Appendix E

Characteristics and Earthquake Performance of RVS Building Types 

E.1 Introduction 

For the purpose of the RVS, building structural framing types have been 
categorized into fifteen types listed in Section 3.7.1 and shown in Table 3-1. 
This appendix provides additional information about each of these structural 
types, including detailed descriptions of their characteristics, common types of 
earthquake damage, and common seismic rehabilitation techniques.

E.2 Wood Frame (W1, W2) 

E.2.1 Characteristics 

Wood frame structures are usually detached residential dwellings, small 
apartments, commercial buildings or one-story industrial structures. They are 
rarely more than three stories tall, although older buildings may be as high as 
six stories, in rare instances. (See Figures E-1 and E-2).
 
Wood stud walls are typically constructed of 2inch by 4-inch wood members 
vertically set about 16 inches apart. (See Figures E-3 and E-4). These walls are 
braced by plywood or equivalent material, or by diagonals made of wood or steel. 
Many detached single family and low-rise multiple family residences in the 
United States are of stud wall wood frame construction. 

Post and beam construction, which consists of larger rectangular (6 inch by 6 
inch and larger) or sometimes round wood columns framed together with large wood 
beams or trusses, is not common and is found mostly in older buildings. These 
buildings usually are not residential, but are larger buildings such as 
warehouses, churches and theaters.

Timber pole buildings (Figures E-5 and E-6) are a less common form of 
construction found mostly in suburban and rural areas. Generally adequate 
seismically when first built, they are more often subject to wood deterioration 
due to the exposure of the columns, particularly near the ground surface. 
Together with an often-found "soft story" in this building type, this 
deterioration may contribute to unsatisfactory seismic performance.

In the western United States, it can be assumed that all single detached 
residential houses (i.e., houses with rear and sides separate from adjacent
structures) are wood stud frame structures unless visual or supplemental 
information indicates otherwise (in the Southwestern U.S., for example, some
residential homes are constructed of adobe, rammed earth, and other non-wood 
materials). Many houses that appear to have brick exterior facades are actually
wood frame with nonstructural brick veneer or brick patterned synthetic siding.

In the central and eastern United States, brick walls are usually not veneer. 
For these houses the is real brick. Second, the thickness of the exterior
wall is estimated by looking at a window or door opening. If the wall is more 
than 9 inches from the interior finish to exterior surface, then it may be a
brick wall. Third, if header bricks exist in the brick pattern, then it may be a 
brick wall. If these features all point to a brick wall, the house can be 
assumed to be a masonry building, and not a wood frame.

In wetter, humid climates it is common to find homes raised four feet or more 
above the outside grade with this space totally exposed (no foundation walls). 
This allows air flow under the house, to minimize decay and rot problems 
associated with high humidity and enclosed spaces. These houses are supported
on wood post and small precast concrete pads or piers. A common name for this 
construction is post and pier construction.

E.2.2 Typical Earthquake Damage 

Stud wall buildings have performed well in past earthquakes due to inherent 
qualities of the structural system and because they are lightweight and low-
rise. Cracks in any plaster or stucco may appear, but these seldom degrade the 
strength of the building and are classified as nonstructural damage. In fact, 
this E: Characteristics and Earthquake Performance of RVS Building Types FEMA 
154 type of damage helps dissipate the earthquake-induced energy of the shaking 
house. The most common type of structural damage in older buildings results from 
a lack of adequate connection between the house and the foundation. Houses can 
slide off their foundations if they are not properly bolted to the foundations. 
This movement (see Figure E-7) results in major damage to the building as well 
as to plumbing and electrical connections. Overturning of Figure E-7 House off 
its foundation, 1983 Coalinga earthquake. 

the entire structure is usually not a problem because of the low-rise geometry. 
In many municipalities, modern codes require wood structures to be adequately 
bolted to their foundations. However, the year that this practice was adopted 
will differ from community to community and should be checked. 

Many of the older wood stud frame buildings have no foundations or have weak 
foundations of unreinforced masonry or poorly reinforced concrete. These 
foundations have poor shear resistance to horizontal seismic forces and can 
fail. Another problem in older buildings is the stability of cripple walls. 
Cripple walls are short stud walls between the foundation and the first floor 
level. Often these have no bracing neither in-plane nor out-of-plane and thus 
may collapse when subjected to horizontal earthquake loading. If the cripple 
walls collapse, the house will sustain considerable damage and may collapse. In 
some older homes, plywood sheathing nailed to the cripple studs may have been 
used to rehabilitate the cripple walls. However, if the sheathing is not 
nailed adequately to the studs and foundation sill plate, the cripple walls will 
still collapse (see Figure E-8). 

Homes with post and pier perimeter foundations, which are constructed to provide 
adequate air flow under the structure to minimize the potential for decay, have 
little resistance to earthquake forces. When these buildings are subjected to 
strong earthquake ground motions, the posts may rotate or slip of the piers and 
the home will settle to the ground. As with collapsed cripple walls, this can be 
very expensive damage to repair and will result in the home building "red-
tagged" per the ATC-20 post-earth-quake safety evaluation procedures (ATC, 1989, 
1995). See Figure E-9. 

Garages often have a large door opening in the front wall with little or no 
bracing in the remainder of the wall. This wall has almost no resistance to 
lateral forces, which is a problem if a heavy load such as a second story is 
built on top of the garage. Homes built over garages have sustained damage in 
past earthquakes, with many collapses. Therefore the house-over-garage 
configuration, which is found commonly in low-rise apartment complexes and 
some newer suburban detached dwellings, should be examined more carefully and 
perhaps rehabilitated. 

Unreinforced masonry chimneys present a life-safety problem. They are often 
inadequately tied to the house, and therefore fall when strongly shaken. On the 
other hand, chimneys of reinforced masonry generally perform well. Some wood-
frame structures, especially older buildings in the eastern United States, have 
masonry veneers that may represent another hazard. The veneer usually consists 
of one wythe of brick (a wythe is a term denoting the width of one brick) 
attached to the stud wall. In older buildings, the veneer is either 
insufficiently attached or has poor quality mortar, which often results in 
peeling of the veneer during moderate and large earthquakes. Post and beam 
buildings (not buildings with post and pier foundations) tend to perform 
well in earthquakes, if adequately braced. However, walls often do not have 
sufficient bracing to resist horizontal motion and thus they may deform 
excessively.

