NIST GCR 11-917-11REV-1 © j p NEHRP Seismic Design Technical Brief No. 6 Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams A Guide for Practicing Engineers Jack P. Moehle Tony Ghodsi John D. Hooper David C. Fields Rajnikanth Gedhada NUST National Institute of Standards and Technology US. Department of CommerceNEHRP Seismic Design Technical Briefs ‘The National Earthquake Hazards Reduction Program (NEHRP) ‘Technical Briefs are publshed by the National Insitute of Standards and Technology (NIST), a8 aids tothe efficient transfer of NEHRP and other research into practice, thereby helping to reduce the nation’s losses fom earthquakes. NIST National Institute of Standards and Technology ‘The National Instute of Standards and Technology (NIST) i federal technology agency within the U.S. Department of Commerce that promotes U.S. nnavaton and industrial competitiveness by advancing measurement science, standards, and technology in ways that enhance ‘economic security and improve our qualty of fe. Its the lead agency ofthe National Earthquake Hazards Reduction Program (NEHRP), Dr. John (Jack) R. Hayes, J isthe Director of NEHRP, within NIST's Engineering Laboratory (EL). Dr. Jefery J. Dragovich managed the project to produce this Technical Brief for EL NEHRP Consultants Joint Venture ‘This NISTunded publication is one of the products of the work of the NEHRP Consultants Joint Venture carried out under Contract ‘9813410700019, Task Order 10254, The partners in the NEHRP Consultants Joint Venture are the Apolied Technology Couneil (ATC) and the Consortium of Universtes for Research in Earthquake Engineering (CURE). The members of the Joint Venture Management Commitee are James R. Harris, Robert Reltherman, Christopher Rojahn, and ‘Andrew Whitaker, and the Program Manager is Jon A Heintz About The Authors ‘Jack P, Moehle, Ph.D., PE. is TY, and Margaret Lin Professor of Engineering atthe University of California, Berkeley, where he teaches ‘and conducts research on earthquake-resistant concrete construction. He is a Fellow of the American Concrete Institute, and has served on the ACI Code Committee 318 since 1989 and as chair of the seismic subcommittee since 1995, He is a Felow ofthe Structural Engineers ‘Association of California and Honorary Member of the Structural Engineers Association of Northern California, ‘Tony Ghodsi, RE, S.E, is Principal at Englekik Structural Engineers, a stuctural engineering frm headquartered nLos Angeles, Calforia, He isa member ofthe Los Angeles Tall Buldings Structural Design Council {and on the Board of Advisors atthe University of Souther California Department of Civil and Environmental Engineering John D, Hooper, PE., SE, is Director of Earthquake Engineering at Magnusson Klemencic Associates, a structural and civil engineering frm headquartered in Seattle, Washington. He is a member ofthe Building ‘Seismio Safety Councis 2014 Provisions Update Committee and chair ofthe American Society of Civil Engineers Seismic Subcommittee for ASCE 7-10, David C. Fields, PE, S.E,,is a Senior Project Manager at Magnusson Klemencic Associates, a structural and civil engineering firm headquartered in Seattle, Washington. He is a member of American Concrete Institute Committee 374: Performance Based Design of Concrete Buildings and the Structural Engineers Association of Washington Earthquake Engineering Committee, Rajnikanth Gedhada, PE, S.E., is a Project Structural Engineer at Englokirk Structural Engineers, a structural engineering firm headquartered in Los Angeles, California. He is a member of the ‘Structural Engineers Association of Souther California. About the Review Panel ‘The contributions of the three review panelists for this publication are gratefully acknowledged, D.E, Lehman, Ph.D, Is the John R. Kiely Associate Professar of Civil and Enviormental Engineering at the University of Washinton, She conducts research on seismic response of engineered structures with ‘an emphas's on the use of large-scale experimental methods. She is also the Director ofthe Structural Research Laboratory at the University of Washington John W. Wallace, Ph.D., PE,, is Professor of StructuralEarthauake Engineering at the University of California, Las Angeles, where he teaches and conducts research on earthquake-esistant concrete construction. He is a Fellow of the American Concrete institute and has served on the ACI 318 seismic subcommittee since 1996, He also has served as a member of the American Society of Cl Engineers ‘Seismic Subcommittee for ASCE 7-05 and the ASCE 41-06 Supplement 1 concrete provisions update committee. He is a member ofthe Los ‘Angeles Tall Buildings Structural Design Councl. Loring A. Wyle, J is @ Structural Engineer and Senior Principal of Degenkal Engineers in San Francisco, Calfornia. He is the 2007 recpient of the American Society of Givi Engineers Outstanding Projects and Leaders (OPAL) design award. He is a pas president of the Structural Engineers Association of California andthe Earthquake Engineering Research Insite. He is a member ofthe Structural Concrete Builing Code Committee 318 ofthe American Concrete Insite and an Honorary Member of ACL ‘Applied Technology Council (ATC) 201 Redwood Shores Parkway - Suite 240, Redwood City, California 94065 (650) 595-1542 wivnatcounciorg email: ate@atcounciorg QTc CConsorium of Universities for Research in Earthquake Engineering (CUREE) 1301 South 46th Street - Building 420 Richmond, CA 94804 (610) 685-3529 vwww.curee.org ‘email curee@euree.orgNIST GCR 11-917-11REV-1 Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams A Guide for Practicing Engineers Prepared for USS. Department of Commerce Engineering Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899-8600 By Jack P. Moohle, Ph.D., P-E University of California, Berkeley Tony Ghodsi, PE., S.E. Englekik Structural Engineers John D. Hooper, PE., S.E. Magnusson Klemencic Associates David C. Fields, PE. S.E. Magnusson Klemencic Associates Rajnikanth Gedhada, PE, $.E. Englokrk Structural Engineers August 2011 Revised March 2012 ro fa Vee, : USS. Department of Commerce John Bryson, Secretary National Institute of Standards and Technology Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and DirectorContents Introduction... The Use of Special Structural Walls. Principles for Special Structural Wall Desigh.......e:c0:seesseseeseeneree Building Analysis Guidance......cccsesssesssseese Design Guidance. Additional Requirements ....ccessseesssesseene