Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanBEHAVIOR AND DESIGN OF STRUCUTRAL WALLS –LESSONS FROM RECENT LABORATORY TESTS &EARTHQUAKESJohn WALLACE1 and Jack MOEHLE2
1

Professor, Department of Civil Engineering, University of California, Los Angeles,
Los Angeles, CA, USA, wallacej@ucla.edu
2Professor, Department of Civil Engineering, University of California, Berkeley,
Berkeley, CA, USA, moehle@berkeley.edu

ABSTRACT: Design and construction practice for structural walls has evolved significantly

over the last 20 years and engineers have pushed design limits in recent years, optimizing
economy and design, and in many practices producing walls with higher demands and more
slender profiles than have been verified in past laboratory testing or field experience. Wall
performance in recent earthquakes and laboratory tests is reviewed and US code provisions
are reassessed to identify possible shortcomings and improvements.
Key Words: 2010 Chile earthquake, structural wall, shear wall, testing, splice, ACI 318,
detailing, displacement-based, slenderness, instability

INTRODUCTION
Design and construction practice for special structural walls (ACI 318 designation) has evolved
significantly since the system was introduced in the 1970’s. Throughout the 1970s and 1980s, it wascommon to use so-called barbell-shaped wall cross sections, where a “column” was used at each wallboundary to resist axial load and overturning along with a narrow wall web. In the late 1980s and early
1990s, use of rectangular wall cross sections became common to produce more economical designs.
Use of walls with boundary columns is still common in Japan; however, based on information
available in the literature, the AIJ Standard for “Structural Calculations of Reinforced Concrete
Buildings” was revised in 2010 to show RC walls with rectangular cross-sections. Engineers aroundthe world have pushed design limits in recent years, optimizing economy and design, and in many
practices producing walls with higher demands and more slender profiles than have been verified inpast laboratory testing or field experience. The trend towards more slender profiles has been
accelerated by use of higher concrete strengths.
Observed wall damage in recent earthquakes in Chile (2010) and New Zealand (2011), wheremodern building codes exist, exceeded expectations. In these earthquakes, structural wall damage1132

