NEHRP Seismic Design Technical Brief No. 2

Seismic Design of Steel 

Special Moment Frames

A Guide for Practicing Engineers

NIST GCR 16-917-41

Ronald O. Hamburger 

James O. Malley

SECOND EDITION

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.GCR.16-917-41 
About the Authors

Ronald O. Hamburger, P.E., S.E., (First and Second Edition),

is Head of Structural Engineering, Western Region, for Simpson, 

Gumpertz, & Heger, Inc. in San Francisco, California. He chaired 

the American Institute of Steel Construction (AISC) Connection 

Prequalification Review Panel from 2002-2015, is a member of the AISC 

Committee on Specifications, and received the Institute’s Higgins award 

in 2006. He has chaired the ASCE 7 committee, the joint USGS-Building 

Seismic Safety Council (BSSC) Project 17 and the BSSC’s Provisions 

Update Committee. Following the 1994 Northridge earthquake, he served 

as Product Director for Product Development of the SAC Joint Venture.

James O. Malley, P.E., S.E., (First and Second Edition), is Senior 

Principal with Degenkolb Engineers in San Francisco, California. 

He chaired the American Institute of Steel Construction (AISC) Task 

Committee on Seismic Design from 1995-2015. Since 2000 he has 

served on the AISC’s Committee on Specifications, becoming its chair 

About the Reviewers

The contributions of the review panelists for this publication are gratefully 

acknowledged.

John D. Hooper, P.E., S.E., (Second Edition) is a Senior Principal and 

Director of Earthquake Engineering at Magnusson Klemencic Associates, a 

structural and civil engineering firm headquartered in Seattle, Washington. 

He is a member of the BSSC’s 2015 Provisions Update Committee and 

chair of the American Society of Civil Engineers Seismic Subcommittee 

for ASCE 7-16.

Thomas A. Sabol, Ph.D., S.E., A.I.A, (First and Second Edition), is 

Principal with Englekirk in Los Angeles, California and Adjunct Professor 

of Civil and Environmental Engineering at the University of California in Los 

Angeles. He is both a registered structural engineer and architect. He has 

served as chair of the seismic systems technical committee of the AISC.

C. Mark Saunders, P.E., S.E., (First and Second Edition), is retired

from Rutherford+Chekene Consulting Engineers where he was formerly

President and Chairman. He has been a member of the AISC Task

Committee 9 – Seismic Provisions for more than 20 years, most recently as

Vice Chairman, and as Chairman of Subcommittee 4 on Moment Frames.

He is a past president of both the Structural Engineers Association of

Northern California and ATC.

Andrew W. Taylor, Ph.D., P.E., S.E., FACI, (Second Edition) is an 

Associate with KPFF Consulting Engineers in Seattle. He is the chair of 

the American Concrete Institute (ACI) 318 Building Code sub-committee 

H on Seismic Provisions, a member of the ACI Technical Activities 

Committee, and chair of the Earthquake Engineering Committee of the 

Structural Engineers Association of Washington. He is also a member of 

the ASCE 41 seismic rehabilitation standard committee, and a member 

of subcommittees of the ASCE 7 seismic design standard.

Acknowledgments

This Second Edition supersedes the original edition of NIST GCR 09-

917-3, published in June 2009. It is based on information in the original

publication and includes updated references to codes and standards,

updated guidance, and current best practices. ATC, CUREE, and NIST

would like to acknowledge the following additional authors and reviewers

who contributed to developing the original publication, which served as

the basis of this Second Edition:

Helmut Krawinkler, formerly with Stanford University, Stanford, CA

Scott M. Adan, formerly with Simpson, Gumpertz, & Heger, San Francisco, CA

Douglas Foutch, University of Illinois at Urbana-Champaign, Urbana, IL

Roberto Leon, Virginia Tech, Blacksburg, VA

National Institute of 

Standards and Technology

NIST is a federal technology agency within the U.S. Department of 

Commerce that promotes U.S. innovation and industrial competitiveness 

by advancing measurement science, standards, and technology in ways 

that enhance economic security and improve our quality of life. NIST 

is the lead NEHRP agency. Dr. Steven L. McCabe is the Earthquake 

Engineering Group (EEG) Leader within the Materials and Structural 

Systems Division of the Engineering Laboratory at NIST. The EEG 

supports the work of the NEHRP program at NIST.

Applied Technology Council

The Applied Technology Council (ATC) is a nonprofit corporation 

advancing engineering applications for hazard mitigation. This 

publication is a product of Task Order 15-264 under Contract SB1341-

13-CQ-0009 between ATC and NIST. Jon A. Heintz served as the

Program Manager for work conducted under this contract, and Scott

Schiff serves as ATC Associate Program Manager on this Task Order.

Consortium of Universities for Research 

in Earthquake Engineering

The Consortium of Universities for Research in Earthquake 

Engineering (CUREE) is a nonprofit organization advancing earthquake 

engineering research, education, and implementation. This publication 

was produced under a cooperative agreement between ATC and 

CUREE. Robert Reitherman served as Associate Program Manager 

overseeing production. Reed Helgens and Darryl Wong served as 

report production and report preparation consultants for this work. 

National Earthquake Hazards Reduction Program (NEHRP) Technical 

Briefs are published by the National Institute of Standards and 

Technology (NIST) as aids to the efficient transfer of NEHRP and other 

research into practice, thereby helping to reduce the nation’s losses 

resulting from earthquakes.

in 2016. He received the Institute’s T. R. Higgins award in 2010 and a 

Lifetime Achievement award in 2012. He also serves as a member of the 

BSSC’s Provision’s Update Committee. Following the 1994 Northridge 

earthquake, he served as Product Director for Topical Investigations of 

the SAC Joint Venture.

Consortium of Universities for Research in 

Earthquake Engineering (CUREE)

1301 South 46th Street, Building 420

Richmond, CA 94804

(510) 665-3529 | curee@curee.org

www.curee.org

Applied Technology Council (ATC)

201 Redwood Shores Parkway, Suite 240

Redwood City, CA 94065

(650) 595-1542 | email: atc@atcouncil.org

www.atcouncil.org

CUREE

NEHRP Seismic Design Technical Briefs
By

Applied Technology Council

In association with the 

Consortium of Universities for Research in Earthquake Engineering

and

Ronald O. Hamburger

James O. Malley

June 2016

Prepared for

U.S. Department of Commerce

National Institute of Standards and Technology

Engineering Laboratory

Gaithersburg, MD 20899-8600

Seismic Design of Steel Special 

Moment Frames

A Guide for Practicing Engineers

NIST GCR 16-917-41

U.S. Department of Commerce

Penny Pritzker, Secretary

National Institute of Standards and Technology

Willie E. May, Under Secretary of Commerce for 

Standards and Technology and Director

SECOND EDITION

With references to ASCE 7-16, AISC 341-16, and AISC 358-16

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.GCR.16-917-41
Disclaimers

This Technical Brief was prepared for the Engineering Laboratory of the National Institute of Standards and Technology (NIST) under 

the National Earthquake Hazards Reduction Program (NEHRP) Earthquake Structural and Engineering Research Contract SB1341-

13-CQ-0009, Task Order 15-264. The contents of this publication do not necessarily reflect the views and policies of NIST or the 

U.S. Government.

This report was produced by the Applied Technology Council (ATC) in association with the Consortium of Universities for Research in 

Earthquake Engineering (CUREE). While endeavoring to provide practical and accurate information, ATC, CUREE, the authors, and the 

reviewers assume no liability for, nor express or imply any warranty with regard to, the information contained herein. Users of information 

in this report assume all liability arising from such use.

Unless otherwise noted, photos, figures, and data presented in this report have been developed or provided by ATC staff, CUREE 

staff, or consultants engaged under contract to provide information as works for hire. Any similarity with other published information is 

coincidental. Photos and figures cited from outside sources have been reproduced in this report with permission. Any other use requires 

additional permission from the copyright owners. 

Certain commercial software, equipment, instruments, or materials may have been used in preparing information contributing to this 

report. Identification in this report is not intended to imply recommendation or endorsement by NIST, nor is it intended to imply that such 

software, equipment, instruments, or materials are necessarily the best available for the purpose.

Cover photo—Steel special moment frame under construction.

NIST (2016). Seismic design of steel special moment frames: A guide for practicing engineers, Second Edition, GCR 16-917-41, NEHRP 

Seismic Design Technical Brief No. 2, produced by the Applied Technology Council and the Consortium of Universities for Research in 

Earthquake Engineering for the National Institute of Standards and Technology, Gaithersburg, MD.

Introduction ..............................................................................................1

The Use of Special Moment Frames................................................................3

Principles for Steel Special Moment Frame Design.........................................10

Analysis Guidance...................................................................................19

Design Guidance.....................................................................................21

Additional Requirements...........................................................................26

Detailing and Constructability Issues...........................................................29

References.............................................................................................33

Notations and Abbreviations......................................................................34

Credits....................................................................................................36

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5. 

6. 

7. 

8.

9.

10.

Contents

How to Cite This Publication
1

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

Structural steel special moment frames are commonly 

used as part of the seismic force-resisting systems in 

buildings designed to resist severe ground shaking and 

are permitted by ASCE/SEI 7-16, Minimum Design 

Loads and Associated Criteria for Buildings and 

Other Structures (ASCE 2016), referred to hereafter 

as ASCE 7, for any Seismic Design Category without 

limitation as to height. Beams, columns, and beamcolumn connections in Steel Special Moment Frames are 

proportioned and detailed to resist flexural, axial, and 

shearing actions that result as the building sways through 

multiple inelastic displacement cycles during strong 

earthquake ground shaking. Special proportioning and 

detailing requirements enable these structures to safely 

resist strong earthquake shaking while experiencing 

substantial inelastic behavior. These moment frames 

are called special moment frames by ASCE 7 because 

of these additional requirements, which improve the 

inelastic response characteristics of these frames in 

comparison with less stringently detailed intermediate 

and ordinary moment frames.

Design requirements for steel special moment frames 

are contained in a series of standards. ASCE 7, sets the 

basic loading criteria for steel special moment frames 

together with associated lateral drift limits. ANSI/

AISC 341-16, Seismic Provisions for Structural Steel 

Buildings (AISC 2016a), referred to hereafter as AISC 

341, provides detailed design requirements relating to 

materials, framing members (beams, columns, and 

beam-column joints), connections, and construction 

quality assurance and quality control. In addition, AISC 

341 presents requirements for columns that are not 

designated as part of the seismic force-resisting system. 

The numerous interrelated requirements for steel special 

moment frames are covered in several sections of AISC 

341, with the primary requirements covered in Section E3. 

Section E3.6c of AISC 341 references ANSI/AISC 358-16 

Prequalified Connections for Special and Intermediate 

Steel Moment Frames for Seismic Applications (AISC 

2016b), referred to hereafter as AISC 358, which 

facilitates and standardizes the selection and design of 

steel special moment frame connections to allow their 

use without the need for project-specific testing. A series 

of different moment connection details are presented in 

AISC 358, and additional connections are anticipated to 

be added in future editions.

1. Introduction

Both AISC 341 and AISC 358 are applied in conjunction 

with the ANSI/AISC 360-16, Specification for Structural 

Steel Buildings (AISC 2016c), referred to hereafter as 

AISC 360, and AISC 303, Code of Standard Practice 

for Steel Buildings and Bridges (AISC 2016d), referred 

to hereafter as AISC 303. AISC 360 is the main AISC 

specification that provides the design and detailing 

requirements for all steel buildings. In addition to these 

standards, American Welding Society (AWS) standards 

AWS D1.1 Structural Welding Code Steel (AWS 2015) 

and (AWS) Standard D1.8/D1.8M: 2009, Structural 

Welding Code—Seismic Supplement (AWS 2009) presents 

requirements for welding and fabrication that pertain to 

steel special moment frames.

This Guide is intended for use by practicing structural 

engineers to assist in their understanding and application 

of the ASCE 7, AISC 341, AISC 358, and AWS D1.8 

standards to steel special moment frame design. The 

material is presented in a sequence that practicing 

engineers have found useful, with historic and general 

principles for seismic design discussed first, followed by 

system-specific analysis and design requirements. This 

Guide is also intended to be of value to building officials, 

educators, and students.

This Guide follows the requirements of the 2016 editions 

of AISC 341 and AISC 358, along with the pertinent 

design load requirements specified in ASCE 7. AISC 

341 primarily addresses the seismic design of systems 

in Seismic Design Categories D, E, and F, as defined 

in ASCE 7. The International Building Code, or IBC, 

(ICC 2015), which is the code generally adopted 

throughout the United States, references ASCE 7 for the 

determination of seismic loads. AISC 341 was developed 

in parallel with ASCE 7, so the documents are well 

coordinated regarding terminology, system definition, 

application limitations, and other issues.
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

2

Code Requirements versus 

Guide Recommendations

Building codes present minimum requirements 

for design and construction of buildings and are 

legal requirements where adopted by the authority 

having jurisdiction. Thus, where adopted, AISC 341, 

AISC 358 and AISC 360, with ASCE 7, must as a 

minimum be followed. This Guide is written mainly 

to clarify requirements of the building code and 

these referenced standards, and it presents other 

recommendations for good analysis, design, and 

construction practices that may not be specifically 

required by the codes or standards. The Guide clearly 

differentiates between building code requirements and 

other recommendations.

The main body of text in this Guide emphasizes 

code requirements and accepted approaches to their 

implementation. It includes background information 

and figures to illustrate the requirements. Sections 3 

through 6 present analysis, behavior, proportioning, 

and detailing requirements for steel special moment 

frames and other portions of the building that interact 

with these frames. Section 7 presents a discussion of 

detailing and constructability issues to highlight unique 

features of steel special moment frame construction. Cited 

references, notations and abbreviations, and credits are 

in Sections 8, 9, and 10.

Edition of Codes and Standards

At the time of this publication the International Building 

Code (ICC 2015) adopts ASCE 7-10, AISC 360-10, 

and AISC 341-10, which reference AISC 358-10. 

However, the 2016 editions of these standards have 

been completed, are anticipated to be adopted by the 

2018 International Building Code, and will be available 

for use in design projects in the near term. 

To enable long-term relevance, this publication 

references the 2016 editions of these standards and 

the 2018 International Building Code. Substantial 

changes in these documents from earlier editions 

affecting the design of steel special moment frames are 

noted in sidebars. Design engineers are responsible 

for verifying the currently applicable building code 

provisions adopted by the authority having jurisdiction 

over their project. Discussion with and approval by 

the building official should occur to verify that a later 

version of a code or standard not yet adopted locally 

may be used.