Detailing & Constructability Issues References... eocsseneteeees Notation and Abbreviations... Credits. Sweenonnena Errata to GCR 11-917-11 Updated: March 2012 The following errors were contained in the August 2011 Edition of Technical Brief No. 6, ‘The error was in the last paragraph of Section S.1, where it said: “For coupling beams, § ~ 0.85 for shear and 0.9 for flexure.” The correction in this version says: “For diagonally reinforced coupling beams, $~ 0.85 for shear. For conventionally reinforced coupling beams, 4 ~ 0.75 for shear and 0.9 for flexure.” In the paragraph following Figure S-1 on page 14, Ye = 0.11," should be “e—0.1J.". Also the first full paragraph in the second column on page 20 includes the term (480+0.8f 214... It should be 480+0.08/ 214... On page 22, the bold text following bullet b. should read “Coupling beams with l/h <2 and ¥,> 4277 Aca” ‘The term A.,was missing Disclaimers This Technical Brief was prepared forthe Engineering Laboratory ofthe National Insitute of Standards and Technology (NIST) under the National Earthquake Hazards Reduction Program (NEHRP) Eartquake Structural and Engineering Research Contract SB134107CQ0019,, Task Order 10254, The statomonts and conclusions contained herein are those ofthe authors and do not necessarily reflect the views and policies of NIST or the U.S. Government, ‘This report was produced by the NEHRP Consultants Joint Venture, a partnership of the Applied Technology Council (ATC) and the Consortium of Universes for Research in Earthquake Engineering (CURE). While endeavoring to provide practical and accurate information, the NEHRP Consultants Joint Venture the authors, and the reviewers assume no labilty for, nor express or imply any warranty with rogar to, the information contained herein, Users ofthe information contained in this report assume all ability arsing from such use ‘The policy of NIST is to use the Intemational System of Units (metric units) inal of ts publications. However, in North America inthe constuction and building materials industry, certain non-Sl units are so widely used instead of SI units that itis more practical and loss confusing to include measurement values for customary units ony inthis publication. Cover photo — Reinforcing of special reinforced concrete walls, Engineering § Building, UCLA. How to Cite this Publication Moehle, Jack, Ghadsi, Tony Hooper, John D., Fields, David C., and Gedhada, Rajnikanth (2011), “Seismicdesign of cast.n-place concrete spacial structural walls and coupling beams: A guide fr practicing engineers,” NEHRP Seismic Design Technical Brie No, 6, produced by the NEHRP Consultants Joint Venture, a partnership ofthe Applied Technology Council and the Consortium of Univers for Research in Earfnquake Enginoering, fr the National Institute of Standards and Technology, Gaithersburg, MD, NIST GCR 11-917. 11REV-11. Introduction The basic structural elements of an earthquake-resistant building are diaphragms, vertical framing elements, and the foundation. In reinforced concrete buildings, the vertical elements are usually either moment-resisting frames or structural walls (sometimes referred to as shear walls). Special reinforced concrete structural walls are walls that have been proportioned and detailed to meet special code requirements for resisting combinations of shear, moment, and axial force that result as a building sways through multiple displacement cycles during strong earthquake ground shaking. Special proportioning and detailing requirements result in a wall capable of resisting strong earthquake shaking without ‘unacceptable loss of stiffness or strength. Although special structural walls can be used in any building, the International Building Code (IBC 2009) only requires them wherever cast-in-place or precast walls are used to resist seismic forces in new buildings assigned to Seismic Design Category D, E, or E. The design force levels are speci Minimum Design Loads for Buildings and Other Structures (ASCEISEI 7-10) (ASCE 2010), and the design proportions and details are defined in the Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary (ACI 2011). This Guide uses units of measure consistent with these codes and standards, (eg., inches, pounds, pounds per square inch). The design requirements for special structural walls are governed by numerous interrelated requirements in these three building codes or standards, making their application challenging for even the most experienced designers. ‘This Guide first deseribes the use of structural walls, then clarifies intended behavior, and finally lays out the design steps and. details so that design and construction can be accomplished efficiently. The Guide is intended especially for the practicing structural engineer, though it will also be useful for building officials, educators, and students, This Guide emphasizes the most common types of special reinforced concrete structural walls, which use cast-in- place, normalweight aggregate concrete and deformed, non- prestressed reinforcement, Wall configurations vary depending ‘onthe application, and may include coupling beams, Building codes permit the use of special walls using precast concrete, lightweight aggregate concrete, or prestressed reinforcement. Building codes also permit the use of ordinary cast-in-place structural walls in buildings assigned o Seismic Design Category A, B, or C, and intermediate precast walls in some buildings assigned to Seismic Design Category A,B, C, D, E, or E, The interested reader is referred to ACI 318 for specific requirements for these other systems, which ate outside the scope of this Guide. This Guide emphasizes code requirements and accepted approaches to their implementation. It also identifies good practices that go beyond the code minimum requirements ‘Background information and illustrative sketches clarify the requirements and recommendations. Sections 2 and 3 describe the use of structural walls in buildings and discuss intended behavior. Section 4 provides analysis guidance. Section 5 presents the design and detailing requirements of ACI 318 along with guidance on how to apply them. Section 6 presents additional requirements for wall buildings assigned to Seismic Design Category D, E, or F, and Section 7 presents detailing and constructability challenges for special structural walls with illustrative construction examples. Codes Referenced in this Guide USS. building codes are continually undergoing