E.2.3 Common Rehabilitation Techniques 

In recent years, especially as a result of the Northridge earthquake, emphasis 
has been placed on addressing the common problems associated with light-wood 
framing. This work has concentrated mainly in the western United States with 
single-family residences. The rehabilitation techniques focus on houses with 
continuous perimeter foundations and cripple walls. The rehabilitation work 
consists of bolting the house to the foundation and providing plywood or other 
wood sheathing materials to the cripple walls to strengthen them (see Figure E-
10). This is the most cost-effective rehabilitation work that can be done on a 
single-family residence. 

Little work has been done in rehabilitating timber pole buildings or post and 
pier construction. In timber pole buildings rehabilitation techniques are 
focused on providing resistance to lateral forces by bracing (applying 
sheathing) to interior walls, creating a continuous load path to the ground. For 
homes with post and pier perimeter foundations, the work has focused on 
providing partial foundations and bracing to carry the earthquake loads. 

E.3 Steel Frames (S1, S2) 

E.3.1 Characteristics 

Steel frame buildings generally may be classified as either moment-resisting 
frames or braced frames, based on their lateral-force-resisting systems. Moment-
resisting frames resist lateral loads and deformations by the bending stiffness 
of the beams and columns (there is no diagonal bracing). In concentric braced 
frames the diagonal braces are connected, at each end, to the joints where beams 
and columns meet. The lateral forces or loads are resisted by the tensile and 
compressive strength of the bracing. In eccentric braced frames, the bracing is 
slightly offset from the main beam-to-column connections, and the short section 
of beam is expected to deform significantly in bending under major seismic 
forces, thereby dissipating a considerable portion of the energy of the 
vibrating building.   Each type of steel frame is discussed below.

Moment-Resisting Steel Frame 

Typical steel moment-resisting frame structures usually have similar bay widths 
in both the transverse and longitudinal direction, around 20-30 ft (Figure E-
11). The load-bearing frame consists of beams and columns distributed throughout 
the building. The floor diaphragms are usually concrete, sometimes over steel 
decking. Moment-resisting frame structures built since 1950 often incorporate 
prefabricated panels hung onto the structural frame as the exterior finish. 
These panels may be precast concrete, stone or masonry veneer, metal, glass or 
plastic. 

This structural type is used for commercial, institutional and other public 
buildings. It is seldom used for low-rise residential buildings. 

Steel frame structures built before 1945 are usually clad or infilled with 
unreinforced masonry such as bricks, hollow clay tiles and terra cotta tiles and 
therefore should be classified as S5 structures (see Section E.6 for a detailed 
discussion). Other frame buildings of this period are encased in concrete. Wood 
or concrete floor diaphragms are common for these older buildings. 

Braced Steel Frame 

Braced steel frame structures (Figures E-12 and E-13) have been built since the 
late 1800s with similar usage and exterior finish as the steel moment-frame 
buildings. Braced frames are sometimes used for long and narrow buildings 
because of their stiffness. Although these buildings are braced with diagonal 
members, the bracing members usually cannot be detected from the building 
exterior.  

From the building exterior, it is usually difficult to tell the difference 
between steel moment frames, braced frames, and frames with shear walls. In most 
modern buildings, the bracing or shear walls are located in the interior or 
covered by cladding material. Figure E-14 shows heavy diagonal bracing for a 
high rise building, located at the side walls, which Figure E-14 Chevron bracing 
in steel building under construction. will be subsequently covered by finish 
materials and will not be apparent. In fact, it is difficult to differentiate 
steel frame structures and concrete frame structures from the exterior. Most 
of the time, the structural members are clad in finish material. In older 
buildings, steel members can also be encased in concrete. There are no positive 
ways of distinguishing these various frame types except in the two cases listed 
below: 

1. If a building can be determined to be a braced frame, it is probably a steel 
structure. 

2. If exposed steel beams and columns can be seen, then the steel frame 
structure is apparent. (Especially in older structures, a structural frame which 
appears to be concrete may actually be a steel frame encased in concrete.) 

E.3.2 Typical Earthquake Damage 

Steel frame buildings tend to be generally satisfactory in their earthquake 
resistance, because of their strength, flexibility and lightness. Collapse in 
earthquakes has been very rare, although steel frame buildings did collapse, for 
example, in the 1985 Mexico City earthquake. In the United States, these 
buildings have performed well, and probably will not collapse unless subjected 
to sufficiently severe ground shaking. The 1994 Northridge and 1995 Kobe 
earthquakes showed that steel frame buildings (in particular S1 moment-frame) 
were vulnerable to severe earthquake damage. Though none of the damaged 
buildings collapsed, they were rendered unsafe until repaired. The damage took 
the form of broken welded connections between the beams and columns. Cracks in 
the welds began inside the welds where the beam flanges were welded to the 
column flanges. These cracks, in some cases, broke the welds or propagated into 
the column flange, "tearing" the flange. The damage was found in those buildings 
that experienced ground accelerations of approximately 20% of gravity 
(20%g) or greater. Since 1994 Northridge, many cities that experienced large 
earthquakes in the recent past have instituted an inspection program to 
determine if any steel frames were damaged. Since steel frames are usually 
covered with a finish material, it is difficult to find damage to the joints. 
The process requires removal of the finishes and removal of fireproofing just to 
see the joint. Possible damage includes the following. 

1.	Nonstructural damage resulting from excessive deflections in frame 
structures can occur to elements such as interior partitions, equipment, and 
exterior cladding. Damage to nonstructural elements was the reason for the 
discovery of damage to moment frames as a result of the 1994 Northridge 
earthquake. 