The First Edition of this document (NIST CGR 09-917-3) 

was published in June 2009. The codes and standards 

referenced in that edition were current as of then but 

have been updated by the documents referenced in this 

Second Edition. The First Edition, which may still be 

relevant in some engineering applications with regards 

to buildings constructed under the earlier editions of 

the codes and standards, referenced the 2006 edition 

of the International Building Code, ASCE 7-05, ANSI/

AISC 341-05, ANSI/AISC 358-05, ANSI/AISC 360-05, 

AISC 303-05, AWS D1.1-2004, and AWS D1.8-2005.
3

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

2.1 Historic Development

Although the concept of a steel special moment frame 

is a relatively recent development in the building codes, 

steel moment frames have been in use for more than one 

hundred years, dating to the earliest use of structural 

steel in building construction. A notable example of 

steel building construction in the United States with the 

frame carrying the vertical loads is the Home Insurance 

Building in Chicago, a 10-story structure constructed in 

1884 with a height of 138 feet (42 m), carrying its gravity 

loads with a framework of iron and steel members, it is 

often credited with being the first skyscraper (Figure 

2-1). This and other tall buildings in Chicago, New 

York, and other U.S. cities spawned an entire generation 

of tall buildings, constructed with load bearing steel 

frames supporting concrete or wood floors and nonload bearing, unreinforced masonry infill walls at their 

perimeters. Framing in these early structures typically 

used “H” shapes built up from plates, “L,” and “Z” 

sections. Starting with the Manhattan Building (1889), 

perimeter framing connections usually incorporated 

large stiffened triangular gusset plates, joined to the 

beams and columns with angles and rivets (Figure 

2-2). Typically, steel framing was completely encased 

by masonry, concrete, or a combination of these, to 

provide fire resistance. Anecdotal evidence suggests 

that designers of these early moment frame structures 

neglected the structural contributions of concrete and 

masonry encasement and further assumed that framing 

connections acted as “pinned” connections for gravity 

loading and “fixed” connections for lateral loading. 

Despite these assumptions, the steel framing in these 

structures was substantially stiffened and strengthened 

by composite behavior with their encasements.

This basic construction style remained popular for 

high-rise construction through the 1930s, though by the 

early 1900s, rolled “I” and “H” shape sections began to 

see increasing use in place of the built-up sections, in 

particular for lighter framing. Many very tall structures, 

including New York’s Empire State Building, for many 

years the world’s tallest structure, are of this construction 

type.

Following World War II, constructing perimeter walls 

out of infill unreinforced masonry, particularly for tall 

buildings became uneconomical, and more modern glass 

and aluminum curtain wall systems were adopted as part 

2. The Use of Special Moment Frames

Figure 2-1. The Home Insurance Building – Chicago, IL, 1884,

 an early skyscraper.

Figure 2-2. Typical early moment connection, consisting of heavy 

triangular gusset plates, angles, and rivets connecting built-up 

columns and beams.

of the new modern architectural style. The larger windows 

possible with these new curtain wall systems made large 

gusseted framing connections undesirable, and engineers 

began to design connections without gussets, using angles 

or split tees to connect top and bottom beam flanges to 

columns (Figure 2-3).

In the 1950s, as welding became increasingly common 

in building construction, the angles and split tees were 

replaced by flange plates that were shop-welded to the 

column flanges and then riveted to the beam flanges. By 

the 1960s, riveting had become uneconomical and was 
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

4

replaced by high strength bolting. Finally, in the early 

1970s, engineers began to use the connection type known 

today as the welded unreinforced flange—bolted web 

(Figure 2-4), incorporating field-welded, complete joint 

penetration groove welds to join beam flanges to columns, 

with shop-welded, field-bolted shear plates joining beam 

webs to columns.

Almost from their inception as a means of building 

construction, engineers began to observe that steel 

moment frames seemed to exhibit superior performance 

in earthquakes. More than 20 such structures were 

subjected to and survived the 1906 San Francisco 

earthquake and the fires that followed it, while few 

other buildings in the central commercial district of 

San Francisco remained standing (Figure 2-5). Many 

of these steel frame buildings are still in service today. 

For nearly 90 years, as additional earthquakes shook 

steel structures with little apparent damage, a reputation 

of superior earthquake-resisting capability was created. 

Much of the seismic and fire resistance possessed by these 

structures was a result of the composite interaction of the 

steel framing with the encasing masonry and concrete. 

Modern steel structures, with lightweight fireproofing 

applied to steel members, do not have the benefit of this 

composite behavior.

As a result of the apparent superior performance of 

these structures, building codes of the 1960s adopted 

preferential seismic design criteria for steel moment 

frames. Under these codes, buildings having complete 

vertical load-carrying space frames as their lateral forceresisting systems could be designed for two-thirds of 

the seismic forces specified for braced frames and half 

the forces specified for bearing wall structures. Further, 

these codes required such moment frames in buildings 

exceeding 160 feet (49 m) in height.

In the 1960s and 1970s, Professor Popov at the University 

of California at Berkeley and other researchers began to 

perform cyclic laboratory testing of steel moment framing 

and discovered that some control on the proportioning 

and detailing of these structures was necessary to obtain 

superior inelastic behavior in strong earthquakes. Slowly, 

throughout the 1970s and 1980s, the building codes began 

to adopt these researchers’ recommendations and require 

special design, configuration, and detailing of steel 

moment frames used for seismic resistance in regions 

of high probability of strong ground motion. Frames 

conforming to these design criteria were first designated 

in the Uniform Building Code (UBC) as ductile momentFigure 2-3. Riveted, unstiffened seat angle connection.

Figure 2-4. Welded unreinforced flange—bolted web connection 

popularly used from 1970 to 1994.
5

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

Figure 2-5. Steel frame buildings in downtown San Francisco performed well in the 1906 earthquake.

resisting space frames, and then finally, in the 1988 UBC 

(ICBO 1988), as special moment-resisting space frames, 

which were assigned the highest response modification 

factor, Rw. The term “special” was adopted, both because 

special criteria applied to the design of these structures 

and because they were expected to provide special, that 

is superior, performance in strong earthquakes.

Initially, the special design criteria were limited to a 

requirement that connections be capable of developing 

the strength of the connected members, with the welded 

unreinforced flange—bolted web connection identified 

as a deemed-to-comply standard. Later, requirements 

were introduced to provide for strong-column/weak-beam 

behavior, regulate panel zone shear strength (defined on 

page 7), and institute section compactness and lateral 

bracing criteria. Building codes of this era required 

the use of ductile moment-resisting space frames in all 

structures exceeding 160 feet (49 m) in height in regions 

with a high probability of experiencing strong ground 

motion, as a result, nearly every tall building constructed 

in the western United States in this era was of steel 

moment-frame construction. Such structures designed in 

the 1960s and 1970s tended to employ moment-resisting 

connections at every beam-column joint, providing 

great redundancy and distribution of lateral force 

resistance. However, by the 1980s, engineers had begun 

to economize their designs and minimize expensive field 

welding by using fewer bays of moment-resisting framing 

that employed heavier beams and columns, resulting in 

less redundant structures with more concentrated lateral 

force resistance. In extreme cases, some tall structures 

were provided with only a single bay, or perhaps two bays, 

of moment-resisting framing on each side of the building.

Figure 2-6. Fracturing of W14 column at welded 

beam-to-column connection in Northridge earthquake.

Following the 1994 Northridge earthquake in the Los 

Angeles area, engineers were surprised to discover 

that more than 20 modern special steel moment frame 

structures had experienced brittle fracturing of their 

welded beam-to-column connections. Figure 2-6 shows 

one example of such damage; however, many different 

types of fractures were also discovered, the majority 

initiating where the bottom beam flange joined the 

column flange. Similar damage occurred one year later 

in the 1995 Kobe earthquake in Japan. Following these 

discoveries, a consortium of professional associations and 

researchers known as the SAC Joint Venture engaged in 

a federally funded, multiyear program of research and 

development to determine the causes of this unanticipated 

behavior and to develop recommendations for more 

robust moment frame construction. The SAC research, 

conducted at a cost of $12 million over eight years, 

resulted in the basis for the current design provisions for 

moment frames contained in AISC 341, AISC 358, and 

AWS D1.8.
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

6

2.2 Steel Moment Frame Seismic Behavior 

Even in regions of very high seismic risk severe 

earthquakes are rare events, affecting typical building 

sites at average intervals of hundreds of years. It is usually 

economically impractical to design structures to resist 

such severe but rare earthquakes without damage. Instead, 

the building codes have adopted a design philosophy 

intended to provide safety by minimizing the risk of 

collapse given the occurrence of severe shaking, termed 

Risk-targeted Maximum Considered Earthquake (MCER) 

shaking, while permitting extensive structural and 

nonstructural damage in these and more moderate events.

Inelastic behavior in steel special moment frame 

structures is intended to be accommodated through the 

formation of plastic hinges in beams at beam-column 

joints, as well as at column bases. Plastic hinging in beams 

and columns can be accompanied by local buckling of 

beam and column flanges, as well as large deformations 

of the webs (Figure 2-7). In recognition of the highly 

ductile inelastic behavior of panel zones and the ability 

of this behavior to minimize the damage to beams, AISC 

341 encourages design to accommodate balanced yielding 

between plastic hinge zones in beams and the panel zones.

In addition to these desired behaviors, a number of other 

less desirable behavior modes can occur. AISC 341 design 

procedures seek to minimize the potential for these less 

desirable modes, which include the following.

Beam behavior. When buckling becomes excessive, 

strength loss and ultimately fractures associated with 

low-cycle fatigue will occur. The use of highly compact 

sections for members intended to experience hinging 

minimizes the potential for strength loss and fracturing 

at deformation levels likely to occur in response to 

MCER shaking. Provision of lateral bracing in zones of 

anticipated plastic hinging is required to avoid lateraltorsional buckling and the strength loss associated with 

that behavior mode.

Beam-to-column connections. Connections must be 

capable of transferring the yield-level stresses and strains 

that develop in the beam or connection components to 

the column during plastic hinging and do so for multiple 

cycles. Depending on the type of connection used, this 

might trigger any of the following failure modes: 

• Fractures in or around welds

• Fractures in highly strained base material

• Fractures at weld access holes

• Net section fractures at bolt holes

• Shearing and tensile failures of bolts

• Bolt bearing and block shear failures

AISC 341 requires demonstration by conformance with 

prequalified details or through prototype testing that 

connections used in steel special moment frames are 

capable of accommodating at least +/- 0.04 radians of 

total rotation without exhibiting strength loss associated 

with these or other failure modes when subjected to 

a specified loading consisting of repeated cycles of 

increasing displacement.

Figure 2-7. Typical local buckling of beam flanges and web 

in zone of plastic hinging at high levels of inelastic rotation.

1994 Northridge Earthquake and the 

SAC Steel Project 

In the aftermath of the 1994 Northridge, California 

earthquake, damage to welded steel moment frame 

connections in the Los Angeles area spawned 

concern about the reliability of established design 

and construction procedures. A number of buildings 

experienced damage in beam-to-column connections 

that underwent only moderate inelastic demands. 

Failures included fractures of the bottom beam flangeto-column-flange complete joint penetration groove 

welds, cracks in beam flanges, and cracks through 

the column section. The fractures were a result of the 

basic connection geometry, lack of control of base 

material properties, the use of weld filler metals with 

inherently low toughness, uncontrolled deposition 

rates, inadequate quality control, and other factors. 

The SAC Steel Project research conducted by the 

SAC Joint Venture, published in the FEMA 350, FEMA 

351, FEMA 352, FEMA 353, and FEMA 355 series 

of reports, underpins current code requirements for 

steel special moment frame design (FEMA 2000 a-e).
7

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

Joint panel zones. The joint panel zone, consisting 

of that portion of the column bounded by the top and 

bottom beam flanges, resists significant shear, tension, 

and compression forces from the beams framing into the 

column. Potential failure modes include web compressive 

buckling opposite beam compression flanges, web shear 

buckling, and, if doubler plates are used to reinforce 

the panel zone, fracture at welds. AISC 341 design 

procedures control these behaviors through requirements 

for minimum shear strength, provision of stiffener plates 

opposite beam flanges, and control of welding details.

Columns. Except at restrained column bases, where 

plastic hinging is likely to occur, columns are designed 

to behave in an essentially elastic manner to minimize 

potential formation of single-story mechanisms (Figure 

2-8). This is accomplished through requirements that 

columns be stronger in flexure than beams connected 

to the columns at the same joint. Nonetheless, columns 

can experience significant inelastic rotations in response 

to severe shaking, resulting in excessive local buckling 

and lateral-torsional buckling. Global buckling can also 

occur. To minimize this potential, columns must have 

adequate axial strength, compactness, and lateral bracing 

to withstand the axial forces associated with formation 

of full frame yield mechanisms.

Figure 2-8. Formation of a single story frame mechanism, 

also termed a “weak story” mechanism.

well as seismic force-resisting frames in steel special 

moment frame structures.

Column bases. Potential failure modes depend on the 

connection between the column and the foundation. They 

can include anchorage stretching or pull-out, fracture in 

base plates or in column-to-base plate connections, and/or 

excessive local and lateral-torsional buckling if inelastic 

deformations are concentrated in the region above the 

base connection.

Structural Stability. Amplification of internal forces and 

lateral displacements, known as the P-D effect, occurs 

when a structure is simultaneously subjected to gravity 

loads and lateral sidesway. This reduces frame lateral 

resistance and stiffness and can cause a negative effective 

lateral tangent stiffness once a mechanism has formed, 

leading to collapse.

Much of the guidance presented here focuses on design 

principles and analysis checks intended to reduce the 

likelihood of the aforementioned failure modes.

2.3 When To Use Steel Special Moment 

 Frames

The principal architectural advantage of moment 

frame structures is that they do not have structural 

walls or diagonal braces. They therefore provide 

architectural freedom in design, permitting open bays 

and unobstructed view lines. The tradeoff for these 

benefits is that moment frame structures can be more 

costly to construct than braced frame or shear wall 

structures. The added cost results from the use of larger 

and heavier sections in moment frames than is common in 

braced structures and more labor-intensive connections. 

However, moment frames typically impose smaller forces 

on foundations than do other structural systems, resulting 

in somewhat more economical foundation systems. 

Another consideration is that because moment frames are 

inherently flexible structures, when earthquakes do affect 

them, drift-sensitive nonstructural components, such as 

cladding and glazing, can experience more damage in 

these structures compared with other structural types.

Once a steel moment frame solution is selected for a 

project, designers may be able to choose from several 

types, including special moment frames, intermediate 

moment frames, ordinary moment frames, and moment 

frames not specifically designed for seismic resistance. 

The following is a general discussion of limitations on 

Undeformed 

shape

Deformed 

shape

Plastic 

hinges

Column splices. Potential failure modes at column 

splices are similar to those enumerated for beam-tocolumn connections. Failure of column splices will not 

only reduce or eliminate bending and tension resistance 

but also reduce or eliminate the ability of the column 

to transfer shear forces. Because gravity load-carrying 

columns in steel special moment frame structures can 

experience substantial lateral deformations and related 

seismic forces, AISC 341 specifies the required strength 

of splices in the columns of gravity-resisting frames as 
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

8

the use of these various systems. ASCE 7 §12.2.1, Table 

12.2-1 states the limitations on use of each of these 

systems, based on the assigned seismic design category 

and the height of the structure. There are no restrictions 

on system use in Seismic Design Category A.