revisions to introduce improvements in design and construction practices. At the time ofthis writing, the building code ditions most commonly adopted by state and local jurisdictions include the 2009 edition of the IBC, the 2005 edition of ASCE 7, and the 2008 edition of ACI 818, This Guide is written, however, according to the latest editions of each of these documents, that is, IBC (2009), ASCE 7 (2010), and ACI 318 (2011). In general, the latest editions of these three documents are well coordinated regarding terminology, system definition, application limitations, and overall approach. The most significant changes relative to the previous editions include: + ASCE 7 (2010) introduces Risk-targeted Maximum Considered Earthquake (MCEx) ground motions. and replaces “occupancy categories” with “risk categories.” + ACI318 (2011) introduces provisions for wall piers and modifies requirements for anchorage of wall horizontal reinforcement in wall boundaries. Hereafter, this Guide uses IBC to refer to IBC 2009, ASCE 7 to refer to ASCE 7 2010, and ACI 318 to refer to ACI 318-2011 ‘Sidebars in the Guide ‘Sidebars are used in this Guide to illustrate key points ‘and to provide additional guidance on good practices and ‘open issues in analysis, design, and construction. Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Praciicing Engineers 12. The Use of Special Structural Walls 2.1 Structural Walls in Buildings ‘Walls proportioned to resist combinations of shears, moments, and axial forces are referred to as structural walls. A special structural wall is one satisfying the requirements of ACI 318, Chapter 21, intended to result in strength and toughness required to resist earthquake effects in buildings assigned to Seismie Design Categories D, E, or F. In buildings, they are used in many different configurations; some are illustrated in Figure 2-1. Solid walls are widely used to brace low-rise buildings. Sometimes walls are perforated with openings. In taller buildings, walls cantilever from a foundation to provide bracing over the building height. Isolated walls can be connected using coupling beams extending between window and door openings, creating a coupled wall system thats stiffer and stronger than the isolated pair of walls Structural Walls and Shear Walls ACI 318 refers to structural walls and, with regard to ‘Seismic Design Categories D through F, special structural walls. The equivalent terms used by ASCE 7 are shear ‘walls and special shear walls. | F (a) Low-ise wall Qo o oogog 5 oo00 7 C)Perandwer—eenirvak | (@)empetwat Figure 2-4 - Some istrative structural wal elevations. 2.2 When to Use Structural Walls Selection of special structural walls as primary seismic force- resisting elements is influenced by considerations of seismic performance, functionality, constructability, and cost. Forlow= tomid-rise buildings, structural walls typically are more cost- effective than other systems such as conerete special moment frames. Structural walls are used in concrete buildings with limited floorto-floor heights or other architectural constraints that cannot accommodate frame beam depths. Stairway and elevator cores are natural locations for structural walls, which serve a dual purpose of enclosing vertical shafts while providing efficient axial and lateral resistance, ASCE 7 imposes height limits for buildings in which special structural walls compose the seismic force-resisting system, specifically 160 ft in Seismic Design Category D and E and 100 ft in Seismic Design Category F. These heights can be increased to 240 ft and 160 fi, respectively, if the building does not have an extreme torsional irregularity and the shear in any line of walls does not exceed 60 % of the total story shear (ASCE 7 § 12.2.5.4). There iso height limit for a dual system combining walls with special moment frames capable of resisting at least 25 % of prescribed seismic forces. 2.3 Wall Layout Structural walls are generally stiff structural elements ‘whose placement in # building can strongly affect building performance. Walls should be proportioned and located considering the range of loads the building will experience during its service life. The engineer and architect should work together to arrive at a building configuration in which walls are located to meet structural, architectural, and programmatic requirements of the project. 2.3.4 Plan Layout ‘Walls should be well distributed within the building plan, with ‘multiple walls providing resistance to story shears in each principal direction. Preferably, long diaphragm spans are avoided, Furthermore, the walls should be positioned such that their center of resistance is close to the center of mass, thereby avoiding induced torsion (Figure 2-2). Walls located near the perimeter may be preferred because they maximize torsional resistance, ‘Tributary gravity loads help resist wall overturning moments, reducing reinforcement and foundation uplift demands. ‘Therefore, it may be desirable to move walls inward from the perimeter and away from adiacent columns so that they support \g Torsion ASCE 7 contains provisions that quantify torsional Irregular, including penalties forlarge irregularities. The ‘code requirements refer only to linear-elastic response, If a building is expected to respond inelastically, the ‘center of resistance ideally should coincide with center ‘of mass for both linear response and for response at strength level. Where identical walls are symmetrically placed (e.g., walls a and b in Figure 2-2a), this objective is relatively easy to achieve, Additional design effort is required where walls are asymmetrically arranged. ‘Seismic Design of Gastin-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Practicing Engineers: 2‘more gravity Toads, as in Wall ¢ in Figure 2-2a, even though this reduces plan torsion resistance, Too much axial force ean result n undesirable compression-controlled flexural response. A good plan layout balances these competing objectives, In buildings with post-tensioned slabs, stiff in-line walls ean act to resist slab elastic and creep shortening, sometimes with deleterious effect. Walls ¢ and d (Figure 2-2) would resist slab shortening along line ed, such that post-tensioning force ‘would tend to transfer from the slab and into the walls. Walls 2, b, and @ are well positioned to allow slab shortening. (a) Wat plan eyout (BINS rss Figure 2:2 - Example plan layout CM. refers to center of mass 2.3.2 Vertical