2.	Cladding and exterior finish material can fall if insufficiently or 
incorrectly connected. 

3.	Plastic deformation of structural members can cause permanent 
displacements.
 
4.	Pounding with adjacent structures can occur. 

E.3.3 Common Rehabilitation Techniques 

As a result of the 1994 Northridge earthquake many steel frame buildings, 
primarily steel moment frames, have been rehabilitated to address the problems 
discovered. The process is essentially to redo the connections, ensuring that 
cracks do not occur in the welds. There is careful inspection of the welding 
process and the electrodes during construction. Where possible, existing full 
penetration welds of the beams to the columns is changed so more fillet welding 
is used. This means that less heat is used in the welding process and 
consequently there is less potential for damage. Other methods include reducing 
welding to an absolute minimum by developing bolted connections or ensuring that 
the connection plates will yield (stretch permanently) before the welds will 
break. One other possibility for rehabilitating moment frames is to convert them 
to braced frames. 

The kind of damage discovered was not limited to moment frames, although they 
were the most affected. Some braced frames were found to have damage to the 
brace connections, especially at lower levels. 

Structural types other than steel frames are sometimes rehabilitated using steel 
frames, as shown for the concrete structure in Figure E-15. Probably the most 
common use of steel frames for rehabilitation is in unreinforced masonry 
bearing-wall buildings (URM). Steel frames are typically used at the storefront 
windows as there is no available horizontal resistance provided by the windows 
in their plane. Frames can be used throughout the first floor perimeter when the 
floor area needs to be open, as in a restaurant. See Figure E-16. When a 
building is encountered with this type of rehabilitation scheme, the building 
should be considered a frame type building S1 or S2.

E.4 Light Metal (S3) 

E.4.1 Characteristics 

Most light metal buildings existing today were built after 1950 (Figure E-
17).They are used for agricultural structures, industrial factories, and 
warehouses. They are typically one story in height, sometimes without 
interior columns, and often enclose a large floor area. Construction is 
typically of steel frames spanning the short dimension of the building, 
resisting lateral forces as moment frames. Forces in the long direction are 
usually resisted by diagonal steel rod bracing. These buildings are usually clad 
with lightweight metal or asbestos-reinforced concrete siding, often corrugated. 
To identify this construction type, the screener should look for the 
following characteristics: 

1.	Light metal buildings are typically characterized by industrial corrugated 
sheet metal or asbestos-reinforced cement siding. The term, "metal building 
panels" should not be confused with "corrugated sheet metal siding." The former 
are prefabricated cladding units usually used for large office buildings. 
Corrugated sheet metal siding is thin sheet material usually fastened to 
purlins, which in turn span between columns. If this sheet cladding is present, 
the screener should examine closely the fasteners used. If the heads of sheet 
metal screws can be seen in horizontal rows, the building is most likely a light 
metal structure (Figure E-18). 

2.	Because the typical structural system consists of moment frames in the 
transverse direction and frames braced with diagonal steel rods in the 
longitudinal direction, light metal buildings often have low-pitched roofs 
without parapets or overhangs (Figure E-19). Most of these buildings are 
prefabricated, so the buildings tend to be rectangular in plan, without many 
corners. 

3.	These buildings generally have only a few windows, as it is difficult to 
detail a window in the sheet metal system. 

4.	The screener should look for signs of a metal building, and should knock 
on the siding to see if it sounds hollow. Door openings should be inspected for 
exposed steel members. If a gap, or light, can be seen where the siding meets 
the ground, it is certainly light metal or wood frame. For the best indication, 
an interior inspection will confirm the structural skeleton, because most of 
these buildings do not have interior finishes. 

E.4.2	 Typical Earthquake Damage 

Because these building are low-rise, lightweight, and constructed of steel 
members, they usually perform relatively well in earthquakes. Collapses do not 
usually occur. Some typical problems are listed below: 

1.	Insufficient capacity of tension braces can lead to their elongation or 
failure, and, in turn, building damage. 

2.	Inadequate connection to the foundation can allow the building columns to 
slide. 
3.	Loss of the cladding can occur. 

E.5	Steel Frame with Concrete Shear Wall (S4) 

E.5.1	Characteristics 

The construction of this structural type (Figure E-20) is similar to that of the 
steel moment-resisting frame in that a matrix of steel columns and girders is 
distributed throughout the structure. The joints, however, are not designed for 
moment resistance, and the lateral forces are resisted by concrete shear walls. 

It is often difficult to differentiate visually between a steel frame with 
concrete shear walls and one without, because interior shear walls will often be 
covered by interior finishes and will look like interior nonstructural 
partitions. For the purposes of  an RVS, unless the shear wall is identifiable 
from the exterior (i.e., a raw concrete finish was part of the architectural 
aesthetic of the building, and was left exposed), this building cannot be 
identified accurately.  Figure E-21shows a structure with such an exposed shear 
wall. Figure E-22 is a close-up of shear wall damage.

E.5.2	 Typical Earthquake Damage 

The shear walls can be part of the elevator and service core, or part of the 
exterior or interior walls. This type of structure performs as well in 
earthquakes as other steel buildings. Some typical types of damage, other than 
nonstructural damage and pounding, are: 

1.    Shear cracking and distress can occur around openings in concrete shear 
walls. 

2.	Wall construction joints can be weak planes, resulting in wall shear 
failure at stresses below expected capacity. 

3.	Insufficient chord steel lap lengths can lead to wall bending failures. 

E.6	Steel Frame with Unreinforced Masonry Infill (S5) 

E.6.1	Characteristics 

This construction type (Figures E-23 and E-24) consists of a steel structural 
frame and walls "infilled" with unreinforced masonry (URM). In older buildings, 
the floor diaphragms are often wood. Later buildings have reinforced concrete 
floors. Because of the masonry infill, the structure tends to be stiff. Because 
the steel frame in an older building is covered by unreinforced masonry for fire 
protection, it is  easy to confuse this type of building with URM bearing-wall 
structures. Further, because the steel columns are relatively thin, they may be 
hidden in walls.  An apparently solid masonry wall may enclose a series of steel 
columns and girders. These infill walls are usually two or three wythes thick. 
Therefore, header bricks will sometimes be present and thus mislead the screener 
into thinking the building is a URM bearing-wall structure, rather than infill. 
Often in these structures the infill and veneer masonry is exposed. Otherwise, 
masonry may be obscured by cladding in buildings, especially those that have 
undergone renovation. When a masonry building is encountered, the screener 
should first attempt to determine if the masonry is reinforced, by checking the 
date of construction, although this is only a rough guide. A E: Characteristics 
and Earthquake Performance of RVS Building Types FEMA 154 clearer indication of 
a steel frame structure with URM infill is when the building exhibits the 
characteristics of a frame structure of type S1 or S2. One can assume all frame 
buildings clad in brick and constructed prior to about 1940 are of this type. 

