites where the 
ground slopes upward or downward beyond the limits of the perimeter foundation to ensure 
that adequate resistance to foundation sliding and settlement are provided and to protect 
against the effects of drainage, erosion, and shallow failures of the sloping surface. 
Prescriptive rules for setbacks from either an ascending or descending slope are given in IRC 
Chapter 4. Smaller (or greater) setbacks may be approved (or required) based on a soil 
investigation report prepared by a qualified engineer. Because landslides can be triggered 
by earthquakes, any building site having natural or man-made sloping terrain above or 
below should be thoroughly evaluated for landslide potential even when the prescriptive 
setbacks of the code are met. 

3.7 SPECIAL CONSIDERATIONS FOR CUT AND FILL SITES 

A hillside site can result in foundations being supported on soils having very different bearing 
capacities. Figure 3-12 shows a situation where a portion of the foundation is supported on rock 
and the other side is supported on a fill that extends above the existing rock grade. This 
condition often occurs when soil is removed (cut) from the high side of a lot and the lower side 
has fill material installed to create a level building pad. Although the IRC requires all fill soils to 
be designed, installed and tested in accordance with accepted engineering practice, there is no 
specific guidance given in the IRC regarding what should be addressed. Site-specific guidance 
on the design and placement of fill material is a particularly important concern in high seismic 
areas. 


Figure 3-12 Foundation supported on rock and fill. 

Studies of damage to houses located on cut and fill sites as a result of the 1971 San Fernando 
(McClure, 1973, Slosson, 1975) and the 1994 Northridge (Stewart, et al., 1994 and 1995) 
earthquakes document the consequences of improperly installed and compacted fill. Figure 3-13 
shows an example of the type of damage that occurred to slab-on-grade houses during the 
Northridge earthquake. Generally, these sites experienced settlement and extension of the fill 
portion of the site and vertical displacement along the line of transition from cut to fill. 


Above-code Recommendation: To meet the intent of the IRC requirement for all fill 
soils to be designed, installed and tested in accordance with accepted engineering 
practice, it is recommended that the reporting requirements conform to those specified 
in IBC Section 1803.5. 

Figure 3-13 Example of damage 
caused in building on cut and fill 
site. 


Above-code Recommendation: Even with proper installation and compaction of engineered 
fills, earthquakes are expected to result in some differential settlement and consequent 
damage when a house has a portion of its foundation on a cut pad and other portions on fill. 

Therefore, in SDCs C, D1 and D2, sites that require a cut and fill approach should be 
avoided, particularly if a slab-on-grade foundation is used. Where cut and fill grading 
of a site is unavoidable, increased levels of quality control should be used to ensure the 
optimum installation of the fill, and foundations should be designed to either 
accommodate or resist the expected settlement. 

3.8 FOUNDATION WALLS 

Foundation walls are typically basement walls but also include foundation stem walls that extend 
from the top of a footing to the bottom of a wood framed floor and enclose a crawl space as 
shown in Figure 3-6. Depending on the difference in ground level on each side of a foundation 
wall, certain minimum requirements for reinforcing and wall thickness will apply. Foundation 
wall construction is dependent to some extent on Seismic Design Category, particularly for 
SDCs D1 and D2 sites. 


Foundation walls can be constructed of concrete, masonry, or preservatively treated wood or by 
using insulating concrete form (ICF) systems. As mentioned earlier, treated wood systems are 
not discussed in this guide. ICF foundation walls are likewise not discussed because IRC Section 
R404.4.1 limits use of ICF foundation walls to SDCs A, B and C; however, future IRC editions 
may allow adequately reinforced ICF foundation walls in SDCs D1 and D2 if proposed code 
changes on this subject are approved. Rubble stone masonry foundation walls also are not 
permitted in SDCs D1 and D2 for reasons similar to those that prohibit this materials use as a 
footing in SDC D1 and D2. 

Regardless of Seismic Design Category, all foundation walls must extend a minimum of 6 inches 
above the adjacent grade or, when brick veneer is used on a wall, the foundation wall must 
extend a minimum of 4 inches above the adjacent grade. 

Concrete and masonry foundation walls must conform to the prescriptive requirements of the 
IRC or may be based on other recognized structural standards such as ACI 318 for concrete or 
either NCMA TR68-A or ACI 530/ASCE 5/TMS 402 for masonry. When concrete or masonry 
walls are subject to hydrostatic pressure from groundwater or support more than 48 inches of 
unbalanced fill, without permanent lateral support at the top and bottom, they must be designed 
using accepted engineering practice. The discussion below relates only to the IRC prescriptive 
methods for concrete or masonry foundation walls. 

Several terms used in this discussion warrant definition. Plain concrete and plain masonry 
are not necessarily devoid of all reinforcing; however, they have less reinforcing than is required 
to be officially designated as being reinforced. Unbalanced backfill is defined as the 
difference in height between the exterior and interior finish ground levels except that, when an 
interior concrete slab floor is present, the height is measured from the top of the slab to the 
exterior finished ground level. 

3.9 FOUNDATION WALL THICKNESS, HEIGHT, AND REQUIRED REINFORCING 

The minimum thickness of plain concrete and plain clay or concrete masonry foundation walls 
ranges from 6 inches to 12 inches depending on several variables. These variables include: 

 
The height of the wall, 
 
The sites soil classification, and 
 
The height of any unbalanced backfill. 


Foundation wall thickness also must be at least equal to the width of the supported wall. 

For plain masonry foundation walls, the minimum thickness also is dependent on the use of solid 
masonry units or hollow units that can be either grouted or ungrouted. Generally, the minimum 
thicknesses for solid masonry and grouted hollow masonry are identical whereas the minimum 
thickness for ungrouted hollow masonry is greater. Minimum wall thickness for plain concrete 
and plain masonry walls, based on specific combinations of these variables, are listed in IRC 
Table R404.1.1(1). 


For SDCs D1 and D2, IRC Section R404.1.4 imposes the following additional limitations on plain 
concrete and plain masonry foundation walls that are not specified in IRC Table R404.1.1(1): 

 
Wall height is limited to 8 feet. 
 
Unbalanced fill height is limited to 4 feet. 
 
A single horizontal No. 4 reinforcing bar shall occur in the upper 12 inches of the wall. 
 
Plain masonry walls shall be a minimum of 8 inches thick. 
 
Plain concrete walls shall be a minimum of 7.5 inches thick except that a 6-inch 
minimum thickness is permitted when the wall height does not exceed 4 feet 6 inches. 
 
Vertical reinforcing of masonry stem walls shall be tied to horizontal reinforcement 
located in the footing. 


In SDCs D1 and D2, when the foundation exceeds 8 feet in height or supports more than 4 feet of 
unbalanced fill, additional minimum reinforcing requirements apply. First, two No. 4 horizontal 
reinforcing bars are required at the top of the wall rather than just one. In addition, a concrete or 
masonry walls vertical reinforcing is required to meet additional prescriptive minimums. The 
vertical reinforcing size and maximum spacing are dependent on wall thickness, wall height, 
unbalanced fill height, and soil classification. Three IRC Tables  Tables R404.1.1(2), (3) and 

(4)  specify the minimum vertical reinforcing based on specific combinations of these variables. 
The IRC also allows alternative sizes and spacing of reinforcing up to a maximum spacing of 6 
feet; however, the alternate size and spacing must result in an equivalent cross-sectional area of 
reinforcing per lineal foot of wall as prescribed in the tables. 

Above-code Recommendation: As described earlier, reinforcing should always be 
continuous; therefore, lap splices are needed where discontinuities occur in either 
horizontal or vertical reinforcing. Lap lengths should not be less than 24 inches for 
No. 4 bars or 30 inches for No. 5 bars. 

Regardless of Seismic Design Category, all vertical reinforcing for these reinforced masonry or 
concrete foundation walls must be at least ASTM Grade 60 (yield strength of 60,000 psi). This 
is important to note because No. 4 bars are commonly available in Grade 40, which has a lower 
yield strength. In addition, the distance from the soil side face of the wall to the centerline of the 
vertical reinforcing must be 5 inches in an 8-inch-thick wall, 6.75 inches in a 10-inch-thick wall, 
and 8.5 inches in a 12-inch-thick wall. 

3.10 WOOD-FRAMED WALL BOTTOM PLATE AND FOUNDATION SILL PLATE 
ANCHORAGE 

For the purpose of the following discussion, the terms wall bottom plate and foundation sill 
plate are used to distinguish between two different locations of wood members along a 
foundation. A wall bottom (sole) plate either is directly supported by a slab-on-grade foundation 
as shown in Figure 3-5 or is part of a cripple wall supported directly on a foundation when a slab 
on grade does not occur. A foundation sill plate is different in that it occurs without wall studs 


framing on top of it; instead, a foundation sill plate is the bearing support between floor joists or 
floor beams and the foundation as shown in Figure 3-6. 

In all Seismic Design Categories, where anchor bolts are required to connect a bottom plate or 
foundation sill plate to a foundation, the bolts must have a minimum embedment of 7 inches into 
a concrete or masonry foundation, and they must have a nut and washer tightened on each bolt. 
Typically, anchor bolts must be a minimum of 1/2-inch diameter and be spaced not to exceed 6 
feet on center. However, in SDCs D1 and D2, the maximum bolt spacing is limited to 4 feet for 
buildings with more than two stories. Because the IRC prescribes bracing only for houses in 
SDC D2 up to a maximum of two stories, the closer spacing is actually only applicable in SDC 
D1 where three stories is permitted. This decreased bolt spacing provides 50 percent more shear 
capacity at the sill-plate connection in recognition that earthquake loads generated in a three-
story building will be greater than those in a one- or two-story building. 

Some builders may choose to use 5/8-inch diameter bolts at the standard 6 feet on center spacing 
in place of using 1/2-inch diameter bolts at a required 4 feet on center spacing. Based on bolt 
capacities listed in 2001 NDS Table 11-E, for most species of wood, 5/8-inch bolts at 6 feet on 
center spacing provide approximately 95 percent of the capacity of 1/2-inch bolts at 4 feet on 
center spacing. However, the approval of local officials should always be obtained prior to 
making any such substitution. 

In SDCs D1 and D2, the washer to be installed under the anchor bolt nut must be a 3-inch by 3inch 
square plate with a minimum thickness of 1/4 inch. This increased washer size limits the 
potential for splitting of bottom plates. 

Above-code Recommendation: Splitting of sill plates can occur when the ends of braced 
wall segments are lifted vertically during the rocking motion braced walls undergo while 
resisting earthquake loads. Cyclic testing of shear walls has shown that a standard round 
cut washer is too small to provide any appreciable resistance to this uplift and allows sill 
plates to split but, when larger square washers are used, walls can sustain much higher 
lateral loads before sill plate splitting will occur. For this reason, the use of plate 
washers is also recommended on anchor bolts connecting to the foundation in SDC C. 

3.11 REQUIRED LOCATIONS FOR ANCHOR BOLTS ALONG EXTERIOR WALLS 

Independent of Seismic Design Category, anchor bolts are required to connect a wall bottom 
(sole) plate or foundation sill plate to foundations located along all exterior wall lines. A 
minimum of two anchor bolts are required in each individual length of plate, with one bolt 
located not more than 12 inches nor less than 7 bolt diameters from each end. 

3.12 REQUIRED ANCHORAGE ALONG INTERIOR BRACED WALLS 

The anchorage of wall bottom plates along interior wall lines is a bit more complex than it is for 
exterior walls. The general rule is that when interior braced walls are supported directly on a 


continuous foundation, the bottom plate of the wall must be anchored using the same bolting 
pattern as required for exterior walls. However, the IRC currently requires a continuous 
foundation along interior braced wall lines in only a few situations. Consequently, bolting of the 
sole plate of an interior braced wall to a foundation occurs only rarely because a foundation is 
not normally required. 

When an interior braced wall frames onto a raised wood framed floor, the code is clear that the 
anchorage connection of the braced wall sole plate uses nails. However, the IRC is silent 
regarding exactly what kind of anchorage is required for connecting an interior braced wall to a 
slab-on-grade when a continuous foundation is not required and not provided. One common 
example is a slab-on-grade house that has its exterior walls less than 50 feet apart and therefore 
does not require its interior braced walls be supported on a continuous foundation. In such a 
case, the slab usually will not have the thickness necessary to allow installation of anchor bolts 
with the standard 7 inches of embedment. However, it is reasonable to assume that the braced 
wall bottom-plate-to-slab connection should provide lateral load resistance at least equivalent to 
the typical anchorage to a foundation using bolts. For the 1/2-inch diameter bolts at 6 feet on 
center, the bolt shear capacity from NDS Table 11E, adjusted by 1.6 for earthquake loads, ranges 
from 152 plf in Spruce-Pine-Fir lumber, to 176 plf in Southern Pine lumber. Therefore, the 
connection to be provided to the slab should provide at least the same capacity as the 1/2-inch 
anchor bolts. For 5/8-inch bolts at 6 feet on center, the capacity ranges from 219 plf in Spruce-
Pine Fir to 248 plf in Southern Pine. 

When anchor bolts cannot be used because of limited slab-on-grade thickness, interior wall 
bottom plates often are connected to a slab-on-grade with powder-driven nails or pins as shown 
in Figure 3-14. However, because of their small diameter and shallow embedment length into 
the slab, each powder-driven nail or pin has significantly less lateral capacity than a single 1/2inch 
diameter bolt. The nails or pins would need to be spaced at a much closer center-to-center 
spacing to be equivalent to a 1/2-inch diameter bolt at 6 feet on center. The actual spacing 
needed to achieve equivalence with anchor bolts generally depends on the diameter and length of 
the nail or pin. 

Figure 3-14 Interior braced wall on slab-on-grade. 



Many manufacturers of powder-driven nails or pins have published shear and tension capacity 
values for their specific products when connecting wood members to concrete. However, it 
should be noted that powder-driven nails or pins used for this purpose generally have very little 
embedment depth into the concrete and, as a result, have very little tension capacity to resist the 
uplift that can occur at the ends of braced walls. Because of this limited tension capacity, they 
may not perform nearly as well as bolts. In addition, when pins are used, there is no way to 
install the square plate washers required on anchor bolts in SDCs D1 and D2 and recommended 
for use in SDC C. 

Above-code Recommendation: In SDCs C, D1 and D2, when interior braced walls use 
wood structural panel sheathing, it is recommended that bottom plates be anchored to a 
thickened slab-on-grade using bolts as shown in Figure 3-5. Instead of the 18-inch 
thickness shown in Figure 3-5, only a minimum 10-inch thickness is necessary for the slab-
on-grade to allow for 7-inch minimum embedment of the bolt and 3 inches of additional 
clearance from the end of the bolt to the bottom of the thickened slab. 

3.13 ANCHORING OF INTERIOR BRACED WALLS IN SDCs D1 AND D2 

In SDCs D1 and D2, anchor bolts are required for interior first-story walls when: 

 
The first-story interior wall is a bearing wall (which may or may not be a braced wall 
line) and it is supported on a continuous foundation. 
 
The interior first story-wall is a braced wall line and that wall line is required to be 
supported on a continuous foundation. 


When applying the two rules listed above, one must differentiate between interior walls that are 
bearing walls and those that are braced walls. Although interior bearing walls can be braced 
walls, this is not always the case. Therefore, to apply the rules it must be determined that: 

 
An interior wall is only a bearing wall and not a braced wall, 
 
An interior wall is a braced wall but is not a bearing wall, or 
 
The interior wall is both a bearing wall and braced wall. 


Summaries of the 2003 IRC minimum requirements for continuous foundations and for installing 
anchor bolts to a foundation along braced wall lines and bearing walls are presented in Tables 31 
and 3-2, respectively. When an interior wall is both a bearing wall and a braced wall, the most 
restrictive requirement from Tables 3-1 and 3-2 applies. 


Table 3-1 Summary of 2003 IRC Continuous Foundation and Anchor Bolt Requirements 
for Braced Wall Lines in One- and Two-family Houses 

Table 3-2 Summary of 2003 IRC Continuous Foundation and Anchor Bolt Requirements 
for Bearing Walls in One- and Two-family Houses 


Chapter 4 
FLOOR CONSTRUCTION


Woodframe floor systems and concrete slab-on-grade floors are discussed in this chapter. 
Although cold-formed steel framing for floor systems also is permitted by the IRC, it is not 
covered here; rather, the reader is referred to the AISI Standard for Cold-Formed Steel Framing 

 Prescriptive Method for One- and Two-Family Dwellings (AISI, 2001) for guidance. Also 
permitted but not discussed here are pressure-treated wood floor systems on ground; information 
on the use of these systems is provided in IRC Chapter 5. 

4.1 GENERAL FLOOR CONSTRUCTION REQUIREMENTS 

Woodframe floor systems form a horizontal diaphragm at each level where they occur and 
transfer earthquake lateral loads to braced walls below that floor level or directly to the 
foundation when the lowest floor is supported on a foundation. When a floor supports walls 
above and is supported on walls below as shown in Figure 4-1, the lateral loads in the floor 
system are based on the mass of the floor itself and a portion of the mass of all the walls in the 
stories immediately above and below the floor. (See Chapter 2 of this guide for a discussion of 
the complete load path.) 

Figure 4-1 An unblocked floor diaphragm. 



Concrete slab-on-grade floors typically are constructed with a concrete perimeter foundation and 
together these elements form the base of the building. Lateral forces from exterior and interior 
braced wall lines are transferred to a slab-on-grade via connections between the bottom plate of a 
braced wall and the slab. In turn, the concrete slab and foundation transfer those forces directly 
to the ground as shown in Figure 4-2. (For more information on anchorage of braced walls to 
slab-on-grade construction, see Chapter 3 of this guide.) 

Figure 4-2 Slab-on-grade and perimeter footing transfer loads into soil. 


4.2 WOODFRAME FLOOR SYSTEMS 

Woodframe floors typically consist of repetitive joists or trusses, at a prescribed spacing, 
sheathed with either boards or wood structural panels attached to the top surface. Finish 
materials such as gypsum board typically are applied to the bottom surface where it serves as the 
ceiling for a room below. Blocking between joists or trusses is used at the ends of the floor joists 
or trusses (or a continuous band joist can be used at the ends) and where walls occur above or 
below. Floor systems also include beams, girders, or headers where needed to support joists. 
Joists can be sawn lumber, end jointed lumber, or a variety of prefabricated (engineered) 
members. Examples of engineered lumber include wood I-joists, trusses, and solid rectangular 
structural composite members such as parallel strand lumber (PSL), laminated veneer lumber 
(LVL), and laminated strand lumber (LSL). Beams, girders, or headers and blocking also can be 
either sawn lumber or engineered lumber. 


The primary design consideration in choosing the minimum size and the maximum span and 
spacing of floor joists, trusses, beams, girders, and headers is adequate support for dead and live 
vertical loads as prescribed by the code depending on the uses that a floor must support. Vertical 
deflection of a floor is another design consideration that can limit the maximum span of floor 
members. Tables in IRC Chapter 5 and similar tables in other documents such as those 
published by the American Forest and Paper Association (AF&PA) or engineered lumber 
manufacturers are available for use in selecting the proper combination of minimum size and 
maximum span and spacing of floor framing members. 

4.3 CANTILEVERED FLOORS 

When floor joists cantilever beyond a support, joist size and spacing are limited by prescriptive 
tables in IRC Chapter 5. IRC Table R502.3.3(1) addresses support of a roof and one story of 
wall for roof spans up to 40 feet and snow loads up to 70 psf. IRC Table R502.3.3(2) addresses 
cantilever joists supporting an exterior balcony. When a floor is supporting more than a roof and 
one story of wall, the maximum prescriptive cantilever distance is limited by the IRC to the 
depth of the joist. If longer cantilevers are desired, a registered design professional must design 
that portion of the floor system. 

In Seismic Design Categories D1 and D2, when cantilevered floor joists support braced wall 
panels in the story above, the cantilevered floor is limited by several additional prescriptive 
requirements in IRC Chapter 3. This is because the braced wall above and braced wall below are 
offset out-of-plane. When a floor cantilever supporting a braced wall does not meet the IRC 
limits, that portion of the house is defined as having an irregularity that prevents the use of 
prescriptive wall bracing where the irregularity occurs. In such a case, engineering must be 
applied to resolve the out-of-plane offset of the braced walls located in the stories above and 
below that floor. The maximum permitted cantilever of a second floor supporting a braced wall 
and roof is illustrated in Figure 4-3. (Also see the discussion of load path in Chapter 2 of this 
guide.) 

Figure 4-3 Cantilevered floor restrictions. 



The specific limits and requirements in IRC Chapter 3 for cantilevered floors in SDCs D1 and D2 
that support braced walls are not particularly difficult to meet and appear to omit addressing the 
uplift restraint that may be necessary at the back span support of cantilever joists. In SDCs D1 
and D2, cantilever floor joists supporting a braced wall panel may not extend more than four 
times the nominal depth of the joist when the following set of rules is met: 

 
Joists must be 2x10 nominal or larger at 16-inch maximum spacing. 
 
The back span of the cantilever joist must be at least twice the cantilever distance. 
 
Joists must be doubled at the ends of the braced wall panel above. 
 
A continuous rim joist is connected to the end of each cantilevered joist. If that rim joist 
is spliced along its length, the splice must be made with either: (a) a 16-gage strap 
having 6  16d common nails on each side of the splice or (b) by using wood blocking 
having the same size as the rim joist, installed between the cantilevered joists, and nailed 
to the rim with 8 16d common nails on each side of the splice. 
 
The cantilever end of the joist is limited to supporting uniform loads from a roof and wall 
above and, if supporting a header above, the header span is limited to 8 feet. 


These rules are illustrated in the framing plan shown in Figure 4-4. What is not mentioned in 
this list of rules is the need for connections to resist uplift at the back-span (interior) end of a 
cantilever joist as noted in Figure 4-4. In IRC Table R502.3.3(1) for cantilever joists supporting 
a roof and wall, the uplift is determined using a back-span distance that is three times the 
cantilever distance (3:1). Because the minimum back span specified in the IRC Chapter 3 (see 
Item 2 above) is only twice the cantilever distance (2:1), the uplift values in IRC Table 
R502.3.3(1) would need to be increased by a factor of 1.5 just to address the gravity loads. 

When the downward earthquake overturning load from the ends of a braced wall panel supported 
by cantilever joists are considered in addition to gravity loads, the uplift load at the back-span 
end of the joist obviously will increase. Therefore, depending on the actual back-span-tocantilever-length ratio, the back-span end of the double cantilever joists supporting the ends of a 
braced wall may need to provide uplift restraint as much as twice than that listed in IRC Table 
R502.3.3(1). However, because the magnitude of the uplift load at the back-span end of a 
cantilevered joist reduces as the back-span length increases, it is possible that a cantilever joist 
that is continuous over its interior support will result in zero uplift at the back-span end. When 
cantilever joists are continuous over an interior support, the back span increases and the uplift at 
the end of the joist is greatly reduced. Therefore, the specific cantilever floor joist layout and 
ratio of the length of the back span to the cantilever will determine if and how much uplift may 
need to be resisted. 

Above-code Recommendation: In SDCs C, D1 and D2, when a braced wall is supported 
at the ends of cantilever joists, the back-span uplift connection capacity should be 
determined using engineering principals for the specific back-span and cantilever 
distances involved. 


Figure 4-4 Cantilever joist at braced wall above. 


4.4 REQUIREMENTS FOR BLOCKING 

It is important in floor framing construction to prevent joists (or trusses) from rotating or 
displacing laterally from their intended vertical position. Rotation loads occur because, when 
floor sheathing is resisting lateral loads oriented perpendicular to the joist, those lateral loads are 
actually trying to move the top edge of the joist sideways. 

Preventing rotation is often accomplished by installing full-depth solid blocking at the ends of 
joists. The ends of joists also can be restrained by attaching the joist to a continuous rim or band 
joist or a header or, in balloon framed walls, by attaching the joist to the side of a stud. In SDCs 
D1 and D2, additional solid blocking between joists (or trusses) is necessary at each intermediate 
support even when that location is not at the end of the joist. For example, intermediate support 
should be located at an interior girder or bearing wall where joists are continuous over that 
support. Blocking installed between joists supported by an interior floor girder is illustrated in 
Figure 4-5. 

Blocking also is required below an interior braced wall line in all Seismic Design Categories 
when joists are perpendicular to the braced wall. Although the IRC is silent regarding minimum 


depth and width for these blocks, the intent of this added blocking is to provide a nailing surface 
for the 16d common nails used to connect the bottom plate of the braced wall to the floor. This 
nailing is an important part of the lateral load path; therefore, the blocking should be of a depth 
sufficient to allow full embedment of the 16d common nails and of sufficient width to prevent 
the nails from missing the block. 

Assuming floor sheathing is at least 1/2- inch thick, the minimum depth of the blocking should 
be 1-1/2 inches. Therefore, a flat 2-inch by 4-inch block as shown in Figure 4-6 can provide 
sufficient depth and, when accurately placed below a wall, can provide a width that greatly 
reduces the potential for bottom plate nails missing the block. 

Figure 4-5 Interior bearing line. 


Figure 4-6 Blocking below interior braced wall where floor joists are perpendicular to wall. 



When an interior braced wall also is a bearing wall and joists below the wall are parallel to the 
wall, a double joist or a beam typically is provided in the floor below the wall. Occasionally this 
pair of joists may be spaced apart to allow for piping or vents passing vertically from the wall 
above through the floor. When this occurs, the double joists cannot be located directly below the 
walls bottom plate. To provide a nailing surface for the bottom-plate connection of the braced 
wall above, 2x flat blocking should be installed in line with the braced walls bottom plate 
between and parallel to these spaced joists as shown in Figure 4-7. 

Figure 4-7 Blocking for floor joists spaced apart for piping in floor. 


4.5 CONNECTION OF FLOOR JOISTS TO WALL TOP PLATE OR FOUNDATION 
SILL PLATE BELOW 

Floor joists (or trusses) are required to be connected to the top plate of supporting walls or to a 
foundation sill plate as specified in IRC Table R602.3(1). Each of these connections provides a 
load path to transfer loads from the floor diaphragm into the braced walls or the foundation 
below. Nailed connections must meet the following minimum requirements: 

 
Rim or band joists parallel to a wall or foundation require a toe-nailed connection to the 
wall top plate or foundation sill plate using 8d box or common nails at 6 inch spacing. 
 