included boundary crushing, reinforcement fracture, and global wall buckling. Recent tests of isolated
structural walls in the US and tests of two, full-scale 4-story buildings with high-ductility structural
walls at E-Defense in December 2010 provide vital new data. A particularly noteworthy aspect ofthese recent tests is the failure of relatively thin wall boundaries to develop ductile behavior incompression, even though they complied with building code provisions and recommendations of ACI
and AIJ.
The observed performance following recent earthquakes and in recent laboratory tests strongly
suggest that the problems observed are not isolated and that analysis and design provisions need to bereassessed. In particular, the quantity and configuration of transverse reinforcement required at wallboundaries needs to be reassessed to address issues associated with wall thickness, slenderness, axialload, and configuration, as well as expected displacement demands and load history. Preliminary
studies indicate that greater amounts of transverse reinforcement may be required for thin walls orwalls with large cover and that tighter spacing of transverse reinforcement may be required to suppress
buckling of vertical reinforcement, especially for walls with light axial load or walls with flanges.
These findings apply to both high ductility (ACI Special) and low ductility (ACI Ordinary) walls.
OBSERVED PERFORMANCE OF STRUCTUAL WALLS
Recent Earthquake Reconnaissance
Recent earthquakes in Chile (Mw 8.8, February 2010), New Zealand (February 2011, ML=6.3), andJapan (Mw 9.0, March 2011) have provided a wealth of new data on the performance of modernbuildings that utilize structural walls for the primary lateral-force-resisting system. Although complete
building collapse was rarely observed, damage was widespread and generally exceeded expectations.
In 1996, Chile adopted a new code (NCh 433.Of96, 1996) based on ACI 318-95 and produced animmense inventory of progressively more slender buildings corresponding essentially to the US
reinforced concrete code provisions, except boundary element confinement was not required. The
2010 Mw 8.8 earthquake caused serious damage to many of these buildings, including
crushing/spalling of concrete and buckling of vertical reinforcement, often over a large horizontal
extent of the wall (Fig. 1). Damage tended to concentrate over a relatively short height of one to threetimes the wall thickness, apparently because buckling of vertical bars led to concentration of damage.
Closer inspection of the wall boundary regions (Figure 1) revealed the relatively large spacing ofhoops (20 cm) and horizontal web reinforcement (20 cm), as well as the 90-degree hooks used onhoops and horizontal web reinforcement, which may have opened due to concrete crushing and/or
buckling of vertical reinforcement (Fig. 1d). Some of the failures are attributable to lack ofclosely-spaced transverse reinforcement at wall boundaries, which was not required by the Chilean
code based on the good performance of buildings in the 1985 M7.8 earthquake; however, many of the
failures are not yet understood, and many suggest that there are deficiencies in current US design
provisions (Wallace, 2011; Massone and Wallace, 2011). In some cases, lateral instability (buckling) ofa large portion of a wall section was observed (Fig. 2); prior to the Chile and New Zealandearthquakes, this global buckling failure had been primarily observed in laboratory tests (e.g.,Thomsen and Wallace, 2004). Detailed surveys conducted as part of ATC-94 (2011) indicate that
global wall buckling was not driven by prior yielding in tension (as had originally been suspectedbased on past research, e.g., Corley et al., 1981; Paulay and Priestley, 1993; Chai and Elayer, 1999)
but instead was the result of lateral instability of previously crushed boundary zones. Furthermore, theATCI-94 study has been unable to establish through analysis the role of pre-emptive longitudinal bar
buckling as a trigger for compression failure of lightly confined boundary zones. Laboratory testing isrequired to understand these behaviors; preliminary studies are underway in Chile and the US toinvestigate these issues.
The 2011 Christchurch earthquake (EERI, 2011; NZRC, 2011) shows many similar wall failures,
suggesting the deficiencies observed in the 2010 Chile earthquake are not isolated (Fig. 3a). All of thewalls depicted in Fig. 2 and 3 have either T-shaped (Fig. 2, 3b) or L-shaped (Fig. 3a) cross sections,which leads to large cyclic tension and compressive demands at the wall web boundary (Wallace, 1996).

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The wall web boundaries are apparently susceptible to out-of-plane buckling following cover concrete
spalling. Although current ACI 318-11 provisions require consideration of an effective flange width,
the provisions do not restrict use of narrow walls and do not address this out-of-plane failure mode, i.e.,there are no restrictions on wall thickness or wall slenderness. Failure of diaphragm-to-wallconnections were observed in Christchurch, potentially contributing to the collapse of the several
buildings (Elwood, 2011). In Chile, typical buildings have a large number of walls thatwell-distributed in plane; therefore, diaphragm failures were not observed.
(a) Viña del Mar(b) Santiago(c) Concepción(d) Boundary zone details

Figure 1 Typical wall damage in Chile Earthquake

Figure 2 Wall lateral instability

Recent laboratory
conventional walls

studiesof

Recent laboratory testing ofstructural walls in the US has
focused on addressing concerns
related to behavior of walls withrectangular and T-shaped crosssections subjected to uniaxial andbiaxial loading (Waugh et al., 2008;
Waugh and Sritharan, 2010;
Brueggen and French, 2010), withcouplers and splices in the plastic
hinge region (Johnson, 2010, Birely
et al., 2010), with higher shear
demands (Birely et al., 2008; Birely
et al., 2010; Sriram and Sritharan,
2010), and with coupling beams
Figure 3a Wall failure in
Fig. 3b Specimen TW2 web(NaishandWallace,2010;2011 Christchurch earthquake boundary failure (Thomsen
Para-Montisenos et al, 2012;(Elwood, 2011)
and Wallace, 2004)
Lehman and Lowes, 2011). All ofthese studies involved quasi-static testing. Shake table testing of walls has been limited, except for
7-story “building slice” tests of walls with rectangular and T-shaped cross sections conducted by
Panagiotou and Restrepo (2007). The overwhelming majority of quasi-static and shake table tests
conducted in Japan have been conducted on barbell-shaped walls and low-rise buildings with “wingwalls” (e.g., Kabeyasawa et al., 2008, 2010(a), 2010(b)), which are not common in the US. Only
recently have the Japanese Building Standard Law and Architectural Institute of Japan (AIJ, 2010)
recommendations been modified to allow the use of rectangular walls with boundary elements, but1134