Older frame buildings may be of several types- steel frame encased with URM, 
steel frame encased with concrete, and concrete frame. Sometimes older buildings 
have decorative cladding such as terra cotta or stone veneer. Veneers may 
obscure all evidence of URM. In that case, the structural type cannot be 
determined. However, if there is evidence that a large amount of concrete is 
used in the building (for example, a rear wall constructed of concrete), 
then it is unlikely that the building has URM infill. 

When the screener cannot be sure if the building is a frame or has bearing 
walls, two clues may help- the thickness of the walls and the height. Because 
infill walls are constructed of two or three wythes of bricks, they should be 
approximately 9 inches thick (2 wythes). Furthermore, the thickness of the wall 
will not increase in the lower stories, because the structural frame is carrying 
the load. For buildings over six stories tall, URM is infill or veneer, because 
URM bearing-wall structures are seldom this tall and, if so, 
they will have extremely thick walls in the lower stories.

E.6.2 Typical Earthquake Damage 

In major earthquakes, the infill walls may suffer substantial cracking and 
deterioration from in-plane or out-of-plane deformation, thus reducing the in-
plane wall stiffness. This in turn puts additional demand on the frame. Some of 
the walls may fail while others remain intact, which may result in torsion or 
soft story problems. 

The hazard from falling masonry is significant as these buildings can be taller 
than 20 stories. As described below, typical damage results from a variety of 
factors. 

1.	Infill walls tend to buckle and fall out-of-plane when subjected to strong 
lateral forces. Because infill walls are non-load-bearing, they tend to be thin 
(around 9") and cannot rely on the additional shear strength that accompanies 
vertical compressive loads. 

2.	Veneer masonry around columns or beams is usually poorly anchored to the 
structural members and can disengage and fall. 

3.	Interior infill partitions and other nonstructural elements can be 
severely damaged and collapse. 

4.	If stories above the first are infilled, but the first is not (a soft 
story), the difference in stiffness creates a large demand at the ground floor 
columns, causing structural damage. 

5.	When the earthquake forces are sufficiently high, the steel frame itself 
can fail locally. Connections between members are usually not designed for high 
lateral loads (except in tall buildings) and this can lead to damage of these 
connections. Complete collapse has seldom occurred, but cannot be ruled out. 

E.6.3	 Common Rehabilitation Techniques 

Rehabilitation techniques for this structural type have focused on the expected 
damage. By far the most significant problem, and that which is addressed in most 
rehabilitation schemes, is failure of the infill wall out of its plane. This 
failure presents a significant life safety hazard to individuals on the exterior 
of the building, especially those who manage to exit the building during the 
earthquake. To remedy this problem, anchorage connections are developed to tie 
the masonry infill to the floors and roof of the structure. 

Another significant problem is the inherent lack of shear strength throughout 
the building. Some of the rehabilitation techniques employed include the 
following. 

1.	Gunite (with pneumatically placed concrete) the interior faces of the 
masonry wall, creating reinforced concrete shear elements. 

2.	Rehabilitate the steel frames by providing cross bracing or by fully 
strengthening the connections to create moment frames. In this latter case, the 
frames are still not sufficient to resist all the lateral forces, and reliance 
on the infill walls is necessary to provide adequate strength. 

For concrete moment frames the rehabilitation techniques have been to provide 
ductile detailing. This is usually done by removing the outside cover of 
concrete (a couple of inches) exposing the reinforcing ties. Additional ties are 
added with their ends embedded into the core of the column. The exterior 
concrete is then replaced. This process results in a detail that provides a 
reasonable amount of ductility but not as much as there would have been 
had the ductility been provided in the original design.

E.7	Concrete Moment-Resisting Frame (C1) 

E.7.1	Characteristics 

Concrete moment-resisting frame construction consists of concrete beams and 
columns that resist both lateral and vertical loads (see Figure E-25). A 
fundamental factor in the seismic performance of concrete moment-resisting 
frames is the presence or absence of ductile detailing. Hence, several 
construction subtypes fall under this category: 

a.	non-ductile reinforced-concrete frames with unreinforced infill walls, 
b.	non-ductile reinforced-concrete frames with reinforced infill walls, 
c.	non-ductile reinforced-concrete frames, and  
d.    ductile reinforced-concrete frames. 

Ductile detailing refers to the presence of special steel reinforcing within 
concrete beams and columns. The special reinforcement provides confinement of 
the concrete, permitting good performance in the members beyond the elastic 
capacity, primarily in bending. Due to this confinement, disintegration of the 
concrete is delayed, and the concrete retains its strength for more cycles of 
loading (i.e., the ductility is increased). See Figure E-26 for a dramatic 
example of ductility in concrete. Ductile detailing (Figure E-27) has been 
practiced in high-seismicity areas since 1967, when ductility requirements were 
first introduced into the Uniform Building Code (the adoption and enforce-Figure 
E-26 

In many low-seismicity areas of the United States, non-ductile concrete frames 
of type (a), (b), and (c) continue to be built. This group includes large 
multistory commercial, institutional, and residential buildings constructed 
using flat slab frames, waffle slab frames, and the standard beam-and-column 
frames. These structures generally are more massive than steel-frame buildings, 
are under-reinforced (i.e., have insufficient reinforcing steel embedded 
in the concrete) and display low ductility. 