Floor joists perpendicular to a wall top plate or foundation sill plate require a toe-nailed 
connection using three 8d box or common nails. 
 
When blocking is installed between the floor joists, the blocking requires a toe-nailed 
connection to the wall top plate or foundation sill plate using a minimum of three 8d box 
or common nails in each block. 



Where toe nailing is used, it must be done correctly so that it can transfer the intended loads and 
so that the nails do not split the wood when being installed. Toe-nailed connections prescribed 
by the IRC should be acceptable when connecting joists to wall top plates or foundation sills that 
are perpendicular to the joists because these connections are not highly loaded by lateral loads. 
The primary lateral load transfer in a floor system occurs through the rim or band joists and 
through blocking that is parallel to braced walls or foundation sill plates. 

Information on proper toe-nail installation is presented in Figure 4-8; however, that idealized 
picture of nail inclination and location is difficult to achieve in actual construction. 
Consequently, many toe-nailed connections that must transfer lateral loads may not actually 
perform very well. 

Above-code Recommendation: In SDCs C, D1 and D2, connections between joists or 
blocking and wall top plates or foundation sill plates that are parallel to the joist or 
blocking should use commercially available light-gage steel angles and nails of the 
correct diameter and length for the product. Many of the toe-nailed connections specified 
in the IRC also can be made using light-gage steel angles through which face nails are driven 
into the two wood framing members being connected. Although light-gage angles may 
require more time to install than toe nails, the angle connections should reduce splitting of the 
wood and can provide a more reliable connection capacity for lateral loads compared to toe 
nails. 

Figure 4-8. Toe nail configuration requirements. 



4.6 FLOOR SHEATHING 

Floor sheathing can be either wood boards installed perpendicular or diagonally to the joists or 
wood structural panels (as subfloor or combination subfloor-underlayment) installed with the 
long direction of the panel perpendicular to the joists. The minimum thickness for wood board 
sheathing depends on joist spacing and the orientation of the boards to the joist (e.g., perpendicular or diagonal). For wood structural panels, the minimum thickness is based on joist 
spacing and the grade of sheathing panels selected. IRC Chapter 5 contains tables to use in 
determining the required minimum thickness for sheathing materials based on a variety of joist 
spacings. 

Above-code Recommendation: Wood boards are rarely used in modern house construction 
unless the underside of the floor is intended to be visible to the interior space below to 
achieve a specific architectural effect. As an above-code measure, wood boards installed 
perpendicular to joists should not be used in SDC C, D1, or D2 unless wood structural 
panels are installed over the boards because the wood board sheathing alone provides 
little resistance to lateral loads. In contrast, diagonally placed wood boards provide greater 
lateral capacity and should be acceptable for small rectangular-shaped floor areas. 

For modern construction, floor sheathing typically will be wood structural panels (OSB or 
plywood). These panels are fastened to the joists based on a schedule prescribed in tables in IRC 
Chapter 6. 

4.7 LATERAL CAPACITY ISSUES FOR WOOD FRAMED FLOORS USING WOOD 
STRUCTURAL PANELS 

The lateral capacity of a floor diaphragm sheathed with wood structural panels is based on five 
factors: 

 
Sheathing thickness, 
 
Sheathing fastener type and size, 
 
Fastener spacing along supported sheathing edges, 
 
Presence or absence of blocking along all edges of each piece of sheathing, and 
 
Layout of the sheathing joints with respect to direction of lateral loading. 


Below is a discussion of how differences in lateral capacity can result depending on how each of 
these is applied to the construction of a floor. 

Sheathing thickness usually is selected based on the spacing of joists and, for floors, will never 
be less than 7/16 inch but normally is at least 5/8 inch. Generally, thicker sheathing will provide 
a more comfortable floor for the occupants to walk on and will have a greater lateral capacity 
compared to thinner sheathing using the same fastener size and spacing. 

The most common sheathing fasteners used are nails with a minimum size of 6d common (0.113 
inch x 2 inches) for a floor sheathing thickness of up to 1/2 inch. The minimum fastener size 


increases with increasing sheathing thickness to a minimum of 10d common nails (0.148 inch x 3 
inches) for sheathing that is 1-1/8 inches thick. Larger diameter nails will provide greater lateral 
capacity than smaller nails in the same thickness of sheathing because the lateral capacity of a 
nail is directly proportional to its diameter. Therefore, using box nails or gun nails that have a 
smaller diameter than common nails will reduce the lateral capacity of a floor diaphragm. 

Staples also can be used to fasten sheathing to framing members. Although not commonly used, 
IRC Table R602.3 (2) has information for specifying alternative sheathing fasteners including 
staples. Generally, staples of either 15 or 16 gage can be used in place of most nails at the same 
spacing as those nails. However, when using staples, it is important to understand that they must 
be installed with the crown parallel to the framing member below the sheathing edge being 
fastened. 

Fastener spacing for floor sheathing is typically 6 inches along continuously supported panel 
edges and 12 inches along supporting members not located at panel edges. Greater lateral 
capacity can be obtained when fastener spacing along supported edges is reduced from 6 inches 
to 4 or 3 inches. 

The IRC only requires floor diaphragms to be fastened along continuously supported panel 
edges. This includes where panel edges are located parallel to and over a joist and at the floor 
framing members forming the perimeter of the floor. The unsupported panel edges that are 
spanning perpendicular to the joists only need to be fastened at each joist. In engineering terms, 
this is called an unblocked diaphragm. See Figure 4-1 for the sheathing layout and nailing 
pattern for a portion of an unblocked diaphragm. 

In contrast, a fully blocked floor diaphragm means that all edges of each sheathing panel that are 
not located on a joist are supported on and fastened to blocking. A blocked diaphragm will have 
significantly greater lateral capacity than an unblocked diaphragm having the same thickness of 
sheathing and attached with identical fasteners because the extra fasteners along the blocked 
edges provide additional capacity. Figure 4-9 shows the layout of sheathing and nailing of a 
portion of a blocked floor diaphragm. Fastening the sheathing to joists or blocking along all 
panel edges allows the shear loads being carried in the sheathing to be transferred from one panel 
to the next much more effectively. This, in turn, ties the floor together better and allows the 
braced walls below that floor to resist an earthquake more as a system than as individual walls. 

Above-code Recommendation: The IRC requires that wood structural panel sheathing be 
installed with the long dimension of the panel perpendicular to joists, but it does not specify 
staggering of panel joints along the short direction of the panels. Although not specifically 
required by the IRC, sheathing should be installed as shown in Figures 4-1 and 4-9 to 
achieve the greatest capacity. This staggered sheathing layout pattern causes the individual 
sheathing panels to interlock and makes the whole floor act as a unit. 


Figure 4-9 Blocked diaphragm configuration. 


Lateral loads in a floor diaphragm also are affected by the distance between braced wall lines or 
between foundations located below the floor. The loads increase with increasing distance 
between lines of parallel braced walls or foundations. Therefore, a long and narrow floor 
diaphragm as shown in Figure 4-10 will have to transfer a greater load per foot along its short 
sides than along its long sides. In order to limit the maximum load along a short side, IRC 
Chapter 6 places limits on the maximum spacing between braced wall lines or foundations. 

Figure 4-10 Diaphragm loads on long and short sides. 



The size and location of floor openings such as for stairs or a two-story entry foyer can create 
concentrations of lateral loads in a floor diaphragm. To address this, IRC Chapter 3 limits 
openings through a floor or roof to the lesser of either 12 feet maximum or 50 percent of the least 
dimension of the floor. When openings exceed these limits, engineering of the floor or roof 
diaphragm is required. 

Above-code Recommendation: In SDCs C, D1 and D2 , when floor openings exceed 50 
percent of the IRC prescriptive opening size limits, it is recommended that 16-gage 
straps be installed along the edges perpendicular to the joists and extended beyond the 
opening by at elast 2 feet at each end as shown in Figure 4-11. The straps can be nailed 
with 10d nails into the framing members forming the perimeter of the opening and into 
blocking beyond the corners. The straps and additional nailing act to reinforce the diaphragm 
and provide a dedicated path for lateral loads to be transferred around the opening to the 
portions of the floor beyond. Smaller openings such as those for chimneys or duct shafts do 
not require any special reinforcing. 

Figure 4-11 Reinforcing straps at large diaphragm openings. 



4.8 CONCRETE SLAB-ON-GRADE FLOORS 

A concrete slab-on-grade can be used as the base of a first-floor level or of a basement level. 
The minimum thickness for a concrete slab-on-grade is 3-1/2 inches except where expansive soil 
is present. Where expansive soils are encountered, a design for the slab-on-grade must conform 
to IBC Chapter 18 (see Chapter 3 of this guide for a discussion of the effects of expansive soil). 

Concrete Strength Requirements  The minimum concrete compressive strength is 2,500 pounds 
per square inch (psi). Floor slabs having an exterior surface exposed to the weather in areas of 
moderate to severe concrete weathering must have higher compressive strength as specified in 
IRC Table R402.2. A map in IRC Chapter 3 identifies locations where moderate and severe 
weathering of concrete is expected to occur. 

Reinforcing of Concrete Slab-on-Grade Floors  In the absence of expansive soils, the IRC does 
not require reinforcing of concrete slabs. Reinforcing typically is used to provide tension 
capacity in concrete and thereby reduce cracking caused by a variety of loads including 
temperature variations. Concrete alone has very good compression capacity but has a very low 
capacity for tension. Therefore, adding reinforcing bars to a slab-on-grade will provide much 
greater resistance to tension loads originating from earthquake loads and other soil conditions 
that could induce tension stress in the slab. 

Reinforcing is required only where a slab-on-grade is thickened along its perimeter edge or 
below an interior bearing wall in SDCs D1 and D2. When exterior braced walls are spaced more 
than 50 feet apart, an interior braced wall also needs a foundation as part of the slab-on-grade in 
SDCs D1 and D2. (See Chapter 3 of this guide for information on where and how much 
reinforcing is needed in foundations provided with a slab-on-grade.) 

Above-code Recommendation: When a slab-on-grade is placed separately from the 
perimeter footing below, the IRC does not specify any vertical reinforcing across this joint. 
When this condition occurs, the possibility of earthquake loads causing sliding along that 
joint is very real. Therefore, as an above-code measure in SDCs C, D1 and D2, when the 
footing and the slab concrete are separately placed, it is recommended that vertical 
reinforcing dowels be placed across the joint between the slab and footing. These 
vertical dowels should be No. 4 steel reinforcing bars at 48-inch maximum spacing. 

(More information on this subject is presented in Chapter 3 of this guide.) 


Chapter 5 
WALLS


5.1 WOOD LIGHT-FRAME CONSTRUCTION 

5.1.1 General Components 

In residential construction, the walls provide the primary lateral resistance to wind and 
earthquake loads. Even in frame type houses (e.g., post-beam construction), the exterior walls 
provide most of the lateral stability to the house. Although this guide focuses on wood light-
frame construction, alternatives such as cold-formed steel, masonry, and concrete construction 
are used in many regions of the country. The reader is referred to the sections on masonry and 
concrete construction later in this chapter for some guidance on the use of these materials. For 
cold-formed steel construction, the reader is referred to the American Iron and Steel Institutes 
(AISI) industry standard for prescriptive cold-formed steel construction, Standard for Cold-
formed Steel Framing Prescriptive Method for One- and Two-Family Dwellings (2001). 

Light-frame walls provide resistance to sliding, overturning, and racking loads induced in the 
house by an earthquake as illustrated in Figure 5-1. The walls are the principle element for 
transmitting the loads from the upper stories and roof to the foundation. The concept of how 
these loads are transferred between the major components of the house is illustrated in Figure 52, and the action of the individual wall segments resisting the lateral loads is illustrated in Figure 
5-3. Wood light-frame walls typically consist of the lumber framing covered by sheathing 
material that is attached to the wood framing with nails, staples, or screws. Figure 5-4 illustrates 
the components of a wall that is sheathed with wood structural panels (OSB or plywood) on the 
outside and gypsum wallboard on the inside. The figure also shows the addition of hold-down 
connectors to the framing, which are required by the IRC for some specific bracing 
configurations. When used, hold-down connectors increase the strength and stiffness of the wall 
segment. 

Four different bracing wall configurations and eight methods (materials) are recognized by the 
IRC. The bracing wall configurations include: 

 
IRC Section R602.10.3 braced wall panels (Figure 5-5a), 
 
IRC Section R602.10.5 continuous (wood) structural panel sheathing (Figure 5-5b), 
 
IRC Section 602.10.6 alternate braced wall panels (similar to Figure 5-5c), and 
 
Wood structural panel sheathed walls with hold-down connections as required by the 
exceptions in IRC Section R703.7 when stone or masonry veneer is used (Figure 5-5c). 



Figure 5-1 Sliding, overturning, and racking action resisted by walls and foundation. 



Figure 5-2 Exploded view of house illustrating load paths. 



Figure 5-3 Wall action for resisting lateral loads 


Figure 5-4 Exploded view of typical residential wall segment. 


Differences in these bracing wall configurations include sheathing materials, minimum bracing 
length, extent of sheathing, and anchorage at the wall base. Differences in overturning 
anchorage for walls are shown in Figure 5-5. 

Figure 5-5 Detailing differences for three options when using 
wood structural panel sheathed walls. 


The braced wall panel (IRC Section R602.10.3) is the most commonly used approach. Eight 
different methods (materials) are recognized by the IRC as acceptable bracing. These are called 
braced wall panel construction methods in the IRC and are listed in Table 5-1. Method 1, let-
in bracing, is not allowed to be used in regions of high earthquake hazard because it often will 
fail as the walls are racked during an earthquake; therefore, this method is not discussed further 
in this guide. 

Of the acceptable braced wall panel materials, wood structural panels and diagonal lumber 
sheathing are known to perform better than others (i.e., withstand higher deformations while 
supporting higher loads). Wall panel bracing is required by IRC Section R602.10.4 to be 
provided in 4-foot minimum lengths sheathed on one face for other than Method 5 and either 4foot lengths sheathed on both faces or 8-foot lengths sheathed on one face for Method 5. Other 
than with Method 5, this bracing often is provided in 4-foot-long isolated segments along the 
wall length. 


Table 5-1 Braced Wall Panel Construction Methods (Materials) Recognized by the IRC 

Construction Method Designation / Sheathing Material 
1 / Nominal 1x4 inch continuous let-in bracing 
2 / 5/8-inch minimum thickness boards applied diagonally to studs 
3 / Wood structural panels (OSB or plywood) 5/16-inch minimum thickness 
4 / 1/2- or 25/32-inch thick structural fiberboard 
5 / 1/2-inch gypsum wallboard 
6 / Particleboard sheathing 
7 / Portland cement plaster 
8 / Hardboard panel siding 

Where braced wall panels use wood structural panel (Method 3) or diagonal lumber (Method 2) 
sheathing, the panel base anchorage to the supporting floor framing or foundation limits the 
bracing strength and stiffness. The braced wall panel anchorage includes two critical weak links 
for uplift: the panel end stud connection and the bottom plate (sole plate) connection. Braced 
wall panel bottom plates are specified by IRC Table R602.3(1) to be attached to the floor 
platform using three 16d common (0.162 x 3.5 inch) or 16d box (0.148 x 3.5 inch) nails every 16 
inches or are specified by IRC Section R403.1.6 to be anchored to the foundation with 1/2-inchdiameter anchor bolts at not more than 6 feet on center. Together, these two weak links cause 
the wall to fail along the bottom of the wall under relatively low loads (these walls have a 
capacity of approximately 150 to 400 plf, which translates to an allowable design value of about 
50 to 140 plf maximum). 

IRC Section R602.10.5, continuous structural panel sheathing, requires that all exterior wall 
surfaces on a given story level other than door and window openings be sheathed with wood 
structural panel sheathing (Figure 5-5b). This bracing wall configuration has greater strength 
and stiffness than braced wall panels with minimum wall base anchorage. The increased 
strength and stiffness are due in part to the continuity provided by additional sheathing above 
and below windows and doors (which makes the wall-base connection capacity less critical) and 
increased overturning capacity due to required corner framing details or hold-downs at wall ends. 
In recognition of the improved strength and stiffness provided with continuous structural panel 
sheathing, IRC Section R602.10.5 permits the minimum length of bracing to be reduced and IRC 
Table R602.10.5 permits use of individual wall bracing panels that are more slender than would 
otherwise be permitted. 

Wood structural panel wall bracing with uplift anchorage provided at each end, per IRC Sections 
R602.10.6 and R703.7, is the strongest and stiffest option for resisting lateral loads. This wall 
configuration is illustrated in Figure 5-5c. Although sheathing, fastening, and hold-down loads 
for these walls are prescribed, the walls are essentially equivalent to engineered shear walls. The 
alternate braced wall panel provisions of IRC Section R602.10.6 were developed to allow use of 
braced wall panels narrower than the 4-foot minimum required by IRC Section R602.10.4 (e.g., 
at the side of garage doors); however, the IRC permits an alternate braced panel to be substituted 
for each 4 feet of bracing throughout the house. The provisions of IRC Section R703.7 use 


increased wall strength and stiffness to compensate for the increased earthquake loads that occur 
due to the weight of stone or masonry veneer. 
As discussed above, where wood structural panel bracing is used, the strength and stiffness of the 
wall bracing is very dependent on the extent of sheathing and anchorage at the wall base. The 
configuration shown in Figure 5-5a, using IRC minimum braced wall panel anchorage, is the 
weakest and least stiff of the wood structural panel bracing configurations. The configuration 
shown in Figure 5-5b adds strength and stiffness by providing continuous structural panel 
sheathing and added detailing or tie-downs at wall ends. The strongest and stiffest configuration 
is illustrated in Figure 5-5c where overturning anchors are provided at each end of each wall 
segment. 

Above-code Recommendation: Use of the configurations shown in Figures 5-5b or 5-5c 
can significantly increase the strength and stiffness of braced wall panels sheathed with 
wood structural panel or diagonal lumber sheathing and may be used to provide 
improved performance whether or not specifically required by the IRC. Use with 
sheathing materials other than Methods 2 or 3 would provide less benefit and is not 
recommended. 

When the wood structural panel wall bracing option is used in an engineered design, the walls 
are designed according to empirical tables that provide allowable design loads in pounds per foot 
of wall depending on the thickness and grade of the sheathing and the size and spacing of the 
sheathing nails. These walls rely on hold-down anchor connections to resist the overturning 
loads and substantial connections (nails, lag screws, bolts, etc.) along the top and bottom plates 
to transmit the lateral loads between the wall framing and the floor platform or foundation. This 
type of wall can resist allowable design loads up to 870 plf when sheathed on one face with 
wood structural panels. 

5.1.2 Design Requirements 

The 2003 IRC requires that wall bracing be provided both at exterior walls and at interior braced 
wall lines. IRC Section R602.10.1.1 specifies that interior braced wall lines must be added such 
that the distance between braced wall lines does not exceed 35 feet; however, allowance is made 
for spacing up to 50 feet. IRC Section R602.10.11 reduces the maximum spacing to 25 feet in 
SDCs D1 and D2. Within each braced wall line, IRC Table R602.10.1 specifies the minimum 
bracing as a percentage of the length of the wall line based on the wind and earthquake exposure 
and the story level under consideration (the lower in the house, the more sheathing is required to 
resist the higher loads). Some interpretations of the IRC would allow a house located in SDC C 
to be braced according to the amounts required for SDCs A and B due to the exemption for 
detached houses in SDC C from all earthquake requirements. 

Above-code Recommendation: Houses in SDC C should be braced according to the 
requirements of SDC D1. Use of the required percentage of wall bracing in each 25-foot 
length of wall will provide a distributed resistance system of walls rather than a concentrated 
wall system. Experience has shown that a distributed wall system performs better in an 
earthquake than concentrated walls in only a few locations. 


Table 5-2 excerpts a portion of this bracing information. As previously noted, the amount of 
bracing may be modified in accordance with IRC Section R602.10.5 when continuous wood 
structural panel sheathing is provided. In addition, an alternate braced wall panel (per IRC 
Section R602.10.6) can be substituted for each 4 feet of braced wall panel. The sheathing must 
be located at the ends of each braced wall line at least 25 feet on center. The idea is to provide a 
distributed resisting system rather than to concentrate the resistance in a few highly loaded wall 
segments. Overall, this provides a more robust structure that can resist the loads induced into the 
house close to the source of the load and not require the floor and roof diaphragms to transmit 
the loads long distances. Let-in bracing is allowed only for the top story of houses in SDC C due 
to its tendency to fail at relatively low lateral load levels. 

Table 5-2 IRC Sheathing Requirements for Seismic Design Categories C, D1, and D2 


Although the basic IRC bracing concept is reasonably straightforward, a number of other 
adjustments may modify the required length of bracing. Close attention is required to ensure that 
the IRC requirements are met. IRC Section R301.2.2.2.1 limits the dead load of assemblies for 
houses in SDCs D1 and D2. The maximum permitted for roof plus ceiling dead load is 15 psf 
(typical asphalt shingle roofs with gypsum ceilings); however, IRC Table R301.2.2.2.1 permits 
this assembly weight to be increased to 25 psf (for heavier roofing materials) provided that the 
length of wall bracing is increasd as specified. Footnote d to IRC Table R602.10.1 notes that the 
earthquake bracing requirements are based on 15 psf (exterior) wall dead load and permits the 
required earthquake bracing length to be multiplied by 0.85 for walls with dead loads of 8 psf or 


less, provided that the length is not less than required by IRC Section R602.10.4 (4 feet for all 
but let-in bracing and gypsum board, which require a length of 8 feet) nor less than required for 
wind loading. 

In addition to the adjustments to required bracing length, the second paragraph of IRC Section 
R602.10.11 (errata, second printing) permits the wood structural panel wall sheathing to begin up 
to 8 feet from the corner in SDCs D1 and D2 provided that: 

 
A 2-foot braced panel is applied in each direction at the house corner or 
 
Tie-downs are provided at the end of the braced wall panel closest to the corner. 


This is more stringent than for low SDCs where braced wall panels are permitted to be located 
up to 12 feet 6 inches from wall corners without providing tiedowns or sheathed corners and 
farther from corners if collectors are provided to carry loads to the braced wall panels. However, 
a house that is tied together at the corners will resist the loads expected from an earthquake much 
better than if the corners are not connected well. When using bracing other than wood structural 
panels in SDCs D1 and D2, braced wall panels must be located at each end of braced wall lines. 

Above-code Recommendation: Braced wall panels should extend to every corner. 

IRC Section R703.7 presents additional bracing modifications where exterior veneer is used. 
These requirements are discussed in Section 5.2 of this guide. For the guides model house, 
Figures 5-6a through 5-6c provide plan views identifying the bracing required when the house is 
in SDC C, has a light finish system such as vinyl siding, and has a crawl space. The bracing 
requirements for the same house located in SDC D2 are shown in Figures 5-7a through 5-7c. 
Notice the significant increase in designated wall bracing due to the higher level loads expected 
in SDC D2. 


Figure 5-6a Plan view of crawl space for model house with light-weight finish material located in SDC C. 


Chapter 5, Walls 


Figure 5-6b Plan view of first floor for model house with light-weight finish material located in SDC C. 


Figure 5-6c Plan view of second floor for model house with light-weight finish material located in SDC C. 



Chapter 5, Walls 


Figure 5-7a Plan view of crawl space for model house with light-weight finish material located in SDC D2. 


Figure 5-7b Plan view of first floor for model house with light-weight finish material located in SDC D2. 



Chapter 5, Walls 


Figure 5-7c Plan view of second floor for model house with light-weight finish material located in SDC D2. 


5.1.3 Cripple Walls 

Cripple walls are short frame walls that extend from the foundation to the bottom of the first 
floor. They are most often found in the western United States. These walls often enclose a crawl 
space or serve as walls for a stepped foundation. Historically, these walls have been the cause of 
significant failures in residential construction during earthquakes primarily due to inadequate in-
plane strength or inadequate anchorage to the foundation. These walls are the most highly 
loaded of all the light-frame walls in a house because they have to resist the entire load from the 
house above. For cripple walls, IRC Section R602.10.2.1 specifies the length of bracing as 1.15 
times the bracing required for the story above and indicates that the spacing should be 18 feet 
instead of 25 feet. This is not applicable in SDC D2, however, where the IRC requires that the 
cripple wall bracing be a minimum of 75 percent of the wall length and be constructed using 
wood structural panels. When interior braced wall lines are not supported on a continuous 
foundation, cripple wall bracing lengths in SDCs D1 and D2 must be modified to increase the 
sheathing length by 50 percent at exterior braced wall lines or to decrease the nail spacing along 
sheathing edges to 4 inches on center (per IRC Section R602.10.11.1). 

5.1.4 Quality Assurance 

Quality assurance for the typical light-frame wall is really quite simple. There are essentially 
three areas to inspect: 

 
The sheathing nails, 
 
The anchorage of the framing to the floor framing or foundation below, and 
 
The anchorage of the wall framing to the roof or floor framing above. 


The most common problem that adversely affects the performance of all wall types is the 
overdriving of the nails attaching the sheathing to the studs; this is especially a problem when 
pneumatic or power-driven nail guns are used. All of the nails used to attach the sheathing to the 
wall framing should be driven only to where the nail head is flush with the surface of the 
sheathing as shown in Figure 5-8; an improperly driven (overdriven) nail also is shown in Figure 
5-8. 

Figure 5-8 Properly and overdriven nails. 

The most important nails are those around the perimeter of each panel of sheathing. These nails 
govern the strength and stiffness of the panel. If the nails are overdriven (as in Figure 5-8), the 
strength of the connection is severely compromised. For instance, if 3/8-inch wood structural 
panel sheathing is used and the nails are overdriven 1/8 inch (typical for many pneumatic nail 
tools), the strength of the wall is reduced as much as 40 to 50 percent. It is therefore imperative 
that the sheathing nails be inspected to ensure that they are properly driven. Many pneumatic 
nail tools have default driving pins that overdrive the nails but changing the driving pin costs 
little (less than 10 percent of the original cost of the tool). Many tool manufacturers now provide 
an adjustment on the tool to allow the user to change the depth of the driving pin without 
replacing the part. (It is recommended that where nail heads occasionally are more than 1/16inch below the surface, an additional nail should be provided between existing nails. If a 
substantial number of nails are overdriven, the sheathing should be removed and the framing 
checked for splitting before replacing the sheathing with proper nailing.) 