Moment frames not specifically detailed for seismic 

resistance have no special detailing criteria and need 

comply only with the strength and drift limits of ASCE 7 

and the design requirements of AISC 360. These frames 

are not permitted as seismic force-resisting systems in 

Seismic Design Categories D, E, or F. Ordinary moment 

frames, designed in accordance with limited requirements 

specified by AISC 341 §E1, are permitted in light, singlestory structures and low-rise residential structures in all 

Seismic Design Categories and are permitted without 

restriction in Seismic Design Categories A, B, and C.

Intermediate moment frames, designed to somewhat 

more restrictive criteria specified in AISC 341 §E2, 

are permitted without restriction in Seismic Design 

Categories A, B, and C. In Seismic Design Category D, 

intermediate moment frames are permitted for structures 

up to 35 feet (11 m) in height. In Seismic Design Categories 

E and F, intermediate moment frames are permitted for 

light, single-story structures only.

Steel special moment frames must conform to the criteria 

in AISC 341 §E3. Special moment frames are permitted 

without restriction in all seismic design categories and are 

required as part of the seismic force-resisting system in 

Seismic Design Categories D, E, and F for most structures 

exceeding 160 feet (49 m) in height. For structures that 

meet certain regularity criteria, the requirement to 

incorporate special moment frames is triggered at a height 

of 240 feet (73 m).

Code-specified seismic strength as a fraction of building 

weight decreases progressively from ordinary moment 

frames to intermediate moment frames to special moment 

frames. However, the added level of detailing required for 

the better-performing systems can significantly increase 

construction cost. In addition, because considerations of 

lateral drift often control the selection of moment frame 

member sizes, the reduced required strength associated 

with the more ductile systems do not necessarily translate 

to savings in member sizes or frame weight. A common 

strategy for tall buildings in Seismic Design Categories 

D, E, and F has been to use dual systems, in which steel 

special moment frames capable of providing at least 

25 percent of the required lateral strength are used in 

combination with shear walls or braced frames. The dual 

system allows economical control of lateral drift while 

permitting design for reduced forces relative to those 

required for pure braced frame systems. 

2.4 Frame Proportioning 

Except for a steel special moment frame used as part of a 

dual system, base shear strength is not usually the primary 

design consideration. The primary factors affecting steel 

special moment frame member size selection are the need 

to control design drifts below specified limits, the need 

to avoid P-D instabilities, and the need to proportion 

structures to comply with the strong-column/weak-beam 

criteria of AISC 341 §E3.4a. Although many designers 

find that the use of deep section columns (W24s, W36s, 

and built-up box sections) is an economical choice that 

facilitates achievement of both drift control and strongcolumn/weak-beam requirements, deep wide flange 

sections, particularly those with lighter weights, are 

susceptible to undesirable local and lateral-torsional 

buckling. The performance of deep column sections is 

the subject of ongoing research.

It is usually advantageous to limit the dimensions of 

bays in moment frames, as long-span frames tend to be 

flexible, driving up section sizes required to control drift. 

Frame spans exceeding 40 feet (12 m) are rarely practical. 

However, short bay widths result in larger plastic rotation 

demands at a given level of inelastic drift, result in higher 

shear demands on connections, and, in extreme cases, 

can result in inelastic behavior dominated by shear, as 

opposed to flexural yielding, of beams. Most connections 

prequalified for use in steel special moment frames have 

limits on the beam span-to-depth ratio that prevent use 

of excessively short bays. Bay widths less than 20 feet 

(6 m) are rarely economical.

The ability of steel framing members to accommodate 

large inelastic deformations is in part dependent on 

section depth and weight. Lighter, shallower sections and 

their connections that meet AISC 341 §D1.1 compactness 

requirements tend to have larger inelastic deformation 

capacity than do deep, heavy sections. For this reason, 

it is desirable to distribute lateral resistance in steel 

special moment frame structures among multiple bays 

of framing, providing high redundancy and reduced 

framing sizes. In some cases, use of smaller members in 

multiple bays can offset the cost of the additional number 

of connections associated with more bays of framing.
9

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

2.5 Strength and Drift Limits 

Although drift control and stiffness considerations 

usually control the proportioning of most steel 

special moment frame members, strength also must 

be considered. ASCE 7 §12.2.1, Table 12.2-1 allows 

design of steel special moment frames using a response 

modification coefficient, R, of 8. That is, they are allowed 

to be designed for a base shear equal to 1/8 that obtained 

from elastic response analysis, so long as this base 

shear does not fall below minimum levels applicable 

to all structures. Base shear calculations are frequently 

controlled by the approximate upper limit period defined 

in ASCE 7 §12.8.2. 

Wind loads also must be checked and may govern strength 

requirements, particularly in taller structures. It is not 

uncommon for seismic loads to govern drift requirements 

while wind loads govern strength requirements. 

Regardless of whether gravity, wind, or seismic forces 

govern, proportioning and detailing provisions for steel 

special moment frames apply wherever these frames are 

used.

Frame stiffness must be sufficient to control lateral drift 

at each story within specified limits. ASCE 7 §12.1, 

Table 12.12-1 provides the allowable story drift, Da, as a 

function of structure type. The redundancy coefficient, 

ρ, determined in accordance with ASCE 7 §12.3.4.2, also 

affects the permissible drift. ASCE 7, §12.12.1.1 limits 

the design story drift, D, to Da/ρ.

Regardless of whether Allowable Strength Design or 

Load and Resistance Factor Design procedures are used 

to evaluate strength, drift is calculated using strengthlevel seismic forces, factored by the ASCE 7 deflection 

amplification coefficient, Cd. ASCE 7 does not specify 

drift limits for wind loads; however, many designers 

of tall buildings limit wind-induced drift to enhance 

occupant comfort during wind storms. In some buildings, 

it may be desirable to limit design seismic drift to reduce 

damage of cladding, stairs, and other nonstructural 

elements that span vertically from one level to another.
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

10

3. Principles for Steel Special Moment Frame Design 

The ASCE 7 procedures for determining required frame 

strength incorporate a seismic response modification 

coefficient, R, that reflects the degree of inelastic response 

expected for design-level ground motions, as well as the 

ductility capacity of the framing system. Steel special 

moment frames are permitted to be designed using a 

value of R = 8 and are expected to be able to sustain 

multiple cycles of significant inelastic response when 

subjected to design-level ground motion. However, many 

steel special moment frame structures have substantial 

overstrength. This overstrength results from a number of 

factors, including oversizing of columns to meet strongcolumn/weak-beam criteria, use of oversize sections 

to provide sufficient stiffness for drift control, and 

variability in the strength of the steel material itself. As 

a result, although the R value of 8 specified by the code 

would imply initiation of inelastic behavior at shaking 

with an intensity 1/8 that of the design earthquake, many 

steel special moment frame structures will remain elastic 

for shaking with an intensity approximately 1/3 that of the 

design earthquake, or even with more intense shaking.

The AISC 341 proportioning and detailing requirements 

are intended to provide ductile inelastic response. The 

primary goals are (1) achieve a strong-column/weakbeam condition that helps distribute inelastic response 

over several stories, (2) avoid P-D instability under 

gravity loads and anticipated lateral seismic drifts, and (3) 

incorporate details that enable ductile flexural response 

in yielding regions.

The design criteria in ASCE 7 §12.2.1, Table 12.2-1 for 

steel special moment frames, including the R, Cd, and 

the overstrength factor, Ωo, coefficients, were established 

based on historical precedent and the past performance of 

frames having essentially full-strength, fully restrained 

connections between the beams and columns. ASCE 7 

§12.2.1.2 establishes equivalency criteria by which 

substitutions for specified components or details for a 

structural system can be qualified as equivalent to the 

specified components, permitting the use of the same R, 

Cd, and Ωo coefficients when these substitutions are made. 

In recognition of this, AISC 341 §E3.2 modified the basis 

of design for steel special moment frames to permit the 

use of partial strength, partially restrained connections 

when this is justified by testing and analysis, such as that 

permitted by ASCE 7 §12.2.1.2. 

At present, one partial strength connection technology, 

the Simpson Strong-Tie Strong FrameTM connection, 

has been demonstrated to be adequate for use as a 

substitute for fully restrained connections in steel special 

moment frames. In frames incorporating this connection 

technology, inelastic behavior is accommodated through 

controlled yielding of the connection elements adjacent 

to the column. The effects of this yielding on frame 

behavior are similar to the formation of plastic hinges in 

beams having full-strength connections. For simplicity, 

the balance of this Guide does not distinguish between 

fully restrained and partially restrained connections, 

except where it is significant to design considerations.

Partially Restrained Connections

Prior to publication of ASCE 7-16 and ASCE 341-16, steel 

special moment frames were required to incorporate full 

strength, fully restrained beam-to-column connections. 

ASCE 7-16 establishes equivalency criteria that have 

been used to qualify the use of one partial strength 

moment connection technology. Additional partial 

strength technologies may be qualified for such use in 

the future.

3.1 Design a Strong-column/Weak-beam 

 Frame 

To avoid development of P-D instability in multi-story 

structures, achieving a relatively uniform distribution 

of drift over the height of the structure is desirable. To 

achieve this distribution, avoiding early formation of 

single-story mechanisms in which inelastic response is 

dominated by formation of plastic hinges at the tops and 

bottoms of columns within a single story (Figure 2-8) 

is important. When such single-story mechanisms form, 

most of the inelastic portion of a structure’s drift will 

occur within these stories, resulting in very large P-D

effects at those locations. To avoid these effects, building 

codes require designs intended to promote formation of 

multi-story sidesway mechanisms dominated by hinging 

of beams, as opposed to columns, like the idealized 

sidesway mechanism of Figure 3-1. These requirements 

are termed strong-column/weak-beam design.

AISC 341 §E3.4 adopts a strong-column/weak-beam 

design approach that requires the sum of column 

flexural strengths at each joint to exceed the sum of 

beam flexural strengths. To determine available column 

flexural strength, considering the axial loads that will 

be simultaneously present in the column along with 

flexural demands is important. The provisions provide 
11

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

Figure 3-1. Idealized sidesway mechanism intended for 

columns with strong-column/weak-beam design.

an expression to determine the column-beam strength 

ratio and acknowledge that the design requirement is 

not adequate to completely avoid flexural hinging of 

columns. Conformance to the strong-column/weakbeam requirement applies except for columns in the top 

stories of frames, columns with required axial strength 

substantially less than their design strength, and columns 

that are exempted because they provide only a limited 

portion of the lateral resistance for the frame or structure 

at the floor considered.

AISC 341 provisions require supplemental lateral bracing 

of beam-column connections unless it can be shown that 

the columns will remain elastic. Section 5.4 of this Guide 

discusses this additional bracing requirement. Analytical 

research has demonstrated that the AISC 341 §3.4 strongcolumn/weak-beam provisions are not adequate to avoid 

formation of story mechanisms in all cases. Designers 

may wish to increase column sizes beyond the code 

requirements to obtain better performance in severe 

earthquake events. When the column-beam moment ratio 

is two or greater, AISC 341 §E3.4c permits an assumption 

that columns will remain elastic. This strategy frequently 

has the advantage of reducing the need to provide costly 

web stiffener and doubler plates; thus, this strategy may 

be more cost effective despite the increase in the total 

weight of steel used on the project.

3.2 Proportion for Drift 

Sizing of beams in steel special moment frames 

typically is controlled by the consideration of drift. 

As a consequence, the sizes of many columns also are 

drift-controlled because the strong-column/weak-beam 

provisions discussed earlier demand larger columns if 

larger beams are used. An exception is end columns in 

tall steel special moment frames, which often have high 

axial load demands and, in most cases, are controlled by 

strength design criteria.

ASCE 7 permits several types of lateral analysis including 

Equivalent Lateral Force Analysis (ASCE 7 §12.8), 

Modal Response Spectrum Analysis (ASCE 7 §12.9.1), 

Linear Response History Analysis (ASCE 7 §12.9.2), and 

Nonlinear Response History Analysis (ASCE 7, Chapter 

16). Except when nonlinear response history analysis is 

performed, ASCE 7 §12.12.1, Table 12.12-1 limits design 

story drift to a specified fraction of the story height, 

depending on Risk Category and other factors. Design 

story drift is determined by factoring the story drift 

obtained from the lateral analysis by the quantity Cd /I. 

For purposes of determining design story drift, using 

the computed building period without consideration 

of the upper limit (CuTa) is permitted. However, if the 

design base shear is controlled by the near-fault criterion 

(ASCE 7 §12.8.6, Equation 12.8-6), scaling the computed 

drift such that the total base shear for the scaled analysis 

is not less than that obtained from Equation 12.8-6 is 

necessary. 

Necessary amplifications of story drift because of real 

and accidental torsion and because of P-D effects (see 

Section 3.3) are stated explicitly in ASCE 7 §12.8.7 and are 

treated equally in linear response history, modal response 

spectrum, and equivalent lateral force designs. Design 

for stiffness because of story drift limitations is often an 

iterative process because the design lateral forces depend 

on the computed fundamental period of the structure.

When nonlinear response history analysis is performed, 

design story drift is determined at the MCER, rather than 

design earthquake shaking level. The mean value of story 

drift obtained from the suite of analyses is limited to twice 

the values specified in Table 12.12-1.

Analysis Procedures

ASCE 7-16 expands the lateral analysis procedures 

available under earlier editions of the standard to 

include linear response history analysis. In addition, 

Chapter 16 of the standard includes a completely 

rewritten procedure for nonlinear response history 

analysis. Except for Risk Category IV structures, 

the nonlinear response history procedure permits 

somewhat more flexible structures than the linear 

methods of analysis. Specifically, the procedure 

permits drifts evaluated at the MCER level twice those 

permitted by ASCE 7 §12.12. This may permit more 

economical designs in some cases. However, the 

procedure is computationally intensive and requires 

specialized designer knowledge. 

Undeformed 

shape

Deformed 

shape

Plastic 

hinges
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

12

Story drifts can be considered as composed of two 

components: shear drift, caused by flexural and shear 

deformations in beams and columns and their connections 

(Figure 3-2a), and flexural drift, caused by axial 

deformations in the columns (Figure 3-2b).

3.3 Frame Stability

In a severe earthquake, frame structures have the 

potential to collapse in a sidesway mode because of P-D

effects. These effects are caused by vertical gravity loads 

acting on the deformed configuration of the structure. 

For design purposes, P-D effect is assessed in codes by 

means of elastic, static concepts, even though the response 

of the structure in a severe earthquake is inelastic and 

dynamic. The simple P-D provisions in ASCE 7 §12.8.7 

provide some protection against sidesway failures but do 

not provide accurate information on the susceptibility of 

a structure to such failure.

ASCE 7 §12.8.7 requires explicit consideration of P-D

effects in each story in which the elastic story stability 

coefficient, θ, given by Equation 12.8-16 exceeds 0.1. 

This evaluation is supplemental to the frame stability 

evaluation required by AISC 360. In Seismic Design 

Categories D, E, and F, this is typically a more severe 

requirement. When computer analysis is performed, 

these elastic P-D effects usually can be accounted for 

automatically in the analysis; however, the user usually 

must specify that the software performs this calculation.