This building type is difficult to differentiate from steel moment-resisting 
frames unless the structural concrete has been left relatively exposed (see 
Figure E-28). Although a steel frame may be encased in concrete and appear to be 
a concrete frame, this is seldom the case for modern buildings (post 1940s). For 
the purpose of the RVS procedures, it can be assumed that all exposed concrete 
frames are concrete and not steel frames.

E.7.2 Typical Earthquake Damage 

Under high amplitude cyclic loading, lack of confinement will result in rapid 
disintegration of non-ductile concrete members, with ensuing brittle failure and 
possible building collapse (see Figure E-29). Causes and types of damage 
include: 

1.	Excessive tie spacing in columns can lead to a lack of concrete 
confinement and shear failure. 

2.	Placement of inadequate rebar splices all at the same location in a column 
can lead to column failure. 

3.	Insufficient shear strength in columns can lead to shear failure prior to 
the full development of moment hinge capacity. 

4.	Insufficient shear tie anchorage can prevent the column from developing 
its full shear capacity. 

5.	Lack of continuous beam reinforcement can result in unexpected hinge 
formation during load reversal. 

6.	Inadequate reinforcing of beam-column joints or the positioning of beam 
bar splices at columns can lead to failures. 

7.	The relatively low stiffness of the frame can lead to substantial 
nonstructural damage. 

8. Pounding damage with adjacent buildings can occur. 

E.7.3 Common Rehabilitation Techniques 

Rehabilitation techniques for reinforced concrete frame buildings depend on the 
extent to which the frame meets ductility requirements. The costs associated 
with the upgrading an existing, conventional beam-column framing system to meet 
the minimum standards for ductility are high and this approach is usually not 
cost-effective.  The most practical and cost-effective solution is to add a 
system of shear walls or braced frames to provide the required seismic 
resistance (ATC, 1992).  

E.8 Concrete Shear Wall (C2) 

E.8.1 Characteristics 

This category consists of buildings with a perimeter concrete bearing-wall 
structural system or frame structures with shear walls (Figure E-30). The 
structure, including the usual concrete floor diaphragms, is typically cast in 
place. Before the 1940s, bearing-wall systems were used in schools, churches, 
and industrial buildings. Concrete shear-wall buildings constructed since the 
early 1950s are institutional, commercial, and residential buildings, ranging 
from one to more than thirty stories. Frame buildings with shear walls tend to 
be commercial and industrial. A common example of the latter type is a warehouse 
with interior frames and perimeter concrete walls. Residential buildings of this 
type are often mid-rise towers. The shear walls in these newer buildings can be 
located along the perimeter, as interior partitions, or around the service core. 

Frame structures with interior shear walls are difficult to identify positively. 
Where the building is clearly a box-like bearing-wall structure it is probably a 
shear-wall structure. Concrete shear wall buildings are usually cast in place. 
The screener should look for signs of cast-in-place concrete. In concrete 
bearing-wall structures, the wall thickness ranges from 6 to 10 inches and is 
thin in comparison to that of masonry bearing-wall structures.

E.8.2 Typical Types of Earthquake Damage 

This building type generally performs better than concrete frame buildings. The 
buildings are heavy compared with steel frame buildings, but they are also stiff 
due to the presence of the shear walls. Damage commonly observed in taller 
buildings is caused by vertical discontinuities, pounding, and irregular 
configuration. Other damage specific to this building type includes the 
following. 

1.	During large seismic events, shear cracking and distress can occur around 
openings in concrete shear walls and in spandrel beams and link beams between 
shear walls (See Figures E-31 and E-32.) 

2.	Shear failure can occur at wall construction joints usually at a load 
level below the expected capacity.
 
3.	Bending failures can result from insufficient vertical chord steel and 
insufficient lap lengths at the ends of the walls. 

E.8.3 Common Rehabilitation 

Reinforced concrete shear-wall buildings can be rehabilitated in a variety of 
ways. Techniques include: (1) reinforcing existing walls in shear by applying a 
layer of shotcrete or poured concrete; (2) where feasible, filling existing 
window or door openings with concrete to add shear strength and eliminate 
critical bending stresses at the edge of openings; and (3) reinforcing narrow 
overstressed shear panels in in-plane bending by adding reinforced boundary 
elements (ATC, 1992). 

E.9 
Concrete Frame with Unreinforced Masonry Infill (C3) 

E.9.1 
Characteristics 

These buildings (Figures E-33 and E-34) have been, and continue to be, built in 
regions where unreinforced masonry (URM) has not been eliminated by code. These 
buildings were generally built before 1940 in high-seismicity regions and may 
continue to be built in other regions. 

The first step in identification is to determine if the structure is old enough 
to contain URM. In contrast to steel frames with URM infill, concrete frames 
with URM infill usually show clear evidence of the concrete frames. This is 
particularly true for industrial buildings and can usually be observed at the 
side or rear of commercial buildings. The concrete columns and beams are 
relatively large and are usually not covered by masonry but left exposed. 

A case in which URM infill cannot be readily identified is the commercial 
building with large windows on all sides; these buildings may have interior 
Figure E-34 Blow-up (lower photo) of distant view of URM partitions.  Another 
difficult case occurs when C3 building (upper photo) showing the exterior walls 
are covered by decorative tile or concrete frame with URM infill (left wall),  
stone veneer. The infill material can be URM or a thin concrete infill.

E.9.2 Typical Earthquake Damage 

The hazards of these buildings, which in the western United States are often 
older, are similar to and perhaps more severe than those of the newer concrete 
frames. Where URM infill is present, a falling hazard exists. The failure 
mechanisms of URM infill in a concrete frame are generally the same as URM 
infill in a steel frame.

E.9.3 Common Rehabilitation Techniques

Rehabilitation of unreinforced masonry infill in a concrete frame is identical 
to that of the URM infill in a steel frame. See Section E.6.3. Anchorage of the 
wall panels for out-of-plane forces is the key component, followed by providing 
sufficient shear strength in the building.

