IT4 – Shear Wall Design

Update April 13, 2017

Voting Members Michel Bruneau, University of Buffalo, Buffalo, NY

Kelly Cobeen, Wiss, Janney, Elstner, Emeryville, CA

Jason Collins, PCS Structural Solutions, Seattle, WA

David Fields, Magnusson Klemencic, Seattle, WA

S. K. Ghosh, S. K. Ghosh Associates, Palatine, IL (Chair)

Gino Kurama, University of Notre Dame, Notre Dame, IN

Andy Taylor, KPFF, Seattle, WA

Corresponding Members Jeffrey W. Berman, University of Washington, Seattle, WA Dan Dolan, Washington State University, Pullman, WA John Hooper, Magnusson Klemencic, Seattle, WA Joe Farzli, Cary Kopczynski & Company, Bellevue, WA Dawn Lehman, University of Washington, Seattle, WA Phil Line, American Wood Council, Washington, D.C. Laura Lowes, University of Washington, Seattle, WA Joe Maffei, Maffei Structural Engineering, Oakland, CA Bonnie Manley, AISI, Norfolk, MA Jack Moehle, University of California, Berkeley, CA Larry Novak, Portland Cement Association, Skokie, IL Jose Restrepo, University of California, San Diego, La Jolla, CA Rafael Sabelli, Walter P. Moore, San Francisco, CA Siamak Sattar, NIST, Gaithersburg, MD John Wallace, University of California, Los Angeles, CA Andrew Whittaker, University of Buffalo, Buffalo, CA Tom Xia, DCI Engineers, Seattle, WA

Corresponding Members Additions since the last meeting:

Jack Moehle, University of California, Berkeley, CA

Larry Fahnestock, University of Illinois, Urbana-Champaign, IL

Purpose Consider the effects of external loads (gravity as well as lateral) on shear walls of concrete, steel, masonry, and wood.

Consider possible failure modes resulting from those load effects or internal forces.

Determine failure modes that are critical in design, leading to possible areas of improvement in current design practice.

Two of those areas are anticipated to be:

Purpose 1. Coupled shear wall systems are recognized as distinct from isolated shear wall systems in Canadian and New Zealand codes; they are also accorded higher response modification factors in view of their superior seismic performance. The IT will examine whether ASCE 7 should go in the same direction. (suggested by PUC: different R factors for coupled and isolated walls need definition of coupled shear wall; ACI 318 will develop the definition; new line items will be added in in Table 12.2-1. Ron Hamburger suggested P 695 study).

Purpose 2. Tall buildings in the 400-ft height range are increasingly being built in highly seismic areas, with seismic forces being resisted entirely by cores consisting of reinforced concrete shear walls. The shear design of these shear walls is absolutely crucial to the safety of these structures. However, we have at best an imperfect understanding of the maximum shear that can develop and how it is transferred at the base. It will be very beneficial for the IT to look into this design aspect. (suggested by PUC: cooperate with multi-period spectra team)

Expected Work Product(s)

The IT will develop Part 1 proposals and accompanying Part 2 commentary dealing with any topic on which the IT is able to progress to that point. All other findings of the IT and work leading to those findings will be recorded in a Part 3 Resource Paper.

Meetings Held August 29-30, 2016, KPFF, Seattle, WA (two-day meeting) Gracious host: Andy Taylor, KPFF

Topics to be pursued tentatively chosen

November 2-3, 2016, KPFF, Seattle, WA (two-day meeting) Gracious host: Andy Taylor, KPFF

First day devoted to concrete-related topics

April 3, 2017 (web meeting)

Spent two hours discussing progress and planning actions for the near future

Upcoming Meetings May 12, 2017 (web meeting)

June 29, 2017 web meeting)

August 15-16, 2017, KPFF, Seattle, WA (two-day meeting) Gracious host: Andy Taylor, KPFF

Masonry Shear Walls Partially grouted shear walls.

Indications are that performance may be improved with improvements in detailing.

Review all available information.

Develop recommended improved detailing requirements for adoption into the TMS 402 Standard.

Steel Plate Shear Walls Coupled SPSW Configuration

• Steel Plate Shear Wall with Coupling (SPSW-WC) – Adjacent SPSW

linked by coupling beams (CB)

– Mobilize adjacent walls to act compositely

Exterior Vertical Boundary Element (eVBE)

Web Plate

Horizontal Boundary Element (HBE)

Coupling Beam (CB)

SPSW

Interior Vertical Boundary Element (iVBE)

Steel Plate Shear Walls Objectives

• Comprehensively characterize behavior and performance of SPSW-WC system

• Develop design guidelines that enable adoption of SPSW-WC configuration

Wood Shear Walls 1. Methods for computing deflections in stacked shear walls.

This topic is currently of interest to designers due to observed differences in shear wall deflection calculation methods for purposes of complying with code specified drift limits.

Initial focus would be on a Part 3 white paper capturing current recommendations.

Having cross-material discussion regarding computation of deflection and intent of seismic drift limitations would be helpful for this topic.

Wood Shear Walls 2. Combined compression and shear failures in shear walls.

The discussion of combined compression and shear failure has, to date, centered on concrete shear walls. Persons involved in high gravity loaded wood light-frame structures would like to explore whether or not this is a potential issue for wood frame buildings.

This is anticipated to be a Part 3 white paper.

Wood Shear Walls 3. Capacity based design.

Implicit in the design for seismic force resistance is that the designated seismic force resisting system has a load path (e.g. load transfer into and out of shear walls) that is capable of developing the seismic force-resisting element capacity. The intent is to use shear wall, wood element, and connection strength estimates from data to determine whether more in-depth evaluation is needed.