their use is not widespread.
Johnson (2010) reports test results isolated, slender (hw/lw and Mu/Vulw = 2.67) cantilever walls toinvestigate the behavior of anchorage details. Three walls were tests, one each with continuous (RWN),
coupled (RWC), and spliced (RWS) vertical reinforcement. The wall cross sections were 6 in. x 90 in.
(152.4 mm x 2.29m), and the walls were subjected to horizontal lateral load approximately 20ft or
6.1m above the base). Although the wall cross-sections were rectangular, different amounts ofboundary vertical reinforcement were used to simulate the behavior of T-shaped wall cross sections;4-#6 (db=19mm) and 2-#5 (db=15.9mm) at one boundary and 8-#9 (db=28.7mm) at the other boundary.
Horizontal wall web reinforcement, of #3 @7.5 in. or ρt = 0.0049 (db=9.5mm @ 19cm), was selected
to resist the shear associated with the expected moment strength (including overstrength). Wall web
vertical reinforcement consisted of #4 @18 in. or ρv = 0.0037 (db=12.7mm @ 45.7cm). It is noted thatthe 18 in. (45.7cm) spacing of vertical web reinforcement is the maximum spacing allowed by ACI
318-11 21.9.2.1. It is questionable whether such a large spacing (45.7cm) in such a thin wall (15.2cm),
satisfies the intent of R21.9.4, which states that wall we reinforcement should be “appropriately
distributed along the length and height of the wall… should be uniform and at a small spacing.”
Because a construction joint exists at the foundation-wall interface, shear friction requirements of ACI
318-11 11.6.4 should be satisfied. Given that 4 pairs of #4 vertical bars are used, the shear friction
strength Vn is 96k (427kN) and 58k (256kN) for a coefficient of friction μ = 1.0 and 0.6, respectively(neglecting the boundary vertical reinforcement). Lateral load versus top lateral displacement relations
for RWC and RWS are plotted in Fig. 4a; since results for RWC and RWN are very similar. For RWC,
the wall reach rotations exceeding +0.035 (#5 in tension) and -0.02 (#9 in tension), whereas for RWS,
the wall reach rotations of approximately +0.02 (#5 in tension) and -0.012 (#9 in tension). Damage
was concentrated at a single, large crack at the foundation-wall interface, which accounted for about
0.015 rotation at a top rotation of 0.02. The crack width reached more than It is noted that the applied
shear is close to or exceeds the shear friction capacity Vn of the walls, depending on the direction of
the applied load and the value assumed for the coefficient of friction. Significant horizontal cracking
also was observed for specimens RWN and RWC, suggesting that the quantity (and large spacing)vertical web reinforcement was insufficient to restrain sliding, and damage concentrated at thefoundation-wall interface for specimen RWS (Fig. 4b). However, the test results do indicate that thepresence of the splice significantly reduced the wall lateral deformation capacity.