E.10 Tilt-up Structures (PC1) 

E.10.1 Characteristics 

In traditional tilt-up buildings (Figures E-35 through E-37), concrete wall 
panels are cast on the ground  and then tilted upward into their final 
positions. More recently, wall panels are  and have become increasingly popular 
in the western and central United States since the 1940s. They are typically one 
and sometimes two stories high and basically have a simple rectangular plan. The 
walls are the lateral-force-resisting system. The roof can be a plywood 
diaphragm carried on wood purlins and glue-laminated (glulam) wood beams or a 
light steel deck and joist system, supported in the interior of the building on 
steel pipe columns. The wall panels are attached to concrete cast-in-place 
pilasters or to steel columns, or the joint is simply closed with a later 
concrete pour. These joints are typically spaced about 20 feet apart. 

The major defect in existing tilt-ups is a lack of positive anchorage between 
wall and diaphragm, which has been corrected since about 1973 in the western 
United States. In the western United States, it can be assumed that all one-
story concrete industrial warehouses with flat roofs built after 1950 are tilt-
ups unless supplementary information indicates otherwise.

E.10.2 Typical Earthquake Damage 

Before 1973 in the western United States, many tilt-up buildings did not have 
sufficiently strong connections or anchors between the walls and the roof and 
floor diaphragms. The anchorage typically was nothing more than the nailing of 
the plywood roof sheathing to the wood ledgers supporting the framing. 

During an earthquake, the weak anchorage broke the ledgers, resulting in the 
panels falling and the supported framing to collapse. When mechanical anchors 
were used they pulled out of the walls or split the wood members to which they 
were attached, causing the floors or roofs to collapse. See Figures E-38 and E-
39. The connections between the concrete panels are also vulnerable to failure. 
Without these connections, the building loses much of its lateral-force-
resisting capacity. For these reasons, many tilt-up buildings were damaged in 
the 1971 San Fernando, California, earthquake. Since 1973, tilt-up construction 
practices have changed in California and other high-seismicity regions, 
requiring positive wall-diaphragm connection. (Such requirements may not have 
yet been made in other regions of the country.) However, a large number of these 
older, pre-1970s-vintage tilt-up buildings still exist and have not been 
rehabilitated to correct this wall-anchor defect. Damage to these buildings was 
observed again in the 1987 Whittier, California, earthquake, 1989 Loma Prieta, 
California earthquake, and the 1994 Northridge, California, earthquake. These 
buildings are a prime source of seismic hazard. 

In areas of low or moderate seismicity, inadequate wall anchor details continue 
to be used. Severe ground shaking in such an area may produce major damage in 
tilt-up buildings.

E.10.3 Common Rehabilitation Techniques 

The rehabilitation of tilt-up buildings is relatively easy and inexpensive. The 
most common form of rehabilitation is to provide a positive anchorage connection 
at the roof and wall intersection. This is usually done by using pre-fabricated 
metal hardware attached to the framing member and to a bolt that is installed 
through the wall. On the outside of the wall a large washer plate is used. See 
Figure E-40 for examples of new anchors. 

Accompanying the anchorage rehabilitation is the addition of ties across the 
building to develop the anchorage forces from the wall panels fully into the 
diaphragm. This is accomplished by interconnecting framing members from one side 
of the building to the other, and then increasing the connections of the 
diaphragm (usually wood) to develop the additional forces.

E.11 Precast Concrete Frame (PC2) 

E.11.1 Characteristics 

Precast concrete frame construction, first developed in the 1930s, was not 
widely used until the 1960s. The precast frame (Figure E-41) is essentially a 
post and beam system in concrete where columns, beams and slabs are 
prefabricated and assembled on site. Various types of members are used. 
Vertical-load-carrying elements may be Ts, cross shapes, or arches and are 
often more than one story in height. Beams are often Ts and double Ts, or 
rectangular sections. Prestressing of the members, including pretensioning and 
post-tensioning, is often employed. The identification of this structure type 
cannot rely solely on construction date, although most precast concrete frame 
structures were constructed after 1960. Some typical characteristics are the 
following. 

1.	Precast concrete, in general, is of a higher quality and precision 
compared to cast-in-place concrete. It is also available in a greater range of 
textures and finishes. Many newer concrete and steel buildings have precast 
concrete panels and column covers as an exterior finish (See Figure E-42). Thus, 
the presence of precast concrete does not necessarily mean that it is a precast 
concrete frame. 

2.	Precast concrete frames are, in essence, post and beam construction in 
concrete. Therefore, when a concrete structure displays the features of a post-
and-beam system, it is most likely that it is a precast concrete frame. It is 
usually not economical for a conventional cast-in-place concrete frame to look 
like a post-and-beam system. Features of a precast concrete post-and-beam system 
include: 

a.	exposed ends of beams and girders that project beyond their supports or 
project away from the building surface, 

b. the absence of small joists, and 

c.	beams sitting on top of girders rather than meeting at a monolithic joint 
(see Figure E-43) 

The presence of precast structural components is usually a good indication of 
this system, although these components are also used in mixed construction. 
Precast structural components come in a variety of shapes and sizes. The most 
common types are sometimes difficult to detect from the street. Less common but 
more obvious examples include the following. 

a.	Ts or double Ts-These are deep beams with thin webs and flanges and with 
large span capacities. (Figure E-44 shows one end of a double-T beam as it is 
lowered onto its seat.) 

b. Cross or T-shaped units of partial columns and beams - These are structural 
units for constructing moment-resisting frames. They are usually joined together 
by field welding of steel connectors cast into the concrete. Joints should be 
clearly visible at the mid-span of the beams or the mid-height of the columns. 
See Figure E-45. 
c. Precast arches-Precast arches and pedestals are popular in the architecture 
of these buildings. 

d. Column-When a column displays a precast finish without an indication that it 
has a cover no vertical seam can be found), the column is likely to be a precast 
structural column. It is possible that a precast concrete frame may not show any 
of the above features, however.