Nails often are located too close to the edge of the sheathing panel, which can result in the nails 
tearing out the side of the panel and weakening the wall. Therefore, the minimum edge distance 
for nailing sheathing is 3/8 inch. The larger the edge distance is, the stronger the wall will be. 
This is especially true for the row of nails at the bottom of the wall. If the bottom row of nails is 
located at the mid-height of the bottom plate for the wall, the displacement capacity of the wall is 
almost double that when the nails are spaced at 3/8 inch. When the edges of two sheathing 
panels meet on a common stud (2x nominal), the maximum edge distance should be 3/8 inch to 
prevent the nail from splitting the edge of the stud behind the sheathing. 

Another common problem is that the nails are driven through the sheathing but miss the stud 
behind. Although it is obvious that nails in the air provide no strength or stiffness, this problem 
is so common it merits emphasizing that sheathing nails must be driven into the studs and top 
and bottom plates of the wall and not miss the framing. The use of pneumatic nail guns makes it 
very difficult for the operator to feel whether or not the nail has been driven into the stud 
behind; therefore, all such nailing must be visually inspected from behind to ensure that the nails 
did not miss the studs. 

Finally, the sheathing nailing around the perimeter of each sheet of sheathing should be 
symmetric about the center of the sheathing panel. This means that there should be 
approximately the same number of nails along each parallel side of the panel as illustrated in 
Figure 5-9. 

The second area to inspect for quality assurance is the anchorage at the bottom of the wall. For 
prescriptive walls, the nails must pass through the bottom plate into the floor framing. If the 
nails miss the floor joists, they have no capacity to resist withdrawal and sliding. If the wall is 
attached directly to the foundation, 1/2-inch-diameter anchor bolts should be spaced at a 
maximum of 6 feet on center, and the end bolts should be not more than 12 inches nor closer 
than 7 bolt diameters from the end of the plate. If the house is located in SDC D1 or D2, the 
anchor bolts are required to have a steel plate washer between the wood sill plate and the nut. 
(See Section 3.10 of this guide for more discussion of the use of plate washers.) If the walls are 
segmented (engineered), the hold-down connector needs to be attached to the end stud (usually a 


double stud) and the bolt or threaded rod holding the connector to the foundation or story below 
needs to be tightened to a snug one-quarter turn condition. Tightening the nut more does not 
provide higher reliability due to the fact that wood is viscoelastic and the tension in the bolt will 
not be sustained over time. A common practice to prevent the nuts from loosening during an 
earthquake is to use a double nut or slack take-up device. The hold-down should connect the end 
stud either directly to the foundation or to the top of a stud in a wall of the story below in order to 
effectively transmit the overturning loads into an active resisting element. 

Figure 5-9 Illustration of concept of equal numbers of fasteners in line 
for symmetric nailing schedule. 


Finally, the connection between the top plate and the roof framing or floor framing for the story 
above should be checked. The lateral loads are transmitted through nails or other fasteners 
connecting the top plate of the wall to the floor or ceiling joists above. (For vaulted ceilings, the 
connection might be between the top plate and the rafters.) If there is a wall on the story above 
that has an overturning anchor attached to the end stud, there should be an equivalent connection 
near the top of a stud in the story below that is upside down from the one on the wall above. A 
threaded rod should connect the two hold-down anchors to transmit the loads between them. If a 
stud is being used to transmit overturning loads from the story above, it will need to have an 
overturning anchor at its base to transmit the loads to the next story or foundation below. 


Above-code Recommendations: Improving the detailing of the braced wall system is the 
most effective way to obtain earthquake performance levels higher than the code minimums. 
It is also most effective to concentrate on the lower stories of the house since this is the area 
that typically has the fewest walls and experiences the highest loads during an earthquake. 

Use of continuous structural panel sheathing is recommended as an above-code 
measure (see Section 5.1.1 and Figure 5-5b of this guide). The analyses of the model house 
used in this guide indicated that use of continuous wood structural panel sheathing with 
overturning anchors at corners significantly reduced the drift in all SDCs and improved the 
performance category from significant damage to moderate damage in SDC D2. The 
cost of making this change would be approximately 9 to 10 percent of the cost of the 
structural portion of the model house. The percentage of the total cost of the house would 
depend on the level of finishes, fixtures, windows, etc., and the cost of the land relative to 
the cost of the structural portion of the house. The structural portion of house costs ranges 
from 15 to 25 percent of total cost; therefore, the cost of fully sheathing a typical house 
would be approximately 1.5 to 2.5 percent of its total cost. 

Use of wood structural panel sheathing extending over and nailed to the floor rim joist 
or blocking is recommended as an above-code measure. This is illustrated in Figure 5-10 
and can be accomplished either by sheathing the wall with oversized panels (9-foot panels 
on an 8-foot wall) or by cutting and blocking standard size sheets. It is important to leave a 
vertical gap between sheathing panels at the mid-height of the band joist (approximately 3/4 
inch for green solid sawn floor framing) to allow for shrinkage of the wood floor member 
without causing the sheathing to buckle. Analytical studies of the model house used in this 
guide indicated that use of sheathing spliced on the rim joist increased the approximate 
performance category from significant damage to moderate damage in SDC D2. 
Improvements identified in the analytical study were less significant in SDCs C and D1. For 
the model house, the cost of implementing this improvement would be approximately 0.5 
percent of the cost of the structural portion of the house, essentially a no-cost item when the 
total cost of the house is considered. 

Use of hold-down anchors at each end of each wood structural panel wall segment is 
recommended as an above-code measure (see Section 5.1.1 and Figure 5-5c of this guide). 
Analytical studies of the model house used in this guide indicated that the addition of hold-
down anchors significantly reduced drift in all SDCs and the approximate performance 
category was increased from significant damage to moderate damage in SDC D2. For 
the model house, the cost of this improvement would be 18 percent of the structural cost of 
the house or 2.5 to 4.5 percent of its total cost. This option provides a house with the best 
connectivity and is the basis for engineered shear wall construction. Further, of all the 
options, it has the greatest effect on the stiffness and strength of the wall. 


Above-code Recommendation: Although not addressed in the analytical studies, use of 
the above-code measures in combination is thought to have a cumulative effect and is 
recommended. This level of connectivity will further improve earthquake performance by 
stiffening and strengthening the walls. Additional above-code options for increasing 
strength and stiffness include spacing sheathing nails closer than the standard 6 inches 
on center (additional detailing requirements may be applicable at 3-inch or closer 
spacing; see the IBC) and placing wood structural panel sheathing on both faces of 
walls. Use of these above-code recommendations will be most effective where the highest 
loads are present, such as lower stories and cripple wall levels. 

Figure 5-10 Sheathing detail for extending the sheathing over the band joist. 



5.2 STONE AND MASONRY VENEER 

Masonry and stone veneer are popular exterior finish materials for residential construction 
(Figure 5-11). Veneer provides a durable finish for the house but, unfortunately, the added 
weight of these veneers increases the loads experienced during an earthquake. The IRC permits 
use of stone and masonry veneer installed over concrete or masonry walls and over a backing of 
wood light-frame or cold-formed steel construction. 

Figure 5-11 Applications of masonry and stone veneer. 


This guide section discusses general principles of earthquake-resistant design for houses with 
veneer, specific IRC requirements important to earthquake performance, and above-code 
measures for improved earthquake performance. 

Stone and masonry veneer is addressed in IRC Sections R702.1 (interior) and R703.7 (general 
and exterior). For earthquake-resistant construction using veneer, there are two major areas of 
concern: 

 
The increased earthquake loading on the house due to the weight of the veneer and 
 
Adequate anchorage of the veneer. 


The weight of stone and masonry veneer permitted under the IRC provisions can vary from as 
little as 20 pounds per square foot installed for adhered veneer to as much as 70 psf installed for 
a 5-inch-thick anchored veneer with 1 inch of grout. The weight of stone and masonry veneer 
greatly increases the overall weight of a light-frame house and, as a result, the earthquake loads. 
The IRC provisions rely exclusively on the strength and stiffness of the light-frame bracing 
systems to resist wind and earthquake loads, discounting any strength and stiffness that might be 
provided by the veneer. This is primarily because there is only limited understanding of the 
ability of the veneer to resist cyclic earthquake loading while acting in combination with the 
light-frame bracing systems and because the veneer cracks and breaks at smaller displacements 
than required for the light-frame system to achieve its capacity. As a result, additional 
requirements apply to light-frame systems with veneer in areas with higher earthquake risk. 


Masonry and stone veneers have been damaged in historic earthquakes. Figure 5-12 illustrates 
damage that occurred during the Northridge earthquake that appears to be due to inadequate 
connection of the wall studs to the top and bottom plates. In failing, the stone veneer pulled the 
wall sheathing and many of the studs out of the wall perpendicular to the pictured wall. 

Figure 5-12 Stone veneer damaged during the earthquake in Northridge, California. 


5.2.1 IRC Earthquake Requirements 

The IRC permits use of veneer above the first story above grade in SDCs D1 and D2 only for one-
and two-family detached houses of wood light-frame construction without cripple walls. 

If a house does not meet these requirements, a design professional should design the house 
following the requirements of the International Building Code or NFPA 5000. For veneer 
installed over a light-frame system, requirements are incrementally more restrictive for higher 
SDCs (IRC Section R703.7). References to the veneer provisions for SDCs C, D1, and D2 appear 
in IRC Sections R301.2.2.3.1 and R301.2.2.4.2. 

For light-frame systems, the general rule is that stone and masonry veneer not exceeding 5 inches 
in thickness is permitted in the first story above grade across all SDCs without any further 
requirements (IRC Section R703.7). 

Although this veneer will have some impact on earthquake performance, the increase in 
earthquake load close to the house base is not judged to have significant life-safety implications. 

In SDCs A and B, veneer on wood light-frame or cold-formed steel houses can be up to 5 inches 
thick and extend up to 30 feet above a noncombustible foundation with an additional 8 feet 
permitted for gable end walls. The 30-foot limit corresponds to the height at which additional 


vertical support of the veneer would be required in an engineered design. Veneer in SDCs A and 
B has no imposed earthquake limitations, leaving wind provisions to control. 

In SDC C, veneer on wood light-frame or cold-formed steel houses can be up to 5 inches thick 
and extend up to 30 feet above a noncombustible foundation with an additional 8 feet permitted 
for gable end walls. It is required, however, that the length of bracing walls in other than the 
topmost story be 1.5 times the length required if veneer is not used. (Note that this provision is 
applicable to all houses in SDC C.) 

In SDC D1, wood light-frame houses are permitted to have veneer up to 4 inches in thickness for 
a height of 20 feet above a noncombustible foundation with an additional 8 feet allowed for 
gable end walls or up to 30 feet where the bottom 10 feet (first story) is anchored to a concrete or 
masonry wall. However, IRC Section R301.2.2.2.1 assembly weight limits for concrete and 
masonry leave little allowance for veneer weight. Where assembly weight limits cannot be met, 
engineered design of the concrete or masonry walls may be required. 

Veneer on wood light-frame walls in SDC D1 (see IRC Section R703.7) is dependent on the use 
of wood structural panel sheathing with specified nail size and spacing, increased bracing 
lengths, use of hold-downs with specified capacities at each end of each braced wall panel, and 
the prohibition of cripple walls as previously mentioned. The minimum sheathing must be at 
least 7/16 inch thick and it must be fastened with at least 0.131 x 2.5 inch (8d common) nails that 
are spaced no more than 4 inches on center at the panel edges. The braced wall length for the top 
story of the house must be at least 45 percent of the total wall length. Finally, the first two 
stories must have hold-down connectors installed. The hold-down connectors for the transfer of 
loads from the second to the first story must have a minimum capacity of 2100 lb and from the 
first floor to foundation must have a minimum capacity of 3700 lb. Load path requirements with 
hold-down devices are discussed in Chapter 2 of this guide. 

In SDC D2, veneer provisions are similar to those for SDC D1, except for a couple of small 
changes: 

 
The maximum thickness of the veneer is limited to 3 inches, 
 
The top story must be sheathed to 55 percent of the total wall length, 
 
The second-to-first story overturning anchors must have a capacity of 2300 lb, and 
 
The first-story-to-foundation overturning anchors must have a minimum capacity 
of 3900 lb. 


Anchorage requirements for anchored stone or masonry veneer are given in IRC Section 
R703.7.4. Use of galvanized corrugated sheet metal ties or metal strand wire ties is required. 
The minimum gage required is No. 9 U.S. gage wire for strand wire ties and No. 22 U.S. gage by 
7/8 inch corrugated sheet metal ties. The maximum supported veneer area is 2-2/3 square feet 
per tie for SDCs A, B, and C. The supported area is reduced to 2 square feet per tie for houses in 
SDC D1 and D2. 


The intent of using ties for masonry veneer is not to prevent the veneer from cracking but rather 
to prevent the veneer from pulling away from the supporting wall system. In earthquakes, the 
desire is to prevent damaged veneer from becoming a falling hazard and the use of ties should 
accomplish this by holding the cracked veneer to the supporting wall. 

Above-code Recommendations: The sheet metal ties or wires used to fasten veneers 
should be corrosion resistant, should penetrate the house paper and sheathing, and 
should be embedded into the wall studs. Fasteners that do not penetrate the studs (i.e., only 
the sheathing is penetrated) have low withdrawal resistance and significantly reduce the ties 
ability to hold the veneer to the wall. 

Where veneer is limited to the first story above grade, increase the length of wood 
structural panel bracing and use hold-down devices on the braced wall panels in the 
first story to increase both the strength and stiffness of the first story above grade. 

These measures will help make the deformation behavior of the light-frame system more 
compatible with the veneer and reduce cracking of the veneer. 

5.2.2 Quality Control 

All mortar joints should be completely filled and well tooled for water tightness. This can have a 
significant effect on the strength and durability of the masonry walls and veneers. To augment 
the tooling of joints for improving moisture control (which directly affects the longer-term 
strength of the wall), flashing and weep holes need to be placed at the bottom of the veneer so 
that any water trapped between the veneer and the structural wall behind can be directed to the 
outside of the house. The flashing and weep holes frequently are omitted in residential 
construction, resulting in expensive repairs to the structural walls of the house. 

Veneer should be placed such that a 1-inch cavity is maintained between the veneer and the 
supporting wall. This cavity provides a drainage plane for channeling the moisture that will 
migrate from the outside surface of the veneer to the inside and then to the bottom of the wall 
where it can escape to the outside through the weep holes. This drainage plane is very important 
to prevent the wall sheathing from being constantly wet, which will result in mold, mildew, and 
decay. 

Anchor ties that hold the veneer to the wall need to be placed at the proper spacing to ensure that 
the area of veneer attributed to each tie does not exceed the maximum allowed for the particular 
Seismic Design Category. For SDCs D1 and D2, the spacing should be such that each tie is 
supporting no more than 2 square feet of veneer. The ties also need to be placed such that the 
nail holding the tie to the wall is embedded into a stud and not just the sheathing material. 

Finally, if horizontal reinforcing wire is used, it should be a minimum size of W1.7 and should 
be placed at a maximum vertical spacing of 18 inches according to the 2002 Masonry Standards 
Joint Committee Code (ACI 530/ASCE 5/TMS 402, Section 6.2.2.10.2.4.). This wire is 
embedded in the horizontal mortar joints between the courses of brick. The brick ties that hold 


the veneer onto the wall behind it should be bent around the wire to improve the effectiveness of 
the tie in preventing the veneer from falling off the wall during an earthquake. 

5.3 COLD-FORMED STEEL HOUSES 

The IRC permits cold-formed steel framing prescriptive construction. This guide section 
discusses the general principles of earthquake-resistant design for cold-formed steel houses, 
specific IRC requirements important to earthquake performance, and above-code measures for 
improved earthquake performance. 

Cold-formed steel construction is, in most respects, like wood light-frame construction. The IRC 
contains provisions similar to the wood light-frame provisions for cold-formed steel floors, roofs, 
and walls. The weights of cold-formed steel floor, roof, and wall assemblies are the same or 
slightly lower than those with wood light-frame construction and result in very similar 
earthquake loads. The system resisting wind and earthquake loads most commonly consists of 
floor and roof assemblies acting as horizontal beams carrying loads to shear walls. Cold-formed 
steel shear walls carry wind and earthquake loads to the foundation. The IRC provides extensive 
illustrations of minimum construction requirements for cold-formed steel framing. One 
significant difference between steel and wood light-frame construction is that steel construction 
assumes that the framing is in-line  that is, that the rafters, studs, and floor joists line up so that 
the top channels of the walls and other elements are not subjected to compression and bending 
loads. Figure 5-13 shows a cold-formed steel house under construction. 

Figure 5-13 Cold-formed steel house under construction. 
Photo Courtesy of Dietrich Metal Framing -- a Worthington Industries Company 




5.3.1 Scope Limitations 

The IRC limits the scope of cold-formed steel houses permitted under the prescriptive provisions. 
Some scope limitations are found in IRC Chapter 3 while others are found in IRC Section R603. 

For all SDCs, steel light-frame houses are limited to two stories above grade; the maximum 
permitted plan dimensions are 60 feet perpendicular to truss or joist span and 36 feet parallel to 
truss or joist span (IRC Sections R505.1.1, R603.1.1, R804.1.1). For all SDCs, story height is 
limited to a 10 foot stud clear height plus a height of floor framing not to exceed 16 inches (IRC 
Section R301.3). For houses in SDCs D1 and D2, IRC Section R301.2.2.2.1 limits cold-formed 
steel wall assembly weights to 14 psf for exterior walls and 5 psf for interior walls. Floor and 
roof plus ceiling assembly weights are also limited to 15 psf by IRC Section R301.2.2.2.1. 

Although some interpretations of the IRC assume the code does not directly require that the 
irregularity limits of IRC Section R301.2.2.2.2 be followed, the same limits are imposed by 
AISIs Standard for Cold-formed Steel Framing  Prescriptive Method for One- and Two-
Family Dwellings (2001) for SDCs D1 and D2. Attention to the issues associated with 
irregularities is important to prevent torsional response overloading of critical elements in the 
wall. 

As for wood houses, the design of cold-formed steel houses is typically controlled by wind rather 
than earthquake loading through SDC C; as a result, there are no added IRC earthquake load 
requirements for SDC C. Per IRC Section R301.2.2.4.5, houses in SDCs D1 and D2 must 
conform to the requirements of the 2001 AISI Standard for Cold-Formed Steel Framing  
Prescriptive Method for One- and Two-Family Dwellings (AISI/COFS/PM). 

5.3.2 IRC Wind and Earthquake Requirements 

Unlike wood light-frame systems, the cold-formed steel provisions rely exclusively on bracing at 
exterior walls for wind and earthquake load resistance. IRC Section R603.7 requires that the 
exterior walls be fully sheathed. IRC Table R603.7 specifies minimum percentages of full-
height sheathing as a function of wind speed and exposure, roof slope, and story being braced. 
Per IRC Section R603.7, the minimum braced length is not permitted to be less than 20 percent 
of braced wall length and bracing panels must be a minimum of 4 feet in length to be counted 
towards the required percent with a minimum 4-foot-long bracing panel required at each corner 
of each exterior wall. The minimum wood structural panel sheathing thickness is specified as 
7/16 inch for oriented strand board and 15/32 inch for plywood. The minimum edge fastening is 
spaced 6 inches on center using No. 8 screws. All wood structural panel edges are to be blocked. 

A number of modifying factors are required to be applied to the tabulated percentage of full-
height sheathing. The tabulated bracing percentages are based on walls with an 8-foot clear 
height and must be multiplied by 1.10 for a 9-foot clear height and 1.20 for a 10-foot clear height 
(IRC Section R603.7). The required sheathing percentage is permitted to be multiplied by 0.6 
where hold-down devices having 4,300 lb capacity are provided at each end of each exterior wall 
(IRC Section R603.7.2). The minimum percentages of sheathing are based on a house aspect 
ratio (ratio of greater plan dimension to lesser plan dimension) of 1:1. The minimum 


percentages for the shorter walls must be multiplied by 1.5 and 2.0 for house aspect ratios of 

1.5:1 and 2:1 respectively (IRC Table R603.7, footnote c). The minimum percentages are 
permitted to be multiplied by 0.95 or 0.90 for hipped roofs, depending on roof slope (IRC Table 
R603.7, Footnote d). 

AISI/COFS/PM provisions for bracing in SDCs D1 and D2 are significantly different from IRC 
requirements for areas of less earthquake risk with respect to bracing approach, percentage of 
full-height sheathing, and detailing. The IRC basic bracing provisions (SDCs A, B, and C) do 
not require use of hold-downs and permit reduction of the percentage of full-height sheathing if 
hold-downs are provided. In contrast, AISI/COFS/PM basic bracing provisions for SDCs D1 and 
D2 require use of hold-downs at each end of each required full-height sheathing segment. An 
alternative approach allows hold-downs to only occur at each end of a braced wall line provided 
that the length of full-height sheathing is increased (up to 3x length otherwise required). This 
alternative approach is similar to the continuous structural panel sheathing (perforated shear 
wall) method used in wood light-frame construction. The user is referred to the AISI/COFS/PM 
document for full details of bracing requirements. 

IRC Section R505 addresses floor construction using cold-formed steel framing. The use of 
wood structural panel floor sheathing is implied by details and fastening requirements. IRC 
Section R503 controls selection of floor sheathing depending on the spacing of joists and 
material used. IRC Table R505.3.1(2) requires a minimum fastening of No. 8 screws at 6 inches 
on center on the perimeter of the sheathing panels and 10 inches on center in the field. This is 
assumed to be at supported edges with no requirements for blocking at other edges. Similarly, 
IRC Section R804 implies wood structural panel roof framing, while IRC Table R804.3 requires 
minimum No. 8 screws at 6 inches on center at edges, 12 inches on center in the field, and 6 
inches on center at gable end trusses. 

It is very important to use appropriate framing materials and fasteners for the performance of the 
wall bracing and floor and roof systems to be acceptable. IRC Sections R505.2, R603.2, and 
R804.2 specify material grade, corrosion protection, and identification of the materials required. 
IRC Sections R505.2.4, R603.2.4, and R804.2.4 specify fastener requirements. Steel to steel and 
wood structural panel sheathing to steel connections are required to be made with self-drilling 
tapping screws conforming to SAE J78 with a Type II coating in accordance with ASTM B633. 
Screws are required to extend through the steel a minimum of three exposed threads. Steel to 
steel connections are to be made with self-drilling tapping screws at a minimum edge and center-
to-center distances of 0.5 inches. Connections are typically specified with No. 8 screws, but 
adjustments are given for use of No. 10 and No. 12 screws. Wood structural panel sheathing to 
steel framing connections are to be made with No. 8 self-drilling tapping screws at minimum 
head diameter of 0.292 inches, countersunk heads, and a minimum edge distance of 3/8-inches. 
Gypsum wallboard ceilings are to be attached with minimum No. 6 screws conforming to ASTM 
C 954. 


Above-code Recommendations: There are no IRC maximum spacing limitations between 
lines of cold-formed steel bracing walls corresponding to the wood light-frame braced wall 
line spacing limits of 25 and 35 feet. With wood light-frame construction, these spacing 
limitations are intended to help distribute the bracing walls in proportion to the house mass 
(dead load) as well as to limit the loading to the floor and roof. This helps to reduce concerns 
regarding house rotation due to torsional irregularities and concerns regarding irregular floor 
and roof system shapes. For cold-formed steel houses, the maximum 60-foot house 
dimension provides some limit for earthquake loads in the floor and roof; however, for 
improved earthquake performance, it is recommended that interior cold-formed steel 
braced wall lines be added such that the distance between braced wall lines does not 
exceed 35 feet. This will help lower the loads within any given wall segment and better 
distribute the earthquake loads throughout the house. The cost associated with adding 
additional bracing walls would be consistent with the percentage increase in wall length. 

For cold-formed steel houses in all SDCs, the irregularity limitations developed for 
wood light-frame houses should be applied. 

5.3.3 Quality Control 

Quality control for cold-formed steel construction can play a major role in the satisfactory 
performance of the house. Important steps in steel construction include ensuring proper load 
path connections. Just as in wood light-frame construction, the connections between the end 
studs and the floor or foundation become critical for the strength and stiffness to be the highest 
possible. Strong connections ensure the load path will function properly. 

Adequate lateral bracing of all floor, roof, and wall framing members is important to prevent the 
individual members (joists or studs) from buckling. Cold-formed members have to be braced 
well before they can support significant compression loads. Attaching sheathing to the stud or 
joist typically will provide this lateral support. 

Screws that attach the sheathing to the framing should be inspected to ensure that they are not 
overdriven (the resulting loss in capacity will be similar to that experienced in wood light-frame 
construction when nails are overdriven). Just as in wood framing, the power tools used to install 
the screws can easily over drill the screws, leaving the head of the screw below the surface of the 
sheathing material and significantly increasing the chances of pull through. 

5.4 MASONRY WALL HOUSES 

While many houses use masonry walls for foundations, some houses are built using masonry for 
the walls above grade. Figure 5-14 shows a house constructed using masonry for the walls. The 
IRC permits construction of houses with masonry walls in accordance with prescriptive 
provisions for concrete, brick, and stone masonry. General provisions for masonry construction 
appear in IRC Section R606. Additional details of construction for concrete unit masonry, brick 
masonry, and masonry grouting appear in IRC Sections R607 through R609. However, it is 


important that all masonry cells with reinforcement be grouted, and care should be taken to 
ensure that the grout is consolidated. Discussed below are the general principles of earthquake-
resistant design for masonry wall houses, specific IRC requirements important to earthquake 
performance, and above-code measures for improved earthquake performance. 

Figure 5-14 House constructed with masonry walls. 