ASCE 7 §12.8.7, Equation 12.8-17, places an upper 

limit on the value of θ given by θmax = 0.5/(β Cd) < 0.25, 

where β is ratio of shear demand to shear capacity for 

the story under consideration. Shear demand is the Load 

and Resistance Factor Design story design shear force, 

and shear capacity is the maximum shear force that 

can be resisted by the story. This shear capacity cannot 

be defined uniquely because the capacity in one story 

depends on the load pattern applied to the full structure. 

An estimate of the story shear capacity can be obtained 

by dividing the average of the “floor moment” capacities 

of the two floors bounding the story by the story height. 

The “floor moment” capacity is the sum of the maximum 

beam or column moments that can be developed at the 

intersection of all beam-to-column centerlines at the 

floor level. 

For connections that follow the strong-column/weakbeam concept, this amounts to the quantity ΣM*pb

employed in AISC 341 §E3.4a, Equation E3-1, divided 

by 1.1 to eliminate presumed overstrength inherent in 

the computation of ΣM*pb. For connections with weak 

columns, the quantity ΣM*pc from AISC 341 §E3.4a, 

Equation 3.1, should be used.

The contributions to the shear mode of drift vary with 

configuration, however, beam bending is generally the 

largest contributor, with column bending and panel zone 

deformation also contributing. ASCE 7 §12.7.3b requires 

that the contribution of panel zone deformation to story 

drift be included when checking drift limits. Section 4.2 

of this Guide provides additional discussion on this topic.

In linear analysis, the flexural mode of drift becomes 

important for relatively slender frames with a height-towidth (aspect) ratio of about 1.5 or larger. For symmetrical 

frames, the portion of the total story drift because 

of flexure is approximately equal to the rotation of a 

cantilevered steel column having a moment of inertia 

I=A D2, where A is the area of a single end column in the 

frame, and D is the distance between the end columns of 

the moment frame.

The total story drift is the sum of shear and flexural 

mode drifts. If the flexural mode of drift contributes 

significantly to the story drift, the remedy is to increase 

the size of the exterior steel special moment frame 

columns. If the shear mode controls, use of deeper beam 

sections is the most effective method of reducing drift, 

although use of deeper beams will also require larger 

columns to satisfy strong-column, weak-beam criteria. 

For slender steel special moment frames, optimal sizing 

of members to meet drift requirements can lead to the 

use of larger beam sections near the frame’s mid-height 

than at lower levels.

(a) Shear drift (b) Flexural drift

Figure 3-2. Shear drift and flexural drift.
13

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

Figure 3-3. Reduced beam section connection.

It is not uncommon for the story stability coefficient to 

exceed 0.1, particularly in regions in which the design 

spectral values, SDS and SD1, are relatively small. In 

such cases, the seismic design forces are small, and 

steel special moment frames may become very flexible 

unless wind criteria control member design. It is also 

common in such cases that the θmax criterion controls and 

P-D considerations will require an increase in member 

stiffness. Most computer analysis programs will not check 

for θmax, so this criterion must be checked manually.

When the stability coefficient, θ, exceeds a value of 

0.1, ASCE 7 §12.8 requires evaluation of P-D effects, 

either using a first-order approach, in which computed 

deflections are amplified by the quantity 1/(1-θ), or by a 

second-order analysis, in which geometric nonlinearities 

are explicitly considered. Many structural software 

packages commonly used by engineers to analyze and 

design steel structures have the ability to perform these 

second-order analyses. However, as noted above, this 

software generally does not evaluate whether θ exceeds 

θmax as required by ASCE 7 §12.8. In that case, engineers 

must manually check that this condition is satisfied.

P-D evaluations should be performed for each frame 

so that torsional effects, which cause displacement 

amplification, are considered. 

3.4 Strength Verification

Columns and beams are required to have adequate 

strength to resist the load combinations of ASCE 7 §2.3 

or §2.4, considering axial-flexural interaction effects. In 

addition, columns and their splices are required to have 

adequate strength to avoid global buckling or tensile 

fracture under maximum axial forces, and beam-column 

connections are required to have adequate strength to 

develop the probable flexural strength of the beams. 

The provisions of AISC 341 and AISC 360 govern the 

calculation of design strength for both Allowable Strength 

and Load and Resistance Factor Design procedures.

3.5 Connection Type Selection

Since the 1994 Northridge earthquake, AISC 341 has 

required that detailing and design of steel special moment 

frame moment connections be demonstrated through 

qualification testing to be capable of developing at least 

(+/- 0.04) radians of interstory drift without excessive 

strength loss when subjected to the cyclic loading protocol 

specified in AISC 341, Chapter K. Qualification testing 

must be conducted on full-size specimens using sections, 

materials, and fabrication procedures comparable to those 

to be incorporated in the actual construction. Relatively 

few laboratories have the capability to perform such 

tests, which are expensive. If initial connection designs 

fail the testing, performing multiple iterations of the 

design and testing may be necessary, adding months of 

delay and hundreds of thousands of dollars of expense 

to projects. To avoid these difficulties, AISC 341 

established a series of requirements to permit the use of 

prequalified connections. Prequalified connections have 

been demonstrated to be acceptable, based on extensive 

testing and analysis, to be capable of reliable service when 

used within specified application limits. Several sources 

of connection prequalifications are described below.

3.5.1 AISC Prequalified Connections

AISC maintains a Connection Prequalification Review 

Panel that develops an American National Standards 

Institute (ANSI)-approved standard, AISC 358 

Prequalified Connections for Special and Intermediate 

Steel Moment Frames for Seismic Applications. AISC 

358 presents materials, design, detailing, fabrication, 

and inspection requirements for a series of prequalified 

moment connection details. This standard is referenced 

by AISC 341, and connection prequalifications contained 

in the standard are acceptable to most building officials. 

Each prequalified connection has unique limits of 

applicability associated with member type, depth, and 

weight. Thus, not every connection can be used in the 

same applications. Figure 3-3 through Figure 3-11 show 

the configurations of connection technologies included 

in AISC 358-16. 
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

14

Figure 3-5. Bolted flange plate connection. Figure 3-6. Welded unreinforced flange—welded web connection.

Figure 3-7. Proprietary Kaiser bolted bracket connection.

(a) Welded to beam (b) Bolted to beam

Figure 3-4. Types of end plate connections.

(a) Bolted unstiffened 

extended end plate

(b) 4-bolt stiffened 

extended end plate 

(c) 8-bolt stiffened 

extended end plate 
15

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

Figure 3-8. Proprietary ConXLTM connection.

Figure 3-9. Proprietary SidePlateTM ® connection.

Figure 3-10. Proprietary Simpson Strong-Tie 

Strong FrameTM connection.

Figure 3-11. Double-tee connection.

AISC 358 prequalifications generally apply to planar 

frames consisting of wide flange beams connected to the 

major axis of wide flange columns. In addition to wide 

flange columns, some connections are also prequalified 

for use with HSS columns, box columns, and cruciform 

wide flange columns. The ConXLTM and SideplateTM ® 

connections have been specifically qualified for use in 

orthogonal intersecting frames, in which beams frame 

into both axes of the columns. 

As noted earlier, all of the depicted connections except 

the Simpson Strong-Tie Strong FrameTM Connection are 

full-strength, fully restrained connections. The use of 

the partial strength connection offers some advantages in 

that limit states associated with local flange buckling and 

lateral-torsional buckling of beams become less likely. 

However, additional design requirements pointed out 

later in this Guide are associated with this connection.

The AISC 358 prequalifications are specifically intended 

for use at connections with columns extending above 

and below the horizontal framing and do not specifically 

apply to top story conditions. However, these connections 

are routinely used in such applications, typically by 

extending a cover plate over the column top or by 

extending a column stub beyond the top level. Similarly, 

the connections have been prequalified assuming that the 

beams and columns align within a vertical plane and that 

the alignment of the beams is orthogonal to the alignment 

of the columns. Engineers designing for conditions 

other than these must exercise engineering judgment 

and possibly modify the prequalified design procedures 

to accommodate the design conditions. Some building 

officials may require project-specific qualification testing 

when these conditions are encountered.
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

16

The reduced beam section, bolted end plate, bolted 

flange plate, welded unreinforced flange-welded web, 

and double-tee connections shown in these figures are 

in the public domain. Other connections shown are 

proprietary and may be subject to licensing fees. AISC 

358 contains more information on these issues. From 

time to time, AISC updates AISC 358 with supplements 

as new connection technologies are prequalified and as 

existing prequalifications are extended or modified in 

applicability.

3.5.2 Other Prequalified Connections

Several independent evaluation services, such as the 

ICC-Evaluation Service, IAPMO Evaluation Service, 

and City of Los Angeles Evaluation Service, offer 

qualification of proprietary products and procedures as 

meeting the criteria in the building code. Some of these 

evaluation services publish connection prequalifications 

for proprietary connection technologies in the form of 

evaluation reports. Most building officials accept these 

reports as evidence of code conformance, as long as 

they are maintained current with the latest building code 

edition. However, engineers relying on these evaluation 

reports should be aware that the rigor of review by these 

evaluation services does not always match that performed 

by the AISC Connection Prequalification Review Panel. 

Therefore, the performance capability of connections that 

have been included in these evaluation reports may not 

match that of connections in AISC 358.

Some individual patent holders for proprietary connections 

not included in AISC 358 maintain their own library of 

test data and analysis to substantiate the performance 

capability of their connections. Strictly speaking, these 

connections are not prequalified. However, some building 

officials will accept their use, sometimes requiring 

independent review as a condition of such use.

3.5.3 Project-Specific Qualification 

In some cases, the prequalifications available in AISC 

358 and evaluation service reports may not be adequate 

to cover the design conditions for a particular project. One 

reason this may occur is that the sizes of frame elements 

selected for a steel special moment frame may fall outside 

the limits contained within the prequalifications. Other 

reasons this may occur include use of connections in 

geometries other than those for which prequalifications 

exist, such as connections to the minor axis of wideflange columns or skewed connections. If no prequalified 

connection meets the requirements of a particular design 

condition, AISC 341 §E3.6c(c)(2) permits project-specific 

testing. At least two specimens must be tested and 

must pass the criteria specified in AISC 341, Chapter 

K. Because the required size of specimens needed to 

comply with the AISC 341, Chapter K requirements can 

be quite large, often only a limited number of university 

laboratories have the capability to perform such testing. 

Scheduling use of these facilities can be difficult. 

Therefore, if project-specific testing will be required, 

early planning for this effort is recommended. Because 

of specimen fabrication, shipping, and set-up costs, 

testing can be expensive. Therefore, whenever possible, 

using framing configurations that will enable the use of 

prequalified connections should be considered.

3.6 Details for Ductile Behavior 

As a highly ductile system, steel special moment frames 

may undergo significant inelastic behavior in numerous 

members when subjected to severe seismic shaking. The 

primary source of this inelastic behavior is intended to 

occur in the form of plastic hinging in the beams adjacent 

to the beam-column connections. In a properly configured 

system, this hinging should occur over multiple stories 

to spread the total displacement demand and limit the 

local deformations and member strains to a level that 

the members can withstand. In addition to the hinging 

of beams, inelastic behavior can be expected to occur 

at column bases and to a more limited extent in beamcolumn connections.

A number of features are incorporated into steel special 

moment frame design to achieve the intended ductility 

level. One primary feature is the level of compactness 

required of beam and column members. In addition, steel 

special moment frame members must be laterally braced 

for stability. AISC 341 §E3.4b prescribes a maximum 

spacing distance for lateral bracing of steel special moment 

frame beams and specifies stiffness and strength criteria 

for this bracing to avoid lateral-torsional buckling. In most 

applications where the framing supports a concrete floor 

slab, the lateral bracing is provided for only the bottom 

beam flange. Lateral bracing of columns at the floor levels 

is also required. This bracing is especially important for 

deep column sections that, although efficient for frame 

stiffness because of their high moment of inertia to weight 

per foot ratio, are more susceptible to lateral-torsional 

buckling than stockier W14 column shapes.

As mentioned in previous sections, implementing a strongcolumn/weak-beam design philosophy is important to 

good steel special moment frame performance. Although 
17

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

it is desirable to avoid column hinging, under very intense 

shaking, columns will invariably form hinges at the frame 

base and other locations. Frame design should explicitly 

consider this inelastic demand. Generally, the design 

of steel special moment frame column bases should be 

strong enough so that inelastic deformation is limited 

to a region that can exhibit significant ductility, such 

as the column member just above the base connection. 

Another approach, if the steel special moment frame 

extends to the foundation, is to design and detail anchor 

bolts to yield as a means of limiting demand on other 

elements of the connection or through the formation 

of yielding in supporting foundation elements. In some 

cases, engineers may wish to design columns assuming 

the bases are “pinned.” In those cases, detailing the bases 

to accommodate the large anticipated rotations without 

failing the anchorage and attachment to foundations is 

important.

3.6.1 Section Compactness 

Reliable inelastic deformation requires limiting the 

width-thickness ratios of compression elements to a range 

that provides a cross section resistant to local buckling 

under inelastic straining. AISC 360 §B4 uses the term 

“compact” for steel cross sections that are able to achieve 

the full plastic section capacity and maintain strength 

through moderate ductility demands. AISC 341 §D1.1 

defines two levels of compactness, one for moderately 

ductile members and one for highly ductile members. 

Although the section limitations for moderately ductile 

members are similar to those for some sections defined as 

compact in AISC 360, they are not identical and in some 

cases are much more severe because of the cyclic nature of 

seismic loading. Members of steel special moment frames 

are required to meet the compactness requirements for 

highly ductile members. Such highly compact sections 

are expected to be able to achieve significant deformation 

ductility. To meet the compactness criteria for highly 

ductile members, AISC 341 §D1.1a requires member 

flanges to be continuously connected to the web(s), 

and §D1.1b requires width-thickness ratios less than or 

equal to those that are resistant to local buckling when 

stressed into the inelastic range. AISC 341 §D1.1b, 

Table D1.1 specifies limiting width-thickness ratios for 

compression elements for highly ductile and moderately 

ductile members.

3.6.2 Demand-Critical Welds 

Demand-critical welds are those welds that are anticipated 

to experience yield-level or higher strains, the failure of 

which would result in critical impairment of the safety 

of the structural system. To perform acceptably, such 

welds require increased quality and toughness relative 

to other welded joints. AWS D1.8 specifies the special 

requirements associated with demand-critical welds. A 

user note in AISC 341 §A3.4b repeats these requirements 

for information; however, specification that welding 

conform to AWS D1.8 is necessary to assure appropriate 

execution of demand-critical welds.

AISC 341 §E3.6a requires demand-critical welds for 

groove welds at column splices and for connections of 

column bases to base plates unless it can be demonstrated 

that neither column hinging nor net tension will occur 

at the base. In addition, unless otherwise permitted by 

AISC 358, or as determined in either prequalification or 

qualification testing, complete joint penetration groove 

welds of beam flanges and of beam webs to columns are 

required to be demand critical. 