Discontinuities Considerations of function and cost sometimes lead to wall ‘openings and other wall discontinuities. Under lateral load- ing, these irregularities can lead to stress concentrations and localized lateral drift that may be difficult to quantify and accommodate in design, and in some cases may result in undesirable seismic response, Some irregularities should be avoided without further consideration; other cases will require additional analysis and design effort. In the past, demand for open space in the first story led to many older buildings in which walls from upper stories were discontinued in the first story, creating a weak first story (Figure 2-3a). ‘These have performed poorly in past earthquakes (Figure 2-4). ‘This configuration, classified by ASCE 7 as an Extreme Weak Story Itregularity, is no longer permitted in new buildings assigned to Seismic Design Categories DE, of F. (@)Woak story () Discontinuous (6) mlompton of Figure 2:3 - Wall verica regulars, Figure 24 Wak story damage, 1971 San Ferando earthquake, Walls extending from the foundation and discontinued at some intermediate level (Figure 2-3b) are permitted by ASCE 7, but the design is penalized by increased seismic design forces. Itis preferred to have more gradual reduction in wall section (either length, thickness, or both), as illustrated by Figure 2-3e. ‘Openings in walls disrupt the flow of forces and are best located in regular pattems that produce predictable force transfers. Figures 2-Ib and d show examples of regularly located wall ‘openings. For such buildings, good design practice keeps ‘vertical wall segments (piers) stronger than beams so that story failure mechanisms are avoided, Sometimes programmatic demands require openings in a less regular pattern (Figure 2-3e). These should be avoided where feasible. Where ‘unavoidable, they require additional design and detailing effort to develop force transfers around openings. See Section 5.9, 2.3.3 Diaphragm Connectivity In a building braced by structural walls, inertial forces generated by building vibration ate transmitted through diaphragms to the walls, which in turn transmit the forces to the foundation, Good connections between diaphragms and structural walls are essential to the seismic force path. ‘This subject is discussed in depth by Mochle etal. (2010). Programmatic requitements often locate diaphragm openings adjacentto structural walls, complicating the seismic force path This ean be especially acute at podium slabs where large wall forces may be transferred through the diaphragm to other stiff ‘elements (Figure 2-Sa), Good diaphragm transfer eapacity is facilitated by solid diaphragms surrounding walls, rather than significantly perforated diaphragms (Figure 2-Sb). Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Praciicing Engineers 3setae (@)Elevaton (by sexton Figure 2-5— Force ransers between walls and laphragms, 2.4 Wall Foundations In low-tise buildings with long walls supporting sufficient gravity loads, spread footings may be adequate to resist design overturning moments, For higher overturning demands, pile foundations, possibly including tension tie-down capacity, ean beused. More commonly, foundation elements are extended to pick up additional gravity loads, Figure 2-6a shows a grade beam acting as a foundation outrigger, Basement walls also can be proportioned to act as outrigger elements (Figure 2-60), Alternatively, a wall extending into subterranean levels ean use a horizontal force couple formed between the grade-level diaphragm and diaphragms below to transfer the overturning ‘moment to adjacent basement walls (Figure 2-66). boundary columns that contain longitudinal reinforcement for moment resistance, improve wall stability, and ereate an ‘element to anchor beams framing into the wall. The boundary columns, however, might create an architectural impediment and increase forming costs, Intersecting wall segments can bbe combined to create flanged walls, including T, L, C, and configurations. Core walls enclose elevators, stairways, and other vertically extruded areas, with coupling beams connecting wall components over doorways. In these walls, any wall segment aligned parallel to the lateral shear force acts as a web element resisting shear, axial force, and flexure, while orthogonal wall segments act as tension or compression. flanges. Walls with openings are considered to be composed of vertical and horizontal wall segments (Figure 2-8). A vertical wall segment is bounded horizontally by two openings or by an ‘opening and an edge. Similarly, a horizontal wall segment is. bounded vertically by two openings or by an opening and an edge. Some walls, including some tilt-up walls, have narrow vertical wall segments that are essentially columns, but whose dimensions do not satisty requirements of special moment frame columns. In consideration of these, ACI 318 defines a wall pier as a vertical wall segment having lb, < 6.0 and Jiglle > 2.0. ‘The lower left vertical wall segment in Figure 2-8b might qualify as a wall pier. Special provisions apply to wall piers (Section 5.7), Emma (@) Rectang snape =F (0)"Bar at snape (@)Founastion (6) Coupte wats on (9 overigeies Figure 2-6 - Various ways lo spread overturn rsstance. If none of these solutions work, foundation rocking may need tobeaccepted. US. building codes do not recognize uplifting walls as an accepted seismic foree-resisting system; either special approval is required or the wall cannot be counted on toprovide seismic force resistance. Regardless, uplifting walls can impose large deformation demands on adjacent framing, ‘members that should be accommodated through design, 2.5 Wall Configurations Special structural walls can be configured in numerous ways (Figure 2-7). Rectangular cross sections are relatively easy to design and construct; very thin sections can have performance problems and should be avoided. “Bar bell” walls have Li (€) Flanged watsin common LC ans Eshapes (ei Possible contguatin ofa cre-all Figure 2-7 - Various wall cross setons (a) Horizontal wal sogmonts (6) Vora wal sopmenis Figure 2-8 -Vertcal and horizontal wall segments (hatched), ‘Seismic Design of Gastin-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Practicing Engineers: 4TThe term coupled wall refers toa system in which cantilever, walls are connected by coupling beams aligned vertically over wall height (Figure 2-9). ‘The design goal is to develop ‘a ductile yielding mechanism in the coupling beams over the height of the wall followed by flexural yielding at the base of the individual cantilever walls. Depending on geometry and design forces, a coupling beam can be detailed as either 1 conventionally reinforced beam or diagonally reinforced beam, See Section 5.8. In taller buildings, outriggers can be used to engage adjacent columns, thereby increasing building stiffness and reducing. pper-story drifts. Outriggers can be incorporated conveniently in floors housing mechanical equipment or atthe roof level. 