Masonry wall construction is significantly heavier than light-frame wall construction; however, 
in most respects, the same principles of earthquake-resistant construction apply. The system 
resisting wind and earthquake loads most commonly consists of floor and roof assemblies acting 
as horizontal beams carrying loads to bracing walls. Masonry bracing walls carry wind and 
earthquake loads down to the foundation with masonry walls primarily resisting loads acting in 
their strong direction  that is, parallel to the wall. Because masonry walls are heavier than stud 
framing, it follows that the earthquake loads resisted by each wall element and connection will 
be greater. As a result, the design of the connections is as important as the proportioning of the 
floor and roof systems and bracing walls. Significant earthquake loads also develop 
perpendicular to the wall surface due to the walls weight. These loads tend to pull the walls 
away or push them toward the floor or roof, making wall to floor or roof anchorage critical. 
Damage from insufficient anchorage or connectivity similar to that shown in Figure 5-12 can 
result if these connections are not strong enough. 

5.4.1 Scope Limitations 

One of the primary approaches the IRC uses to deal with the increased earthquake loading 
associated with masonry wall houses is to limit the scope of houses permitted under the 
prescriptive provisions. IRC Section R301.2.2 introduces earthquake provisions applicable to 
houses in SDCs D1 and D2. The first scope limitation is found in IRC Section R301.2.2.2.1, 
Weights of Materials, which limits masonry walls to 8 inches thick and 80 pounds per square 
foot, which essentially prohibits the use of rubble stone masonry (IRC Section R606.2.2) as well 
as relatively thick concrete or brick masonry walls. IRC Section R301.2.2.4.3 triggers the 
reinforcement and configuration requirements of IRC Section R606.11.3 for all houses in SDC 
D1 and of Section R606.11.4 for all houses in SDC D2. In SDCs D1 and D2, the scope of 


prescriptive construction of houses with masonry walls is effectively limited to walls one story in 
height with up to 9 feet between lateral supports; beyond this, engineered design is required. A 
final scope limitation in IRC Section R301.3, Item 3, limits masonry house walls to a clear height 
of 12 feet with a maximum floor framing depth of 16 inches. An additional 8 feet of masonry 
wall height is permitted for gable end walls. It is also worth noting that IRC Section 
R301.2.2.2.2, Item 7, prohibits the mixing of light-frame and masonry construction such that 
light-frame walls would be required to support earthquake loads due to masonry wall 
construction (e.g., mixing masonry and light-frame walls on the same story.) 

5.4.2 Wall Lateral Support, Reinforcing, and Anchorage 

General requirements for lateral support of masonry walls for all SDCs are provided in IRC 
Section R606.8. Walls are permitted to be laterally supported by cross-walls, pilasters, 
buttresses, or structural frame members where walls span horizontally between supports and by 
floors and roofs where walls span vertically between supports. Minimum reinforcing 
requirements apply only for masonry laid in stack rather than running bond and for interior non-
load bearing walls where they intersect with other masonry walls. IRC Section R606.10 requires 
anchorage of masonry walls to floor and roof systems in accordance with specific details. 
However, in recent U.S. earthquakes, similar details have been observed to be susceptible to 
damage. Performance of some of these details for supporting site-built masonry walls also is 
thought to be problematic. Several details on how to comply with the IRC provisions are 
provided in the code itself. 

IRC Section R606.11.1.1 triggers additional requirements for floor and roof systems for houses 
in SDCs D1 and D2. Floor and roof wood structural panel sheathing is required to have all edges 
blocked and nailed at 6 inches on center. Where the floor or roof system is long and narrow 
(more than twice as long as it is wide), the nailing at sheathing edges is required to be reduced 
from 6 to 4 inches on center. 

For houses in SDC D1, IRC Section R606.11.3 places further limits on masonry walls using 
prescriptive provisions: 

 
Masonry walls are limited to one-story, 
 
Additional reinforcing is required, 
 
Reinforcement detailing provisions for masonry columns apply, and 
 
Type N mortar and masonry cement are prohibited. 


Similar additional requirements are triggered by IRC Section R606.11.4 for SDC D2. SDCs D1 
and D2 requirements are illustrated in IRC Figure R606.10(3). Again, specific design is required 
for wall to framing anchorage per IRC Section R606.11.2.2.1; thus, the anchorage to roof 
framing illustrated in IRC Figure R606.10(3) may not be the only alternative. 

In the design of wall anchorage to the floor and roof, it is important that a direct tension tie be 
provided from the wall to the floor or roof framing members if framing is perpendicular or to 
blocking that is continued across the floor or roof if framing is parallel. Detailing in IRC Figures 


611.8(1) to (7) shows the concept of this connection when used with concrete walls, but they are 
equally applicable to masonry construction. 

5.4.3 Parapet walls 

IRC Section R606.2.4 limits masonry thickness and height of parapets in all SDCs and sets the 
requirements for reinforcing walls of houses in SDCs D1 and D2. Parapet walls are easily 
damaged in an earthquake and attention to placement of reinforcement and bracing to stabilize 
the parapet is important. 

5.4.4 Problematic Gaps Between Prescriptive and Engineered Construction 

The current IRC requirements for masonry construction reveal significant gaps between 
prescriptive and engineered construction, creating a significant opportunity to employ above-
code measures to improve performance. 

For areas of high earthquake risk, one notable omission is the lack of a limit on the amount of 
opening in masonry walls. Light-frame provisions require a percentage of the wall to be solid to 
provide bracing so that the bracing wall strength is in proportion to the earthquake load, but the 
same requirement is not imposed for masonry walls. This is of particular concern when the 
house layout results in front and back walls with a large percentage of openings for doors and 
windows. 

Above-code Recommendation: Each exterior wall and each interior bracing wall should 
have at least one, and preferably two, sections of solid wall not less than 4 feet in length. 
Further, sections of solid wall should not be spaced more than 40 feet on center and 
should be placed as symmetrically as possible (Figure 5-15). The provisions of IRC 
Section R611.7.4 for ICF walls illustrate a reasonable approach to regulation of minimum 
wall length. The improvement requires only reasonable planning and should not result in 
higher construction costs. 

IRC Section R403.1 requires that exterior walls be supported on continuous concrete or masonry 
footings and regulates minimum footing width and depth, but nothing in the IRC appears to 
require that other masonry walls be supported on foundations. 

Above-code Recommendation: It is vital that all masonry walls be supported on 
substantial continuous footings extended to a depth that provides competent bearing. If 
this is not done the walls have a high probability of being damaged due to uneven settlement. 

There are no IRC maximum spacing limitations between lines of masonry bracing walls 
corresponding to the light-frame limits of 25 and 35 feet. In light-frame construction, these 
spacing limitations are intended to help distribute the bracing walls in proportion to the house 
mass (dead load) as well as to limit the loading to the floor and roof, which helps prevent 
damage to the house from rotation due to torsional irregularities caused by irregular floor and 
roof system shapes. 


Figure 5-15 Symmetric layout of walls to distribute loads uniformly and 
thereby prevent torsion. 

Above-code Recommendations: Because prescriptive detailing is not available for the 
addition of interior masonry braced wall lines, it is recommended that the distribution 
of bracing walls be carefully balanced and that the floor and roof plans use simple 
rectangular shapes without indentions, bump-outs, or openings. The cost associated with 
this improvement is proportional to the length of wall added in relation to the initial total wall 
length. 

The IRC also does not regulate house irregularities for masonry construction. The concepts 
behind IRC Section R301.2.2.2.2, Irregularities 1 through 6, should apply equally to a 
masonry wall house. The exceptions to Irregularities 2 and 5 also can be applied; 
however, the rest of the exceptions are not applicable. 

Above-code Recommendation: Solid portions of wall should be stacked from floor to 
floor and masonry walls should be continuous from the top of the structure to the 
foundation. Masonry walls not directly supported on walls below require engineered 
design for gravity load support. Design for earthquake and wind loads also should be 
provided. The cost of this improvement is negligible; it requires only proper planning when 
laying out the wall positions for the house. 


Above-code Recommendation: Running bond lay up of masonry units is inherently 
much stronger than a stack bond lay up. For concrete masonry, use of open end units at 
locations of vertical reinforcement and use of bond beam units for horizontal 
reinforcement also help to increase the interlocking of the masonry construction, 
thereby increasing strength. The cost associated with using running bond rather than 
stacked bond masonry is minimal. The cost of bond beam masonry units for placing 
horizontal reinforcement at the top of each wall segment would be significantly less than 1 
percent of the cost of the original structural system but would dramatically improve the 
performance of the wall and connection to the floor or roof framing. 

Above-code Recommendation: Any of the many required or recommended measures 
for areas of high earthquake risk would improve the performance of masonry wall 
houses in areas of lower earthquake risk as well as in high-wind areas. Priorities 
include provision for reinforcing such as that shown in IRC Figure R606.10(2), wall 
anchorage using details developed to resist out-of-plane wall loads such as those shown 
in IRC Figures R611.8(1) to (7) for walls, minimum length of bracing walls, and a 
maximum spacing of bracing wall lines. 

5.4.5 Quality Control 

Quality control during masonry construction can play a major role in the satisfactory 
performance of the masonry. The following should be monitored: 

 
Proper placement of reinforcing in masonry. Unless specifically designed otherwise, 
reinforcing should be located as near the centerline of the masonry cavity as possible. In 
no case should reinforcing be closer than 5/8 inch to a masonry unit wall. 
 
Anchor bolts should be secured into place before grout placement (not placed into grout 
following the pour). 
 
Excess mortar and other obstructions should be cleaned from the cavity to allow free 
placement of grout. 
 
Consolidation of the grout after it is placed into the cavity is necessary to eliminate voids. 


5.5 INSULATING CONCRETE FORM (ICF) WALL HOUSES 

The IRC permits construction of houses with concrete walls in accordance with the prescriptive 
provisions of IRC Section R611 for ICF (insulating concrete form) walls and Section R612 for 
conventionally formed walls. ICF walls are concrete that is cast into forms that remain in place 
to serve as house insulation. The code covers three geometries of ICF forms: flat, waffle-grid, 
and screen grid forms. Both the waffle and screen grid consist of concrete cast into a series of 
interconnected horizontal and vertical cores within the insulation. Interior and exterior wall 


finishes are applied over the ICF wall form. ICF walls may be used for the full height of the 
house or for the lower story only with light-frame walls above. The provisions of IRC Section 
R611 are based on the use of light-frame floor, roof, and ceiling assemblies in ICF wall houses. 
The light-frame construction can be either wood or steel light-frame; however, current detailing 
developed for areas of high earthquake demand provides solutions for wood light-frame floors 
and roofs. This section discusses general principles of earthquake-resistant design for ICF wall 
houses, specific IRC requirements important to earthquake performance, and above-code 
measures for improved earthquake performance. An example of ICF construction is shown in 
Figure 5-16. 

Figure 5-16 Insulated concrete form 
house under construction. 


ICF wall construction, like masonry, is significantly heavier than light-frame wall construction. 
In most respects, however, the same principles of earthquake-resistant construction apply. The 
system resisting wind and earthquake loads consists of floor and roof assemblies acting as 
horizontal beams carrying loads to bracing walls. ICF bracing walls carry wind and earthquake 
loads down to the foundation with ICF walls primarily resisting loads acting in their strong 
direction, parallel to the wall (shear walls). Because ICF walls are heavier than light-frame 
walls, the earthquake loads to be resisted by each wall element and connection will be greater. 
As a result, proportioning of the floor and roof systems and bracing walls is important as are the 
connections between the walls and the floor and roof framing. Significant earthquake loads also 
develop perpendicular to the wall surface due to the wall weight. These loads tend to pull the 
walls away or push them toward the floor or roof, making wall to floor and wall to roof 
anchorage very important. 

5.5.1 Scope Limitations 

One of the primary approaches the IRC uses to address the increased earthquake loading with 
ICF wall houses is to limit the scope of houses permitted under the prescriptive provisions. For 
ICF, some scope limitations are found in Chapter 3 while others are found in IRC Section R611. 
For all Seismic Design Categories, ICF wall houses are limited to two stories above grade. The 
maximum permitted house plan dimension is 60 feet, and framing clear spans are limited to 32 
feet for floors and 40 feet for roofs (IRC Section R611.2). For houses in SDCs D1 and D2, IRC 
Section R301.2.2.2.1 limits the dead weight of concrete walls to 85 pounds per square foot for 
walls that are 6 inches of solid concrete. IRC Section R611.7.4 limits the minimum nominal 
thickness of the wall to 5-1/2 inches. Application of these criteria and the maximum unit 
weights in IRC Table R611.2 permits use of 5-1/2-inch flat ICF walls, 6- and 8-inch waffle-grid 


walls, and 6-inch screen grid walls. The weight of interior and exterior wall finishes is limited to 
8 psf (IRC Section R611.2). Floor and roof plus ceiling assembly weights also are limited by 
IRC Section R301.2.2.2.1 as for the other materials. 

For houses in SDCs D1 and D2, IRC Section 611.2 limits the scope to rectangular houses with a 
maximum floor and roof aspect ratio (length to width ratio) of 2:1 and requires that the ICF walls 
be aligned vertically (stacked) without cantilevers or setbacks. An additional limitation is that 
the gable end portion of walls must be of light-frame rather than ICF construction. Further, the 
top of the ICF wall must be anchored to a required attic floor as described in IRC Section 
R611.9. 

For houses in SDCs D1 and D2, IRC Section R301.2.2.2.2 requires engineered design for 
irregular portions of houses. The irregularities are discussed in detail in an earlier section of this 
guide. The exceptions to irregularities are mostly applicable to wood-frame construction and so 
do not apply. The 2003 IRC as printed refers to irregularities in IRC Sections R301.2.2.7 and 
R301.2.2.9, which do not exist; the intent was to reference IRC Section R301.2.2.2. IRC Section 
R611.8.3.1 repeats the prohibition of vertical offsets in floor framing (such as split levels) in 
SDCs D1 and D2. 

IRC Section R301.2.2.4.4 triggers the requirements of IRC Sections R611 and R612 for all 
houses in SDCs D1 and D2. These requirements include all of the reinforcement and detailing 
provisions for concrete walls that provide the continuity and connectivity for the system to 
function effectively under wind or earthquake loads. 

5.5.2 Wall Reinforcing and Anchorage 

Reinforcing steel is of primary importance for concrete and masonry house performance. The 
reinforcing steel is what ties the components together and provides toughness to the concrete 
when it cracks. General requirements for reinforcing of ICF walls are provided in IRC Sections 
R611.3 through R611.5. Reinforcing is required for all ICF walls across all SDCs. IRC Section 
R611.7.1.2 imposes additional minimum reinforcing size and spacing limits for houses in SDCs 
D1 and D2. IRC Section R611.7.1.3 provides similar limits for horizontal reinforcing and 
includes requirements to provide for continuity of reinforcing around house corners and 
termination of horizontal reinforcement. Reinforcement is required to be doweled into the 
foundation and extend for the full height of the ICF wall, including the parapet if applicable. 
Lap splicing of reinforcing is permitted. 

Like light-frame wall bracing provisions, ICF wall provisions require a minimum length or 
percentage of the wall to be solid (without openings) so that the bracing wall strength is in 
proportion to the earthquake load. The minimum required lengths of bracing wall for ICF 
construction in each Seismic Design Category are defined in IRC Section R611.7.4. Minimum 
bracing lengths must be provided for wind load parallel and perpendicular to the ridge per IRC 
Tables 611.7(9A) through (10B). The required bracing length must be provided using wall 
segments not less than 2 feet in width. For all houses in SDCs D1 and D2, the bracing length 
requirements of IRC Table R611.7(11) also must be met using wall segments that are not less 


than 4 feet in width. The increase from 2 to 4 feet provides a wall that is stronger and much 
better able to withstand cyclic earthquake loading. 

Also of primary importance for house performance is the connection of the ICF walls to the 
floors and roofs, both for bracing loads parallel to the wall and out-of-plane loads perpendicular 
to the wall. The IRC addresses three types of connections to wood light-frame floors and roofs: 

 
Top bearing connections where the light-frame floor bears on the top of the ICF wall with 
light-frame walls above (IRC Section R611.8.1), 
 
Ledger-bearing connections where the floor or roof is supported off the face of the ICF 
wall by a wood ledger (IRC Section R611.8.2), and 
 
Top-bearing connections where the light-frame roof bears on top of the ICF wall (IRC 
Section R611.9). 


It is important that a direct tension tie be provided from the wall to the floor or roof framing or 
blocking to resist loads perpendicular to the wall (this connection keeps the wall from pulling 
away from the floor or roof framing). This does not necessarily occur for lower Seismic Design 
Categories. For top bearing connections, sill plate to framing connections rely on minimum 
prescriptive fastening at framing perpendicular to the wall and no provision exists for a tension 
connection at framing parallel to the wall. For ledger-bearing connections, the direct tension ties 
shown in IRC Figures R611.8(2) through R611.8(5) are not required for lower SDCs, leaving the 
remaining ledger connection susceptible to cross-grain tension failure as illustrated in Figure 5

17. Very specific prescriptive anchorage requirements capable of resisting perpendicular-to-wall 
loads are given for sill or ledger to wood framing connections for houses in SDCs D1 and D2. 
Included are angle clips from sill to joist or blocking, continuous straps tying blocking together, 
and direct tension ties at ledger-bearing connections. Detailing is illustrated in IRC Figures 
R611.8(1) to 611.8(7) and R611.9. The specification of a maximum bolt size of 3/8-inch 
diameter for top-bearing connections is intended to favor bending of the bolt as a failure method 
over a brittle failure of the concrete or splitting of the wood. Larger bolt sizes should not be 
substituted. 

Figure 5-17 Illustration of cross-grain bending of wood ledger. 



IRC Section R611.8.3 addresses requirements for floor and roof systems. The floor and roof 
systems act as horizontal beams carrying wind and earthquake loads to the bracing walls. The 
increased weight of the ICF walls results in increased earthquake load in the floor and roof 
systems. For SDCs A through C, use of wood structural panel sheathing is required, and the IRC 
minimum fastening schedules of 6 inches around the perimeter of each sheet and 12 inches along 
intermediate supports are referenced. IRC Section R611.8.3.1 triggers additional requirements 
for houses in SDCs D1 and D2. Minimum panel thicknesses are specified for floor and roof 
wood structural panel sheathing, minimum nail diameter and penetration are specified, all panel 
edges are required to be blocked, and the panel edge nailing is required to be 4 inches on center 
in SDC D1 and 3 inches on center in SDC D2. 

IRC Section R611.9.1 has similar requirements for roof construction but also contains two 
notable variations. First, the requirements apply to houses in SDCs D1 and D2. Second, where 
gable end wall conditions occur, it is required that a wood structural panel sheathed attic be 
provided to support the gable end wall (for truss roofs, the wood structural panel sheathing can 
be applied to the bottom of the truss between the truss chord and the ceiling finish material) and 
that the wall be anchored in accordance with requirements for anchorage to floor and roof 
diaphragms. IRC Section R611.2 requires that the gable-end portion of this wall be light-framed. 
The writers of the ICF provisions preferred this solution over allowing increased ICF wall 
heights at the gable end due to the increased loads associated with concrete and the additional 
bracing requirements. 

IRC Section R403.1 requires that exterior walls be supported on continuous concrete or masonry 
footings and regulates minimum footing width and depth. If interior ICF walls are provided, 
these too should be supported on substantial continuous footings extended to a depth that 
provides competent bearing. 

5.5.3 Problematic Gaps Between Prescriptive and Engineered Construction 

The current IRC requirements for ICF wall construction reveal some gaps between prescriptive 
and engineered construction creating an opportunity for above-code measures to improve 
performance in areas of high earthquake risk. 

There are no IRC maximum spacing limitations between lines of ICF bracing walls 
corresponding to the light-frame limits of 25 and 35 feet. In light-frame construction, these 
spacing limitations are intended to help distribute the bracing walls in proportion to the house 
mass (dead load) as well as to limit the loading to the floor and roof. This helps to reduce 
concerns regarding house rotation due to torsional irregularities and concerns regarding irregular 
floor and roof system shapes. For ICF wall houses, the scope limitation to rectangular houses 
reduces the likelihood of rotational behavior. 

Above-code Recommendation: Careful balancing of bracing walls around the house 
perimeter is recommended to further limit torsional behavior. The maximum 60-foot 
house dimension will provide some limit for earthquake loads in the floor and roof. The cost 
of distributing the wall segments around the perimeter of the house should not result in any 
increased cost for construction. 


For ICF wall houses, some of the irregularity limitations developed for light-frame houses have 
been made applicable. The concepts behind IRC Section 301.2.2.2.2, Irregularities 1 through 6, 
apply equally to an ICF wall house. 

Above-code Recommendation: Any of the many required or recommended measures 
for areas of high earthquake risk would improve performance of ICF wall houses in 
areas of lower earthquake risk as well as in high-wind areas. Most of the 
recommendations would improve house performance in high-wind events. Priorities 
include wall anchorage using details developed to resist out-of-plane wall loads such as 
those shown in IRC Figures R611.8(2) to R611.8(7). 

5.5.4 Quality Control 

Quality control for ICF wall construction can play a major role in the satisfactory performance of 
the house. Important steps in ICF construction include: 

 
Reinforcing should be placed properly. Unless specifically designed otherwise, 
reinforcing should be located as near the centerline of the ICF cavity as possible but at 
least within the middle third of the wall. 
 
Anchor bolts should be secured into place before concrete placement (not placed into 
concrete following placement of the concrete) to prevent air pockets from forming around 
the bolt which reduces its strength. 
 
Concrete should be consolidated as it is placed into the forms to prevent voids from 
forming. 



Chapter 6 
ROOF-CEILING SYSTEMS


Woodframe roof-ceiling systems are the focus of this chapter. Cold-formed steel framing for a 
roof-ceiling system also is permitted by the IRC but will not be discussed; rather, the reader is 
referred to the AISI Standard for Cold-Formed Steel Framing  Prescriptive Method for One- 
and Two-Family Dwellings (2001) for guidance. Most of the recommendations for improving 
the earthquake performance of woodframe roof-ceiling systems also apply to cold-formed steel 
construction since the systems are very similar. 

6.1 GENERAL ROOF-CEILING REQUIREMENTS 

Woodframe roof-ceiling systems, regardless of the pitch of the roof, form a roof diaphragm that 
transfers earthquake lateral loads to braced walls in the story level immediately below the roof in 
the same manner that floors transfer loads from interior portions of the floor to the braced wall 
lines of the story below. The lateral loads in the roof-ceiling are based on the mass of the roof-
ceiling assembly and a portion of the mass of the walls in the story immediately below the roof. 

Woodframe roof-ceilings typically consist of repetitive rafters and ceiling joists or prefabricated 
(engineered) trusses at a prescribed spacing. They are sheathed with either spaced solid wood 
boards or with wood structural panels attached to the top surface of the rafter or truss. Figure 6-1 
illustrates this type of roof and ceiling framing system. Roof members also can consist of 
repetitive beams spaced further apart than rafters, either with or without ceiling joists. 

Figure 6-1 Typical light-frame roof-ceiling system. 



Depending on the roof shape chosen for a house, hip and valley beam members may be needed 
where intersecting rafters change the direction of their span. Depending on the slope of the roof, 
hip and valley rafters can experience very high loads when supporting long-span rafters. 
Therefore, purlins are sometimes provided below and perpendicular to rafters. The purlins, in 
turn, are supported by a stud or post attached to a wall or beam below. Ridge boards also are 
often provided at the peak of a roof where sloping rafters meet. Blocking between rafters (or 
trusses) is used at the bearing points of rafters and ceiling joists to prevent lateral movement or 
rolling of the rafter. Finish materials such as gypsum board are typically applied to the bottom 
surface of ceiling joists or the bottom chord of a truss if the space below is to be occupied. 

Rafters, purlins, ridge boards, and hip or valley members can be sawn lumber, end-jointed 
lumber, or any one of a variety of prefabricated (engineered) members. Examples of engineered 
lumber include wood I-joists or solid rectangular structural composite members such as parallel 
strand lumber (PSL), laminated veneer lumber (LVL), or laminated strand lumber (LSL). Roof 
beams and blocking can be either sawn lumber or engineered lumber. 

The minimum required size and maximum span and spacing of roof rafters or beams, ceiling 
joists, and trusses is based on providing adequate support for dead and live vertical loads 
prescribed by the code. Snow loads must be considered for rafters, and attic storage must be 
taken into account for ceiling joists. Vertical deflection of rafters and ceiling joists is another 
design consideration that may limit the maximum span of these members. Rafter spans listed in 
prescriptive tables are based on the horizontal projection of the rafter rather than being measured 
along the slope, which would be a greater distance. 

Tables in IRC Chapter 8 and similar tables in other documents such as those published by the 
American Forest and Paper Association (AF&PA) or engineered lumber manufacturers are 
available for use in selecting the proper combination of size, span, and spacing of most roof-
ceiling framing members. Depending on the roof pitch, certain roof members require 
engineering to determine their size. For a roof pitch less than 3:12, the size of ridge boards and 
hip or valley members must be individually determined based on their spans and the span of the 
rafters they support. 

6.2 SPECIAL FRAMING CONSIDERATIONS 

Roof rafters must either be tied together at the ridge by a gusset plate or be framed to a ridge 
board. Ridge boards in roofs having a pitch of 3:12 or greater must be at least 1x nominal 
thickness and at least the same depth as the cut end of the intersecting rafters. Valley and hip 
members in a roof having a pitch of 3:12 or greater must be at least 2x nominal thickness and at 
least the same depth as the cut end of the rafters. Because the cut end depth of a rafter increases 
with increasing roof pitch, a 2x8 rafter will need a 1x10 nominal ridge board and a 2x10 nominal 
hip or valley member for a pitch up to 9:12. At a pitch exceeding 9:12, a 1x12 or 2x12 will be 
needed because the cut end of a 2x8 rafter will be greater than the 9-1/4 inch actual depth of a 
1x10 or 2x10 nominal member. Figure 6-2 shows a 1x12 ridge board for a 12:12 pitch condition 
with the dimension for the cut end of a 2x8 rafter. 


Figure 6-2 Ridge board for 12:12 pitch roof. 