3.6.3 Protected Zones 

AISC 341 §E3.5c requires designation of the region at 

each end of a steel special moment frame beam subject 

to inelastic straining as a protected zone. Protected zones 

must meet the requirements of AISC 341 §D1.3. AISC 358 

designates the location and extent of protected zones for 

prequalified connections. For connections not contained 

in AISC 358, engineers should specify protected zones 

based on the inelastic behavior exhibited in connection 

assembly qualification tests. In beams carrying heavy 

gravity loads, plastic hinging may occur within beam 

spans remote from connections. When such conditions are 

anticipated, engineers should designate protected zones 

in these additional areas of anticipated plastic hinging.

Protected Zone 

Testing conducted by the SAC Steel Project 

demonstrated that the regions of beams undergoing 

significant inelastic strains are sensitive to 

discontinuities caused by welding, rapid change 

of section, penetrations, or construction-related 

flaws. Connections, attachments, notches, or flaws 

may initiate a fracture. For this reason, areas of 

anticipated inelastic straining are designated as 

protected zones and are not to be disturbed by other 

building construction operations, including tack 

welds, welded shear studs, or bolted or screwed 

attachments. However, AISC 341 §I2.1 does permit 

arc spot welds and power-actuated fasteners up to 

0.18 inch (4.6 mm) diameter used to fasten metal 

deck to top beam flanges. 
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

18

3.6.4 Panel Zones 

Panel zones experience large shear forces because of 

the transfer of moments from beams to columns. Shear 

yielding of panel zones is a very ductile behavior; 

however, the accompanying shear deformation depicted 

(greatly amplified) in Figure 3-12 can impose potentially 

harmful secondary stresses on welded beam-flange-tocolumn joints and has resulted in premature fractures 

in some laboratory tests. The best inelastic behavior 

of beam-column connections is obtained when limited 

yielding occurs in the panel zone simultaneously with 

beam plastic hinging.

AISC 341 §E3.6e1 requires panel zones to have adequate 

strength per AISC 360 §J.10.6 to develop the expected 

plastic moment strengths of the beams at the intended 

zone of plastic hinging. If panel zone deformation is 

explicitly considered in frame stability evaluation, which 

is not typical, the AISC 360 formulation for shear strength 

includes a significant contribution from the column 

flanges. When this design approach is used, it can help 

to provide balanced yielding between the panel zone 

and beam, which has been shown to provide enhanced 

ductility for some connection types. Other column 

strength considerations, however, can lead to panel zones 

that are stronger than required and that will result in less 

panel zone yielding. Section 4.2.1 of this Guide provides 

further discussion of panel zone modeling considerations.

3.6.5 Lateral Bracing and Stability 

Stability bracing is required to inhibit lateral buckling or 

lateral-torsional buckling of primary framing members. 

Section 5.2 discusses detailing issues associated with 

beam flange bracing.

3.6.6 Beam Flange Continuity Plates

In addition to their role in controlling web buckling 

opposite concentrated beam flange forces, continuity 

plates can also serve an important role in stiffening and 

strengthening column flanges and assisting in transfer of 

beam flange forces from the column flanges to the column 

web. Analytical research conducted as part of the SAC 

Steel Project indicated that a significant contributing factor 

in the premature fracture of pre-Northridge style welded 

connections was that beam flange stresses and strains were 

significantly larger at the centers of beam flanges, opposite 

the column webs, than at the beam flange edges. To 

minimize this effect, the SAC Steel Project recommended 

use of beam flange continuity plates matching the beam 

flange in thickness for columns with beams on both faces 

This requirement carried forward into the first edition of 

AISC 358. More recently, research has shown that this 

requirement was excessively conservative. AISC 341 

§E3.6f sets the minimum requirements for beam flange 

continuity plates for most types of special steel moment 

frame beam-to-column connections. AISC 358 sets 

additional criteria for these plates for some prequalified 

connections.

Figure 3-12. Panel zone inelastic deformation (exaggerated).
19

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

4.1 Analysis Procedure 

ASCE 7 §12.6 permits five different analysis procedures 

to determine required member strengths and design 

drifts: (1) equivalent lateral force, (2) a simplified version 

of the equivalent lateral force method, (3) modal response 

spectrum, (4) linear response history, and (5) nonlinear 

response history analysis. The simplified Equivalent 

Lateral Force (ELF) method is not permitted for moment 

frame design. ELF analysis is the simplest procedure 

permitted for use in steel moment design, but will not 

adequately capture higher mode effects when these are 

significant nor will it account for the effects of some 

irregularities. Therefore, ASCE 7 §12.6, Table 12.6-1 

prohibits ELF for structures having fundamental periods 

that are large enough that significant higher mode effects 

are likely or having horizontal or vertical irregularities 

specified in that section.

The ELF procedure permits the use of an approximate 

fundamental period unless the period is determined 

by more exact analysis. In most cases, the more exact 

analysis will determine a substantially longer period than 

that obtained using the approximate methods. As a result, 

substantial reduction in base shear forces often can be 

obtained by calculating building periods using the more 

exact methods. ASCE 7 §12.8.2 places an upper limit on 

the period that can be used. 

Modal response spectrum analysis or linear response 

history analysis are the preferred procedures, as they more 

accurately account for a building’s dynamic behavior, 

take advantage of calculated rather than approximate 

period, and account for modal participation, which 

can result in lower response than that calculated using 

the ELF procedure. ASCE 7 §12.9.4 requires scaling 

the modal base shear and all corresponding element 

forces to a minimum of 100 percent of the base shear 

determined using the ELF procedure. This provision is 

intended to guard against the use of analytical models 

that underestimate stiffness and produce unrealistically 

low estimates of design forces. The scaling requirement 

generally does not apply to drift, as excessively flexible 

models will produce conservatively larger estimates of 

drift. However, if a structure is located close to a major 

active fault and the ELF base shear is controlled by 

ASCE 7 §12.8.1.1, Equation 12.8-6, drift must be scaled 

as well.

For structures with calculated periods that exceed the 

ASCE 7 limits, either the modal response spectrum 

or seismic response history analysis procedures are 

required. Elastic response history analysis is more 

difficult than modal response spectrum analysis, but it 

does have the advantage that the sign of forces obtained 

from the analysis have meaning and that at any instant, 

the structure will be in equilibrium. In modal response 

spectrum analysis this is not the case.

ASCE 7 §12.6, Table 12.6-1 permits the use of nonlinear 

response history analysis for any structure. This analysis 

method is computationally complex. In addition, ASCE 7, 

Chapter 16, which specifies the requirements for 

nonlinear response history analysis, requires independent 

peer review when this technique is used. ASCE 7, Chapter 

16 also requires that designs conducted using nonlinear 

response history analysis meet the requirements for an 

appropriate linear analysis method, with the exception 

that drift criteria are relaxed for structures conforming 

to Risk Categories II and III. Because drift often controls 

member sizes in steel special moment frames, use of 

nonlinear response history analysis can result in more 

economical designs. In addition, as explained below, 

nonlinear response history analysis can be used to reduce 

the required axial design strength for columns.

Linear analysis can use either 2-D or 3-D computer 

models. Three-dimensional models are recommended, 

and sometimes required, because they are effective in 

identifying the effects of any inherent torsion in the 

lateral system, as well as combined effects at corner 

conditions. Nonlinear response history analysis requires 

three-dimensional models.

ASCE 7 §12.5 specifies requirements for the combination 

of seismic forces along different building axes. The 

design forces for beams and columns are calculated 

independently for response in each orthogonal direction. 

It is common to combine the resulting seismic forces 

using the orthogonal combination procedure in which 100 

percent of the seismic force in one direction is combined 

with 30 percent of the seismic force in the perpendicular 

direction. Multiple load combinations are required to 

bound the orthogonal effects in both directions. The 

design of each beam and column is based on an axial and 

biaxial flexural interaction for each load combination. 

4. Analysis Guidance 
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

20

AISC 341§D1.4a also requires that columns have 

adequate axial strength that is independent of flexural 

considerations to resist the axial force resulting from 

development of a full side-sway mechanism in the frame. 

For columns common to intersecting moment frames, 

this section requires consideration of simultaneous 

yielding of both frames. This requirement supplements 

the orthogonal combination rules of ASCE 7 for this load 

consideration. If nonlinear response history analysis is 

used to determine required forces, it is permissible to limit 

the required axial design strength for columns based on 

this analysis. This can result in more economical designs, 

particularly for taller structures.

4.2.2 Beam Stiffness Reductions 

For reduced beam section connections, the beam flange 

width is reduced near the beam ends, where curvature 

effects are at a maximum under lateral frame loading. 

Accounting for the resulting reduction in beam stiffness in 

analytical models is important. Some structural analysis 

software incorporates explicit elements that can model the 

reduced stiffness of beams having reduced beam section 

cutouts. Alternatively, use of 90 percent of the beam 

section properties is typically a reasonable approximate 

representation of this effect when 50 percent reductions in 

flange width are used and is a conservative approximation 

when smaller reductions in beam section are used.

4.3 Base Fixity

Base restraint can have a significant effect on moment 

frame behavior. ASCE 7 §12.7.1 permits consideration 

of the columns to be either fixed or flexible at the base, 

as suits the conditions of construction. Therefore, 

the designer is required to determine the appropriate 

analytical restraint conditions for column bases.

Because most column bases and foundations will 

provide some restraint against rotation, an assumption 

that column bases are pinned will tend to overestimate 

column flexibility, building period, and ground story 

drift, all of which are conservative design assumptions 

for frames with design controlled by drift considerations. 

When pinned bases are assumed, the column base 

anchorage must be designed with adequate capacity 

to transfer the shear and axial forces to the foundation 

while accommodating the rotations that will occur at 

the column bases. Some of this rotation can occur in the 

foundation itself.

Similarly, except when columns extend through basement 

levels and are embedded within the basement walls, few 

column bases provide true fixity. Except in those cases 

where columns embed in basement walls, fixed base 

assumptions tend to result in underestimates of column 

flexibility, building period, and ground story drift, which 

can be unconservative because the model will not provide 

an appropriate prediction of drift demand. Column 

bases should not be modeled as fixed unless the bases 

themselves and the foundation elements they are attached 

to can effectively provide sufficient restraint to provide 

such fixity. The extent of fixity present at column bases 

can be determined by explicit modeling of the base and 

foundation conditions.

Simultaneous Yielding

The requirement to consider simultaneous yielding of 

the frames in orthogonal directions when designing 

columns that are part of intersecting frames is a new 

requirement in AISC 341-16. This requirement was 

introduced because given the large R factors used to 

design steel special moment frames, simultaneous 

yielding in frames providing lateral resistance in both 

orthogonal directions is likely to occur.

4.2 Modeling

4.2.1 Connection Modeling

Most designers model moment frames with fully 

restrained connections using line representations of 

beams and columns, with the lines intersecting at 

dimensionless nodes. This accounts in an approximate 

manner for flexibility inherent in the panel zone. It 

is also possible to explicitly model panel zones using 

any of several 4-node scissor or 8-node quadrilateral 

elements. When this is done, AISC 360 §J10.6 permits 

consideration of increased strength for the panel zones. 

However, the benefits of doing this when fully restrained 

connections are employed are typically small; thus, this 

is not typically done.

Some analysis software permits specification of a rigid 

end offset to simulate stiffness of the panel zone and to 

simplify calculation of forces at the column face. This is 

not an appropriate method to represent panel zones and 

often results in unconservative estimates of frame stiffness 

and underestimation of design drift. 

When partially restrained connections are used, explicitly 

modeling the connection to capture the additional frame 

flexibility introduced by such connections is mandatory. 

AISC 358 §12.9 provides requirements for such modeling 

for the Simpson Strong-Tie Strong FrameTM Connection.
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Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

5. Design Guidance

5.1 Design Procedure 

The three basic steel special moment frame design 

components are beams, columns, and beam-column 

connections. Beams span the horizontal clear distance 

between protected zones, columns span the vertical clear 

distance between panel zones, and the beam-column 

connections encompass both protected and panel zone 

regions at the beam-column intersections.

AISC 341 permits the use of either Load and Resistance 

Factor Design or Allowable Strength Design approaches 

to proportion beams and columns in steel special moment 

frames. However, AISC 358 §1.3 only permits the use 

of Load and Resistance Factor Design procedures for 

design of prequalified connections. The corresponding 

nominal strengths, resistance and safety factors, and 

available strengths of the components must be determined 

in accordance with the provisions of AISC 360 unless 

noted otherwise in AISC 341 or AISC 358.

5.2 Beam Design

5.2.1 Limitations 

To provide for reliable inelastic deformations, AISC 341 

§E3.5a requires beams to conform to the compactness 

requirements for highly ductile members. Beams should 

not be designed as composite with supported concrete 

slabs for seismic resistance because the composite 

behavior is not available when the top flange is in tension 

and because AISC 341 §D1.3 prohibits placement of shear 

connectors in the zone of anticipated plastic hinging 

unless specifically permitted in AISC 358 or via other 

connection conformance demonstration. AISC 341 

§E3.5c requires the designation of the region at each 

end of the beam that is subject to inelastic straining as a 

protected zone. AISC 341 §E3.5b prohibits abrupt changes 

in beam flange area in this region. The drilling of flange 

holes or trimming of the flanges (e.g., as is done in the 

reduced beam section connection) is permitted only 

if qualification testing demonstrates that the resulting 

configuration can develop a stable plastic hinge. Welds 

connecting the web(s) and flanges of built-up members 

in the expected regions of plastic hinging must be made 

using complete joint penetration groove welds with a pair 

of reinforcing fillets in accordance with AISC 358 §2.3.2a.

AISC 358 §2.3 requires both rolled wide-flange and 

built-up beams to conform to the cross-section profile 

limitations applicable to the specific connection type. 

These limitations include restrictions on beam depth, 

weight, flange thickness, and clear span-to-depth ratio. 

The limitations do not apply when project-specific 

qualification testing is performed using beams of the 

proposed cross section.

5.2.2 Lateral-Torsional Buckling 

When subjected to inelastic deformation, steel special 

moment frame beams must resist member instability 

resulting from lateral-torsional buckling. AISC 341 §E3.4b 

requires lateral bracing of top and bottom flanges near the 

following: concentrated forces, changes in cross-section, 

and locations where analysis indicates that a plastic hinge 

will form during inelastic deformation. Bracing must be 

spaced at no more than 0.095ryE/(RyFy), where ry is the 

beam radius of gyration about the weak axis, E is the 

modulus of elasticity, Fy is the specified minimum yield 

stress, and Ry is the ratio of expected to specified yield 

stress. Figure 5-1 shows lateral beam bracing consisting 

of diagonal “kicker” braces extending from the bottom 

flange of the moment frame beam to the top flange of an 

adjacent beam. Although this detail is commonly used, 

it can be problematic because the brace will be stressed 

whenever loading of the two beams produces dissimilar 

deflections, causing the kicker braces to tend to twist the 

moment frame beam by pushing the bottom flange to the 

side as the adjacent beam deflects. Figure 5-2 shows an 

alternate detail in which intermediate horizontal framing 

is used to provide lateral bracing in the form of rotational 

restraint. Figure 5-3 shows another alternative, in which 

kickers are moved one bay from the moment frame beam. 