Fig. 4aLoad-displacement relations

Fig. 4b Wall damage at end of test (RWS)

Tests of walls with splices also were conducted by Birely et al. (2010). The test specimens wereroughly one-half scale replicas of the bottom three stories of a ten-story wall (Fig. 5a. Base shear
versus 3rd story (top) displacement plots are shown in Fig. 5b for three of the tests, PW1 (splice,
Mb=0.71hwVb), W2 (splice, Mb=0.50hwVb), and W4 (no splice, Mb=0.50hwVb). Design wall shear
stresses were 0.23, 0.33, and 0.33 f c' MPa MPa for W1, W2, and W4, respectively (equivalent to 0.7,
0.9, and 0.9Vn). The #4 (db=12.7mm) boundary bars were lapped 0.61m, with spacing of boundary1135

transverse reinforcement of 51mm (s/db =4). The test with lower shear stress was reasonably ductile,achieving 1.08Mn and a 3rd story lateral drift of 1.5% prior to strength loss; however, test PW4, withno splice, reached only 1.0% lateral drift at the third story (top) prior to strength loss. For all tests withsplices, damage initiated with buckling of the interior bar at the wall edge (Fig. 6a) and thenconcentrated at the top of the splices (Fig. 6b), whereas damage was concentrated at thefoundation-wall interface for test PW4 with no splice (Fig. 6c). Even without consideration of theelastic deformations over the top seven stories not included in the test, deformation capacities of thewalls are less than expected, especially for PW4, with no splice.

Fig. 5a NEESR UW wall tests (Lowes et al, 2011)

Fig. 5b Base Shear vs Drift

Figure 6 Wall damage: (a) PW2 @ 1.0% drift; (b) PW2 end of test; (c) PW4 @ 1.0% drift

Nagae, et al. (2011) reports E-Defense tests on two 4-story buildings, one conventionally reinforced
and the other using high-performance RC construction, both with rectangular wall cross sections (Fig.
7a). The conventionally reinforced wall had confinement exceeding U.S. requirements, with axial loadof approximately 0.03Ag f c' , yet the compression boundary zone sustained localized crushing andlateral buckling (Fig. 7b, following Kobe 100% motion). The base overturning moment versus roof
displacement responses are plotted in Fig. 8; base rotations are slightly less than the roof drift ratio(e.g., for Kobe 100%, the base rotation measured over 0.27lw is a little more than 0.02). Following
crushing of boundary regions, sliding shear responses increased substantially during the Kobe 100%
test (Fig. 8). Sliding displacements in the Takatori 60% test reached the limits of the sensor, +45mm
and -60mm with peak shear of +/- 2000 kN. It is noted that the relatively large clear cover over theboundary longitudinal bars was used (~40mm) and the boundary transverse reinforcement wasinsufficient to maintain the boundary compressive load following cover spalling. It is noted that the
crushing/spalling of the boundary region was accompanied by lateral buckling of the compressionzone, as was observed in Chile and New Zealand (Fig. 2).

1136

Figure 7a RC conventional wall (Nagae et al., 2011)

Figure 7b Wall damage

Figure 8 RC conventional building responses (structural wall direction)

Exploratory tests on prisms (Moehle, 2010) also
showed a tendency for thin wall boundaries tobuckle over an extended height of the wall (Fig. 9).
Two buckling mechanisms may occur observed. If awall segment is subjected to plastic tensile straining,
the pre-cracked boundary zone becomes a relatively
flexible element that might buckle globally undercertain conditions. (This type of behavior wasobserved in past laboratory tests, and has beenstudied analytically – see Corley et al., 1981; Paulayand Priestley, 1993; Chai and Elayer, 1999). A
second global buckling mode begins with spallingof cover concrete, leaving a relatively thin core withlongitudinal reinforcement that tends to buckle
laterally, displacing the remainder of the wall. As
noted in the section on recent earthquake
reconnaissance, the latter mode was widely
observed in the 2010 Chile earthquake, and also for
the E-Defense test. This latter buckling mode has
not been studied previously.