One additional consideration when using nominal 1x ridge boards is that ceiling joists or rafter 
ties are needed at the top plate of the supporting walls to prevent the walls from spreading 
outward. The ceiling joists or rafter ties act as a brace to resist the outward thrust of the rafters at 
the wall support ends of rafters. In the absence of ceiling joists or ties at the wall top plate, 
rafters must be supported at the ridge by a beam designed for the support of rafter loads. In 
addition, where a nominal 1x ridge is used and a 2x valley or hip member intersects the ridge, the 
valley or hip member must be supported by a stud or post attached to a bearing wall below to 
transfer the high loads associated with hip and valley rafters. 

Above-code Recommendations: A special condition can occur at the gable ends of a roof. 
When the exterior wall at the gable end has its double top plate level with the low ends of the 
roof framing, the wall studs are not continuous to the roof sheathing along the gable edge. A 
section view of this condition is shown in Figure 6-3. To provide above-code performance, 
the framing extending above the top plate to the level of the sloping gable end roof 
sheathing should be braced at regular intervals of not more than 4 feet on center at both 
the wall top plate level and along the top edge of the sloping roof edge. This bracing 
permits the top of the exterior wall and the framing extending above to resist the lateral loads 
that are acting perpendicular to the wall. Without this bracing, the framing above the double 
top plate could be easily displaced because a hinge can form where the gable end wall 
framing attaches to the top plate. 

Another above-code alternative to the framing shown in Figure 6-3 would be to provide 
wall studs that are continuous to a sloping double top plate located just below the roof 
sheathing (balloon framing). When continuous studs are provided, there is no weak 
location for a hinge to form. However, when this method is used, the required stud size, 
spacing, and maximum height must comply with IRC Table R602.3.1 or the wall studs will 
need to be engineered by a design professional. Bracing a gable end wall also is important for 
providing resistance to high winds, especially for roofs with a steep pitch. 


Figure 6-3 Gable end wall or gable truss bracing. 


6.3 BLOCKING AND LATERAL LOAD PATHS FOR ROOF SYSTEMS 

Rafters and ceiling joists having a nominal depth-to-thickness ratio exceeding 5:1 (e.g., 2x10) 
need blocking at their points of bearing to prevent them from rotating or displacing laterally from 
their intended position. Rotation loads on rafters occur when the roof sheathing is resisting 
lateral loads perpendicular to the rafter because these loads are actually trying to move the top 
edge of the rafter sideways. Preventing rotation is typically accomplished by installing full-
depth solid blocking along wall top plates between rafters and ceiling joists. Figure 6-4 
illustrates blocking installed between adjacent pairs of a ceiling joist and rafter that are bearing 
on an exterior wall. 


Figure 6-4 Blocking at rafters to exterior wall. 


However, when the nominal size of a ceiling joist or rafter is 2x10 or smaller, blocking over the 
exterior wall may be omitted. This would be likely to occur where the roof overhang is enclosed 
by a soffit. Without an enclosed soffit, blocking is typically provided between rafters along the 
exterior wall line regardless of the rafter size to serve as a barrier between the outside and the 
attic space. 

Because attics (or spaces between rafters where ceiling finish is attached to the bottom of the 
rafter) require ventilation openings, the spaces between rafters along exterior walls are typically 
used to provide some portion (if not all) of the required attic ventilation opening. When blocking 
is installed, ventilation openings often are provided by drilling holes in the blocking; when 
blocking is not required, ventilation can be provided through the entire opening space between 
rafters. 

Although blocking may not be required for 2x10 and smaller rafters, there still must be a load 
path for lateral loads in the roof sheathing to reach the exterior braced walls immediately below 
the roof. The most direct load path is for the roof sheathing to be edge nailed to blocking 
between each rafter. That blocking is then nailed to the wall top plate with three 8d nails per 
rafter space as prescribed in IRC Table 602.3(1). This load path is shown in Figure 6-4. 
Although alternate load paths are certainly possible, the one shown is the most direct and is 
essentially the same load path the IRC provides between a floor and the braced walls below that 
floor. 


Above-code Recommendation: In Seismic Design Categories D1 and D2, blocking is 
recommended between rafters along exterior wall lines to provide a surface for the edge 
nailing of roof sheathing and to provide a very direct load path to the top plate of the 
exterior braced walls. When this blocking is provided, it also is necessary to ensure that 
minimum attic ventilation opening requirements are met. Depending on the attic area being 
ventilated, this can be accomplished by drilling holes in the blocking and, when more opening 
area is necessary, by adding gable end wall openings or ridge vents. 

6.4 CONNECTION OF CEILING JOISTS AND RAFTERS TO WALLS BELOW 

Ceiling joists and rafters (or trusses) are required to be connected to the top plate of supporting 
walls as specified in IRC Table R602.3(1). These connections also provide a portion of the load 
path that transfers loads from the roof diaphragm into the braced walls below. Ceiling joists 
require a toe-nailed connection to the top plate using three 8d box or common nails. Rafters also 
require a toe-nailed connection to the top plate using two 16d box or common nails. Blocking 
installed between the rafters or ceiling joists requires a toe-nailed connection to the top plate 
using a minimum of three 8d box or common nails in each block. Toe nailing must be done 
correctly if the needed transfer of loads is to occur; therefore, ensure that the nails do not split the 
wood. In high-wind areas, light-gage steel connectors often are used in place of these toe-nailed 
connections. The use of commercially available light gage steel connectors in place of toe nails 
can reduce wood splitting and provide more reliable load transfer. Additional information on 
proper toe-nailing installation is illustrated in Figure 4-8. 

6.5 ROOF SHEATHING 

Wood boards installed either perpendicular or at an angle to the rafters (or trusses) or wood 
structural panels can be used as roof sheathing. IRC Table R803.1 specifies the minimum 
thickness for wood board roof sheathing for various spacings between roof rafters, trusses, or 
beams. 

Above-code Recommendation: Solid wood board roof sheathing is rarely used in modern 
housing construction except perhaps along roof eave overhangs. When wood boards are used 
and are installed perpendicular to rafters they provide a very weak diaphragm with little 
stiffness. As a result, solid wood board sheathing installed perpendicular to rafters 
should not be used in Seismic Design Categories C, D1, and D2. Wood boards installed 
diagonally would provide much better diaphragm capacity but are very rarely used in modern 
housing construction. 

Wood structural panel roof sheathing is the most common roof sheathing used in current 
construction. The minimum thickness is based on rafter (or truss) spacing and the grade of 
sheathing panels selected. IRC Table R503.2.1.1(1) is used to determine the minimum required 
thickness for wood structural panel roof sheathing materials for a variety of rafter spacings. For 
roofs, the short direction panel joints between wood structural panels can be either staggered or 


not staggered. Typical wood structural panel roof sheathing installation using staggered joints is 
illustrated in Figure 6-5. 

Figure 6-5 Typical roof diaphragm sheathing nailing when wood structural panels are used. 


When the roof covering consists of wood shingles or shakes, IRC Chapter 9 permits the use of 
spaced wood boards as roof sheathing. However, spaced wood board sheathing is not permitted 
in Seismic Design Category D2. This limitation in SDC D2 is intended to provide a stiffer and 
stronger roof diaphragm that will resist the larger lateral loads anticipated in SDC D2. When 
wood boards are used, they typically are nailed to each rafter with only two 8d box or common 
nails. Wood structural panel sheathing results in the stiffest roof diaphragms, and the panels 
typically are fastened to the rafters with 8d box or common nails spaced at 6 inches along 
supported edges and 12 inches in the field of the panel. When spaced wood boards are used, 
even fewer nails are provided than with solid wood board sheathing and even less lateral 
resistance results. 

Above-code Recommendation: Spaced wood board sheathing is not recommended for 
use in Seismic Design Categories C and D1. 


6.6 LATERAL CAPACITY ISSUES FOR WOOD FRAMED ROOFS 

The lateral capacity of a roof diaphragm sheathed with wood structural panels is based upon the 
same five factors as floor diaphragms. (See the discussion in Section 4.7 of this guide for more 
information on the effects of sheathing thickness, fastener size and spacing, use of blocking, and 
layout of wood structural panel sheathing.) 

The effects of distance between braced wall lines below the roof and the effects of large roof 
openings (e.g., skylights) are also similar to those discussed for floors. The recommendations 
for reinforcing floor diaphragms with large openings also apply when roof openings exceed the 
code maximum limits. (See Section 4-7 for more information.) 

Although roof-ceiling systems typically are subjected to the smallest loads when a house 
experiences earthquake ground motion, several simple things can be done to improve the 
performance of the system. 

Above-code Recommendations: 

Reinforce the framing around skylights to provide positive connections to transfer the 
diaphragm loads around the opening. Strapping and blocking as illustrated in Figure 4-11 
can be used to strengthen the roof around large openings and the additional cost for the 
blocking and strapping for a typical skylight opening would add less than 0.4 percent to the 
cost of the structural portion of the project. 

Make sure all rafters and ceiling joists are blocked at all locations where they are in 
contact with a top plate of a wall below (alternately a rim joist may be used). For the 
exterior walls of the model house, this would cost 0.8 percent of the structural portion of the 
project. If interior walls were also blocked, an additional cost of 0.7 percent of the structural 
portion of the project would be incurred. 

Glue the sheathing to the roof framing in the same way most floor sheathing is installed 
and then block the roof as well. Construction adhesives are typically used to prevent 
squeaky floors but they also strengthen and stiffen. The cost of adding adhesive to the roof 
system for the model house in this guide would be 1.8 percent of the structural system, and 
adding blocking to the roof system would add another 3.4 percent to the cost of the structural 
system. 


6.7 QUALITY CONTROL 

The most important item to monitor for quality of the roof diaphragm system is to ensure that the 
nails used to attach the sheathing to the framing are driven flush with the top of the sheathing and 
not overdriven and counter sunk into the sheathing materials. Overdriving sheathing nails has 
been shown to reduce the strength of shear walls and diaphragms. Evidence of a 40 to 60 
percent loss in strength of the shear wall and diaphragm has been observed in laboratory tests of 
assemblies with overdriven nails. (See Section 5.1.4 of this Guide for more discussion on the 
effects of overdriven nails and how pneumatic tools can be altered to correct for this error.) 

To ensure the quality of the roof-ceiling system, make sure blocking is installed correctly so that 
the wood framing is not split by the toe nailing. 

If adhesives are used to attach the sheathing, attention needs to be paid to the time between the 
application of the adhesive and when the nails are driven to hold the sheathing in place. 
Especially in hot weather, the adhesive tends to skin over or cure on the surface quickly, which 
reduces the adhesion between the glue and the sheathing. Check the time allowed for each 
specific product used. 


130 



Chapter 7 
CHIMNEYS, FIREPLACES, BALCONIES, AND DECKS 


This chapter provides an overview of the IRC provisions for earthquake-resistant design and 
construction of chimneys, fireplaces, balconies, and decks in houses. Above-code 
recommendations for improved earthquake performance are provided. 

7.1 CHIMNEYS AND FIREPLACES 

IRC Chapter 10 presents requirements for masonry fireplaces and chimneys and for factory-built 
fireplaces and chimneys enclosed in framing. The provisions of the IRC are intended for the 
moderately sized fireplaces and chimneys commonly found in houses. 

Above-code Recommendation: Where fireplaces or chimneys are large or oddly 
configured, an engineered design is encouraged in order to fully address the design of 
the chimney and fireplace and their influence on the house. 

7.1.1 Masonry Chimneys and Fireplaces 

Although the IRC permits construction of masonry fireplaces and chimneys in earthquake-prone 
regions, masonry chimneys are particularly vulnerable to earthquake damage, and such damage 
has occurred in most moderate to severe U.S. earthquakes (Figures 7-1 and 7-2). Masonry 
fireplaces and chimneys can be heavy and rigid, and many chimneys in existing houses also are 
brittle. The movement of the fireplace and chimney in response to earthquake ground motions 
can be significantly different from the movement of the light-frame house itself, creating the 
potential for damage to both the chimney and the house. 

Figure 7-1 Chimney damage.
Photo Courtesy National
Information Service for 
Earthquake Engineering, 
University of California, Berkeley 




Figure 7-2 Chimney damage in Northridge 
earthquake. 


The IRC triggers requirements for masonry fireplace and chimney reinforcing steel and 
anchorage to floors, roofs, and ceilings for houses in SDCs D1 and D2. Although these 
requirements cannot completely eliminate the possibility of damage to the fireplace and chimney 
in an earthquake, their use permits a chimney to better withstand earthquake loads and should 
lessen the falling hazard posed by a damaged chimney. Although it may be possible with 
systematic engineering design to mitigate the damage often seen in masonry chimneys and 
fireplaces, the lower weight and greater flexibility of factory-built fireplaces and chimneys make 
them the better choice for light-frame houses in earthquake-prone areas. 

A substantial footing is necessary if a fireplace and chimney is to perform well under any type of 
loading. The footing should extend to a depth not less than that of surrounding footings. IRC 
Section R1003 contains minimum footing requirements. 

IRC Section R1003.3 provides requirements for masonry chimney and fireplace reinforcing steel. 
The minimum amount of vertical reinforcing steel is four No. 4 bars for a chimney up to 40 
inches wide (a depth of approximately 24 inches is common). An additional two No. 4 vertical 
bars are required for each additional flue or each additional 40 inches of width. Where the 
reinforcing bars cannot run full height, a lap splice of not less than 24 inches is needed. Grout, 
continuous from the footing to the top of the chimney, must surround the reinforcing steel. For 
horizontal reinforcing, a minimum of 1/4-inch ties at not more than 18 inches on center is 
required in the mortar joints. IRC Section R1003.3 also cites the Section R609 requirements for 
grouted masonry discussed in Chapter 5 of this guide. Proper grouting and consolidation around 
the reinforcing steel is needed in order for the reinforcing and anchorage to contribute to 
earthquake resistance. Lack of grout and poorly consolidated grout are common contributors to 
earthquake damage. It is also important to note that IRC Section R609 prohibits the use of Type 
N masonry mortar in SDCs D1 and D2. 


Anchorage of the masonry chimney to the framing at each above grade floor, roof, and ceiling 
level is necessary. IRC Section R1003.4 provides anchorage requirements applicable in SDCs 
D1 and D2. Steel straps not less than 3/16-inch by 1-inch are required to extend a minimum of 12 
inches into the chimney masonry, hook around outer reinforcing bars, and extend not less than 6 
inches beyond the hook. Chimney anchorage locations are illustrated in Figures 7-3 and 7-4. 
The IRC provisions specify anchorage to a minimum of four ceiling or roof joists with not less 
than two 1/2-inch bolts. This description does not give details of the intended configuration and 
does not address framing parallel to the chimney wall. Figures 7-5 and 7-6 illustrate 
implementation of this anchorage provision with detailing consistent with industry 
recommendations (MIA, 1995) and earlier Uniform Building Code provisions. Anchorage in 
general conformance with Figures 7-5 and 7-6 should be consistent with the intent of the IRC. 

Figure 7-3 Locations for earthquake 
anchorage of masonry chimney at 
exterior house wall. 



Figure 7-4 Chimney section showing earthquake anchorage. 


Figure 7-5 Anchorage detail for framing parallel to exterior wall. 



Figure 7-6 Anchorage detail for framing perpendicular to 
exterior wall. 


To reduce the fire hazard with respect to the surrounding wood structure, IRC Section R1001.15 
requires a clearance of 1 to 2 inches (depending on configuration) between combustible framing 
materials and the masonry chimney. Detailing of earthquake-resistant anchorage at the floor, 
ceiling, and roof levels needs to maintain this required clearance with only the steel straps 
extending across this clearance gap. 

Even when fireplaces and chimneys show no signs of damage after an earthquake, the masonry 
or flue liner may have cracked, and inspection before reuse is recommended. 

7.1.2 Factory-Built Fireplaces and Flues 

Factory-built fireplaces and flues generally are installed within light-frame fireplace enclosures 
and chimneys. During an earthquake, the deflections of the light-frame enclosure are compatible 
with those of the house, which greatly reduces the potential for damage. Detailing of framing 
anchorage, however, is important. At exterior walls, as shown in Figure 7-7, the framing for the 
chimney disrupts the typical wall and roof framing. Wall top plates often are discontinued and 
studs are balloon framed to the top of the light-frame chimney. Measures should be taken to 
restore the continuity of the top plates and to anchor the fireplace/chimney wall framing to the 
floor and roof. Figure 7-8 shows how one light-frame chimney without sufficient connections 
behaved. No specific requirements for this construction currently exist in the IRC. Clearances to 
combustible wood framing remain important with factory-built fireplaces and chimneys and are 
typically addressed in the installation instructions. 


Figure 7-8 Collapsed factory-built chimney, light-frame 
enclosure, and deck after the San Simeon earthquake. 
Figure 7-7 Factory-built flue Photo courtesy of Josh Marrow, Simpson Gumpertz and Heger 
and light-frame enclosure. Inc. 


Above-code Recommendations: Factory-built fireplaces and flues typically are 
installed within light-frame fireplace enclosures and chimneys that generally perform 
well during earthquakes; therefore, their use in the higher Seismic Design Categories is 
recommended. However, special attention should be given to the detailing of the 
framing anchorage and to compliance with clearances to combustible wood framing 
addressed in installation instructions. 

Use of stone or masonry veneer increases the weight of the light-frame chimney, thereby 
increasing earthquake loads proportionately. Particular care should be taken to tie the wood 
framing into the floor and roof when veneer is used. Veneer attachment to the framing should be 
in accordance with IRC Chapter 7. See Section 5.2 of this guide for a discussion of veneer 
attachment. 

Above-code Recommendation: Use of reinforcing steel and chimney anchorage are 
recommended to improve the performance of fireplaces and chimneys across all Seismic 
Design Categories and particularly in SDC C. Adding reinforcement and anchorage for 
the chimney on the model house used in this guide would increase the cost of the structural 
portion of the house by approximately 2 percent, which is approximately 0.5 percent of the 
total cost. 


Above-code Recommendation: Both masonry and factory-built fireplaces and chimneys 
result in increased weight and earthquake loading as well as discontinuities in house 
configuration. Additional bracing walls in both directions in the vicinity of the fireplace 
are recommended to resist the additional earthquake load. This will result in a reduction 
in the amount of window opening available, but the cost of adding the wall will likely be 
offset by the reduced cost for windows. 

7.2 BALCONIES AND DECKS 

Balconies and decks often are prominent features of modern residential construction and, for 
many people, add much desired living space (see Figure 7-9). Although the IRC contains some 
provisions addressing balconies and decks, the earthquake resistance of balconies and decks has 
not been systematically considered in the development of the IRC provisions. This section 
addresses three aspects of balconies and decks that need consideration: 

 
The effect of added floor area beyond braced wall lines, 
 
Anchorage for earthquake loads, and 
 
Vertical support. 


7.2.1 Added Floor Area Beyond Braced Wall Lines 

The bracing provisions of the IRC reflect the need to support the floor area within the exterior 
braced wall lines. The addition of balconies and decks creates additional weight and increases 
earthquake loads, a fact that was not envisioned when required bracing lengths were determined. 
Although the addition of a small balcony or deck is not likely to greatly affect the earthquake 
performance of a house, the addition of a large balcony or deck may. Further, balconies and 
decks tend to concentrate the added earthquake load on one side of the house and one braced 
wall line. This can contribute to rotational behavior (see the discussion of plan irregularities in 
Section 2.3 of this guide) and concentrations of damage. 

Two IRC sections limit balcony and deck size: 

 
IRC Section R502.3.3 addresses floor framing cantilevers, including exterior balconies. 
IRC Table R502.3.3(2) specifies permitted cantilevers as a function of framing size and 
spacing and live load. Permissible cantilevers range up to a maximum of 6 feet (see 
additional discussion of back-span connections in Section 4.3 of this guide). 
 
For SDCs D1 and D2, IRC Section R301.2.2.2.2, Item 2, requires braced wall lines on all 
edges of a floor or roof. The exception to Item 2 permits floors that do not support 
braced wall panels to extend up to 6 feet beyond a braced wall line. 



Figure 7-9 Decks and balconies in residential construction. 


Balconies extending more than 6 feet beyond the exterior house wall in all Seismic Design 
Categories fall outside of the framing provisions of the IRC and require engineered design. 

Where balconies or decks extend more that 6 feet beyond the house exterior, additional lines of 
bracing for earthquake and wind loads are sometimes provided. When this approach is taken, it 
is important that loading in both the longitudinal and transverse directions be considered. In 
addition, when both the house and supplemental bracing are used to support the balcony or deck, 
the load-deformation behavior of the bracing system must be compatible with that of the house. 
Where this is not the case, it is best to completely separate the balcony or deck from the house 
and provide a gap large enough to permit independent movement. 


7.2.2 Anchorage for Earthquake Loads 

Where a balcony or deck is laterally supported by the house, adequate connection to the house is 
key to good earthquake performance. The IRC includes two provisions that address anchorage to 
the house: 

 
IRC Section R311.2.1 requires that the connections used to attach exterior balconies, 
stairs, and similar exit facilities to the rest of the structure provide for resistance to both 
vertical and lateral loads, but the magnitude of loads to be resisted is not specified. Use 
of toe nails or nails subject to withdrawal is prohibited. 
 
IRC Section R502.2.1 provides similar requirements for decks. 


Because balcony framing generally has a back span that extends into the interior of the house, 
adequate connection generally is not an issue; however, care should be taken to ensure that the 
back span is adequately fastened to the floor sheathing or to lapped floor framing and that 
blocking is provided where the cantilever bears on the exterior wall. 

Above-code Recommendation: Deck framing, on the other hand, does not generally have 
inherent continuity into the house. Attachment only to the floor band (rim) joist is not 
sufficient and will result in failures between the band joist and the rest of the floor system. 

Engineered design of connections is recommended as is use of a positive connection, 
such as the hold-down device shown in Figure 7-10. 

Figure 7-10 Hold-down device providing positive connection of deck framing to house framing. 



7.2.3 Vertical Support Issues 

Although beyond the scope of earthquake resistance, two issues  vertical load connections and 
moisture and decay  related to vertical support of decks and balconies are of enough 
significance to warrant discussion. 

Deck construction is notoriously problematic and typically at least one deck collapse occurs 
somewhere in the United States every week. Inadequate connection between the deck and the 
house for vertical loads is the biggest problem. Where an engineered design is not provided, 
connections for vertical loads very often are inadequate. Inadequate connection for lateral 
loading also can be a contributor. Attention to adequacy of vertical load connections is 
imperative for safety. 

Above code Recommendation: Providing a line of vertical support (posts and beams) 
alongside the exterior house wall can help to reduce the load on the deck-to-house 
connection. 

The interface between a balcony or deck and the exterior house wall is critical for the 
waterproofing system. Penetration of moisture at this interface can endanger not only the 
capacity of the connection but also the interior and exterior framing members. IRC Section 
R319.1.2 requires that the joints of exterior balconies be designed such that moisture will not 
collect in the connection area or that the connection be otherwise protected from moisture. 

Above-code Recommendation: The joints of exterior decks should be designed such that 
moisture will not collect in the connection area or that the connection is otherwise 
protected from moisture. 

The resource list provided in Appendix E includes references addressing deck connections and 
moisture and decay issues. 

Above-code Recommendation: As noted, balconies and decks often are subjected to 
significant lateral loads during an earthquake. As an above-code measure for houses 
located in SDCs C, D1 and D2, the connections used to attach the deck to the house 
should be designed by a registered design professional to ensure that the loads acting on 
the deck are properly transferred to the framing of the house. In addition, the lateral 
bracing of the deck for the sides not attached to the house and for all sides of free-standing 
decks must be sufficient to prevent a torsional response. 


Chapter 8 
ANCHORAGE OF HOME CONTENTS 


8.1 GENERAL 

Anchorage of home contents can greatly reduce the risk of injury, property loss, and interruption 
of home use as a result of an earthquake. Anchorage is particularly recommended for large and 
heavy items such as water heaters, bookcases, and file cabinets and for items that could cause 
injury if they fell (e.g., items on shelves above a bed). Other priority items to anchor or 
otherwise restrain include wood stoves, similar heating appliances, and outside fuel tanks, all of 
which pose a fire risk. 

Anchorage of other home contents will further reduce disruption following an earthquake. 
Measures to anchor computers and televisions range from very simple home fixes to specialized 
restraint systems available from a number of manufacturers. Locks on kitchen and china 
cabinets can help reduce the spilling and breakage of contents during an earthquake. Measures 
are available to secure a wide range of other home contents including pictures, mirrors, fragile 
objects, and fire extinguishers. 

Section 8.2 provides guidance on the anchorage of water heaters. Section 8.3 addresses 
anchorage of a number of items using excerpts from FEMA 74, Reducing the Risks of 
Nonstructural Earthquake Damage (FEMA, 1994). 

8.2 WATER HEATER ANCHORAGE 

When not properly anchored, water heaters can fall over, resulting in a fire hazard and water 
damage. Bracing is required for new water heater installations and is recommended as a top 
priority for existing installations. IRC Chapters 20, 24, and 28 contain some provisions for the 
installation of water heaters but do not specifically address anchorage. Kits for bracing water 
heaters are available at many hardware stores. As an alternative to kits, Figures 8-1 and 8-2 
provide example details for anchorage using plumbers tape (24-gage by 3/4-inch minimum steel 
straps) and electrical conduit. These figures are adapted from Guidelines for Earthquake 
Bracing of Residential Water Heaters (California Division of the State Architect, 2002); consult 
this publication for detailed installation instructions and additional cautions and limitations. 

Note that Figures 8-1 and 8-2 are applicable only to water heaters with a maximum capacity of 
52 gallons with two strap locations and with a maximum capacity of 75 gallons with three. 
Some jurisdictions place additional limits on water heater bracing details. Where water heaters 
are installed on a platform, the water heater base should be attached to the platform and the 
platform should be anchored to the floor. Placing water heaters in metal pans to retain any 
spilled water is a possible precaution in addition to bracing. The IRC requires that water heaters 
be placed in pans where loss of water would cause damage. Required clearances to walls and 
combustible construction need to be maintained; the water heater UL listing and local 
jurisdiction requirements should be verified prior to moving or installing a water heater. 


Figure 8-1a and 8-1b Securing a water heater with wall bracing. 