Figure 5-1. Stability bracing using diagonal “kickers”.
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

22

Plastic hinge locations must be consistent with AISC 

358 §2.4.2, or as otherwise determined, with either 

prequalification or qualification testing. AISC 358 

requires placement of lateral bracing just outside 

designated protected zones. Such bracing also is 

required at any other locations where plastic hinging 

can occur, which for beams with heavy gravity loading 

can be within beam spans. In many cases, when a beam 

supports a concrete structural slab that is connected 

to the beam between the protected zones with welded 

shear connectors, supplemental lateral bracing may 

be eliminated at the plastic hinge. Such detail-specific 

exceptions are outlined in the individual connection 

chapters of AISC 358.

If lateral braces are provided adjacent to the plastic hinge, 

AISC 341 §D1.2c requires a brace strength equal to at 

least 6 percent of the expected flange capacity at the 

plastic hinge location. Otherwise, the brace strength must 

meet the provisions of AISC 360, Appendix 6, Equation 

A-6-7. All braces also must meet the stiffness provisions 

of AISC 360, Appendix 6, Equation A-6-8.

5.2.3 Strength

Required beam strength initially is determined using 

the load combinations of ASCE 7 §2.3 or §2.4. Although 

steel special moment frame story drift limits often will 

control the selection of the beams, the flexural and shear 

strengths still must be verified.

Beam nominal flexural strength, Mn , is determined in 

accordance with AISC 360 §F2. Because AISC 341 

requires compact sections meeting the criteria for highly 

ductile members and having adequate lateral bracing, it is 

necessary to evaluate only the yielding limit state (plastic 

moment). When using reduced beam section connections, 

adequacy of beam flexural strength must be evaluated 

both at the column face and at the reduced section. The 

effective reduced beam section plastic section modulus is 

determined in accordance with AISC 35 §5.8, Equation 

5.8-4.

Beam nominal shear strength, Vn , is determined from 

the limit states of shear yielding and shear buckling 

in accordance with AISC 360 §G2. Beam sections are 

designed for flexure and shear such that design strengths 

including corresponding resistance or safety factors are 

at least equal to required demand moments and shears.

5.3 Column Design 

5.3.1 Limitations 

As with beams, AISC 341 §3.5a, requires column sections 

to conform to the compactness criteria for highly ductile 

members. AISC 358 §2.3 also requires that both rolled 

wide-flange and built-up columns conform to the cross 

section profile limitations applicable to the specific 

connection type. These limitations include restrictions 

on depth, weight, and flange thickness.

AISC 358 §2.3.2b prequalifies a number of built-up 

column shapes, including (1) I-shaped welded columns 

that resemble standard rolled wide-flange shape in cross 

section shape and profile; (2) boxed wide-flange columns, 

fabricated by adding side plates to the sides of an I-shaped 

cross section; (3) built-up box columns, fabricated by 

welding four plates together to form a closed box-shaped 

cross section; and (4) flanged cruciform columns. The 

flanged cruciform columns are fabricated by splitting 

a wide-flange section in half and welding the webs on 

either side of the web of an unsplit I-shaped section at its 

mid-depth to form a cruciform shape, each outstanding 

leg of which terminates in a rectangular flange. In 

addition, concrete-filled HSS sections can be used with 

the proprietary ConXLTM connection, and unfilled HSS 

sections can be used with the proprietary SideplateTM ® 

connection.

5.3.2 Stability 

In most cases, steel special moment frame columns are 

required to be braced at beam-to-column connections 

Figure 5-2. Stability bracing using transverse framing.

Figure 5-3. Alternate stability bracing detail.

Brace

Floor framing Horizontal strut

Moment 

frame 

beam
23

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

to prevent rotation out of the plane of the moment 

frame, particularly if inelastic behavior is expected 

in or adjacent to a beam-column connection. In some 

special cases, such as when a column spans two or 

more stories without a supporting floor, the potential for 

out-of-plane buckling at the unbraced connection must 

be minimized. In the event such a column containing 

a connection is not laterally braced, AISC 341 §3.4c2 

requires the column to conform to AISC 360 §H1, but 

with a number of additional requirements. For example, 

the unbraced column segment must be designed using the 

distance between adjacent lateral braces as the column 

height for buckling transverse to the plane of the frame, 

and the column must be designed for the ASCE 7 load 

combinations that include consideration of overstrength.

When columns are braced laterally by the floor or 

roof framing, column and beam webs are coplanar, 

and columns remain elastic outside panel zones, AISC 

341 §E3.4c requires only bracing at beam top flanges. 

Otherwise, column flange bracing is required at both 

the top and bottom levels of beam flanges. It is assumed 

that a column will remain elastic outside the panel zone 

when the beam-column moment ratio is greater than 2.0. 

Flange lateral bracing can be direct or indirect. Direct 

lateral support (bracing) can be achieved through the 

use of braces or other members, deck and slab, attached 

to a column flange at or near the desired bracing point. 

Indirect lateral support can be achieved through the 

stiffness of members and connections that are not directly 

attached to column flanges, but rather act through column 

web or stiffener plates.

AISC 341 §E3.4c1(b) specifies required brace strength 

for column flange bracing that is equal to 2 percent of 

available beam flange strength.

5.3.3 Strength 

Required column strength initially is determined using 

the load combinations of ASCE 7 §2.3 or §2.4. Although 

steel special moment frame story drift limits and strongcolumn/weak-beam requirements often will control the 

selection of column sections, the combined axial and 

flexural strengths still must be verified.

Adequacy of column strength for combined flexural 

and axial loads is verified using interaction equations in 

AISC 360 §H1. The interaction equations accommodate 

flexure about one or both principal axes as well as axial 

compression or tension. In these equations, the column 

available axial compressive strength, Pc , is determined in 

accordance with AISC 360 §E as the value obtained by 

the limit state of flexural buckling. The column available 

flexural strength, Mc, is determined in accordance with 

AISC 360 §F2 as the lower value obtained by the limit 

states of yielding (plastic moment) and lateral-torsional 

buckling.

Column nominal shear strength, Vn , is determined from 

the limit states of shear yielding and shear buckling in 

accordance with AISC 360 §G2. Column sections are 

designed for shear such that nominal strength including 

corresponding resistance or safety factors is at least equal 

to the required demand shear.

In addition to design for the load combinations of ASCE 7 

§2.3 or §2.4, AISC 341 §D1.4a requires that columns 

have sufficient axial strength to avoid global buckling 

or tensile fracture under load combinations, including 

the overstrength seismic load. Compliance with this 

requirement can be checked by computations using the 

Ωo factor per ASCE 7 §12.2.1 or by performing a plastic 

mechanism analysis and determining the capacity-limited 

load as permitted by AISC 341 §B2. It is permitted to 

neglect consideration of concurrent bending moments 

when meeting the requirements of AISC 341 §D1.4a.

5.3.4 Column Bases

Column base connections are among the more important 

elements in steel special moment frame design. When 

design assumes a fixed-base condition, the column bases 

must be designed and detailed to develop potential plastic 

hinging in the columns. AISC 341 §D2.6a, §D2.6b, and 

§D2.6c outline the column base requirements for axial, 

shear, and flexural strengths, respectively.

AISC 341 Commentary §D2.6c, Figure C-D2.6 shows 

several examples of rigid base assemblies that employ 

thick base plates, haunches, cover plates, and other 

strengthening mechanisms to develop plastic hinging 

in the column. In addition, AISC 341 §D2.6b, Figure 

C-D2.4 shows examples of base assemblies that employ 

anchor rod bearing, shear key bearing, or grout bearing 

to transfer shear forces into the supporting concrete 

foundation. Friction must not be relied on as a means of 

shear transfer at column bases.

The available strength of concrete elements at column 

bases must be in accordance with ACI 318, Chapter 17. 

ACI 318 §17.2.3.1 requires design of anchorage in Seismic 

Design Categories D, E, and F to be controlled by ductile 

yielding of a steel element and reduces the available 
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

24

also may occur in other locations, such as the column 

panel zone and the beams that have large gravity load 

demands within the beam span.

AISC 358 §2.4.3, Equation 2.4.3-1 specifies computation 

of the probable plastic moment at the assumed plastic 

hinge zone. The probable plastic moment at the plastic 

hinge is intended to be a conservative estimate of the 

maximum moment likely to be developed by the beam 

at the plastic hinge under cyclic inelastic response, 

considering material overstrength and strain hardening.

5.4.2 Column-Beam Moment Ratio

AISC 341 §E3.4 requires, with some exceptions, a check 

on the relative bending strength of columns versus beams, 

using the intersection of beam and column centerlines as 

a reference point. At this intersection, the ratio ΣM*pc /

ΣM*pb should be greater than 1.0, where ΣM*pc is the sum 

of the plastic moment capacities of the columns above 

and below the panel zone, reduced for axial load effects, 

including overstrength load combinations, and ΣM*pb

is the sum of beam moments obtained by “projecting” 

the expected flexural strengths of the beams from the 

plastic hinge locations to the column centerline. The 

term “projecting” implies that each of these moments is 

calculated from the flexural strength at the plastic hinge 

locations and amplified by a moment resulting from shear 

forces computed using the Load and Resistance Factor 

Design load combinations acting between the location of 

the plastic hinge to the column centerline. ΣM*pc is based 

on nominal strengths, and ΣM*pb is based on expected 

strengths (defined as nominal strength times 1.1Ry).

The larger the ratio ΣM*pc /ΣM*pb, the smaller is the 

likelihood that undesirable plastic hinges will form in 

columns. AISC 341 §E3.4c permits the assumption that 

columns remain elastic if this ratio is greater than 2.0. 

When a column cannot be shown to remain elastic outside 

the panel zone, column flanges have to be braced laterally 

as discussed in Section 5.3.2.

In those cases where columns form part of two intersecting 

moment frames, AISC 341 §E3.3 requires that evaluation 

of moment ratios consider the potential for simultaneous 

yielding of the frames in both directions. This evaluation 

can increase the axial load in end columns of frames and 

reduces the available flexural strength because of the 

increased axial load as well as the simultaneous bending 

of the column about both axes.

capacity of groups of anchors by a factor of 0.75. AISC 

341 §D2.6 specifies design of column base connections for 

the ASCE 7 load combinations considering overstrength.

5.3.5 Column Splices

Contrary to the notion that steel special moment frame 

columns typically will bend in double curvature with 

an inflection point near mid-height, nonlinear analyses 

have demonstrated that mid-height column bending 

moments can be substantial, and under some conditions, 

single curvature bending is possible. Accordingly, AISC 

341 §D2.5b requires that the expected flexural strength 

of the smaller column cross section be developed at 

column splices, either through the use of complete joint 

penetration groove welds or through other means that 

can provide similar strength. In addition, it requires the 

shear strength of the splice to be sufficient to resist the 

shear developed when the column nominal plastic flexural 

strength, Mpc , occurs at each end of the spliced column. 

AISC 341 §D2.5d permits column splices to be either 

bolted or welded, or welded to one column and bolted to 

the other. In the case of bolted splices, plates or channels 

must be used on both sides of column webs and singlesided connections are not permitted.

Partial joint penetration groove welded column splices 

can be subject to fracture. The use of such joints has 

been discouraged in the past. Research on this topic 

is ongoing and suggests that such splices can perform 

acceptably under some conditions. AISC 341 §E3.6g 

indicates when such joints can be used. Companion 

requirements for nondestructive evaluation of these 

partial joint penetration splices is given in AISC 341 

§J2.6c. Ultrasonic inspection of partial joint penetration 

welds is not a typical process for building construction. 

Therefore, extra effort and coordination may be required 

to avoid disputes.

5.4 Connection Design

5.4.1 Probable Maximum Moment 

AISC 358 identifies the locations of assumed plastic 

hinge zones in the respective provisions for each of 

the prequalified connection types. These plastic hinge 

locations are specified based on observed hinge formation 

during connection assembly tests. They represent 

the anticipated location of inelastic deformation in 

connection assemblies conforming to the prequalification 

requirements. Although AISC 358 specifies the region of 

primary plastic hinging, some limited inelastic behavior 
25

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

Figure 5-4. Panel zone free body diagram.

5.4.3 Beam-Column Panel Zone 

AISC 341 §E3.6e specifies that the required panel zone 

shear strength be determined from the summation 

of moments at the column faces, as determined by 

projecting the expected moments at the plastic hinge 

points to the column faces. The expected moment is 

the bending strength at the plastic hinge point based on 

expected material yield strength, i.e., based on RyFy. If the 

summation of moments at the column faces is denoted 

as ΣMb, the required panel zone shear strength can be 

estimated as Vpz = ΣMb /db – Vcol, (see Figure 5-4), where 

Vcol is the column shear associated with formation of a 

plastic mechanism in the frame. More elaborate equations 

should be used if Vcol above the panel zone differs 

significantly from Vcol below the panel zone and if the 

depths of the two beams framing into the panel zone are 

different. The design shear strength is φvRv, with φv=1.0 

for Load and Resistance Factor Design. The nominal 

shear strength Rv is determined from AISC 360 §J10-6.

doubler plates. AISC 341 Commentary §E3.6e, Figures 

C-E-3.5 through Figure C-E3.9 show alternative details 

for connection of the web doubler plate(s) to the column 

panel zone, when doubler plates are required. Often, 

using a heavier column section than providing doubler 

plate reinforcement in panel zones is more economical.

5.4.4 Continuity Plates

AISC 341 §E3.6f2c requires continuity plates in steel 

special moment frames consistent with the connection 

qualification designated in AISC 358 or, as otherwise 

determined, with either prequalification or qualification 

testing. 

AISC 360 §J10 and AISC 341 §E3.6f2b govern the design 

of continuity plate thickness. The continuity plates must 

be attached to the column in accordance with AISC 

341 §E3.6f. In addition, to avoid welding in regions of 

potentially low notch toughness in wide-flange sections, 

the corners of the continuity plates are required to be 

clipped in accordance with AISC 341 §I2.4 and AWS 

D1.8, clause 4.1 (see also the discussion in Section 3.6.6 

of this Guide).

5.4.5 Beam Web-to-Column Connection

Two types of web connection details are commonly used 

for steel special moment frame connections: a welded and 

a bolted detail. Most connections use the welded detail, 

with the beam web welded directly to the column flange 

using a complete joint penetration groove weld. A few 

connections use a bolted detail, in which pretensioned 

high-strength bolts attach the beam web to a single 

plate shear connection. AISC 358 provides specific 

requirements for each connection type. 

Fb,r = Mb,r /db

Fb,r = Mb,r /db

Mb,l Mb,r db

Fb,l = Mb,l /db

Fb,l = Mb,l /db

Vcol

Vcol

dc SMb = Mb,r+ Mb,l 

Panel

zone

If the effect of panel zone deformations on frame stability 

is considered (see Section 4.2.1), AISC 360 §J10.6 permits 

an increase in the design shear strength beyond the level 

associated with global shear yielding of the panel zone. 