1137

Figure 9 Exploratory prism tests

The pre-NEESR tests conducted at NEES@Minnesota (Brueggan, 2009; Waugh and Sritharan,
2010; Brueggen and French, 2010) studied the role of biaxial loading by subjecting cantilever walls withT-shaped cross sections to biaxial loading and comparing their results with similar tests subjected toin-plane loading (Thomsen and Wallace, 2004). The 6 in (152.4mm) thick walls exhibited rotations overthe first story (hs = 0.8lw) of approximately 0.02 prior to lateral strength degradation. Their findings
suggest that analytical models validated previously for in-plane loading adequately captured themeasured responses for combined in- and out-of-plane loading. However, based on video and post-test
observations, damage at wall boundaries of the conventional reinforced concrete building tested on the
E-Defense shaking table may have been influenced by simultaneous in-plane and out-of-plane
responses. The New Zealand Royal Commission report (NZRC, 2011) raises the issue of biaxial loading
as a possible contributing factor to the unexpected wall damage in the February 2011 earthquake. Thisissue has not been adequately studied, and the issue is complicated by the observation that out-of-plane
failures are observed at wall boundaries (and prisms) for in-plane loads alone.

Fig. 10 Load vs displacement relations: (a) web direction; (b) Flange direction (Brueggen, 2009)

Recorded Ground Motions

Response Spectra computed using ground motions recorded in recent earthquakes have significantly
exceeded values used for design. For example, spectra for records in Chile (Boroschek et al, 2010) andChristchurch (Elwood, 2011) significantly exceed values used for design (Fig. 11). For Chile, a vast
majority of buildings are designed for the Soil II spectrum, whereas spectral ordinates are generally 2
to 6 times the values for Soil II over a broad period range. Given such large demands it is important tore-evaluate how displacement demands influence design requirements for structural walls.

2

Sa (g)

1.5

Berkeley DBE

1Christchurch 500 yrs

0.5

Sacramento DBE

0
0

0.5

1

1.5

2

2.5

3

3.5

Period (s)(a) Chile displacement spectra(b) Christchurch acceleration spectra (Elwood, 2011)
Fig. 11 Spectra from recent large earthquakes

1138

4

Summary

Wall performance in recent earthquakes and laboratory tests raise a number of design concerns. In
Chile, brittle failures at wall boundaries were likely influenced by the level of axial stress (possibly
leading to compression failures), the larger than expected displacement demands, the use of
unsymmetric wall cross sections, and the lack of closely-spaced transverse reinforcement at wallboundaries. A particularly noteworthy aspect of recent tests (Nagae et al., 2011; Lehman and Lowes,
2011; Moehle et al., 2010) is the failure of relatively thin wall boundaries to develop ductile behavior
in compression, even though they complied with ACI 318 special boundary element requirements, aswell as Japan Standard Building Law and AIJ (2010) requirements. Recent tests to investigate the roleof splices within the plastic hinge region of structural walls suggest that splices will substantiallyreduce wall inelastic deformation capacity. Given these observations, current ACI 318-11 codeprovisions for Special Structural Walls are reviewed to identify possible concerns and to suggest
changes that could be implemented to address these concerns.
ACI 318 CHAPTER 21 CODE PROVISIONS FOR SPECIAL STRUCTURAL WALLS

Provisions for “Special Structural Walls” are contained in ACI 318-11 §21.9 and include provisions for
Reinforcement (21.9.2), Shear Strength (21.9.4), Design for Flexural and Axial Loads (21.9.5), andBoundary Elements of Special Structural Walls (21.9.6). In light of the prior discussion, key aspects ofthese provisions are reviewed and areas of concern are noted. In many cases, insufficient information
is available to develop comprehensive requirements and comments provided here are advisory.
Reinforcement and splices