Figure 8-2a and 8-2b Securing a water heater with corner bracing. 



8.3 SECURING OTHER ITEMS 

The examples included here show representative details for protecting common items from 
earthquake damage. Two different types of details are discussed: 

 
Do-it-yourself methods, which are simple generic methods for securing typical nonstructural 
items found in the home. Enough information is provided to permit a handyman with 
common tools and readily available materials to complete an installation. 
 
Engineered methods, which are schematic details showing common solutions for the items 
in question. These sketches do not contain enough information for installation; they are 
provided here primarily as an illustration of the scope of work required. The designation 
Engineering Required has been used for items where do-it-yourself installation is likely to 
be ineffective. FEMA 74 recommends that design professionals be retained to evaluate the 
vulnerability of these items and design appropriate anchorage or restraint solutions, 
particularly where safety is an issue. 


Cost estimates are provided with the details in this section as a rough guide for planning or 
budgeting purposes. The values are intended to cover the cost of materials and labor. They do 
not include allowance for architectural or engineering fees, permits, special inspection, etc. 
These estimates represent a professional opinion based on information available at the time 
FEMA 74 was published in 1994 so current actual construction costs may vary significantly, 
depending on the timing of construction, changes in conditions, the availability of materials, 
regional cost variations, and other factors. 

FEMA 74 provides several pages of installation notes that will be of assistance to users 
interested in selection and installation of fasteners appropriate to construction materials of the 
house (www.fema.gov/hazards/earthquakes/nehrp/fema-74.shtm). Reproductions of FEMA 74 
pages are shown in Figures 8-3a through 8-3f. 


Figure 8-3a Reproduction of FEMA 74 suggestions for securing drawer and cabinet latches. 


Figure 8-3b Reproduction of FEMA 74 suggestions for securing a freestanding wood stove. 



Figure 8-3c Reproduction of FEMA 74 suggestions for securing desktop computers 
and office equipment. 


Figure 8-3d Reproduction of FEMA 74 suggestions for securing freestanding walls or fences. 



Figure 8-3e Reproduction of FEMA 74 suggestions for securing compressed-gas cylinders. 



Figure 8-3f Reproduction of FEMA 74 suggestions for securing miscellaneous furniture. 



Chapter 9 
EXISTING HOUSES 


Additions or alterations can reduce the earthquake resistance of an existing house. With proper 
consideration, however, earthquake resistance can be maintained or even increased as part of an 
addition or alteration. This chapter discusses the earthquake-resistance implications of additions 
and alterations and provides recommendations and references for earthquake upgrades. 

9.1 ADDITIONS AND ALTERATIONS 

Additions and alterations modify the load-resisting systems of existing houses. Generally, both 
the systems supporting gravity loads and those supporting lateral (wind and earthquake) loads 
are affected. For additions and alterations, IRC Section R102.7.1 requires that any new work 
conform to the IRC, but existing construction is allowed to remain unless it is made unsafe or 
will adversely affect the performance of the house. This wording provides significant 
opportunity for interpretation by the user and building official. IBC Section 3402 provides more 
specific guidance for acceptable reduction in strength or increase in loading, which may be 
appropriate to some additions and alterations. The following discussion of additions and 
alterations highlights issues and concerns that should be considered when interpreting IRC 
requirements. 

9.1.1 Alterations 

Alterations to existing houses often involve modification or removal of existing bracing walls 
and portions of floors and roofs. Figure 9-1 shows two alterations that remove exterior bracing 
walls from a house and disrupt the roof. Interior remodels often remove interior walls that 
provide bracing for earthquake and wind loads. 

Where existing bracing walls are removed or reduced due to alterations, the remaining bracing 
walls should be checked for conformance with the bracing location, length, and bracing type 
requirements of the IRC provisions. The primary focus should be on bracing in the immediate 
vicinity of the alteration. If bracing deficiencies occur in other portions of a house, upgrade of 
those areas is encouraged. 

When skylights, dormer windows, or similar openings are added to existing roofs, the openings 
should be checked for conformance with IRC requirements. For earthquake loading, this would 
include checking the opening size against permitted maximum sizes in the irregularities 
provisions and checking detailing against IRC requirements. The framing around the opening 
also should be checked for gravity load requirements such as doubled rafters and headers. If a 
significant rebuilding of the roof is occurring, a broader range of IRC provisions require 
checking as does the completeness of the load path for gravity and lateral loads. 


Figure 9-1 Alterations to existing house: bracing wall and roof modifications 
(left) and modification for the addition of sun room (right). 


9.1.2 Additions 

An addition to an existing house often results in both the removal of some existing bracing wall, 
roof, and floor areas and the addition of weight and, therefore, increased earthquake loading. 
Most additions can be categorized as horizontal additions, vertical additions, or a combination of 
the two. Horizontal additions generally are built along the side of an existing house and often 
require reframing of the roof. Vertical additions generally involve the addition of an upper story. 
Figure 9-2 illustrates horizontal and vertical additions. 

Horizontal additions may create irregularities or make existing irregularities worse. Thus, the 
IRC building configuration irregularity provisions should be reviewed to assess the post-addition 
configuration. 

Horizontal additions include the construction of new bracing walls at the new exterior of the 
house (and sometimes on the interior). A significant reconfiguration of bracing walls at the 
interface of existing and new construction also often occurs. All bracing walls in the addition 
and interface should be checked for conformance with the IRC as should any portions of the 
existing house where framing and bracing modifications have been made. 


Figure 9-2 Horizontal addition (above) and vertical addition (below). 


Finally, it is very important that the addition and the existing house be tied together very well. 
Ideally the level of interconnection should be the same as would occur if they had been built at 
the same time, but this generally cannot be practically achieved. Top plates and sill plates should 
be strapped between the new and existing construction to provide continuity. 

Sheathing should be continuous and fastened to the same or interconnected framing members 
where possible. Where not possible, strapping of framing members should occur at a regular 
interval. Figure 9-3 shows the earthquake behavior of a house with failed cripple walls and 
adjoining (very small) slab-on-grade addition; a combination of cripple wall upgrade and 
strapping between the house and the addition would have greatly improved the performance of 
this house. 


Figure 9-3 Cape Mendocino earthquake damage to cripple 
wall house with slab-on-grade additions. 


A vertical addition demands significantly greater consideration of both gravity and lateral 
(earthquake and wind) loads. This is because the story that is added often will more than double 
both the gravity and lateral loads on the existing lower story. Thus, when adding an upper story, 
the entire house, including the lower story, should be brought into conformance with current 
code requirements. Although it should be possible to meet IRC requirements, use of the 
engineered design requirements of the IBC or NFPA 5000 may result in a more practical design. 

With any addition or alteration, it is important that the gravity and lateral load paths be checked 
in detail to ensure that they are complete and meet the requirements of the IRC, IBC, or NFPA 
5000. Additions and alterations often create nontypical load path details, and it is important that 
these details result in a complete load path with load-carrying capacity that is not less than would 
have resulted had typical details been used. In some cases, the new detailing deviates enough 
from that which is typical that engineered design should be employed. 

Adding to or altering an existing house offers a clear opportunity to voluntarily upgrade existing 
portions of the house to better resist earthquake forces. The following section describes where 
such upgrades might be employed. 

9.2 EARTHQUAKE UPGRADE MEASURES 

As noted elsewhere in this guide, the life-safety performance of houses in past earthquakes has 
been good with only a few exceptions. There are, however, certain conditions or portions of 
houses that have repeatedly resulted in earthquake damage, loss, and, in some cases, life loss or 
injury. Among these are: 

 
Missing or inadequate bolting to the foundation, 
 
Inadequate cripple wall bracing, 



 
Damage to bracing and finish materials, 
 
Excessive drift at garage fronts, 
 
Partial or complete collapse of hillside houses, 
 
Separation and loss of vertical support at split-level floor offsets, and 
 
Damage and collapse of masonry chimneys. 


Because existing houses vary widely in configuration and construction based on age, region, 
siting, etc., it is necessary to identify upgrade measures appropriate to the particular house. In 
deciding on voluntary upgrade measures, take into account the configuration of the house and the 
potential benefit of the upgrade. Based on the principles discussed in this guide, some simple 
upgrade measures for existing houses are described below. This discussion addresses when the 
various upgrade measures are appropriate and suggests approximate levels of priority. Note, 
however, that the primary objective of most published upgrade measures for houses is reduction 
of hazard to life. 

If an upgrade is being undertaken on a voluntary basis, the house generally will not be required 
to conform to all of the code requirements for new construction. The building official or 
authority having jurisdiction should be consulted regarding minimum requirements. It is 
recommended that a basis for the upgrade work (i.e., published prescriptive method or 
earthquake load level) be established and clearly documented. When a house is being remodeled 
or extensively renovated, a systematic upgrade to meet the IRC requirements for new 
construction may be reasonable and may be required by the authority having jurisdiction. 

The remainder of this section provides an overview of common upgrade measures. Those 
interested in implementing specific measures are referred to the list of references and resources 
in Appendix E for further information on implementation. 

9.2.1 Foundation Bolting 

Many houses constructed in California before the 1950s and even later in other parts of the 
United States do not have anchor bolts that attach the wall bottom plate or foundation sill plate to 
the concrete or masonry foundation. During an earthquake, this can allow the wood framing to 
slide off the foundation, causing loss of vertical support and sometimes cripple wall or partial 
basement story collapse. Where the headroom and foundation configuration permit, this 
situation can be remedied by adding new anchor bolts. Common anchorage configurations are 
illustrated in Figure 9-4. Expansion-type anchors can be used in strong concrete and masonry 
foundations. Adhesive anchors are recommended for use with unreinforced masonry or weaker 
concrete foundations but can be used with all foundation types. When anchor bolts are added, 
use of steel plate washers in accordance with the IRC is recommended. 

If there is insufficient overhead room or the foundation configuration does not permit the use of 
anchor bolts, a wide variety of proprietary anchors are available from manufacturers. The 
primary purpose of this anchorage is to transmit horizontal earthquake loads acting parallel to the 
foundation from the foundation sill plate to the foundation. Where used, proprietary anchors 
should be designed for this use and loading direction by the manufacturer. 


A high priority is suggested for addition of foundation bolting to houses that are not bolted based 
on the relatively low upgrade cost and generally high benefit. Applicable references include the 
International Residential Code (ICC, 2003a), the International Existing Building Code Appendix 
Chapter A3 (ICC, 2003c), Training Materials for Seismic Retrofit of Wood-Frame Homes 
(ABAG), Homeowners Handbook (City of San Leandro, California), and Project Impact (City 
of Seattle). 

Figure 9-4 Common anchorage configurations. 


9.2.2 Cripple Wall Bracing 

Cripple walls are partial-height wood light-frame walls that extend from the top of the 
foundation to the first framed floor. Cripple walls are very susceptible to damage during an 
earthquake but are also one of the easiest portions of a house to upgrade for improved earthquake 
performance. Many of the cripple walls of existing houses (especially those constructed prior to 
1960) have inadequate bracing capacity due to the type of sheathing used, inadequate attachment 
of the sheathing, inadequate attachment of the framing to the foundation and first floor, or decay 
of the system. Figure 9-5a shows a house with collapsed cripple walls (note that the porch floor 
is still at the original house floor elevation). As a minimum, this house will have to be jacked up 
to be repositioned on the foundation and have utilities reconnected. The house in Figure 9-5b 
suffered more severe damage due to cripple wall collapse. 


Figure 9-5a House with collapsed 
cripple walls. Photo Courtesy of 
National Information Service for 
Earthquake Engineering, University of 
California, Berkeley 

Figure 9-5b A house with 
severe damage due to cripple 
wall collapse. Photo Courtesy 
of FEMA. 


Prior to an upgrade, the existing cripple wall system should be inspected and any sections of the 
framing that show signs of decay should be replaced. Framing materials in areas where moisture 
is present or in contact with the foundation should be replaced with preservative treated or 
decay-resistant materials. The upgrade should include anchorage of the foundation sill plate to 
the foundation, framing anchorage to the first-floor framing, and sheathing of the cripple walls 
with wood structural panel sheathing applied to either the exterior or interior face of the crawl 
space walls. Sheathing and connections may be installed to meet the requirements of the IRC or 
in accordance with other provisions developed specifically for upgrades. The basic elements of 
cripple wall bracing are illustrated in Figure 9-6. 


Figure 9-6 Cripple wall bracing. 


Upgrade of interior cripple walls also is recommended where a crawl space is large. Where 
bracing is to be installed only at the perimeter of a large crawl space, performance can be 
improved by providing additional bracing length, reducing nail spacing from 6 inches to 4 
inches, or providing sheathing on both faces of the cripple wall. A high priority is suggested for 
upgrading inadequately braced cripple walls based on the relatively low cost and generally high 
benefit. Applicable references include the International Residential Code (ICC, 2003a), the 
International Existing Building Code Appendix Chapter A3 (ICC, 2003c), Training Materials 
For Seismic Retrofit of Wood-Frame Homes (ABAG), Homeowners Handbook (City of San 
Leandro), and Project Impact (City of Seattle). 

9.2.3 Weak- and Soft-Story Bracing 

Earthquake damage often is concentrated in the first story of multistory houses because the first 
story experiences higher loads while usually having the least amount of bracing. To reduce this 
damage, the first-story walls can be upgraded to increase their strength and stiffness. One 
method of accomplishing this is to remove the interior finish material (usually gypsum wall 
board or interior plaster) at the bottom of the wall in the corners of the house and to add hold-
down anchors for overturning resistance. The anchors should be attached to the end studs (or 
other studs that have sheathing edge nailing) and the vertical rod or bolt should be attached to the 
foundation below (see Figure 9-7). This upgrade does not require that the entire interior finish 
be removed but only a section in each corner that is one stud spacing in width and several feet in 
height. This is an effective upgrade measure where continuous reinforced foundations exist. 


Hold-down anchorage to isolated footings or unreinforced masonry footings is likely to be much 
less effective and engineering review is recommended. 

Figure 9-7 Weak- and soft-story bracing. 


If the interior finish material is being removed for other reasons, then additional upgrade 
opportunities present themselves. These include additional anchorage of the top of the wall to 
the floor framing above, attachment of the bottom plate of the wall to the floor framing below, 
and the addition of blocking at the floor framing for the story above if not present at all locations 
that bear on walls. In addition, before gypsum wallboard or another finish material is attached to 
the walls, wood structural panel sheathing can be applied to the interior of the walls using 4-inch 
nail spacing around the perimeter of each sheet of sheathing and to the stud or post with the hold-
down attached. A moderate priority is suggested for soft- and weak-story bracing upgrades in 
one- and two-family detached houses based on their moderate costs. The benefit can vary widely 
depending on the configuration of the existing house. 

9.2.4 Open-Front Bracing 

An open-front configuration occurs when bracing walls are omitted (or of grossly inadequate 
length) along one edge of a floor or roof. This is applicable to stories braced by light-framed 
walls. A number of apartment buildings with open front first stories were severely damaged or 
collapsed in the Loma Prieta and Northridge earthquakes. In this apartment building type, called 
tuck-under parking, significant lengths of first-story bracing wall were omitted in order to 
provide access to under-building parking. One- and two-family houses with open-front 
configurations also are vulnerable to earthquake damage. Two common occurrences of open


front configurations in one- and two-family houses are the fronts of attached garages with 
inadequate bracing length and window walls with no bracing. These conditions are illustrated in 
Figure 9-8. 

The open-front garage condition is of most concern when there is living space over the garage. 
In newer houses, narrow wall segments at the side of garage door openings may contain prefabricated bracing wall systems or engineered bracing walls that can be identified by the use of 
steel hold-down connectors or straps. Where there is no indication of such bracing systems, the 
installation of bracing is recommended. In detached one- and two-family houses, wood 
structural panel sheathing and anchorage connectors can be added in accordance with IRC 
provisions using adhesive anchors to existing foundations (Figure 9-9). The performance of 
narrow bracing walls with hold-down devices relies on the continuity of the existing foundation. 
If the existing foundation is not continuous, shows signs of damage or is constructed of 
unreinforced masonry or post-tensioned concrete, an engineering evaluation should be 
undertaken. Steel moment frames or collectors transferring loads to other portions of the house 
are alternative upgrade measures where use of bracing walls is not possible. 


Figure 9-8 Common open-front occurrences in one- and two-family detached houses. 


Figure 9-9 Detailing of narrow bracing wall piers at open fronts. 


A high priority is suggested for upgrade of open fronts in the first stories of multistory houses. A 
moderate priority is suggested in single-story houses or the top story of multistory houses. 

9.2.5 Hillside House Bracing 

A number of houses on steep hillsides collapsed or were severely damaged in the Northridge 
earthquake. Where damage occurred, the lot sloped downward from the street level, and the 
main floor of the house was located at or near street level with either a stilt system or tall wood 
light-frame walls between the house and grade. Many of the failures began with floor framing 
pulling away from the uphill foundation or foundation wall. Typical damage to these types of 
houses is shown in Figure 9-10. As a result of Northridge earthquake damage, the City of Los 
Angeles has developed provisions for anchoring such houses to their uphill foundations to reduce 
the risk of failure. 

There are no prescriptive upgrade measures currently available for these hillside houses so 
upgrade requires an engineered design. Upgrade measures to improve the response of the 
structure and reduce the amount of damage that occurs during an earthquake include: 

 Securely anchoring the floor framing to the uphill foundation. This will require that 
anchors (i.e., hold-down connectors) be used to attach the floor joists to the foundation 


using adhesive anchors (see Figure 9-11). This detail should be repeated at each framed 

floor level that attaches directly to an uphill foundation. 

 
Attaching the bottom plates of the framing of the stepped (or sloped) and downhill 
cripple walls to the foundations. Supplemental anchorage is particularly important where 
the top of the side foundations are sloped rather than stepped. 
 
Attaching the stepped or sloped side cripple wall top plates together at all splice joints 
using strap connectors. These straps should be heavy and connected securely to both 
sides of the splice, making the top plate of the stepped cripple wall act as though it were 
one piece along the entire length of the wall. 
 
Continuously sheathing the stepped wall and the down-hill wall with wood structural 
panel sheathing. Adequate shear transfer into and out of the cripple walls should be 
provided. 


These upgrade measures should reduce but will not necessarily eliminate earthquake damage. A 
high priority is suggested for evaluation of hillside house vulnerability. The need for upgrade 
should be determined based on an engineering evaluation. Applicable references are Voluntary 
Earthquake Hazard Reduction in Existing Hillside Buildings (City of Los Angeles, 2002) and 
Framing Earthquake Retrofitting Decisions: The Case of Hillside Homes in Los Angeles (von 
Winterfeldt, Roselund and Kitsuse, 2000). 

Figure 9-10 House located on a hillside site 
damaged during the Northridge, California, 
earthquake. 

Figure 9-11 Anchorage of floor framing to the uphill foundation. 

9.2.6 Split-Level Floor Interconnection 

Split-level houses experienced partial collapse and significant damage in the 1971 San Fernando 
earthquake. These houses had vertical offsets in the floor framing elevation on either side of a 
common wall or other support as shown in Figure 9-12. Earthquake damage occurred when 
sections of floor and roof framing pulled away from the common wall. (See Section 2.3 of this 
guide for additional discussion of irregularities.) The behavior of split-level configurations can 
be improved by adequately anchoring floor framing on either side to the common wall. Where 
offset floors are close enough in elevation that a direct tension tie can be provided between 
levels, an upgrade can be accomplished with installation of steel straps; a strap spacing of not 
more than 8 feet on center is recommended (Figure 9-12). Where direct tension ties are not 
practical, ATC (1976) provides a variety of details for anchorage of framing to the supporting 
wall. Finish removal will often be required in order to install connections, making this upgrade 
most practical when remodel work is occurring. It is difficult to establish a priority for this 
upgrade because significant damage was observed only in the San Fernando earthquake and 
photos suggest that the houses damaged had little or no positive connection provided between 
offset floor levels. An applicable reference is A Methodology for Seismic Design and 
Construction of Single-Family Dwellings (ATC, 1976). 


Figure 9-12 Split-level ties for floor framing. 


9.2.7 Anchorage of Masonry Chimneys 

Fireplaces and chimneys in new construction were discussed in Chapter 7 where it was noted 
that masonry and concrete fireplaces are heavy, rigid, brittle, and very susceptible to earthquake 
damage. IRC requirements for new construction in SDCs D1 and D2 dictate use of horizontal and 
vertical reinforcing and anchorage of the chimney to the framing at floors, ceilings, and roof. 

Chimneys on existing houses generally are even more vulnerable than new chimneys because 
they seldom have reinforcing or are anchored to the house. Common chimney failures range 
from hairline fractures of masonry and flue liners to complete fracture (i.e., the top of the firebox 
and at the roof line) permitting large sections of the chimney to fall away from or into the house, 
shattering into a pile of rubble (see Chapter 7 for discussion and illustrations). Chimneys are 
reported to have caused one fatality in the 1992 Landers earthquake and critically injured one 
person in the 2000 Napa earthquake (Association of Bay Area Governments). 

The upgrading of chimneys is very controversial within the earthquake engineering community. 
Upgrading of existing masonry chimneys most often includes strapping the chimney to the house 
at the roof, ceiling, and floor levels. Where the chimney extends a significant distance above the 
roof line, braces from the top of the chimney down to the roof also may be added. Advocates of 
chimney bracing believe that anchoring the chimney will reduce the hazard posed by falling 
portions of the chimney. Opponents note that even with strapping, a chimney seeing significant 
earthquake loading is likely to be damaged to the point that removal and reconstruction are 
required. Opponents further point out that upgrading does not always improve chimney 
performance. Both arguments deserve consideration and the reader is referred to the reference 
list for additional discussion. Easy and practical approaches to reducing risk to life are provided 
on the Association of Bay Area Governments (ABAG) website and include the suggestion that 
occupants should not sleep in the area immediately surrounding a fireplace with an unreinforced 
or unanchored chimney. 


Where chimneys occur at the house exterior, steel straps similar to those discussed in Chapter 7 
can be wrapped around the outside of the chimney and anchored to floor, ceiling, and roof 
framing in much the same way as was illustrated for new construction. Because this strap will 
have exterior exposure, heavy steel should be used and corrosion protection will need to be 
maintained. It also is important that the IRC Section R1001.15 gap be maintained between the 
chimney face and combustible framing. Although the addition of straps is not likely to keep a 
chimney from being damaged, it may reduce the falling hazard if it is damaged. A recommended 
alternative is removal of the chimney or fireplace and chimney and replacement with a factory-
built fireplace and flue surrounded by light-frame walls. 

Applicable references include Info on Chimney Safety and Earthquakes (ABAG) and 
Reconstruction and Replacement of Earthquake Damaged Masonry Chimneys (City of Los 
Angeles, 2001). 

9.2.8 Anchorage of Concrete and Masonry Walls 

Under earthquake loading, concrete and masonry walls can pull away from roof and floor 
framing. This is primarily a concern where bolted ledgers support framing and no direct 
anchorage of the wall to the framing exists. This condition can be effectively upgraded by 
providing a tension connection between the wall and the floor and roof framing as shown in 
Figure 9-13. The connection should be made to the joists when the joists run perpendicular to 
the wall and should be made to blocking and extend at least 4 or 5 feet into the interior of the 
floor system when the joists run perpendicular. An engineering evaluation of the existing 
condition and engineered design of upgrade measures are recommended. 

Applicable references include International Existing Building Code Appendix Chapter A2 (ICC, 
2003c) and Guidelines for Seismic Evaluation and Rehabilitation of Tilt-up Buildings and Other 
Rigid Wall / Flexible Diaphragm Structures (SEAONC, 2001). 

Figure 9-13 Anchorage of concrete or masonry walls to floor, roof, or ceiling framing. 



Appendix A 


ANALYSIS OF THE MODEL HOUSE USED IN THIS GUIDE 

A model single-family detached house was developed and analyzed in preparing this guide. The 
house is described in Chapter 1, and the results of the analysis are referred to throughout the 
guide. This appendix provides additional details concerning the model house, the analysis, and 
the interpretation of analysis results. 

The analysis of the model house provided an approximate comparison of performance for 
varying wood light-frame house and bracing configurations permitted by the IRC and permitted 
the assessment of improved performance resulting from application of the above-code 
recommendations made in this guide. While the model house and the analysis performed cannot 
represent all houses that may be constructed using IRC provisions, they do provide a specific 
example of relative performance from which trends can been observed. 

A1 MODEL HOUSE 

The model house contained both one-story and two-story portions, three bedrooms, 2-1/2 baths, 
and an area of approximately 2,500 square feet plus garage. The house design is intended to 
reflect current configurations for wood light-frame construction but not necessarily any specific 
region of the United States. Separate analytical models were developed for common variations 
in the design including base conditions, exterior finishes, and earthquake bracing configurations. 

The base conditions are slab-on-grade construction with turned-down footings (Figure A-1), 
continuous exterior footings with level 2-foot-high cripple walls (Figure A-2), a hillside 
condition with cripple walls of varying height (Figure A-3), and a full basement with concrete or 
masonry walls (Figure A-4). Exterior finishes are categorized as light and veneer. The light 
finish is intended to represent low-weight finishes such as vinyl or fiber-cement board siding. 
The veneer is intended to represent a single-wythe anchored brick veneer used for the entire 
house exterior. Bracing requirements were determined for each configuration and Seismic 
Design Category (SDC) in accordance with the 2003 IRC. Chimneys of light-frame construction 
were used for all house configurations. 


Figure A-1 Slab on grade base. 
Figure A-2 Level cripple wall base. 
Figure A-3 Hillside base. 
Figure A-4 Basement. 

 
IRC prescriptive bracing requirements were determined for each combination of base condition, 
exterior finish, and Seismic Design Category. Because use of veneer is not permitted on houses 
with cripple walls in SDC D1 and D2 (IRC Section R703.7, Exceptions 3 and 4), both the level 
and hillside cripple wall configurations with veneer were limited to SDC C. Table A-1 is an 
example of one bracing spreadsheet. The remaining spreadsheets used in determining bracing 
requirements for each of the designs are not included here due to their length; however, they and 
other information used in the analysis are available upon request from the Building Seismic 
Safety Council. 