This is anticipated to be a Part 3 white paper.

Wood Shear Walls It is intended that these topics would be collaboratively considered by IT4 and the AWC Wood Design Standards Committee and/or its seismic task committee. These topics have in common that they are of interest to the designers of wood frame shear wall structures.

Item 1, on deflection calculation, is intended to address the varying methods of calculating deflection for compliance with code prescribed drift limits.

Items 2 and 3 are related in that capacity based design principles will be studied and used to evaluate potential strength limit states of wood frame shear wall structures under combined high gravity and lateral shear loading.

Hybrid Shear Walls

• Precast Concrete Wall Panels with Post-Tensioning Strands and Mild Steel Bars

• During Large Earthquake, Gap Opens at Base Joint

– High Strength Unbonded Post-Tensioning Strands Provide Re-Centering Force

– Mild (E.D.) Steel Bars Provide Energy Dissipation

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Hybrid Shear Walls

Gino Kurama of Notre Dame gave a presentation at Meeting 1 on hybrid concrete shear walls. This brought up some interesting collateral issues, such as splices at wall bases. Should IT4 study hybrid shear walls further? System is already classified. After further discussion it was decided that there is no urgent topic in this area that needs IT4’s attention.

Concrete Shear Walls Coupled Shear Walls

• There are now many forms of coupling beams, e.g. steel, precast, cast-in-place.

• Important first step is to define a coupled shear wall. Canadian code has a definition of coupled shear walls:

• Coupled Shear Wall: A shear wall system with coupling beams where at least 66% of the base overturning moment resisted by the shear wall system is carried by the axial tension and compression forces resulting from shear in the coupling beams.

Concrete Shear Walls Coupled Shear Walls

• Ron Klemencic of Magnusson Klemencic Associates (MKA) suggested that since we are going to ask for higher R-values for coupled shear wall systems, the definition for a coupled shear wall system should probably be based on energy dissipation.

Concrete Shear Walls Coupled Shear Walls

• As the result of quite a bit of work done at MKA, an energy dissipation-based definition for coupled shear walls is close to having been developed. It has been balloted within ACI 318H.

Concrete Shear Walls Coupled Shear Walls

Concrete Shear Walls Coupled Shear Walls

• ACI 318-14

• Section 2.3 Terminology

• Coupled Structural Wall: Special

• Structural Walls linked by coupling beams

• with average aspect ratio (length/height) greater than or equal to 3.0.

Concrete Shear Walls Coupled Shear Walls

• If coupled concrete shear walls are to gain acceptance by ASCE 7 into the R-factor table, it appears a P695 study is required. This may not be a full-scale study, but a more abbreviated study. This needs to be discussed further. Several members have concerns and reservations about the P695 process.

Concrete Shear Walls Coupled Shear Walls

• John Wallace has produced a preliminary proposal for a limited P695 study.

• If such a study is felt to be absolutely necessary, it will be most desirable to form the Peer Review panel at this meeting.

Concrete Shear Walls Shear Design of Special Shear Walls

Vu fVn

ACI 21.9.3 (18.10.3)

Vu is to be obtained from lateral load analysis in accordance with factored load combinations

Concrete Shear Walls Shear Design of Special Shear Walls

Vu fVn

ACI 9.3.4 (21.2.4)

For structures that rely on special reinforced concrete

structural walls to resist earthquake force effects E, f for

shear for any structural member that is designed to resist E is

to be 0.6 if the nominal shear strength of the member is less

than the shear corresponding to the development of the

nominal flexural strength of the member.

Concrete Shear Walls Shear Design of Special Shear Walls

• Nonlinear time history analyses show much higher shear forces developing in shear walls, when compared to the Vu of ACI 318.

Concrete Shear Walls Shear Design of Special Shear Walls

• Moehle and others have split shear amplification into “dynamic amplification” and “overstrength amplification.” Neither has a clear correlation with building height.

Concrete Shear Walls Shear Design of Special Shear Walls

• John Wallace is working on an ACI 318H change proposal that would propose a revised design shear for shear design of shear walls including the effects of overstrength and dynamic amplification. He has researched the background already. Canadian Standard A23.3 already considers dynamic amplification.

Concrete Shear Walls Shear Design of Special Shear Walls

Vu fVn

Capacity side of the inequality

• Bi-axial bending at the corners of core walls

• Rate of loading effect on shear strength

• Effect of compression on shear strength

Concrete Shear Walls Design of Special Shear Walls

Designing for high shear strength leads to nearly elastic behavior of shear walls in flexure in taller buildings. Very little flexural yielding of reinforcement takes place.

Can flexural reinforcement be decreased at the same time the required shear strength is increased?

Concrete Shear Walls Design of Special Shear Walls

• Three classes of behavior: shear governed, transition, and flexure governed.

• Separation based on shear demand? Based on height/length ratio and shear strength?

• There is definitely a transition region between shear governed and flexure governed walls. It is difficult to test in the transition region.

Concrete Shear Walls Design of Special Shear Walls

Another possible scheme for classification:

• Walls of limited ductility

• Ductile walls

– Non-coupled ductile walls

– Coupled ductile walls

• The last two types of walls would have different R values.

Concrete Shear Walls Detailing of Special Shear Walls

Rather than focusing on changes in details, step back and look at the system design approach in general. Each detailed change usually has some merit, but taken together the small changes may discourage the use of concrete shear walls. Furthermore, a broad review of shear wall design may yield more benefit than focusing on detailed changes.

End