A single curtain of web reinforcement is allowed if the wall shear stress is less than 0.17 f c' MPa MPa .
This provision is acceptable for squat walls with low shear stress (e.g., walls with aspect ratio less than1.5); however, for slender walls where buckling of boundary vertical reinforcement and lateral
instability are more likely due to significant tensile yielding of reinforcement under cyclic loading,two curtains should always be used. This recommendation applies to both Special Structural Walls(high ductility) and Ordinary Structural Walls (moderate ductility).
Recent laboratory tests have identified that wall deformation capacity may be compromised incases where splices exist within the wall critical section (plastic hinge) because nonlinear
deformations are concentrated outside of the splice region, either at the wall-foundation interface(large moment gradient; Johnson, 2010) or above the splice (nearly uniform wall moment; Birely et al.,2010). Given these results, it is questionable whether boundary vertical reinforcement should belapped spliced within the plastic hinge region. Test results did indicate that use of ACI 318-11 Type II
couplers performed adequately. The option of staggering splices is not addressed here.
Design displacement and plastic hinge length
The model used to develop ACI 318-11 §21.9.6.2 provisions is shown in Figure 12. Given this model, thedesign displacement δ u ( ACI ) ≡ δ x = Cd δ e I ( ASCE 7) is related to local plastic hinge rotation and extreme
fiber compressive strain as:⎡δ ⎤ ⎡ c ⎤
l ⎞
ε ⎞⎛
⎛
(1)
; θ p = ⎜ φu = c ⎟ ⎜ l p = w ⎟ ∴ ε c = 2 ⎢ u ⎥ ⎢ ⎥
2
c
⎝
⎠⎝
⎠
⎣ hw ⎦ ⎣ lw ⎦If the compressive strain exceeds a limiting value, typically taken as 0.003, then special transverse
reinforcement is required. In ACI 318-11 Equation (21-8), this approach is rearranged to define alimiting neutral axis depth versus a limiting concrete compressive strain as:

θp =

δu

hw

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climit =0.003lwlw
lw=≈
2 (δ u hw ) 667 (δ u hw ) 600 (δ u hw )(2)

In this approach, it is obvious that the result is sensitive to the values used for the design displacement
and the plastic hinge length. Revised formulations, using a detailed displacement-based design
approach (Wallace and Orakcal, 2002) and a plastic hinge length that varies with wall thickness(Wallace, 2011), produces the following relation:

ε sy ⎛ 11 hw⎛ t l ⎞⎛ α t w ⎞
t
t t ⎞
(3)
= ε cu ⎜ α w w ⎟⎜ 1 −−α w +α2 w w ⎟⎟+⎜2 hw ⎠ (1 − c lw ) ⎝ 40 lw
hw
lw
hw lw ⎠
⎝ lw c ⎠⎝Where tw is the wall thickness, c is the neutral axis depth, hw is the wall height, lw is the wall length,δuand ε sy is the tensile reinforcement yield strain. The constant 11/40 results based on the assumed
distribution of lateral force over the height of the wall (Wallace and Moehle, 1992). In (3), therelationship between the wall neutral axis depth, concrete compressive strain, and drift is computed forvarious ratios of lw tw and hw lw for the three assumed values of plastic hinge length. For thispreliminary study, wall aspect ratio hw lw is set to 3.0 and the ratio of lw t w is set to 13.3 for U.S.
construction. Concrete compressive strain is set to 0.003; results presented in Fig. 13 define when
special transverse reinforcement would be required at wall boundaries for three plastic hinge lengths.

δu

φu
lp
lp

θp

Fig. 12 ACI 318-11 §21.9.6.2 modelFig. 13 Influence of plastic hinge length on need for SBEs

According to Fig. 13, if the drift ratio is 0.01, the neutral axis must exceed 0.17lw before SBE arerequired by ACI 318-11. However, for the same neutral axis depth of 0.17lw, if inelastic deformations
are concentrated over a short height ( l p = (a = 2)tw ) , only less than one-half of this drift ratio (0.005),
can be tolerated before SBEs are required. The sensitivity of the results suggests that measures areneeded to ensure appropriate spread of plasticity by requiring walls to be tension-controlled or by
ductile yielding of concrete in compression.
In current US codes the intent is to provide 90% confidence of non-collapse for MCE shaking. In
contrast, the current ACI confinement trigger is based on 50% confidence of not exceeding theconcrete crushing limit in the Design Basis Earthquake (which is much lower shaking intensity thanthe MCE). To address this issue, it is necessary to adjust the Equation (21-8) to be more consistent
with the building code performance intent. Three factors need to be considered: 1) MCE exceeds DBE.
2) There is dispersion about the median response. 3) Damping is likely to be lower than the 5% value
assumed in the ACI provisions. To address these issues, the coefficient of 600 in the denominator of
Equation (21-8) in ACI 318-11 should be increased by a factor of approximately 1.5 to adjust to MCE
level shaking and to consider dispersion, and by approximately 1.2 to 1.3 to account for potential
lower damping ratios; therefore, a coefficient of 1000 to 1200 is more needed.