E.11.2 Typical Earthquake Damage 

The earthquake performance of this structural type varies widely and is 
sometimes poor. This type of building can perform well if the detailing used to 
connect the structural elements have sufficient strength and ductility 
(toughness). Because structures of this type often employ cast-in-place concrete 
or reinforced masonry (brick or block) shear walls for lateral-load 
resistance, they experience the same types of damage as other shear-wall 
building types. 

Some of the problem areas specific to precast frames are listed below. 

1.	Poorly designed connections between prefabricated elements can fail. 

2.	Accumulated stresses can result due to shrinkage and creep and due to 
stresses incurred in transportation. 

3.	Loss of vertical support can occur due to inadequate bearing area and 
insufficient connection between floor elements and columns. 

4.	Corrosion of the metal connectors between prefabricated elements can 
occur. 

E.11.3 Common Rehabilitation Techniques 

Seismic rehabilitation techniques for precast concrete frame buildings are 
varied, depending on the elements being strengthened. Inadequate shear capacity 
of floor diaphragms can be addressed by adding reinforced concrete topping to an 
untopped system when E: Characteristics and Earthquake Performance of RVS 
Building Types FEMA 154 possible, or adding new shear walls to reduce the 
seismic shear forces in the diaphragm.  Corbels with inadequate vertical shear 
or bending strength can be strengthened by adding epoxied horizontal shear 
dowels through the corbel and into the column.  Alternatively, vertical shear 
capacity can be increased by adding a structural steel bolster under the corbel, 
bolted to the column, or a new steel column or reinforced concrete column can be 
added (ATC, 1992). 

E.12	Reinforced Masonry (RM1 and RM2) 

E.12.1 Characteristics 

Reinforced masonry buildings are mostly low-rise structures with perimeter 
bearing walls, often with wood diaphragms (RM1 buildings) although precast 
concrete is sometimes used (RM2 buildings). Floor and roof assemblies usually 
consist of timber joists and beams, glued-laminated beams, or light steel 
joists. The bearing walls consist of grouted and reinforced hollow or solid 
masonry units. Interior supports, if any, are often wood or steel columns, 
wood stud frames, or masonry walls. Occupancy varies from small commercial 
buildings to residential and industrial buildings. Generally, they are less than 
five stories in height although many taller masonry buildings exist. Reinforced 
masonry structures are usually basically rectangular structures (See Figure E-
46). 

To identify reinforced masonry, one must determine separately if the building is 
masonry and if it is reinforced. To obtain information on how to recognize a 
masonry structure, see Appendix D, which describes the characteristics of 
construction materials. The best way of assessing the reinforcement condition is 
to compare the date of construction with the date of code requirement for the 
reinforcement of masonry in the local jurisdiction. The screener also needs to 
determine if the building is veneered with masonry or is a masonry building. 
Wood siding is seldom applied over masonry. If the front facade appears to 
be reinforced masonry whereas the side has wood siding, it is probably a wood 
frame that has undergone facade renovation. The back of the building should be 
checked for signs of the original construction type. 

If it can be determined that the bearing walls are constructed of concrete 
blocks, they may be reinforced. Load-bearing structures using these blocks are 
probably reinforced if the local code required it. Concrete blocks come in a 
variety of sizes and textures. The most common size is 8 inches wide by 16 
inches long by 8 inches high. Their presence is obvious if the concrete blocks 
are left as the finish surface.

E.12.2 Typical Earthquake Damage 

Reinforced masonry buildings can perform well in moderate earthquakes if they 
are adequately reinforced and grouted, and if sufficient diaphragm anchorage 
exists. A major problem is control of the workmanship during construction.   
Poor construction practice can result in ungrouted and unreinforced walls. Even 
where construction practice is adequate, insufficient reinforcement in the 
design can be responsible for heavy damage of the walls. The lack of positive 
connection of the floor and roof diaphragms to the wall is also a problem.

E.12.3 Common Rehabilitation Techniques 

Techniques for seismic rehabilitation of reinforced masonry bearing wall 
buildings are varied, depending on the element being rehabilitated.  Techniques 
for rehabilitating masonry walls include: (1) applying a layer of concrete or 
shotcrete to the existing walls; (2) adding vertical reinforcing and grouting 
into ungrouted block walls; and (3) filling in large or critical openings with 
reinforced concrete or masonry dowelled to the surrounding wall. Wood or steel 
deck diaphragms in RM1 buildings can be rehabilitated by adding an additional 
layer of plywood to strengthen and stiffen an existing wood diaphragm, by shear 
welding between sections of an existing steel deck or adding flat sheet steel 
reinforcement, or by adding additional vertical elements (for example, shear 
walls or braced frames) to decrease diaphragm spans and stresses.  Precast floor 
diaphragms in RM2 buildings can be strengthen by adding a layer of concrete 
topping reinforced with mesh (if the supporting structure has the capacity to 
carry the additional vertical dead load), or by adding new shear walls to reduce 
the diaphragm span (ATC, 1992).

E.13 Unreinforced Masonry (URM) 

E.13.1 Characteristics 

Most unreinforced masonry (URM) bearing-wall structures in the western United 
States (Figures E-47 through E-51) were built  before 1934, although this 
construction type was permitted in some jurisdictions having moderate or high 
seismicity until the late 1940s or early 1950s (in some jurisdictions URM may 
still be a common type of construction, even today). These buildings usually 
range from one to six stories in height and function as commercial, residential, 
or industrial buildings. The construction varies according to the type of use, 
although wood floor and roof diaphragms are common. Smaller commercial and 
residential buildings usually have light wood floor joists and roof joists 
supported on the typical perimeter URM wall and interior, wood, load-bearing 
partitions. Larger buildings, such as industrial warehouses, have heavier floors 
and interior columns, usually of wood. The bearing walls of these industrial 
buildings tend to be thick, often as much as 24 inches or more at the base. Wall 
thickness of residential, commercial, and office buildings range from 9 inches 
at upper floors to 18 inches a lower floors. 