In this instance, Equations J10-11 and J10-12 may be used 

rather than Equations J10-9 and J10-10. If panel zones are 

explicitly modeled in the analysis, it is permissible to use 

these equations. However, as explained in Section 4.2.1, 

this is not common.

Equating required shear strength with design shear 

strength will determine the need for panel zone 

doubler plates. AISC 341 §E3.6e2 presents minimum 

requirements for plate thickness of column web and 
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

26

6. Additional Requirements

In addition to requirements governing proportioning and 

detailing of special steel moment frames the building code 

and AISC specifications also specify quality assurance, 

materials, and construction requirements. This Section 

discusses several of these important requirements. 

Readers are referred to the building code and AISC 

specifications for complete information.

6.1 Special Inspection 

Generally, the special inspector is required to inspect 

work for conformance to approved design drawings 

and specifications. Under IBC §1704.3, the engineer 

of record is required to prepare a Statement of Special 

Inspections indicating the specific inspections and tests 

to be performed; these requirements should be on the 

structural drawings. Some jurisdictions additionally 

require submittal of these inspection requirements on a 

separate form. Contract documents should also specify 

the tests and inspections to be performed and require 

that the special inspector furnish inspection reports to 

the building official, engineer of record, owner, and 

contractor. Discrepancies should be reported to the 

contractor for correction, and, if uncorrected, to the 

engineer and the building official. A final signed report 

should be submitted by the special inspector stating 

whether the work requiring special inspection was 

completed in conformance with the approved plans and 

specifications and applicable workmanship provisions of 

the IBC and its referenced standards.

IBC §1705.12.1 requires special inspection of all structural 

steel elements of the seismic force-resisting system in 

accordance with the quality assurance requirements of 

AISC 341. AISC 341, Chapter J sets the requirements 

for quality assurance including the qualifications of 

agencies and personnel providing quality assurance 

services and the specific quality assurance tasks required 

as a minimum for welding and high-strength bolting 

operations. Individual engineers can specify additional 

special inspection/quality assurance tasks on individual 

projects if they deem this appropriate. When setting 

quality assurance requirements, engineers should 

consider the experience level and competence of the 

fabricator and erector and the adequacy of their internal 

quality control programs.

6.2 Material Properties

Wherever steel special moment frames are used, 

regardless of the seismic design category, AISC 341 

stipulates that material properties conform to specific 

requirements. These requirements are intended to result 

in a frame capable of sustaining multiple inelastic 

deformation cycles without critical degradation.

AISC 341 §A3.1 requires that structural steel used in steel 

special moment frames meet additional requirements 

to those specified in AISC 360 §A3.1 for all structural 

steel. The specified minimum yield stress to be used for 

members in which inelastic behavior is expected cannot 

exceed 50,000 psi (345 MPa) unless the suitability of 

the material is determined by testing in accordance with 

AISC 341, Chapter K or other rational criteria. This same 

limitation does not apply to columns for which the only 

expected inelastic behavior is yielding at the column base.

AISC 360 §A3.1c and AISC 341 §A3.3 require structural 

steel hot-rolled shapes with a flange thickness 11⁄2 inches 

(38 mm) or greater, and plates with a thickness 2 inches 

(51 mm) or greater, that are used in steel special moment 

frames to have Charpy V-Notch toughness of 20 ft-lb at 

70 °F (15 J at 21 °C). The frequency of testing is separately 

specified for each. This is intended to help ensure that 

the material properties of heavier shapes and thicker 

plates are consistent with those assumed for all members 

of the seismic force-resisting system.

6.3 Bolting

AISC 341 §D2.2a requires fasteners used in steel special 

moment frames to be pretensioned high-strength bolts 

meeting the requirements of AISC 360 §J3.8 with a 

Class A surface. The faying surfaces for some types 

of bolted connections are permitted to be painted with 

coatings not tested for slip resistance or with coatings 

with a slip coefficient less than that of a Class A faying 

surface. AISC 358 §4.3 requires bolts in prequalified 

steel special moment frame connections to conform to 

ASTM A325, A490, F1852, or F2280 unless a connection 

prequalification specifically permits the use of bolts 

conforming to other specifications. AISC 360 §M1 and 

AISC 341 §I1.1 require the locations of pretensioned bolts 

to be shown in the shop and erection drawings. There 
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Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

may be connections or applications for which details are 

not addressed specifically by referenced standards. If 

such a condition exists, the shop drawings should include 

appropriate requirements for that application.

Inspection of bolts and bolting operations in steel special 

moment frames must be performed in accordance 

with IBC §1704.3. AISC 341 §J7, Tables J7-1, J7-2, 

and J7-3 suggest that the minimum acceptable level of 

quality assurance is for the special inspector to observe 

high-strength bolting operations, including grade and 

size of fasteners, storage procedures, pre-installation 

verification, installation personnel qualifications, joint fitup and faying surface condition, the installation of bolts, 

and documents both acceptable and rejectable conditions. 

The Special Inspector is not required to torque test high 

strength bolts. The use of load indicator style washers 

and other forms of pretension indicators can facilitate the 

assurance that bolts are properly installed.

6.4 Welding

AISC 341 §I2.3 requires welding of steel special moment 

frames to be performed in accordance with AWS 

D1.1 (AWS 2015) and AWS D1.8. Welding Procedure 

Specifications must be submitted to and approved by the 

engineer of record. The Welding Procedure Specification 

essential variables, including current setting, length of 

arc, angle of electrode, speed of travel, and filler metal 

specification, must be within the parameters established 

by the filler metal manufacturer. AISC 360 §M1 requires 

that the locations of shop and field welds be included in 

the structural design, shop, and erection drawings and in 

the structural specifications. AISC 341 §I1.2 requires that 

joints requiring special welding sequences or techniques 

be identified on the erection drawings. Any welded joint 

specified as demand critical should be included in this 

requirement. When reviewing submittals, engineers 

should ensure these requirements are met. In addition, 

when reviewing Welding Procedure Specifications, 

engineers should ensure that the manufacturer’s weld 

filler metal data sheets have been submitted and that 

the proposed filler metals have the required toughness 

properties.

Inspection of welds and welding operations in steel 

special moment frames must be performed in accordance 

with IBC §1704.3, AISC 341 §J.6, and AWS D1.1. AISC 

341 §J.6 requires visual inspection of all welded joints 

in the seismic force-resisting system before, during, and 

after welding, as specifically indicated in Tables J6-1, 

J6-2, and J6-3. In addition, AISC 341 §J6.2a requires 

ultrasonic testing on 100 percent of complete joint 

penetration groove welds in materials 5/16 inch thick or 

thicker. Magnetic particle testing of 25 percent of all 

beam-to-column complete joint penetration groove welds 

is also required. AISC 341 §J6.2i permits reductions in 

these requirements upon successful demonstration by the 

fabricator and erector that welds of acceptable quality 

are routinely being produced. Ultrasonic testing is also 

required on 100 percent of partial joint penetration groove 

welds in column splices and column to base plate joints.

AISC 341 §J6.2c requires ultrasonic testing of base metal 

at any welded splices and connections with base metal 

thicker than 11⁄2 inch (38 mm) that is subject to throughthickness tension and of tee and corner joints where the 

connected materials are thicker than 3/4 inch (19 mm). 

The purpose of this testing is to detect lamellar tearing. 

AISC 341 §J6.2e requires magnetic particle testing on 

any repairs to damaged sections of reduced beam section 

flange cutouts. This testing can be conducted by the 

contractor’s quality control agency and observed by the 

special inspector.

6.4.1 Filler Metal

Welded joints often include a number of small flaws 

including porosity, slag, and small cracks that are 

permitted under AWS D1.1. However, under conditions 

of high stress, these flaws can act as crack initiators. 

When welded joints have high toughness, they are more 

resistant to formation of such cracks. Recognizing that 

members and connections in the seismic force-resisting 

systems can be subjected to very high stress, AWS D1.8 

requires all welds in these members and connections 

be made using filler metal with a minimum Charpy-V 

Notch toughness of 20 ft-lb at 0 °F (15 J at -17.8 °C) (as 

determined by the appropriate AWS classification test 

method or manufacturer certification.

Additionally, AWS D1.8 also requires that filler metals 

used for welds designated as demand-critical also be 

capable of providing a minimum Charpy V-Notch 

toughness of 40 ft-lb at 70 °F (73 J at 21 °C). This dual 

requirement ensures that the filler metal will provide 

ductile behavior under dynamic loading and inelastic 

demands. These criteria are approrpiate when the 

steel frame is normally enclosed and maintained at a 

temperature of 50 °F (10 °C) or higher. For structures with 

anticipated service temperatures lower than 50 °F (10 °C), 

the qualification temperature must be reduced to 20 °F 

(-7 °C) above the lowest anticipated service temperature. 
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

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6.5 Additional System Design Requirements

6.5.1 Structural Diaphragms 

In steel special moment frame construction, roof and floor 

slabs typically consist of concrete-filled metal deck slabs 

that are connected to the structural framing and provide 

an in-plane diaphragm that collects and distributes 

inertial forces. NEHRP Seismic Design Technical Brief 

No. 5 (Sabelli et al. 2011) provides detailed guidance on 

the design of such diaphragms.

6.5.2 Foundations

ACI 318 §18.13 (ACI 2014) outlines design requirements 

for foundations that transfer earthquake-induced forces 

between the steel special moment frame and the ground in 

Seismic Design Categories D, E, or F. Because foundation 

damage can be extremely difficult to detect and repair, it is 

desirable that inelastic response during earthquake ground 

shaking occurs above the foundation. As previously 

discussed, AISC 341 requires design of the connection of 

the column base to foundation to accommodate the load 

combinations, including overstrength. However, neither 

ASCE 7 nor ACI 318 requires that the foundation itself 

be designed for these forces. Unless a designer chooses 

to design foundations to remain elastic, the foundation 

will probably experience some inelastic behavior. When 

the designer chooses to allow such inelastic behavior, 

combined footings and concrete grade beams, such as that 

shown in Figure 6-1, should be detailed with longitudinal 

and transverse reinforcement that meet the concrete 

special moment frame requirements of ACI 318 §18.6.

Figure 6-1. Concrete grade beam connecting 

adjacent column steel special moment frame bases.

6.5.3 Members Not Designated as Part of the 

 Seismic Force-Resisting System 

Because of the inherent flexibility of steel special moment 

frame systems, columns that are not part of the seismic 

force-resisting system still may develop significant 

bending moments and shears when the frame is subjected 

to the design displacements. Even though the connections 

of beams framing to columns are often considered to 

be pins, the columns must deform to accommodate the 

deflected shape of the moment frame and typically will 

bend in double curvature with the inflection point near 

mid-height. As a result, such columns may develop 

significant shear forces. This behavior is beneficial in that 

it provides steel special moment frame buildings with 

substantial overstrength and helps to inhibit the formation 

of inelastic soft stories. AISC 341 §D2.5a requires that 

the splice location for such columns be located away from 

the beam-to-column connection and near the expected 

inflection point. AISC 341 §D2.5c requires that splices in 

gravity columns have the strength to resist shear forces 

associated with development of their expected plastic 

moment capacity at one end of the column. 
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Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

This section of the Guide addresses a number of issues 

related to documentation of the design engineer’s 

information, connection detailing, and construction 

quality and control that are essential to achieving the 

expected seismic performance of steel special moment 

frames.

7.1 Specifications and Structural, Shop, 

 and Erection Drawings

Clear documentation of the expectations of the design 

engineer is essential to convey the design intent to the 

general contractor, fabricator, and erector. For steel special 

moment frame projects, this is especially important 

because the design is intended to result in significant 

inelastic response when subjected to shaking equal to or 

greater than the design earthquake. This documentation 

manifests itself in the form of complete drawings and 

project specifications, with special emphasis on the 

unique aspects of steel special moment frame connections 

design, details, and joining via welding and/or bolting.

Recognizing the importance of this documentation, 

AISC 341 §A4 and §I1 specifically list items that are 

required for documentation beyond that required for all 

steel structures as listed in the AISC Code of Standard 

Practice for Steel Buildings and Bridges (AISC 2016d).

7.2 Protected Zones

The majority of steel seismic force-resisting systems 

not designated as “ordinary” in AISC 341 have specific 

elements that are intended to be the primary source of 

inelastic response when the structure is subjected to 

severe ground shaking. In steel special moment frame 

structures, the primary inelastic behavior is intended 

to occur in the beams near or within the beam-column 

connections. In most cases, this inelastic behavior can be 

expected to concentrate over a length approximately equal 

to or slightly longer than the beam depth. Because large 

inelastic strains are expected to occur at these locations, 

any discontinuities in the material in the steel beam in 

the hinge zones could become fracture initiation points. 

In an attempt to avoid these fractures, AISC 341 §D1.3 

requires protection of these zones from discontinuities 

to the greatest extent practicable. AISC 341 §I2.1 lists 

the specific discontinuities that are unacceptable in these 

“protected zones.”

One of the discontinuities addressed by AISC 341 §I2.1 

is shear connectors, such as those commonly used 

for composite slab behavior. The limitation on shear 

connectors in the protected zone is the result of a fracture 

that occurred in a connection test with a composite floor 

slab that had headed shear studs in the plastic hinge region 

to connect the deck and slab to the moment frame beam. 

Power actuated fasteners and arc spot welds used to fasten 

metal deck to beam flanges for composite behavior are 

permitted, however.

AISC 341 §A4.1, §I1.1, and §I1.2 require designation 

of the location and dimensions of protected zones on 

structural design, shop, and erection drawings. Engineers 

should be aware that trades that may make attachments to 

structural framing often may not be familiar with these 

requirements. As shown in Figure 7-1, enforcement 

of the protected zone provisions can be a significant 

challenge on construction sites. Preconstruction 

meetings with the general contractor should be used to 

emphasize the importance of these requirements. The 

concept needs to be clear not only to the structural steel 

and decking subcontractors but also to all curtain wall, 

mechanical, electrical, and plumbing subcontractors. 

AISC 303 incorporates language requiring that the 

general contractor paint or otherwise designate these 

regions. If fireproofing is used, the marking should be 

applied after its application. Because not all contractors 

are familiar with AISC 303, repeating this requirement 

in the drawings and specifications is recommended.

7. Detailing and Constructability Issues

Figure 7-1. Unauthorized attachment within the protected 

zone of a reduced beam section connection.
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

30

When repair of a discontinuity within the protected zone 

is required, the repairs are subject to the approval of the 

engineer of record. As a reference, the engineer of record 

can refer to AWS D1.1 and ASTM A6 §9 (ASTM 2014)

for guidance in establishing repair acceptance criteria. 

Outside the protected zone, AWS D1.1 requirements apply 

for the repair of discontinuities.