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Axial load and compression-controlled walls:

As noted above, the provisions of 318-11 §21.9.6.2 assume that nonlinear deformations within thecritical (plastic hinge) region of the wall will spread out over a distance equal to one half the member
depth. ACI 318-11 §9.4 defines tension- and compression-controlled sections; however, no guidance isprovided on how these requirements should be applied to special (or ordinary) structural walls. In
addition, ACI 318 and ASCE 7 do not place limits on wall axial stress. The performance of walls inChile suggests that higher axial stresses and wall cross section shape (e.g., T-shaped) may lead to cases
where concrete compressive strain reaches 0.003 prior to yield of tension steel.
Various approaches could be used to address this issue, such as placing limit on axial stress or
requiring wall critical sections to be tension-controlled. In the 1997 version of the Uniform Building
Code, wall axial load was limited to 0.35P0; for higher axial loads the lateral strength and stiffness of
the wall could not be considered. An alternative to neglecting the lateral-force-resistance ofcompression-controlled walls would be to impose more stringent design requirements, such as always
requiring Special Boundary Elements (SBEs) for wall critical sections that are not tension-controlled,using the same definition of tension-controlled used for beams, i.e., where the reinforcement tensile
strain exceeds 0.005. In addition, it also might be necessary to impose a larger minimum wall
thickness (tw) and a smaller wall slenderness ratio (hs/tw) for compression-controlled walls. The
objective of these requirements would be to maintain a stable compressive zone as the concrete yields
in compression.
Even with more stringent design requirements for compression-controlled wall sections, it may not
be possible to expect significant inelastic deformation capacity (rotation) can be achieved throughcompression yielding. That is, it might be prudent to limit the nonlinear deformations. This objective
can be accomplished by placing a limit on concrete compressive strain using Equation (1). For anassumed neutral axis depth of 0.6lw (for balanced failure), a limiting compression strain of 0.01,
Equation (1) gives: δ u / hw < 0.010 / (2)(0.6) = 0.0083 . Given the simplifying assumptions associatedwith Equation (1), a slightly higher value (0.01) might be appropriate.
Boundary Element Detailing

ACI 318-11 detailing requirements for SBEs are based on requirements that were developed forcolumns; these provisions may be insufficient for SBEs of thin walls. The review of recent walldamage in earthquakes and laboratory tests provides sufficient evidence to raise concerns related todetailing of thin walls. For example, although the quantity of transverse reinforcement provided at theboundaries of the conventional RC wall tested at E-Defense were 1.4 and 2.1 times that required by
ACI 318-11 §21.9.6.4 (for the larger spacing of 100mm used at Axis C), concrete crushing and lateral
instability (Fig. 7b) occurred earlier in the Kobe 100% test, followed by substantial sliding (Fig. 8).
Inspection of the damaged boundary zone revealed that relatively large clear cover was used, on theorder of 40mm (larger than the code minimum in ACI 318, which is 19mm), suggesting that theconfined core was incapable of maintaining stability of the compression zone following loss ofconcrete cover. For smaller columns, ACI 318-11 Equation (21-4), which is based on maintainingcolumn axial load capacity after cover concrete spalling, typically governs the selection of transverse
reinforcement for smaller columns where cover makes up a larger percentage of the gross concrete
section. This equation also was required for wall SBEs prior to ACI 318-99; it was dropped because it
rarely controlled for the thicker walls that were commonly used at that time. For the E-Defense
conventional RC wall, the provided transverse reinforcement is only 0.34 and 0.45 times that requiredby ACI 318-11 Equation (21-4), suggesting that improved performance may have resulted had thisrelation been required. Additional testing is needed to determine if reinstating (21-4) is sufficient toensure ductile behavior of thin boundary zones.
ACI 318-11 §21.6.6.2 allows the distance of 14” (356mm) between adjacent hoops or ties. Use ofsuch a large spacing for thin SBEs is unlikely to provide sufficient confinement (Fig. 14) and isincompatible with use of a vertical spacing one-third the wall thickness. For example, for a 10 in.