The first step in identifying buildings of this type is to determine if the 
structure has bearing walls. Second, the screener should determine the 
approximate age of the building. Some indications of unreinforced masonry are 
listed below. 

1.	Weak mortar was used to bond the masonry units together in much of the 
early unreinforced E: Characteristics and Earthquake Performance of RVS Building 
Types FEMA 154 masonry construction in the United States. As the poor earthquake 
performance of this mortar type became known in the 1930s, and as cement mortar 
became available, this weaker mortar was not used and thus is not found in more 
recent masonry buildings. If this soft mortar is present, it is probably URM. 
Soft mortar can be scratched with a hard instrument such as a penknife, 
screwdriver, or a coin. This scratch testing, if permitted, should be done in 
a wall area where the original structural material is exposed, such as the sides 
or back of a building. Newer masonry may be used in renovations and it may look 
very much like the old. Older mortar joints can also be repointed (i.e., regular 
maintenance of the masonry mortar), or repaired with newer mortar during 
renovation. The original construction may also have used a high-quality mortar. 
Thus, even if the existence of soft mortar cannot be detected, it may still be 
URM. 

2.	An architectural characteristic of older brick bearing-wall structures is 
the arch and flat arch window heads (see Figure E-52). These arrangements of 
masonry units function as a header to carry the load above the opening to either 
side. Although masonry-veneered wood-frame structures may have these features, 
they are much more widely used in URM bearing-wall structures, as they were the 
most economical method of spanning over a window opening at the time of 
construction. Other methods of spanning are also used, including steel and stone 
lintels, but these methods are generally more costly and usually employed in the 
front facade only.

3.	Some structures of this type will have anchor plates visible at the 
floor and roof lines, approximately 6-10 feet on center around the perimeter of 
the building. Anchor plates are usually square or diamond-shaped steel plates 
approximately 6 inches by 6 inches, with a bolt and nut at the center. Their 
presence indicates anchor ties have been placed to tie the walls to the floors 
and roof. 

These are either from the original construction or from rehabilitation under 
local ordinances. Unless the anchors are 6 feet on center or less, they are not 
considered effective in earthquakes. If they are closely spaced, and appear to 
be recently installed, it indicates that the building has been rehabilitated. In 
either case, when these anchors are present all around the building, the 
original construction is URM bearing wall.

4. When a building has many exterior solid walls constructed from hollow clay 
tile, and no columns of another material can be detected, it is probably not a 
URM bearing wall but probably a wood or metal frame structure with URM infill.
 
5.	One way to distinguish a reinforced masonry building from an unreinforced 
masonry building is to examine the brick pattern closely. Reinforced masonry 
usually does not show header bricks in the wall surface. If a building does not 
display the above features, or if the exterior is covered by other finish 
material, the building may still be URM.

E.13.2 Typical Earthquake Damage 

Unreinforced masonry structures are recognized as the most hazardous structural 
type. They have been observed to fail in many modes during past earthquakes. 
Typical problems include the following. 

1.	Insufficient Anchorage-Because the walls, parapets, and cornices are not 
positively anchored to the floors, they tend to fall out. The collapse of 
bearing walls can lead to major building collapses. Some of these buildings have 
anchors as a part of the original construction or as a rehabilitation. These 
older anchors exhibit questionable performance. (See Figure E-53 for parapet 
damage.) 

2.	Excessive Diaphragm Deflection-Because most of the floor diaphragms are 
constructed of finished wood flooring placed over 3/4"-thick wood sheathing, 
they tend to be stiff compared with other types of wood diaphragms. This 
stiffness results in rotations about a vertical axis, accompanying translations 
in the direction of the open front walls of buildings, due to a lack of in-plane 
stiffness in these open fronts. Because there is little resistance in the 
masonry walls for out-of-plane loading, the walls allow large diaphragm 
displacements and cause the failure of the walls out of their plane. Large 
drifts occurring at the roof line can cause a masonry wall to overturn and 
collapse under its own weight. 

3.	Low Shear Resistance-The mortar used in these older buildings was often 
made of lime and sand, with little or no cement, and had very little shear 
strength. The bearing walls will be heavily damaged and collapse under large 
loads. (See Figure E-54) 

4.	Slender Walls -Some of these buildings have tall story heights and thin 
walls. This condition, especially in non-load-bearing walls, will result in 
buckling out-of-plane under severe lateral load. Failure of a non-load-bearing 
wall represents a falling hazard, whereas the collapse of a load-bearing wall 
will lead to partial or total collapse of the structure. 

E.13.3 Common Rehabilitation Techniques 

Over the last 10 years or more, jurisdictions in California have required that 
unreinforced masonry bearing-wall buildings be rehabilitated or demolished. To 
minimize the economical impact on owners of having to rehabilitate their 
buildings, many jurisdictions implemented phased programs such that the critical 
items were dealt with first. The following are the key elements included in a 
typical rehabilitation program. 

1.	Roof and floor diaphragms are connected to the walls for both anchorage 
forces (out of the plane of the wall) and shear forces (in the plane of the 
E: Characteristics and Earthquake Performance of RVS Building Types FEMA 154 
wall). Anchorage connections are placed at 6 feet spacing or less, depending on 
the force requirements. Shear connections are usually placed at around 2 feet 
center to center. Anchors consist of bolts installed through the wall, with 6-
inch-square washer plates, and connected to hardware attached to the wood 
framing. Shear connections usually are bolts embedded in the masonry walls in 
oversized holes filled with either a non-shrink grout or an epoxy adhesive. See 
Figure E-55. 

2.	In cases when the height to thickness ratio of the walls exceeds the 
limits of stability, rehabilitation consists of reducing the spans of the wall 
to a level that their thickness can support. Parapet rehabilitation consists of 
reducing the parapet to what is required for fire safety and then bracing from 
the top to the roof. 
3.	If the building has an open storefront in the first story, resulting in a 
soft story, part of the storefront is enclosed with new masonry or a steel frame 
is provided there, with new foundations. 

4.	Walls are rehabilitated by either closing openings with reinforced masonry 
or with reinforced gunite.