2.6 Wall Reinforcement Figure 2-10 illustrates typical reinforcement for a special structural wal of rectangular eross section, Asa minimum, a special structural wall must have distributed web reinforcement in both horizontal and vertical directions. In many cases, a special structural wall also will have vertical reinforcement concentrated at the wall boundaries to provide additional resistance to moment and axial force. ‘Typically, longitudinal reinforcement is enclosed in transverse reinforcement to confine the concrete and restrain longitudinal bar buckling. 9 » sf Figure 29 ~ Coupled wall gomety and target yield mechanism, fesecs The distributed web reinforcement ratios, p) for vertical reinforcement and p, for horizontal reinforcement, must be at Teast 0,0025, except that py and pare permitted to be reduced if ¥, < AnaA77. Reinforcement spacing each way is not to exceed 18 inches. At least two curtains (layers) of reinforcement are required if ¥,>24,3,77. Reinforcement paso is 10 be designed for wall shear forces, as described in Section 5.4. Finally, i h/ly< 2.0, ps not to be less than the provided p, ACI 318 has no requirements about whether vertical ororizontal distibuted reinforcement shouldbe inthe outer layer, although lap splices of vertical reinforcement will perfor beter if horizontal bars ave placed outside the vertical bars as shown in Figure 2-10 [boundary element foveal horizontal web reinforcement vertical web reinforcement lp , Figure 2-10 — Typical enforcement for rectangular wal ‘A boundary element is a portion along a structural wall edge ‘or opening that is strengthened by longitudinal and transverse reinforcement. Where combined seismic and gravity loading results in high compressive demands on the edge, ACI 318 requires a special boundary element. These have closely spaced transverse reinforcement enclosing the vertical houndary bars. to increase compressive strain capacity of core conerete and to restrain longitudinal bar buckling. See Section 5.3.3. 2.7 Wall Proportioning Walls should be proportioned to satisfy strength and drift imit requirements of ASCE 7, unless an alternative approach is approved. According to ASCE 7, walls are designed for load. combinations in which seismic forces, £ are determined using a force reduction factor, . The value of R depends on whether the wall is part of a Dual System (R = 7), a Building Frame System (R =6), ora Bearing Wall System (R= 5). To qualify as Dual System, the special structural walls must be combined with special moment frames capable of resisting atleast 25 % of prescribed seismic forces. If it dees not qualify as a Dual System, then it ean qualify as a Building Frame System if it has an essentially complete space frame providing support for vertical loads, with structural walls providing seismic force- resistance. If there is not a complete space frame providing support for vertical loads, the system must be designed as a Bearing Wall System, Building Frame System versus Bearing Wall System Different jurisdictions interpret the ASCE 7 provisions differently. San Francisco (DBI, 2009) declares the wall to be a bearing wall fit supports more than 5 % of the entire building floor and roof loads in addition to self-weight. SEAW (2009) recommends designing a frame column into the wall boundary capable of supporting tributary gravity loads, such that R = 6 can be used regardless ofthe tributary loads on the wall. SEAOC (2008) recommends R need to adda frame column where confined boundary elements are provided. This Guide recommends checking with the local jurisdiction. Note that ACI 318 and ASCE 7 define a bearing wall as any wall that supports more than 200 Ibilinear ft of vertical load in addition to self-weight. This definition of bearing wall should not be confused with the Bearing Wall System designation of ASCE 7. Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Praciicing Engineers 5ASCE 7 specifies drift limits a a function of building height and Occupaney Category (Table2-1). Driftis calculated using, the design seismic forces £ amplified by C,(ASCE 7 § 128.6). ais 5 for both Bearing Wall and Building Frame Systems and is 55 for Dual Systems. Structure ‘Occupancy Category Tor TL W. Nea 0.025h., | 0.020%, | 0.015h, All other structures | 0.020h,. | 0.015/.. | 0.010h, Table 2-4 ~ Allowable interstory Dit Ratios per ASCE 7. Although cost considerations might suggest designing ‘minimum-weight sections, such sections may be difficult 10 construct and might not perform well, Once the decision has bbeen made to incorporate a wall in the building, formwork and reinforcement detailing will dominate costs. Selecting a thicker wall section is unlikely to have an appreciable effect on construction cost or Functionality, but will reduce reinforcement congestion and improve earthquake performance. Although ACI 318 has no prescriptive minimum thickness, 8 inches is a practical lower limit for special structural walls. Construction and performance are generally improved if the wall thickness is at least 12 inches where special boundary elements are used and at least 10 inches elsewhere. Walls that incorporate coupling beams require a minimum thickness of approximately 4 inches to accommodate reinforcement, required cover, and bar spacing, although 16 inches isa practical minimum where diagonally reinforced coupling beams are used. Flanges and enlarged boundary sections are helpful to stabilize boundaries and anchor reinforcement from adjacent members. See Section 5 for guidance on wall proportioning. ‘Seismic Design of Gastin-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Practicing Engineers: 63. Principles for Special Structural Wall Design Buildings designed according to the provisions of ACI 318 Chapter 21 and ASCE 7 are intended to resist earthquake motions through ductile inelastic response of selected members. For structural walls, the nature and extent of inelastic response will vary with wall layout and aspect, ratio. A good design anticipates the inelastic mechanism and provides proportions and details in the wall that will enable itto respond as intended, The following sections summarize the key principles forthe design of structural walls. Detailed design guidance is presented later in the Guide. Slender versus squat walls Expected behavior of walls depends partly on wall aspect ratio. Slender walls (i,!