7.3 Weld Access Holes

Many steel special moment frame connections include 

a complete joint penetration groove weld between the 

beam flanges and the column flange. In most cases, this 

joint is made with a single bevel weld that is detailed 

with weld backing across the width of the flange, with 

the weld being made in the flat position. The backing 

is typically a steel bar, 1 inch wide by 3/8 inch thick (10 

mm), although ceramic and copper backing can also be 

used. To accommodate this backing and to provide access 

for the welder to make the weld at the bottom flange, a 

weld access hole is provided. AISC 360 §J1.6 specifies 

the minimum permissible shape of these access holes for 

typical conditions.

One finding of the post-Northridge earthquake research 

was that the configuration and preparation of these 

access holes can play a critical role in the performance 

of steel special moment frame connections. Large 

inelastic strains are concentrated in these regions in 

connections that focus much of the inelastic behavior at 

the beam-column interface, for example in the welded 

unreinforced flange–welded web connection. Both 

experimental and parametric finite element analytical 

studies have confirmed that modifications to the standard 

AWS access hole configurations are needed to achieve 

the levels of inelastic deformation anticipated in steel 

special moment frame designs. AWS D1.8 §6.10.1.2 

specifies the weld access hole configuration required for 

welded unreinforced flange–welded web connections. 

Access holes for reduced beam section connections 

must be detailed according to AISC 360 §J1.6. Complete 

joint penetration groove welds for end plate connections 

fabricated per AISC 358 §6.9.7 are to be detailed without 

weld access holes because extensive testing of this 

connection indicates that eliminating the access holes 

significantly improves the performance.

Similar to protected zones, weld access holes should 

be free of discontinuities that could cause a premature 

fracture. AWS D1.8 §6.10.2 provides the criteria for weld 

access hole surface finishes and repair. 

7.4 Web Doubler Plates

As discussed previously in this Guide, high shear forces 

occur in the joint panel zones of steel special moment 

frames. In many cases, to meet panel zone shear strength 

requirements, a doubler plate is needed to locally 

strengthen the column web. Adding doubler plates is 

expensive because of the significant shop fabrication 

time that is needed to prepare the plate and weld it into 

the column web. A rule of thumb that commonly applies 

for most typical moment frame configurations, story 

heights of approximately 15 feet (5 m), and beam spans of 

approximately 30 feet (10 m), is as follows: if the designer 

can increase the weight per foot of the column by less 

than 100 lb/ft (150 kg/m) and avoid the need for doubler 

plates, the cost of the frame will be reduced. Engineers 

should confirm this approach with the fabricators selected 

for a given project.

Proper detailing of the welding of the doubler plates with 

the column web, column flanges, and/or continuity plates 

is needed to ensure that force transfers through this highly 

stressed region can be achieved. AISC 341 §E3.6e3 states 

the requirements for welding of doubler plates to the 

column flanges and web. In most applications, the doubler 

plate is placed immediately adjacent to the column web 

plate. This location requires welding of the doubler in the 

region of the web-flange junction of the column, sometimes 

known as the “k-area.” Some fabrication-induced cracking 

in this area of the column also has led to the suggestion to 

obtain symmetry in the connection by moving the doubler 

plate, or plates, away from this highly stressed area and 

closer to the midpoint of the flange half-width. This practice 

has not gained widespread acceptance because the need for 

a second plate and the increased thickness necessary for 

plate stability increases the cost of this detailing approach 

over the typical single plate placed adjacent to the column 

web. AISC 341 §E3.e2 requires that all plates have a 

thickness that is larger than 1/90 of the sum of the panel 

zone depth plus width (all terms in inches). Doubler plates 

that are thinner than this limit are typically brought into 

conformance with this requirement by the addition of a 

series of four plug welds at about the quarter points of 

the joint panel zone. AISC 341 Commentary §E3.6e2, 

Figure C-E3.5, depicts the various configurations for 

web doubler plates.
31

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

7.5 Continuity Plates

As shown in Figure 7-2, continuity plates often are 

required between column flanges to help transfer beam 

flange forces through the connection and help to stiffen 

the column web and flanges. The individual AISC 358 

connection prequalifications include specification of 

whether continuity plates are required as well as the 

design procedures for these plates.

plates to avoid the rounded area at the web flange 

junction. The reader is referred to AISC 341 Commentary 

§D2.3, Figure C-D2.3 for a graphical explanation of the 

continuity plate clips in this region.

When both continuity plates and doubler plates are 

included in a steel special moment frame detail, the 

engineer must decide whether the length of the doubler 

plates will be stopped at the face of the continuity plates 

that are adjacent to the joint panel zone. Stopping the 

doubler plate in this way requires careful detailing of 

the two welded joints that would occupy the same space 

at this intersection. The option of extending the vertical 

length of the doubler plate beyond the extent of the 

beam can facilitate welding procedures. Recent research 

suggests that welding of the doubler plates to the column 

web need not fully develop the beam flange or continuity 

plates. AISC 341 §E3.6e3 specifies the requirements for 

welding of these plates.

7.6 Column Splices

Splices in steel special moment frame columns also can 

be critical to system performance. In many cases, the 

primary demand on steel special moment frame columns 

is flexure, or flexure combined with axial tension, 

rather than axial compression. In effect, these columns 

act as “vertical beams” rather than classical columns. 

Nonlinear response history analyses of steel special 

moment frame designs performed for the SAC Steel 

Project demonstrated that column flexural and tension 

demands can approach column capacity (FEMA 2000). 

These studies also indicate that the location of minimum 

moment in a column is not static but moves along the 

length of the column, depending on the characteristics of 

the ground motion and the frame configuration. In some 

instances, no point of inflection occurs over entire story 

heights at various times during seismic response of the 

frames. As a result of these findings and the potentially 

dramatic consequence of column splice failures, AISC 

includes restrictive criteria for the design of steel special 

moment frame column splices. In most cases, complete 

joint penetration groove welds, such as those shown in 

Figure 7-3, will probably be required for these splices. 

Partial joint penetration groove welds are permitted in 

some cases, but AISC 341 places severe limitations on 

their use.

Figure 7-2. Installed connection continuity plates in a bolted 

stiffened extended end plate connection.

Like doubler plates, proper detailing of continuity plates 

is crucial to the anticipated ductile performance of steel 

special moment frame connections. Welds between 

continuity plates and the other elements of a connection 

may be required to develop the capacity of the plate. In 

such instances, yield level strains may be anticipated in 

the plates and their connections. Care should be taken 

to avoid fracture-sensitive details, such as partial joint 

penetration welds or single sided fillet welds. Complete 

joint penetration groove welds and double fillet welds or 

partial penetration groove welds are both options. Also, 

like doubler plates, continuity plates require welding near 

the “k” area of wide flange columns.

AISC 341 §I2.4 references AWS D1.8 for fabrication 

requirements for continuity plates, which requires that 

continuity plates be configured to avoid the welding in the 

k-area of the column, because the straightening process 

used by some mills can cause local embrittlement of the 

wide flange section in this area. When shapes have been 

made brittle by mill straightening, welding can result 

in fracturing of the section during fabrication. Specific 

dimensions are provided for the clipping of continuity 
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

32

The proceeding discussion focused on splices for seismic 

force-resisting system columns. The SAC Steel Project 

research studies also found that columns that were not 

part of the seismic force-resisting system and that are 

intended primarily for gravity load resistance can provide 

beneficial effects to overall system seismic performance. 

Continuity of these columns was found to help vertically 

distribute inelastic demands throughout the building 

height, thereby avoiding focusing inelastic demands 

in a single or small number of stories. This finding 

caused AISC 341 to place a design requirement on shear 

connections of non-frame column splices as a means of 

providing this continuity. This requirement increases 

design forces on this splice substantially but still can be 

accomplished by bolted connections in most cases.

7.7 Concrete Placement

AISC 358 limits the prequalification of some moment 

connections when a concrete structural slab is present 

because slabs will tend to act compositely with the steel 

framing, whether intended to or not, and in the process 

shift the location of the beam neutral axis and alter 

stress distributions in the connection. AISC 358 permits 

some connections to be used with a structural slab only 

if the slab is not restrained by the column or, in some 

cases, by other protruding elements associated with the 

connection. In this manner, the slab will not inhibit the 

expected performance of the connection. As shown in 

Figure 7-4, detailing compressible material against the 

protruding elements prior to the placement of the concrete 

is a sufficient means to address the requirement.

Figure 7-4. Compressible material used to isolate a protruding 

bolted stiffened extended end plate connection prior to 

concrete placement.

Figure 7-3. Steel special moment frame complete 

joint penetration groove weld column splice.
33

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

8. References

ACI (2014). Building code requirements for structural 

concrete, (ACI 318-14), and commentary on 

building code requirements for structural concrete, 

(ACI 318R14), American Concrete Institute, 

Farmington Hills, MI.

AISC (2016a). Seismic provisions for structural steel 

buildings, ANSI/AISC 341-16, American Institute 

of Steel Construction, Inc., Chicago, IL.

AISC (2016b). Prequalified connections for special 

and intermediate steel moment frames for 

seismic applications including supplement no. 1, 

ANSI/AISC 358-16, American Institute of Steel 

Construction, Inc., Chicago, IL.

AISC (2016c). Specification for structural steel 

buildings, ANSI/AISC 360-16, American Institute 

of Steel Construction, Inc., Chicago, IL.

AISC (2016d). Code of standard practice for steel 

buildings and bridges, AISC 303-16, American 

Institute of Steel Construction, Inc., Chicago, IL.

ASCE (2016). Minimum design loads and associated 

criteria for buildings and other structures, ASCE/

SEI 7-16, American Society of Civil Engineers, 

Reston, VA.

ASTM (2014). Standard specification for general 

requirements for rolled structural steel bars, 

plates, shapes, and sheet piling, ASTM A6/A6M14, ASTM International, West Conshocken, PA.

AWS (2009). Structural welding code—seismic 

supplement, ANSI/AWS D1.8/D1.8M: 2009, 

American Welding Society, Miami, FL. 

AWS (2015). Structural welding code stee1, ANSI/AWS 

D1.1: 2015, American Welding Society, Miami, FL.

FEMA (2000a). Recommended seismic design criteria 

for new steel moment frame buildings, FEMA 

350, Federal Emergency Management Agency, 

Washington, DC.

FEMA (2000b). Recommended seismic evaluation and 

upgrade criteria for existing welded steel moment 

frame buildings, FEMA 351, Federal Emergency 

Management Agency, Washington, DC.

FEMA (2000c). Recommended postearthquake 

evaluation and repair criteria for welded steel 

moment frame buildings, FEMA 352, Federal 

Emergency Management Agency, Washington, 

DC.

FEMA (2000d). Recommended specifications and 

quality assurance guidelines for steel moment 

frame construction for seismic applications, 

FEMA 353, Federal Emergency Management 

Agency, Washington, DC.

FEMA (2000e). State of art reports on steel moment 

frame structures (FEMA 355 A through F), Federal 

Emergency Management Agency, Washington, 

DC.

ICBO (1997). Uniform building code, International 

Conference of Building Officials, Whittier, CA.

ICC (2015). International building code, International 

Code Council, Washington, DC.

Sabelli, R., Sabol, T. A., and Easterling, S. W. (2011). 

Seismic design of composite steel deck and 

concrete-filled diaphragms: A guide for practicing 

engineers, NEHRP Seismic Design Technical 

Brief No. 5, NIST GCR 11-917-10, produced by the 

Applied Technology Council and the Consortium 

of Universities for Research in Earthquake 

Engineering for the National Institute of Standards 

and Technology, Gaithersburg, MD.
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

34

area of the end column

deflection amplification coefficient defined in 

ASCE 7

coefficient for upper limit on calculated period 

as defined in ASCE 7

distance between end columns

overall beam depth

overall column depth

modulus of elasticity of steel

beam flange force (tension or compression) 

at the column face

specified minimum yield stress

moment of inertia, in4; also importance factor 

as defined in ASCE 7

column available flexural strength

risk-targeted maximum considered shaking 

nominal flexural strength

column nominal plastic flexural strength

moments at the face of the column

moment at the center of the column

sum of the projected beam moments on either 

side of the panel zone

sum of the projected column moments at the 

top and bottom of the panel zone

total vertical design load as defined in ASCE 7

column available axial compressive strength

radius of gyration about y-axis

9. Notations and Abbreviations

A

Cd

Cu

D

db

dc

E

Fb 

Fy

I

Mc 

MCER

Mn

Mpc

ΣM

ΣM*b

ΣM*pb

ΣM*pc

P

Pc 

ry

R

Rv

Rw

Ry

SDS

SD1

Ta

Vcol

Vn

Vpz

β

D

Da

Ωo

θ

θmax

ρ

φv

response modification coefficient as defined 

in ASCE 7

panel zone nominal shear strength

response modification factor

ratio of expected strength to specified yield 

stress

design, 5 percent damped, spectral response 

acceleration parameter at short periods as 

defined in ASCE 7

design, 5 percent damped, spectral response 

acceleration parameter at a period of 1 s as 

defined in ASCE 7

approximate fundamental period of building 

defined in ASCE 7

column shear force

nominal shear strength

panel zone shear force

ratio of shear demand to shear capacity for the 

story under consideration, as defined in ASCE 7

story drift as defined in ASCE 7

allowable story drift as defined in ASCE 7 

story drift 

overstrength factor

stability coefficient as defined in ASCE 7

upper limit of θ as defined in ASCE 7

redundancy factor as defined in ASCE 7

resistance factor for panel zone shear strength

Notations
35

Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

ACI American Concrete Institute

AISC American Institute of Steel Construction

ANSI American National Standards Institute

ASCE American Society of Civil Engineers

ASTM ASTM International (formerly American Society of Testing and Materials)

ATC Applied Technology Council

AWS American Welding Society

CUREE Consortium of Universities for Research in Earthquake Engineering

FEMA Federal Emergency Management Agency

IAPMO International Association of Plumbing and Mechanical Officials

IBC International Building Code 

ICC International Code Council

NEHRP National Earthquake Hazards Reduction Program

SAC SAC Joint Venture (SEAOC, ATC, CUREE)

SEAOC Structural Engineers Association of California

SEI Structural Engineering Institute

Abbreviations
Seismic Design of Steel Special Moment Frames: A Guide for Practicing Engineers, Second Edition

36

10. Credits

Cover photo

Figure 2-1

Figure 2-5

Figure 2-6

Figure 2-7

Figure 3-4(b)

Figure 3-7

Figure 3-8

Figure 3-9

Figure 3-12

Figure 7-1

Image courtesy of Michael D. Engelhardt.

Image #X539.3/c, courtesy of Images of America, Library of Congress.

Image 524396, courtesy of National Archives and Records Administration NARA.

Image courtesy of the SAC Joint Venture.

Image courtesy of Michael D. Engelhardt.

Copyright © 2016 by American Institute of Steel Construction

Reprinted with permission; all rights reserved.

Image courtesy of Helmut Krawinkler.

Image courtesy of ConXtech, Inc., reprinted with permission.

Image courtesy of SidePlate Systems, Inc., reprinted with permission.

Copyright © 1978 by American Institute of Steel Construction

Reprinted with permission; all rights reserved.

Image courtesy of Thomas A. Sabol.

Images not credited here were provided by the authors.