Because gypsum wallboard is used in almost every U.S. residential building, it was used for the 
structural bracing wherever possible. Since it would be installed as a finish anyway, its use for 
bracing has the least construction cost. Wood structural panel wall bracing was used where 
length and percentage bracing requirements could not be met with gypsum wallboard. 
Alternative braced wall panels conforming to IRC Section R602.10.6 were used for the slender 
walls at the house front and garage front for slab-on-grade and basement base conditions. The 
alternative braced wall panels require support directly on a continuous foundation; therefore, 
they could not be used in combination with cripple walls. The IRC Section R602.10.5 


continuous structural panel sheathing modifications to bracing length and percent were not 
used. See Figure 5-6a through 5-6b for an example bracing plan of a cripple wall base condition 
in SDC C. 

Several interpretations of IRC requirements were made in developing the bracing designs. First, 
it was recognized that the roof-plus-ceiling assembly weight would fall just below the limit of 15 
psf of IRC Section R301.2.2.2.1 if the roof assembly weight were considered based on the unit 
weight on slope (12:12 roof slope) but would exceed 15 psf if the weight were adjusted to 
horizontal projected area. Although adjustment to the horizontal projected area is common 
practice in engineering calculations, it was decided that this calculation is not specifically noted 
in the 2003 IRC provisions so the roof-plus-ceiling assembly weight was deemed to fall within 
the 15 psf IRC limit. 

The second interpretation related to the use of gypsum wallboard bracing (IRC Section 
R602.10.3, Method 5). IRC Section R602.10.4 requires that gypsum wallboard braced wall 
panels applied to one side of a wall be at least 8 feet in width. It was interpreted to mean that a 
continuous length of full-height wall not less than 8 feet wide would have to be available in order 
to use this bracing method. Interruption of the 8-foot length by perpendicular walls was 
interpreted to mean that it was not permitted. Where an 8-foot length of full-height wall was not 
available, wood structural panels were used as bracing instead. Based on this interpretation and 
the configuration of the model house, wood structural panels rather than gypsum wallboard were 
used for a significant portion of the exterior wall bracing. Where gypsum wallboard bracing can 
be applied to both faces of a wall (such as at interior walls), the minimum required length of full-
height sheathing is reduced to 4 feet. While the perforated shear wall method that includes hold-
down anchorage at the ends of the wall line was used as an above-code option for the analysis, 
the continuous sheathed option of IRC Section R602.10.5 that allows a 10 percent reduction in 
the sheathing percentage was not used in the analysis. 

The third interpretation relates to the bracing requirements used for the model house in SDC C. 
IRC Table R602.10.1 specifically identifies sheathing length requirements for SDC C. Some 
IRC users, however, interpret the IRC Section R301.2.2 exception to mean that the table bracing 
length requirements for SDCs A and B can be used for houses in SDC C. Analysis of the model 
house performed for this guide used the SDC C bracing length requirements. 

The resulting bracing configurations are illustrated on a set of bracing plans and elevations for 
each of the designs are available from the Building Seismic Safety Council on a CD-ROM. The 
increased bracing length requirements for higher Seismic Design Categories can be observed to 
have reduced allowable door and window openings. 


Appendix A, Analysis of the Model House Used in This Guide 

Table A-1 Example Wall Bracing per 2003 IRC, Slab-on-grade Base Condition 


A2 ANALYSIS USING STANDARD ENGINEERED DESIGN METHODS 

Prior to evaluation using other methods, earthquake forces and deformations were estimated 
using the linear static methods commonly used in engineering design of new buildings. Included 
were force calculations using the International Building Code (IBC) linear static method, 
estimation of drift using the APA-The Engineered Wood Association four-term shear wall 
deflection equations at strength level forces, and amplification to estimated drifts using IBC 
amplification factors. This approach resulted in the APA shear wall deflection equations being 
used outside of their intended range (based on force per nail limits included with nail slip 
variables). This provided clearly unrealistic shear wall deflections amplified to unrealistic 
estimated drifts (over 36 inch drifts in some cases). Thus, it was concluded that the use of these 
engineered design estimates as predictors of performance for non-engineered buildings was not 
realistic and it was not pursued. Likewise, use of other available deflection equations that 
represent simplifications of the APA equations were not pursued. This issue should not occur 
when using this standard deflection calculation method for engineered buildings. 

A3 ANALYSIS USING NONLINEAR METHODS 

Nonlinear time-history analysis using the Seismic Analysis of Woodframe Structures (SAWS) 
analysis program was chosen as the best available method for estimating force and deformation 
demands based on analytical studies that were verified against shake table results from the 
FEMA-funded CUREE-Caltech Woodframe Project. Analysis models included both designated 
bracing and finish materials. The Woodframe Project analytically predicted forces and 
deflections compared favorably with shake-table results and were clearly differentiated from 
analysis and testing results without finish materials (Folz and Filiatrault, 2002). 

The SAWS analysis program uses rigid diaphragms to represent floor and roof diaphragms. 
Walls are modeled as nonlinear springs with hysteretic parameters developed specifically to 
describe the behavior of woodframe bracing systems. For the example house, rigid diaphragms 
were used to represent the high roof, the low roof plus second floor, and, where appropriate, the 
first floor. A simplified representation of the rigid diaphragms and wall springs for the model 
house is presented in Figure A-5. 

Ten sets of hysteretic parameters were developed from component testing data to describe wall 
bracing and interior gypsum wallboard finishes. Figure A-6 illustrates the meaning of the 
parameters, and a summary of analytical modeling parameter values is provided in Table A-2. 
For each of the bracing materials (with the exception of No. 5 and 6), the hysteretic parameters 
were determined for a 4-foot bracing length. Because widely varying lengths are used in the 
house, the parameters were scaled for varying bracing lengths. 


Figure A-5 Analysis model. 


Figure A-6 Hysteretic parameters for model ((Folz and Filiatrault, 2002). 


Hysteretic parameters currently available from laboratory testing of wall components vary based 
on wall boundary conditions, test set-up, and test protocol. Parameters chosen for the analysis of 


the model house tended towards lower bounds of strength and stiffness. Future analytical studies 
should consider exploring upper and lower bounds. 

In order to simplify interpretation of analysis results, the analysis model uses consistent 
identification of bracing walls across all building configurations. Because of this modeling 
approach, cripple walls have been included in the model for all building configurations; where 
slab-on-grade construction occurs, the cripple walls are modeled as extremely rigid elements 
(Property No. 9) resulting in negligible deflection. In addition, some wall elements occur only in 
limited configurations; a bracing length of 0.1 foot is used where a bracing panel is intended to 
have no effect. 

Table A-2 Hysteretic Parameters Used for Nonlinear Dynamic Model 



The strength and stiffness contribution of exterior wall finishes was not included in the analysis. 
This approach was chosen because it would lead to a lower bound and, therefore, conservative 
estimate of deformation demand. In addition, some exterior finish materials are believed to have 
very little impact on building behavior (e.g., vinyl siding) and information was not available on 
the contribution of some other finishes (e.g., brick veneer). Due to the judgment necessary to 
select appropriate component testing and to derive parameters and the simplification of not 
including exterior finishes, the resulting modeling must be qualified as being approximate. 

Earthquake demand is represented using the larger horizontal acceleration record from Canoga 
Park for the 1994 Northridge, California, earthquake. This record was chosen because it 
corresponds well with the code design spectra over a range of building periods. The peak 
acceleration was scaled for each Seismic Design Category by dividing the maximum SDS value 
for each category by 2.5, resulting in peak accelerations of 0.2, 0.33 and 0.47g for SDCs C, D1, 
and D2, respectively. For comparison, the recorded ground motion has a peak acceleration of 


0.42g and was scaled to 0.50g to represent Zone 4 anticipated ground motions in the CUREE 
shake-table testing. The ground motion scaling used for this analysis represents the demand used 
as a basis for code design. The demand from the maximum considered earthquake (MCE) 
ground motion (MCE) would be approximately 50 percent greater. 

Detailed assembly weights and building weights have been determined for each house 
configuration. The analysis model spreads the resulting mass uniformly over a single rectangle 
used to describe each above-ground diaphragm. The center of the mass rectangle is set at the 
calculated center of mass of the building. This simplification, made necessary by analysis 
limitations, should have a minor effect on results. 

A4 ANALYSIS RESULTS 

The selected ground motion was run once in the horizontal X-direction and once in the 
horizontal Y-direction for each combination of base condition, exterior finish, and Seismic 
Design Category as well as for a series of above-code recommendations. From the nonlinear 
time-history analysis, peak drifts in each of the bracing wall lines and peak reactions to 
supporting foundations were extracted and summarized in tables. These tables are not included 
here for brevity but are available upon request from the Building Seismic Safety Council on the 
analysis CD. The controlling value was the largest absolute value of the X- and Y-directions. 

A4.1 Deformation Demand Relation to Performance 

In order to translate the results of the analysis into an approximation of house performance, three 
ranges of peak transient wall drift and associated approximate descriptions of building 
performance were developed. The choice of range and description of performance are based on 
component and full-building test results combined with the opinions of those participating in the 
development of this guide. 

The approximate performance categories and corresponding drift ranges are: 

 
Minor damage potential  Less than or equal to 0.5% story drift 
The house is assumed to suffer minor nonstructural damage such as 
cracked plaster or gypsum wallboard and hopefully would be greentagged (occupancy not limited) by inspectors after an earthquake, 
which would permit immediate occupancy. Some repairs should still be 
anticipated. 


 
Moderate damage potential  Above 0.5% to 1.5% story drift 


The house is assumed to suffer moderate damage including possible 
significant damage to materials and associated structural damage, but the 
building is assumed to have some reserve capacity in terms of strength and 
displacement capacity. The house hopefully would be green-tagged or, 


more likely, yellow-tagged (limited occupancy) by inspectors after an 
earthquake and may or may not be habitable. Significant repairs should be 
anticipated. 

 Significant damage potential  Greater than 1.5% story drift 

The house is assumed to have significant structural and nonstructural 
damage that could result in its being red-tagged (occupancy prohibited) 
by inspectors after an earthquake. Significant repairs to most components 
of the building should be anticipated, and it may be more economical to 
replace the house rather than repair it. 

Use of these three categories permits an approximate comparison of the relative performance of 
different IRC bracing solutions and above-code recommendations. 

A4.2 Discussion of Results 

Selected results of peak drift values and approximate performance category are provided in 
Tables A-3 and A-4. In most cases, the drift increased with increased SDC in spite of the 
bracing requirements also having increased. The approximate performance often increased from 
minor or moderate to significant as the SDC went from C to D2. The primary reason is the 
inclusion of interior gypsum wallboard in the models for all Seismic Design Categories. As the 
SDC increased, interior walls became required braced wall panels per IRC requirements rather 
than simply nonstructural partition walls; however, the analytical model did not change because 
the interior walls had already been included. The result was application of a higher demand to a 
model with only nominal increases in resistance. 

Table A-3 Selected Results for IRC Bracing Provisions, 
Slab-on-grade Base Condition 


Although the building mass increased significantly with the addition of brick veneer, the increase 
in drift ranged from moderate to slight. This is due to the IRC requirement for wood structural 
panel sheathing and hold-down devices for veneer in SDCs D1 and D2. The analysis model 
differentiated between wood structural panel shear walls with and without hold-down devices so 
the different strength, stiffness, and deformation capacity were accounted for. Because of this, 
the IRC bracing required for brick veneer was seen to partially compensate for the increased 
demand. 

The above-code measures were applied to the slab-on-grade base condition. The measures were 
seen to generally reduce the building drift, although drift increases were seen in a few walls due 
to changes in diaphragm rotation. In SDC D2, the approximate performance was improved by 
one category for all three above-code measures. In SDCs C and D1, significant decreases in drift 
were seen within an approximate performance category. 

Table A-4 Selected Results for Above-code Measures, Slab-on-grade Base Condition 

The cost of implementing each above-code measure during construction of the house was 
estimated in terms of percentage change to the construction cost for the basic house structure. 
Comparison to total house cost was not made because variations in finishes and fixtures can 
dramatically vary the house cost. 

Use of continuous wood structural panel wall sheathing (fully sheathed) with overturning 
anchors in the corners of the house significantly reduced the drift in all Seismic Design 
Categories, and the approximate performance category was increased by one in SDC D2. The 
cost of making this change was estimated to be 9 to 10 percent of the cost of the structural 
portion of the model house used in this guide. 

The addition of hold-down anchors at the ends of each full-height wall segment (at the corners 
and edges of each door and window) significantly reduced the drift in all Seismic Design 


Categories, and the approximate performance category was increased by one step in SDC D2. 
For the model house, the cost of implementing this improvement was estimated to be 18 percent 
of the structural cost of the house. 

Lapping wood structural panel wall sheathing over the band joist of the floors did not have a 
significant effect in SDC C or D1 but did improve the approximate performance category by one 
in SDC D2. The cost of implementing this improvement was estimated to be 0.5 percent of the 
cost of the structural portion of the house. This above-code measure can be accomplished by 
either sheathing the wall with oversized panels (9-foot panels on an 8-foot wall) or cutting and 
blocking standard size sheets. 

Use of the above-code measures in combination is thought to have a cumulative effect in 
improving performance and so is encouraged. 


Appendix B 
EARTHQUAKE PROVISIONS CHECKLIST 
FOR BUILDERS AND DESIGNERS 


General Earthquake-Resistance Requirements 

Load Path 

C NC N/A Priority 

HIGH Foundation anchor bolts. IRC Section R403.1.6 for all SDCs. IRC 
Section R403.1.6.1 and R602.11.1 for foundation anchor bolts and plate 
washers in SDCs D1 and D2 (and townhouses in SDC C). 

Typical 1/2-inch diameter bolts at 6 feet along all exterior walls. Also, 
1/2-inch bolts at 6 feet along interior braced walls and interior bearing 
walls supported on a continuous foundation, in SDCs D1 and D2 (and 
townhouses in SDC C); 1/2-inch at 4 feet along all exterior walls and 
along all interior bearing walls and interior bracing walls supported on a 
continuous foundation for three-story in SDC D1 (and three-story 
townhouses in SDC C). 

3x3x1/4-inch steel plate washers: On all required anchor bolts in SDCs D1 
and D2 (and townhouses in SDC C). 

First anchor bolt should be placed seven bolt diameters minimum (3-1/2 
inches for 1/2-inch diameter, 4-1/2 inches for 5/8-inch diameter) and 12 
inches maximum from each end of a foundation sill plate. 

Sheathing and framing fasteners (except for 1/2-diameter or larger steel 
bolts) used in pressure preservatively treated wood framing members must 
have corrosion-resistant coating or be of corrosion-resistant material. 

HIGH Overturning Anchorage. IRC Section R602.10.6 for alternate braced wall 
panels, all SDCs. IRC Section R602.10.11, second paragraph, Exception 
2, where braced wall panels are not located at corners in SDCs D1 and D2. 
IRC Section R703.7, Exceptions 3 and 4, when veneer is used in SDCs D1 
and D2. Overturning load path to foundation needed where hold-down 
anchors are used. 


Load Path Above-code Recommendations: 

 
Provide 4-foot anchor bolt spacing along all exterior and interior braced wall 
lines for two-story houses in SDCs D1 and D2. 
 
Provide continuous foundation below interior braced walls with anchor bolts at 
spacing of 6 feet of less in all SDCs. 
 
Provide corrosion-resistant coatings for anchor bolts installed through pressure 
preservatively treated foundation sill plates in all SDCs. 
 
Do not wet set anchor bolts; securely place anchor bolts prior to placing 
concrete. 
 
In SDC C, provide overturning anchorage as required in SDCs D1 and D2 for 
braced wall panels not located at corners and for houses with masonry veneer. 
 
Add tie straps between first and second story corner studs to tie the walls 
together in SDCs C, D1 and D2. 
 
Use oversized sheathing panels on exterior walls and lap over rim-joist. Nail 
both into the plates (top and bottom) and the rim-joists in all SDCs. 


MED Minimum fastening. All SDCs per IRC Table R602.3. 

MED Designed collector members aligned with and connected to the top plate of 
braced walls (continuous from the end of a braced wall line to the end of 
the braced wall panel closest to the end of the wall line). In all SDCs 
where the first braced wall panel begins more than 12 feet from the end of 
a braced wall line. (Section IRC R602.10.1) 

Also, in SDCs D1 and D2, where the braced wall panel uses wood 
structural panel sheathing and is not located at the end of the wall line. 
However, when either (a) a minimum 24-inch-wide panel is provided each 
side of the wall corner or (b) the braced panel end closest to the corner is 
provide with a hold-down, a designed collector is not required if the wood 
structural panel braced wall panel is located 8 feet or less from the end of 
the braced wall line. (IRC Section R602.10.11 last paragraph) 


Foundations and Foundation Walls 

Concrete Foundations 

C NC N/A Priority 

HIGH Horizontal reinforcing. In SDCs D1 and D2, typical one No.4 in footing. 
Additional No. 4 in concrete stem wall (if stem wall occurs). One No. 4 
top and bottom in thickened slab footing, with alternate of one No. 5 or 
two No. 4 in middle third of footing height for thickened slab footings cast 
monolithically with slab. (IRC Sections R403.1.3 through R403.1.3.2) 

HIGH Vertical reinforcing. In SDCs D1 and D2, one No. 4 at 48 inches 
maximum spacing where a pour joint occurs between concrete footing and 
concrete stem wall. (IRC Section R403.1.3) 

MED Adequate support of reinforcing and anchor bolts. Reinforcing concrete 
cover distances of 3 inches when cast against earth and 1-1/2 inches when 
concrete will be exposed to weather. 

HIGH Clean footing excavations before casting concrete. Proper concrete 
consolidation. No water added to concrete mix at site. 

MED Minimum concrete strength. 2500 psi, all SDCs. 3000 or 3500 psi in 
moderate or severe weathering probability areas (IRC Section R402.2 and 
Table R402.2). 

MED Rebar lap splice length of 24-inches (straight lap) (IRC Section 
R611.7.1.2). Rebar bend radius (outer) of 2 inches for No. 4 and 2-1/2 
inches for No. 5. Hook at corners and intersections of 8 inches for No. 4 
and 10 inches for No. 5. 

Masonry Foundations 

C NC N/A 

HIGH Horizontal reinforcing. 

HIGH Vertical reinforcing. 

MED Rebar lap splice length of 24-inch (straight lap) (IRC Section 
R606.11.2.2.3). Rebar bend radius (outer) of 2 inches for No. 4 and 2-1/2 
inches for No. 5. Hook at corners and intersections of 8 inches for No. 4 
and 10 inches for No. 5 (IRC Section R606.11.7.4). 

Foundation Walls 

C NC N/A 

HIGH Wall thickness. 

HIGH Horizontal reinforcing. 

HIGH Vertical reinforcing. 


MED Rebar lap splice length of 24-inches (straight lap). Rebar bend radius 
(outer) of 2 inches for No. 4 and 2-1/2 inches for No. 5. Hook at corners 
and intersections of 8 inches for No. 4 and 10 inches for No. 5 

Above-code Recommendations: 

 
Avoid construction of slab on grade homes on cut and fill sites where possible. 
Where this condition cannot be avoided, provide additional quality control for 
fill placement and compaction operations. 
 
Regardless of SDC, provide not less than one continuous horizontal No. 4 
reinforcing bar in concrete footings. Provide a second No. 4 horizontal bar in 
stem wall if occurs. This will provide tension and bending capacity to help 
mitigate foundation damage due to earthquake, wind, soil movement and frost 
heave. 
 
Regardless of SDC, remove lose debris in the construction joint between a 
concrete footing and a separately cast slab-on-grade. 
 
In SDCs C, D1 and D2, provide not less than No. 4 at 4-feet vertical bars as 
dowels between a concrete footing and a separately cast slab on grade. 
 
Regardless of SDC, provide not less than one continuous No. 4 reinforcing bar in 
masonry foundations. 


Floor Construction 
C NC N/A 

HIGH Floor sheathing nailing. Floor sheathing should be edge nailed to blocking 
above all braced wall lines, exterior and interior, as part of the load path 
(IRC Table R602.3(1), Footnote i). Blocking with edge nailing needs to 
have a load path to top of braced wall panels. 

MED SDCs D1 and D2, blocking or lateral restraint. Required at intermediate 
floor framing member supports. (IRC Section R502.7, Exception) 

Light-Frame Wall Construction 

HIGH Overdriven sheathing nails. For wood structural panel sheathing, nails are 
to be driven so that the top of the head is flush with the face of the 
sheathing. (It is recommended that where nail heads occasionally are 
more than 1/16-inch below the surface, an additional nail should be 
provided between existing nails. If a substantial number of nails are 
overdriven, the sheathing should be removed and the framing checked for 
splitting before replacing the sheathing with proper nails.) 


HIGH Sheathing nailing to hold-down posts and studs. Hold-downs cannot carry 
any load unless the wall sheathing is edge-nailed to the hold-down post or 
stud.

 HIGH Threaded rods with properly attached nuts need to be in place before the 
wall sheathing is attached to the second side of the walls. 

Above-code Recommendations: 

 
Increase first-story strength and stiffness to mitigate weak-story irregularity. 
Approaches include: (a) use of wood structural panel wall bracing and hold-
down connectors at each end of each full height wall segment, (b) fully 
sheathing all exterior walls including below windows and above and below 
doors and providing hold-down connectors at building corners, and (c) 
providing more than the minimum braced wall panel length. 
 
Increase cripple wall strength and stiffness to mitigate weak-story irregularity 
by sheathing full length of exterior cripple walls. 
 
Use oversized sheathing panels on exterior walls to increase wall stiffness and 
strength. Lap the sheathing over the floor joists and nail to both the plates (top 
and bottom) and the floor joists. 


Roof Construction 
C NC N/A 
HIGH Sheathing nailing at braced wall lines. Roof sheathing should be edge 
nailed (to blocking where present) above all braced wall lines, exterior and 
interior, as part of the load path (IRC Table R602.3(1), Footnote i). 
Blocking with edge nailing needs to be nailed to the top of the braced 
wall below to provide a complete load path. 
Cold-formed Steel Construction 
C NC N/A 
HIGH Load path connections. Connection of cold-formed steel framing 
members is different than wood light-frame connection. The IRC 
provisions include a significant number of specific connection details. 
Attention to these details is important to the building performance for all 
load types. 


Above-code Recommendations: 

 
Add interior cold-formed steel braced walls such that the distance between braced
wall lines does not exceed 35 feet. 
 
In all SDCs, apply the irregularity limitations developed for wood light-frame houses 
(IRC Section R301.2.2.2.2). 


Masonry Wall Buildings 

C NC N/A 

HIGH Construction quality control. Proper type of mortar for masonry being 
used and proper mortar mixing. (Type N mortar is prohibited in higher 
SDCs.) Proper placement of reinforcing. Adequate support and 
attachment of reinforcing and anchor bolts. Cleaning out of grout space to 
allow proper grout placement, including cleaning out excess mortar if 
necessary. Provide cleanouts if necessary for adequate cleaning. 
Consolidation of grout. 

Above-code Recommendations: 

 
Each exterior wall and each interior braced wall should have one, and preferably 
two, sections of solid wall not less than 4 feet in length. 
 
Sections of solid wall should be spaced no more than 40 feet on center and should be 
placed as symmetrically as possible. 
 
All masonry walls should be supported on substantial continuous footings extended 
to a depth that provides competent bearing. 
 
The distribution of interior masonry braced walls should be carefully balanced and 
floor and roof plans should use simple rectangular shapes without jogs and openings. 
 
Apply the irregularity limitations developed for wood light-frame houses (IRC 
Section R 301.2.2.2.2). The IRC exceptions to Irregularities 2 and 5 can be applied 
but the rest of the exceptions are not applicable. 
 
Solid portions of wall should be stocked from floor to floor and masonry walls 
should be continuous from the top of the structure to the foundation. Masonry walls 
not directly supported on walls below require engineered design for gravity load 
support and design for earthquake and wind loads should be provided. 
 
Running board lay up of masonry units should be used instead of stack bond lay up. 



Above-code Recommendations (continued): 

 
For concrete masonry, use open end units at locations of vertical reinforcement and 
use wood beam units for horizontal reinforcing to increase the interlocking of 
masonry construction. 
 
Apply the measures required or recommended for masonry construction in areas of 
high earthquake risk, in areas of lower earthquake risk, and in high-wind areas. 
Priorities include provisions for reinforcing (IRC Figure R606.10 (2)), wall 
anchorage using details developed to resist out-of-plane wall loads (e.g., IRC Figures 
R611.8 (1) through (7)), minimum length of bracing walls, and a spacing limit for 
braced wall lines. 


Concrete and Insulating Concrete Form Wall Buildings 

C NC N/A 

HIGH Construction quality control. Proper placement of reinforcing. Adequate 
support and attachment of reinforcing and anchor bolts. Cleaning out of 
cells space to allow proper concrete placement. Consolidation of concrete. 

Above-code Recommendations: 

 
Carefully balance bracing walls around the perimeter of the building. 
 
Apply the measures required or recommended for ICF houses in areas of high 
earthquake risk, in areas of lower earthquake risk, and in high-wind areas. 
Priorities include wall anchorage using details developed to resist out-of-place wall 
loads (e.g., IRC Figures R611.8 (2) through (7)). 


Stone and Masonry Veneer 

C NC N/A 

HIGH Veneer thickness limited to 5-inch nominal thickness in SDCs A, B, and 
C, 4-inch nominal thickness in SDC D1, and 3-inch actual thickness in 
SDC D2 except that up to 5-inch nominal thickness can be used in SDC D1 
and D2 if veneer only extends to first story above grade. (IRC Section 
R703.7) 


Above-code Recommendations: 

 
Use corrosion-resistant sheet metal ties or wires to fasten veneer. The ties or wires 
should penetrate the house paper and sheathing and should be embedded in the wall 
studs. 
 
Where veneer can be used only on the first story above grade, increase the length of 
the structural wood panel bracing and use hold-down devices on braced wall panels 
in the first story. 