/,> 2.0) tend to behave much lke flexural cantilevers. The preferred inelastic behavior mode of slender walls is ductile flexural yielding, without shear failure. In contrast, walls. with very low aspect ratios (hin/l, < 0.5) tend to resist lateral forces through a diagonal strut mechanism in which concrete and distributed horizontal and vertical reinforcement resist shear. Wall behavior transitions between these extremes for intermediate aspect ratios. ‘Shear yielding of slender walls generally is considered Unacceptable because itreduces inelastic deformation capacity below expected values. Shear yielding of very squat walls is often accepted because such walls tend to have high inherent strength and low ductility demands. 3.1 Slender Walls 3.1.1 Select Intended Yield Mechanism For slender walls, the design should aim to achieve ductile flexural yielding at the base of the wall, For slender coupled walls, the target mechanism should include ductile yielding of coupling beams over the height ofthe wall plus ductile flexural yielding atthe base of the walls. Wall shear failure and failure Of diaphragms and foundations generally should be avoided. See Figures 2-9 and 3-1 Where the design intent is to have a single critical seetion for flexure and axial force, the designer should provide a distribution of strength over wall height that inhibits yielding at other critical sections. One approach is to design the selected critical section to have strength in flexure and axial closely matching the required strength, with some overstrength provided at other locations (Figure 3-1). Where this approach is used, the special details for ductile response cean be concentrated around the selected critical section, with relaxed detailing elsewhere, By AAC vrei Srtcnce | Mat Oa (a) atteovaion —— (b) Moments Fig ~ Provided versus required exural stengt ina wall wih single rica section. In some cases, alternative mechanisms have to be accepted. In very tall buildings, higher-mode response may cause some wall flexural yielding in intermediate stories in addition to the primary yielding mechanism, Detail such locations so they are capable of moderate ductility capacity. In highly irregular walls, including walls with irregular openings, it ccan be difficult to precisely identify and control the yielding, ‘mechanism, Some conservatism in the design of these systems ccan help achieve the desired performance, 3.1.2 Achieve Ductile Flexural Yielding The intended critical section should be proportioned and detailed to be capable of multiple inelastic cycles. Key factors to improving cyclic ductility are (a) keep global compressive and shear stresses low; (b) design a confined, stable flexural ‘compression zone; and (e) avoid splice failures A good wall design keeps the axial foree well below the balanced point, such that flexural tension reinforcement yields before the flexural compression zone reaches the compressive strain capacity, Using ACI 318 terminology, compression- controlled walls (concrete reaches strain of 0.008 before tension reinforcement yields) should be avoided. It is noteworthy that the 1997 Uniform Building Code § 1921.6.6.4@) (UBC 1997) limited wall axial force to P, = 0.35), which corresponds approximately to the balanced axial force ina symmetric wall ACI 318 does not have any limits on the wall exial force. Although ACI 318 permits factored shear on individual wall segments as high a V. = 109\F? doy the flexural ductility capacity for such walls is reduced compared with identical walls having lower shear. This Guide recommends factored shear, calculated considering flexural overstrength (se Seetion 3.1.3), not exceed approximately 4677 4a, to 66VF7 4. 80 that flexural ductility capacity is not overly compromised Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Praciicing Engineers 7Inelastic lexural response may result in concrete compressive strains exceeding the unconfined crushing strain, typically taken as 0,003. Ifthe flexural compression zone lacks properly. detailed transverse reinforcement, concrete crushing and vertical reinforcement buckling at a section can result in a locally weakened “noteh” where deformations concentrate, leading to relatively brittle behavior (Figure 3-2). Transverse reinforcement is necessary to confine the boundary, thereby enhancing concrete strain capacity and restraining longitudinal bar buckling. ‘The special boundary element transverse reinforcement should comprise closely spaced hoops with crossties engaging peripheral longitudinal bars (Figure 2-10) In excessively thin wals, spalling of cover concrete ean leave a relatively narrow core of confined concrete that can be unstable under compressive loading. This Guide recommends ‘a minimum wall thickness of 12 inches for sections requiring special boundary elements unless tests on representative sections demonstrate adequate performance for thinner sections. Concrete cover over confinement reinforcement should be minimized such that cover spalling, iit occurs, will not result in a large reduction in section area. Good detailing practice also provides lateral support for every longitudinal bar inspecial boundary elements located within the intended hinge region. ACI 318 permits somewhat less stringent detailing (Gee Section 5.3.3) Figure 3-2 Concrete crushing and reinforcement bucking of inadequately conned wal, 2010 Chile earhavake. Lap splices of vertical reinforcement can result in a locally strengthened section, such that yielding, if it occurs, may be shifted above or below the lap splice. Consequences of this shift should be considered. Lap splices subjected to multiple yielding cycles can “unzip” unless they are confined by closely. ‘spaced transverse reinforcement. For such splices, ACI 318 requires splice lengths at Ieast 1,25 times lengths calculated for {Fin tension, with no requirement for confinement. This Guide recommends either that lap splices be moved out of the hinge zone or else be confined by transverse reinforcement. Slender