1141

(254mm) thick wall, such as used in the E-Defense test, the vertical spacing per ACI 318-11 is limited
to 3.33” (84.6mm); however, the horizontal spacing along the wall can reach 356mm (356/84.6 = 4.2).
An additional limit should be considered for wall SBEs, similar to that used for vertical spacing, where
the horizontal distance between legs of hoops or ties is limited to a fraction of the wall thickness, e.g.,
two-thirds tw. No allowing intermediate, unsupported bars at the wall edge, which initiated the section
failure for test PW2 (Fig. 6a), also should be considered.
Wall Slenderness and Lateral Stability

To limit instability failures, limits on wall slenderness should be considered, similar to what wasdone in the Uniform Building Code (1997), which imposed a slenderness limit of tw ≥ hs 16 . Basedon observations in recent earthquakes and tests, a lower limit should probably be used within plastic
hinge zones, a ratio of tw ≥ hs 10 was recently recommended in Moehle et al. (2011). This issue iscurrently under study by ATC 94 (2011).
CONCLUSIONS

Wall performance in recent earthquakes and laboratory tests is reviewed and American Concrete
Institute 318 provisions are reassessed to identify possible shortcomings. The findings suggest anumber of issues require more in-depth study, particularly for thin walls, as well as approaches that
could be implemented to address these issues. In particular, changes are needed to increase the design
displacement used in ACI 318-11 Equation (21-8), a factor of two is suggested, and to ensure spread ofplasticity consistent with the derivation of Equation (21-8). To address this latter issue, walls should
either be tension controlled or be designed and detailed to ensure ductile compression yielding byrequiring that walls be thicker and by imposing a limit on wall slenderness. Limiting wall compression
strain for compression-controlled walls also might be prudent.
ACKNOWLEDGMENTS

This research described in this paper was carried out with funding from various sources, including the
EERI Learning from Earthquakes program (NSF CMMI-0758529), NSF RAPID projects to enhance
US-Japan collaboration related to the E-Defense tests in December 2010 (CMMI-1110860 and
CMMI-1000268; Program Director Joy Pauschke), NSF NEES REU (CMMI-0927178), as well as
support provided to the first author by the Japan Society for the Promotion of Science (JSPS)
Invitation Fellowship Program during the fall 2010. This support is gratefully acknowledged. The
authors would like to thank those researchers who have contributed their research results to NEEShub,which provides an invaluable resource, as well as other researchers, including C French (U Minnesota),
S. Sritharan (Iowa State), L Lowes and Dawn Lehman (U Washington), and K Elwood (U BC). And
finally, the authors would like to express their deep appreciation to our Japanese collaborators within
the reinforced concrete buildings area related to the December 2010 E-Defense tests for sharing theirresearch ideas and results, including: T Nagae (NIED), K Tahara (NIED), T Matsumori (NIED), H
Shiohara (U Tokyo), T Kabeyasawa (U Tokyo ERI), S Kono (Kyoto U), M Nishiyama (Kyoto U); M.
Nakashima (NIED, Kyoto U). The opinions stated here are those of the authors and do not reflect
Opinions, findings, conclusions, and recommendations in this paper are those of the authors, and do
not necessarily represent those of the sponsors or other individuals mentioned here.
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