Fireplaces and Chimneys 

C NC N/A 

HIGH Masonry reinforcing. Vertical reinforcing of not less than four No. 4 bars 
for chimneys up to 40 x 24 inches. Should extend from bottom of 
foundation (3-inch minimum concrete cover) to top of chimney except that 
splices of not less than 24 inches are acceptable. Must be placed such that 
reinforcing can be surrounded in grout. Horizontal ties of 1/4-inch 
minimum at 18 inches maximum on center in mortar joint. SDCs D1 and 
D2. (IRC Section R1003.3) 


Appendix C 


EARTHQUAKE PROVISIONS CHECKLIST 
FOR DESIGNERS AND PLAN CHECKERS 


General Earthquake-Resistance Requirements 

General Earthquake Limitations 

C NC N/A 

Seismic Design Category. Buildings in SDCs A through D2 may be designed per 
the IRC; buildings in SDC E require engineered design unless the alternate 
determination of Seismic Design Category provisions of IRC Sections 
R301.2.2.1.1 or R301.2.2.1.2 are met. (IRC Section R301.2.2) 

Assembly weight. Weight of roof plus ceiling, floor, interior wall and exterior 
wall assemblies are limited in SDCs D1 and D2 and townhouses in SDC C (IRC 
Section R301.2.2.2.1). 

Number of stories. Wood light-frame buildings are limited to two stories plus 
cripple walls in SDC D2 (IRC Section R301.2.2.4.1 and Table R602.10.1). Cold-
formed steel framed buildings are limited to two stories above grade in SDCs D1 
and D2 (Section R301.2.2.41). Masonry walls are limited to one story and 9 feet 
between lateral supports in SDCs D1 and D2 (IRC Section R606.11.3.1 and 
R606.11.4). 

Story height. All SDCs. Building story height is limited by the following limits 
on bearing wall clear height plus a maximum of 16 inches for the floor framing 
depth: 

Wood light frame 12 ft (IRC Section R301.3, Item 1 Exception) 
Cold-formed steel 10 ft (IRC Section R301.3, Item 2) 
Masonry 12 ft plus 8 ft at gable ends (IRC Section R301.3, Item 3) 
ICF 10 ft (IRC Section R301.3, Item 4, and Section 611) 


Load Path 

C NC N/A 

Minimum wood light frame fastening. All SDCs. (IRC Table R602.3) 

Anchor bolts and plate washers. IRC Section R403.1.6 for all SDCs. IRC 
Sections R403.1.6.1 and 602.11.1 for SDCs D1 and D2 and townhouses in SDC C. 

Overturning Anchorage. IRC Section R602.10.6 for alternate braced wall panels, 
all SDCs. IRC Section R602.10.11 Exception 2 ,where braced wall panels are not 
located at corners for SDCs D1 and D2. IRC Section R703.7, Exceptions 3 and 4, 
when veneer is used for SDCs D1 and D2. 


Designed collector members aligned with and connected to the top plate of braced 
walls (continuous from the end of a braced wall line to the end of the braced wall 
panel closest to the end of the wall line). In all SDCs IRC Section R602.10.1. In 
SDC D1 and D2, IRC Section R602.10.11 last paragraph. IRC Section 
R301.2.2.2.2 for SDCs D1 and D2 and townhouses in SDC C. 

Irregularities 

C NC N/A 

Irregularity 1: Exterior braced wall panels not in one plane (stacked) from 

foundation to top most story in which they are required. 

 Irregularity 2: Section of floor or roof not supported by braced wall lines on all 
edges. 

Irregularity 3: End of braced wall panel occurs over opening in wall below, and 
extends more than one foot beyond the edge of the opening. 

Irregularity 4: Opening in floor or roof exceeds lesser of 12 feet or 50% of least 
floor or roof dimension. Figure 2-x. 


Irregularity 5: Portions of floor level are vertically offset (split level). 

 
Irregularity 6: Braced wall lines do not occur in two perpendicular directions. 


Irregularity 7: Stories braced by light-frame walls include concrete or masonry 
construction. 


Above-code Recommendations: 

 
Apply irregularities to all SDCs because they are also applicable for wind load. 
 
Increase first-story strength and stiffness to mitigate weak-story irregularity. 
 
Increase cripple wall strength and stiffness to mitigate weak-story irregularity. 


Foundations and Foundation Walls 

General 

C NC N/A 
Continuous perimeter foundations. All exterior walls are to be supported on 
continuous perimeter foundations. All SDCs. (IRC Section R403.1) 

 Continuous interior foundations. At interior braced wall lines in buildings with 
plan dimensions greater than 50 ft. SDCs D1 and D2 . (IRC Section R403.1.2). 


Special Soils Conditions 

C NC N/A 

Low bearing capacity. Soils investigation required when building official 
determines that soil bearing capacities of less than 1500 psf might be present at 
site. All SDCs. (IRC Table R401.4.1, footnote b) 

Soil testing when expansive, compressible, or shifting soils are encountered or are 
likely. (IRC Section R401.4) 

Frost protection. Footings are to be below the frost line or adequate frost 
protection should be provided. (IRC Section R403.1.4.1) 

Concrete Foundations 

C NC N/A 

Minimum concrete strength. 2500 psi for all SDCs. 3000 or 3500 psi in 
moderate or severe weathering probability areas. (IRC Section R402.2 and Table 
R402.2) 

Horizontal reinforcing. One No.4 in footing and second No. 4 in stem wall. No. 
4 top and bottom in thickened slab footing with alternative of one No. 5 or two 
No.4 in middle third of footing height for thickened slab footings cast 
monolithically with slab. SDCs D1 and D2. (IRC Sections R403.1.3 and 
R403.1.3.2) 

Vertical reinforcing. No. 4 at 48 inches maximum spacing required where a pour 
joint occurs between concrete footing and concrete stem wall. SDCs D1 and D2. 
(IRC Section R403.1.3) 

Masonry Foundations 

C NC N/A 

Masonry foundation type. Solid clay masonry and fully grouted concrete 
masonry permitted in all SDCs (IRC Section R403.1). Rubble stone masonry 
foundation walls limited to SDCs A through C (IRC Section R404.1.1). 

Horizontal reinforcing. One No. 4 in footing and second No. 4 in stem wall. 
SDCs D1 and D2. (IRC Section R403.1.3 and R403.1.3.1) 

Vertical reinforcing. Minimum No.4 at 4 feet on center extending into footing 
with standard hook. SDCs D1 and D2. (IRC Section R403.1.3) 


Foundation Walls 

C NC N/A 

Wall thickness. Six inches minimum up to 12 inches required based on soil type 
at site. SDCs A through D2. (IRC Table R401.1.1(1)). 

Horizontal reinforcing. Dependent upon all thickness and material. Minimum 
No. 4 in upper 12 inches of wall. SDCs D1 and D2. (IRC Sections R404.1.4 and 
R606.11). 

Vertical reinforcing. Varies depending on wall height and soil type at site. 
ASTM Grade 60 minimum. All SDCs. (IRC Tables R404.1.1(2)). 

Above Code Recommendations: 

Avoid construction of slab-on-grade homes on cut and fill sites where possible. Where 
this condition cannot be avoided, provide additional quality control for fill placement 
and compaction operations. 

Regardless of SDC, provide not less than one continuous No. 4 reinforcing bar in 
concrete footings. Provide a second No. 4 in stem wall if present. This will provide 
tension and bending capacity to help mitigate foundation damage due to earthquake, 
wind, soil movement, and frost heave. 

In SDCs C, D1 and D2, provide not less than No. 4 vertical bars at 4 feet as dowels 
between a concrete footing and separately cast slab-on-grade. 

In concrete foundations, lap reinforcing bars not less than 24 inches. Bend radius 
(outer) for No. 4 bar is 2 inches and 2-1/2 inches for No. 5. Hook at corners and 
intersections of 8 inches for No.4 bars and 10 inches for No. 5 bars. 

Regardless of SDC, provide not less than one continuous No. 4 reinforcing bar in 
masonry foundation stem walls. 

In masonry foundation walls and stem walls, lap reinforcing bars not less than 24 
inches. Bend radius (outer) for No. 4 bar is 2 inches and 2-1/2 inches for No. 5. Hook at 
corners and intersections of 8 inches for No. 4 and 10 inches for No. 5. 

Floor Construction 

C NC N/A 

Blocking or lateral restraint. Required at intermediate floor framing member 
supports. SDCs D1 and D2. (IRC Section R502.7, Exception). 


Light-Frame Wall Construction 
C NC N/A 
Braced wall length required for each 25 ft of wall length. (IRC Section R602.10.6) 
Sheathing attachment spacing. (Various IRC sections) 
Cold-formed Steel Construction 
C NC N/A 
Cold-formed steel framing. Buildings in SDCs D1 and D2 need to comply with 
the AISI Standard for Cold-Formed Steel Framing  Prescriptive Method for 
One- and Two-Family Dwellings in addition to the requirements of the IRC 
Section R301.2.2.4.5. 
Masonry Wall Buildings 
C NC N/A 
Limited to one story for SDCs D1 and D2. (IRC Section R606) 
In SDCs D1 and D2, 9 feet between lateral supports. (IRC Section R606.8) 
Light-frame restricted from supporting lateral loads from masonry. (IRC Section 
R301.2.2.2.2) 
Reinforcement detailing (IRC Section R606.11). 
Concrete and Insulating Concrete Form Wall Buildings 
C NC N/A 
Limited to two stories above grade. (IRC Section R611) 
Minimum wall thickness of 5.5 inches for IFC and 6 inches for solid concrete. 
(IRC Section R611.7.4) 
Maximum plan dimension of 60 feet and aspect ratio of 2:1. (IRC Section 
R611.2) 
Reinforcement detailing. (IRC Sections R611.3 - R611.5 and R611.7.1.2 and 
R611.7.1.3) 


Stone and Masonry Veneer 
C NC N/A 
Veneer. In SDCs D1 and D2, veneer is not permitted on buildings with cripple 
walls. (IRC Section R703.7, Exceptions 3 and 4) 
Fireplaces and Chimneys 
C NC N/A 
Vertical reinforcement requirements (four No. 4 Bars). (IRC Section R1003.3) 
Type N mortar prohibited in SDCs D1 and D2. (IRC Section R609) 
Anchorage requirements for SDCs D1 and D2. (IRC Section R1003.4) 


Appendix D 


SIGNIFICANT CHANGES FOR THE 
2006 INTERNATIONAL RESIDENTIAL CODE


During the 2006 cycle of technical updates for the International Residential Code (IRC), a 
number of important technical changes were made that will have an impact on many houses 
around the country. This appendix highlights the most significant of these changes so that the 
designer can continue to use this document when the 2006 IRC is adopted by his or her 
jurisdiction. 

D1 REVISED SEISMIC DESIGN MAPS 

In response to concerns over the perceived increases in earthquake design forces that were 
implemented with the adoption of the 2000 editions of the International Building Code (IBC) 
and IRC, the U.S. Geological Survey (USGS) conducted a detailed evaluation of earthquake risk 
on a county-by-county basis for regions with a high probability of earthquake occurrence. The 
project incorporated significant new information about local geological and geotechnical features 
and local experts for the regions being investigated were consulted. The result of the project is 
revised design maps that are incorporated into the 2006 editions of the IRC and IBC. In general, 
the new maps reduce the amount of geographic area affected by the high seismic risk, but the 
remaining area is also affected so some extent. This is primarily evident in the Charleston, South 
Carolina, region where the Seismic Design Category was raised in the counties closest to 
Charleston, but the rest of the state experiences a reduction in seismic risk level. The seismic 
design map that was adopted for the 2006 IRC is shown in Figure D-1 on the following pages. 

D2 ADDITION OF SEISMIC DESIGN CATEGORY D0 

In addition to provide some relief for the construction of houses using heavier finish materials 
such as masonry veneers and stucco, Seismic Design Category D1 was divided into two SDCs  
D0 and D1. Since design values must be set to the highest value in the range, dividing the 
original D1 into two lowered the earthquake design load for the lower design category. Only the 
brick masonry veneer industry has started to take advantage of this change to date. However, it 
is expected that other materials also will propose changes in the future to take advantage of the 
lower forces associated with Seismic Design Category D0. The geographic area associated with 
the change can be seen in Figure D-1. 


FEMA 232, Homebuilders Guide 



Appendix D, Significant Changes for the 2006 International Residential Code 



FEMA 232, Homebuilders Guide 



Appendix D, Significant Changes for the 2006 International Residential Code 



FEMA 232, Homebuilders Guide 



D3 CHANGE IN APPLICABILITY OF IRREGULAR BUILDING REQUIREMENTS 

During the update cycle resulting in the 2006 IRC, it was noticed that the wording requiring that 
the building conform to the requirements of IRC Section R301.2.2.2.2 only applied to wood and 
ICF concrete construction. Cold-formed steel construction had to conform to these irregularity 
requirements even through the referenced design and construction document (COFS/PM) had 
less stringent requirements and application to masonry wall buildings was not specified. Thus, a 
change was made to require all buildings to conform to the irregularity provisions of the IRC, 
which limit the concentrations of loads and deformations that irregularities cause. 

D4 CLARIFICATION AND ADDITION OF REQUIREMENTS FOR MASONRY 
VENEER 

A significant change was made to the requirements for the use of stone and masonry veneer in 
areas of high earthquake risk. The changes clarify veneer weight limits and stories where veneer 
is permitted. They also clarify and illustrate the required hold-down anchorage of walls and the 
requirements for ties and other reinforcement and attachment of the veneer to the walls. The 
change is too extensive to document here; rather, the reader is referred to the 2006 IRC for 
details. 


Appendix E 
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American Forest & Paper Association. 1996. Load and Resistance Factor Design (LRFD) 
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American Forest & Paper Association. 2001. Details for Conventional Wood frame 
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American Forest & Paper Association. 1993. Span Tables for Joists and Rafters. AF&PA, 
Washington, D.C. 

American Forest & Paper Association. 1987. Basic Requirements for Permanent Wood 
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American Institute of Timber Construction (AITC). 2004. Timber Construction Manual  Fifth 
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American Iron and Steel Institute (AISI). 2001. Standard for Cold-Formed Steel Framing -
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Washington, D.C. 

Anderson, C., F. Woeste, and J. Loferski. 2005. Manual for the Inspection of Residential Wood 
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Applied Technology Council (ATC). 1976. A Methodology for Seismic Design and 
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Association of Bay Area Governments (ABAG). Training Materials for Seismic Retrofit of 
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www.abag.ca.gov/bayarea/eqmaps/fixit/training.html 

Association of Bay Area Governments (ABAG). Info on Chimney Safety and Earthquakes. 
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Association of Bay Area Governments. Training Materials for Seismic Retrofit of Wood-Frame 
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Society of Civil Engineers, Reston, Virginia. 

Structural Engineers Association of California. 2003. Commentary on Chapter 3 Guidelines for 
Seismic Retrofit of Existing Buildings, 1 October 2003 Version. Existing Buildings Committee, 
SEAOC, Sacramento, California. 

Structural Engineers Association of Northern California (SEAONC). 2001. Guidelines for 
Seismic Evaluation and Rehabilitation of Tilt-up Buildings and Other Rigid Wall/Flexible 
Diaphragm Structures. SEAONC, San Francisco, California. 


U.S. Department of Agriculture (USDA). 1999. Wood Handbook  Wood as an Engineering 
Material, USDA Forest Products Laboratory General Technical Report FPL-GTR-113. USDA 
Forest Products Laboratory, Madison, Wisconsin. 

Von Winterfeldt, D., N. Roselund, and A. Kitsuse. 2000. Framing Earthquake Retrofitting 
Decisions: The Case of Hillside Homes in Los Angeles, PEER Report 2000-03. Pacific 
Engineering Research Center, Richmond, California. 

Wood Truss Council of America (WTCA). 1997. Metal Plate Connected Wood Truss 
Handbook  Third Edition. WTCA, Madison, Wisconsin. 


Appendix F 
HOMEBUILDERS GUIDE PROJECT PARTICIPANTS 


BSSC HOMEBUILDERS GUIDE PROJECT COMMITTEE 

Chair  J. Daniel Dolan, PhD, PE, Professor, Washington State University, Wood Materials and 
Engineering Laboratory, Pullman (writing team leader) 

Members 

Kelly Cobeen, Structural Engineer, Cobeen and Associates Structural Engineering, Lafayette, 

California (co-author) 
Gerald Jones, PE, Code Official, Retired, Kansas City, Missouri 
James E. Russell, Building Codes Consultant, Concord, California (co-author) 
Jim W. Sealy, FAIA, Architect/Consultant, Dallas, Texas 
Douglas M. Smits, CBO, City of Charleston, South Carolina 

FEMA Liaison Members 

Robert D. Hanson, Federal Emergency Management Agency, Walnut Creek, California 
Michael G. Mahoney, Federal Emergency Management Agency, Washington, D.C. 

INVITEES/PARTICIPANTS IN HOMEBUILDERS GUIDE WORKSHOPS AND 
REPORT REVIEWERS 

Mamood Abolhoda, City of Freemont, California 
Thomas Ahrens, City of Mill Valley, California 
John C. Anderson, City of San Diego, California 
Paul Armstrong, International Code Council, Whittier, California 
Christopher Arnold, FAIA, RIBA, Building Systems Development Inc., Palo Alto, California 
James Bela, Oregon Earthquake Awareness 
Carol Bloom, State Farm Insurance, Rohnert Park, California 

J. Gregg Borchelt, PE, Brick Industry Association, Reston, Virginia 
Amanda Brady, City of San Diego, California 
Patrick Bridges, Builder, Portland, Oregon 
Mark Caldwell, Apex Engineers, Calvert City, Kentucky 
Lee Campbell, Property Solutions, Inc., Irvine, California 
Cathleen Carlisle, Federal Emergency Management Agency, Washington, D.C. 
Tom Caulfield, Muller and Caulfield Architects, Oakland, California 
Frank Chiu, City and County of San Francisco, California 
Michael Christoffersen, Architectural Designs, Inc., Fort Wayne, Indiana 
John Chrysler, Masonry Institute of America, Torrance, California 
Glen Clapper, Southern Brick Institute, Conyers, Georgia 
Nicolino G. Delli Quadri, SE, City of Los Angeles, California 
Bradford K. Douglas, PE, American Forest and Paper Association, Washington, D.C. 


Ali Fattah, City of San Diego, California 

Jeffrey K. Feid, State Farm Insurance Company, Bloomington, Illinois 

Art Garcia, City of San Diego, California 

Robert S. George, Architect, South San Francisco, California 

Frank Golon, Pulte Homes, Bernardsville, New Jersey 

Rose Grant, State Farm Insurance, Bloomington, Illinois 

Michael Gregor, SGM Architecture Interiors, Mount Pleasant, South Carolina 

Cynthia Hammond, CPCU, State Farm Insurance, Rohnert Park, California 

Isam Hasenn, City of San Diego, California 

Perry A. Haviland, FAIA, Haviland Associates Architects, Oakland, California 

John R. Henry, PE, International Code Council, Sacramento, California 

Fred M. Herman, City of Palo Alto, California 

George Horton, Oakland, California 

Eric N. Johnson, Brick Association of the Carolinas, Charlotte, North Carolina 

Stephan A. Kiefer, City of Livermore, California 

Joseph Knarich, National Association of Home Builders, Washington, D.C. 

Vladimir G. Kochkin, National Association of Home Builders Research, Upper Marlboro, 
Maryland 

Laurence Kornfield, City and County of San Francisco, California 

Ray Kothe, Builder, Baton Rouge, Louisiana 

Jay W. Larson, American Iron and Steel Institute, Bethlehem, Pennsylvania 

Sheila Lee, City of Santa Clara, California 

Adlai Leiby, Berkeley, California 

Philip Line, American Forest and Paper Association, Washington, D.C. 

Jeff Lusk, Federal Emergency Management Agency Region IX, Oakland, California 

Joan O. MacQuarry, City of Berkeley, California 

Bonnie Manley, National Fire Protection Association, Quincy, Massachusetts 

Harry W. Martin, American Iron and Steel Institute, Auburn, California 

Zeno A. Martin, PE, APA-The Engineered Wood Association, Tacoma, Washington 

Joseph J. Messersmith, Jr., PE, Portland Cement Association, Rockville, Virginia 

Greg Mulvey, City of San Diego, California 

Steve Pfeiffer, City of Seattle, Seattle, Washington 

David Pollock, Washington State University, Pullman 

Steven E. Pryor, Simpson Strong-Tie, Dublin, California 

Kevin Reinerston, HCD Division of Codes and Standards, Sacramento, California 

Timothy A. Reinhold, PhD, PE, Institute for Business and Home Safety, Tampa, Florida 

Dennis Richardson, San Jose, California 

Alan Robinson, Tuan and Robinson Structural Engineers, Inc., San Francisco, California 

William R. Schock, City of San Leandro, California 

Roger Sharpe, City of Walnut Creek, Retired, Berkeley, California 

Thomas D. Skaggs, PhD, PE, APA-The Engineered Wood Association, Tacoma, Washington 

Steve Skalko, Portland Cement Association, Macon, Georgia 

Charles A. Spitz, AIA, CSI, NCARB, Architect-Planner-Code Consultant, Wall, New Jersey 

William W. Stewart, FAIA, Stewart-Schaberg/Architects, Chesterfield, Missouri 

Jason Thompson, SE, National Concrete Masonry Association, Herndon, Virginia 

Ray Tu, Hardy Frames, Inc., Ventura, California 


Fred M. Turner, Seismic Safety Commission, Sacramento, California 
David P. Tyree, PE, CBO, American Forest and Paper Association, Colorado Springs, Colorado 
Calvin N. Wong, PE, City of Oakland, California 
Matt Zamani, City of San Diego, California 

2005-2006 BSSC BOARD OF DIRECTION 

Chair --Jim W. Sealy, FAIA, Architect/Consultant, Dallas, TX 

Vice Chair --David Bonneville, Degenkolb Engineers, San Francisco, California 

Secretary --Jim Rinner, Project Manager II, Kitchell CEM, Sacramento, California 

Ex-Officio Member --Charles Thornton, Chairman/Principal, Thornton-Tomasetti Group, Inc., 
New York, New York 

Members 

Edwin Dean, Nishkian Dean, Portland, Oregon 

Bradford K. Douglas, Director of Engineering, American Forest and Paper Association, 
Washington, D.C. 

Cynthia J. Duncan, Director of Specifications, American Institute of Steel Construction, 
Chicago, Illinois 

Henry Green, Executive Director, Bureau of Construction Codes and Fire Safety, State of 
Michigan, Department of Labor and Economic Growth, Lansing, Michigan (representing 
the National Institute of Building Sciences) 

Jay W. Larson, American Iron and Steel Institute, Bethlehem, Pennsylvania 

Joseph Messersmith, Coordinating Manager, Regional Code Services, Portland Cement 
Association, Rockville, Virginia (representing the Portland Cement Association) 

Ronald E. Piester, Assistant Director for Code Development, New York State, Department of 
State, Kinderhook, New York 

James Rossberg, Manager, Technical Activities for the Structural Engineering Institute, 
American Society of Civil Engineers, Reston Virginia 

W. Lee Shoemaker, Director, Engineering and Research, Metal Building Manufacturers 
Association, Cleveland, Ohio 
Howard Simpson, Simpson Gumpertz and Heger, Arlington, Massachusetts (representing 
National Council of Structural Engineers Associations) 

Shyam Sunder, Deputy Director, Building Fire Research Laboratory, National Institute of 
Standards and Technology, Gaithersburg, Maryland (representing Interagency Committee 
on Seismic Safety in Construction) 

Charles A. Spitz, Architect/Planner/Code Consultant, Wall New Jersey (representing the 
American Institute of Architects) 
Robert D. Thomas, Vice President Engineering, National Concrete Masonry Association, 
Herndon, Virginia 


BSSC Staff 

Claret M. Heider, Vice President for BSSC Programs 
Bernard F. Murphy, PE, Director, Special Projects 
Carita Tanner, Communications/Public Relations Manager 


THE BUILDING SEISMIC SAFETY COUNCIL 


The purpose of the Building Seismic Safety Council is to enhance the public's safety by providing a 
national forum to foster improved seismic safety provisions for use by the building community. For the 
purposes of the Council, the building community is taken to include all those involved in the planning, 
design, construction, regulation, and utilization of buildings. 

To achieve its purposes, the Council shall conduct activities and provide the leadership needed to: 

 
Promote development of seismic safety provisions suitable for use throughout the United States; 
 
Recommend, encourage, and promote adoption of appropriate seismic safety provisions in 
voluntary standards and model codes; 
 
Assess implementation progress by federal, state, and local regulatory and construction agencies; 
 
Identify opportunities for the improvement of seismic regulations and practices and encourage 
public and private organizations to effect such improvements; 
 
Promote the development of training and educational courses and materials for use by design 
professionals, builders, building regulatory officials, elected officials, industry representatives, 
other members of the building community and the public. 
 
Provide advice to governmental bodies on their programs of research, development, and implementation; and 
 
Periodically review and evaluate research findings, practice, and experience and make re
commendations for incorporation into seismic design practices. 


The scope of the Council's activities encompasses seismic safety of structures with explicit consideration 
and assessment of the social, technical, administrative, political, legal, and economic implications of its 
deliberations and recommendations. 

Achievement of the Council's purpose is important to all in the public and private sectors. Council 
activities will provide an opportunity for participation by those at interest, including local, State, and 
Federal Government, voluntary organizations, business, industry, the design professions, the construction 
industry, the research community and the public. Regional and local differences in the nature and 
magnitude of potentially hazardous earthquake events require a flexible approach adaptable to the relative 
risk, resources and capabilities of each community. The Council recognizes that appropriate earthquake 
hazard reduction measures and initiatives should be adopted by existing organizations and institutions and 
incorporated into their legislation, regulations, practices, rules, codes, relief procedures and loan requirements, whenever possible, so that these measures and initiatives become part of established activities 
rather than being superposed as separate and additional. 

The Council is established as a voluntary advisory, facilitative council of the National Institute of Building Sciences, a nonprofit corporation incorporated in the District of Columbia, under the authority given 
the Institute by the Housing and Community Development Act of 1974, (Public Law 93-383), Title VIII, 
in furtherance of the objectives of the Earthquake Hazards Reduction Act of 1977 (Public Law 95-124) 
and in support of the President's National Earthquake Hazards Reduction Program, June 22, 1978.