boundary zones can be susceptible to overall buckling, under compressive loading (Figure 3-3). The problem can be exacerbated ifthe section was yielded previously in tension due to loading in the opposite direction, leaving a more flexible pre- cracked section, ACI 318 has no limits on slenderness of special structural walls. This Guide recommends (yb < 10 within the intended hinge region and ly/b <16 (the limit prescribed in the 1997 Uniform Building Code) elsewhere. Figure 3-3 —Wal bucking, 201 Christchurch earthquake, 3.1.3 Avoid Shear Failure Shear failure in a slender structural wall can lead to rapid strength loss at drifts below those anticipated in design. Shear failure also can compromise the wall axial strength, This is ‘especially so for walls resisting high shear forces (exceeding, around 10YF74.)), because shear failure in such walls can occur by web crushing (Figure 3-4), For these reasons, the engineer should design slender walls to avoid shear failure diagonal compression strut oa Figure 3-4 Web cxushing due to high shea force in laboratory tes. Design proceduresin ACI 318and ASCE T require consideration of multiple load combinations, and this invariably leads to flexural strength Macs that, under some load combinations, ‘exceeds the required flexural strength Macs (Figure 3-5), ‘Consequently, the lateral forces required to yield the wall in flexure, and the resulting wall shears, will be higher than the design values. A good practice is to amplify the design shear toaccount for this effect. One approach isto define a flexural overstrength factor 8, = Macs!Macs, which reflects how much ‘Seismic Design of Gastin-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Practicing Engineers: 8flexural overstrength is built into the wall, and to inerease the dosign shears by this same factor. ACI 318 encourages this approach by permitting a higher strength reduction factor ¢ for shear when this approach is used (See Section 5.1) Anticipating that the wall will develop even higher flexural strength due to material overstrength and strain-hardening, SEAOC (2008) recommends 4, = Mp.cs/Mgcs. Note that Macs and My-csdepend on axial force, which varies for different load. combinations and, for coupled walls, with loading direction. This Guide recommends using the load combination producing, the most conservative value of ¢, wall area and concrete compressive strength. See Section 54, Sliding shear failure is evident in horizontal cracks and sliding along construction joints and is controlled by proper treatment of construction joints, including surface roughening, and possibly intermittent shear keys, as well as placement of vertical reinforcement across the potential sliding plane (Geetion 8.5). 3.2 Squat Walls Walls tend to have high inherent flexural strength and thus are prone to inelastic response in shear rather than flexural yielding. Contrary to slender walls, such behavior ean provide sufficient post-yield stiffness and deformation capacity. Squat walls are prone to two types of shear failure. “Shear yielding” within the wall web involves development of inclined ‘cracks (Figure 3-6). Horizontal force equilibrium of segment cede requires distributed horizontal reinforcement providing force F;. Moment equilibrium of segment cde about e, or segment ab about b, requires distributed vertical reinforcement providing force F. Thus, ACI 318 requires both vertical and horizontal reinforcement to resist shear in squat walls, “Shear sliding” tends to occur at construction joints, including the ne% al —— coteterces rr eapactampliiod Ton, Oho Macs Meee (e)Loterattorces (pj Wallelvation (6) Shear (6) Moment Figure 35 — Wal tra forces, shears, and moments; code-prescrived ‘orcas and code-prescribed forces coresponding to development of ‘nominal xual strength In multi-story buildings, dynamic response produces ever changing paticrns of lateral inertial forees. Some prevalent farce patterns shift the centroid of lateral forces downward, further increasing the shear forces corresponding to flexural strength at the critical section. To approximate this effect the design shear can be increased to V,~ ah, where isa dynamic amplification factor. For buildings designed by the equivalent lateral foree procedure (Section 4.1, SEAOC (2008) recommends «= (09+ N/10) for buildings up to 6 stories and (13.30) forbuildings over 6 stories. Ifshears are based on ‘modal response spectrum analysis, need not exceed (1.2 + [NISQ). Nis the numberof stories from base to roof, assuming typical story heights. Equivalent story heights shouldbe used in buildings with unusually tall stores. Eurocode (2004) has analternative formulation. ACI318 and ASCE 7 do not require designing for this dynamic amplification factor. Designing a wall to avoid shear failure requires consideration of several failuro modes. Diagonal tension failure is evident in inclined cracks extending from the flexural tension boundary. through the wall web, and it is controlled by provision of web horizontal and vertical reinforcement (Section $4). Diagonal ‘compression failure is evident in crushing of the web near the flexural compression zane (Figure 3-4) and is controlled by limiting the maximum value of wall shear as a function of wall-foundation interface. Axial force V, and distributed vertical reinforcement Ayr (including added dowels) provide a clamping force actoss the interface that resists sliding. Reinforcement A,sis most effective if distributed. Thus, it may be preferred to distribute the flexural reinforcement uniformly without concentrated boundary elements. Reinforcement diy is more effective in resisting sliding if oriented at an angle of & 45°, although this creates a constructability challenge. When conerete is placed against previously hardened conerete at this interface, ACI 318 requires the surface be clean and free of laitance, Intentional roughening increases sliding resistance. beeeeeeiy a men t, ENN Fe Eel ! MI} coos” hn Figure 3-6 ~ Shea yielding and shear sldingin a squa wal. Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Praciicing Engineers 93.3 Diaphragms and Foundations The intent of U.S, building codes is that significant inelastic response will be limited to vertical framing elements of the seismic foree-resisting system (for example, special moment Frames, and special structural wall) that are detailed for ductile response, Diaphragms, foundations, and their connections, are intended to remain essentially elastic. Sections 6.3.2 and 6.3.3 of this Guide summarize ACI 318 and ASCE 7 requirements Foundation design practices vary. Some engineers design foundations for forces determined from load combinations ineluding E without consideration of the capacity of vertical ements framing into the foundation. Others use capacity design prineiples to determine foundation forees based on the capacity of the vertical elements, Yet another practice for squat walls is to acknowledge the difficulty of tying down, the foundation, and to accept foundation rocking. Rocking cean impose latge deformations on other components of the strueture of the building that must be considered in design, Design requirements for rocking foundations are not included in this Guide. ‘Seismic Design of Gastin-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Practicing Engineers: 104. Building Analysis Guidance 4.1 Analysis Procedures ASCE 7 allows the seismie forces in a structural wall to be determined by three types of analysis: Equivalent Lateral Force Analysis, Modal Response Spectrum Analysis, and Seismic Response History Analysis. The Equivalent Lateral Force Analysis procedure is the simplest and can be used effectively for basic low-rise structures. This analysis procedure is not permitted for long-period structures (Fundamental period 7 greater than 3.5 seconds) or structures with certain horizontal ot vertical irregularities. The seismic base shear V calculated according to Equivalent Lateral Force Analysis is based on an approximate fundamental period, T;, unless the period of the structure is determined by. analysis, Generally, analysis of moderate-to-tall structural wall buildings will show that the building period is longer than the approximate period, although the upper limit on the period (CT,) applies for the base shear calculation. The longer analytical period will result in a reduced calculated base shear when the period is greater than T,, often called the transition period. Per ASCE 7 Equations 12.8-3 and 128-4, the base shear inthis range decreases as the considered period. inereases, up to the point where the minimum base shear ‘equation governs. Modal Response Spectrum Analysis is often preferred to account for the elastie dynamic behavior of the structure and to determine the calculated building periods. Another advantage of Modal Response Spectrum Analysis is that the combined modal base shear response can be less than the base shear caleulated using Equivalent Lateral Force procedure. In such eases, however, the modal base shear must be scaled up to a minimum of 85 % of the Equivalent Lateral Force base shear. Fora Modal Response Spectrum or Seismic Response History Analysis, a 3-D computational model is typically used as an effective means of identifying the effects of inherent torsion in the lateral system as well as the directional interaction of flanged walls. For such analyses, code-preseribed accidental torsion forces typically are applied as statie story torsions combined linearly with the dynamic results, ASCE 7 § 125 specifies the requitements for the directions in which seismic forces are to be applied to the structure. Although the design forces for structural walls often may be based on the seismic forces applied in each orthogonal direction independently, it is common to apply the seismic forces using the orthogonal combination procedure of ASCE 7§ 125.32. This combination considers 100 % of the seismic Force in one direction combined with 30% of the seismic force in the perpendicular direction, Multiple load combinations are required to bound the orthogonal effects in both directions. To avoid excessive conservatism, the resulting structural wall, demands typically are considered for each combination rather than being enveloped. The orthogonal force combination procedure is required for structural wall design only ifthat wall forms part of two or more intersecting seismic force-resisting systems and is subjected to axial load due to seismic forces acting along either principal plan axis equaling or exceeding 20 % of the axial design strength of the wall ACII8 § 21.12.1 requires that the interaction of all structural ‘and nonstructural members that affect the linear and nonlinear response of the structure ta earthquake mations be considered inthe analysis. Important examples include interactions with ‘masonry infill (partial or full height), architectural concrete walls, stairwells, castin-place stairways, and inclined parking ramps. Itis not always necessary to inelude these elements in the global model. Instead, global analysis results can be used to check whether interferences with nonstructural elements ‘occur, and construction details can be modified as needed. 4.2 Stiffness Recommendations ‘When analyzing a structural wall, it is important to model appropriately the eracked section stiffness of the wall and any coupling elements, as this stiffness determines the building periods, base shear, story drifts, and internal force distributions. According to ACI 318 § 8.8.2, wall stiffness ‘can be defined by (a) $0 % of gross-section stiffness; (b) = 0.701, if uncracked or 0.35), if cracked, and 4, = 104, oF (©) ‘more detailed analysis considering the reduced stiffness under loading conditions. Actual stiffness of structural walls depends ‘onreinforcement ratio, slip of reinforcement from foundations, foundation rotation, axial force, and other parameters. The flcxural and axial stiffness values prescribed by ACI 318 are reasonable for many cases; shear stiffness, however, is typically as low as G.A,/10 to G.A/20, ATC 72 (2010) provides additional guidance, ACI 318 provides frame beam effective stiffness values, but these are not appropriate for typical coupling beams. Coupling beams arc expected to sustain damage before significant yielding occurs in walls, leading to faster stiffness reduction. ‘Coupling beam effective stiffness is further reduced because of concentrated end rotations associated with reinforcement, slip from anchorage zones within the wall boundary. ATC 72 (ATC 2010) recommends taking E.1, = 0.15EsI, with shear deformations calculated based on G.=0.4E, for = 2and G. 0.1, for f= 1.4, with linear interpolation for intermediate aspect ratios. ‘The preceding recommendations intend to approximate secant stiffness to onset of yielding, Actual instantancous Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams: A Guide for Praciicing Engineers "