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new trends, new techniquesand current industry issues

INSIGHTS

Jeffrey W. Berman, Ph.D., isTomas &Marilyn Nielsen

Associate Professor of StructuralEngineering and Mechanics atthe University of Washington,Seattle. Jeffrey may be reached atjwberman@uw.edu.

By Jeffrey W. Berman, Ph.D.

Advances in Steel PlateShear Walls

Steel plate shear wall (SPSW) technologyis advancing, making more wide-spreadimplementation of seismic force resistingsystems possible. Tese stiff and ductile

systems have had years of research that has dem-onstrated their excellent seismic performance,explored various details and configurations, andresulted in the design provisions in ANSI/AISC341-10, where they are denoted special plate shear

walls. Te key principal for design is that yieldingis expected in the web plates, at the beams endsand at the column bases.SPSW ductility is superior to braced frame

and even moment frame systems. Figure 1shows the base shear versus story drift behaviorfor a well-detailed SPSW (Li et al. 2014) and aconcentrically braced frame detailed to providebetter ductility than a modern special concentri-cally braced frame (Roeder et al. 2011). Bothare results from two-story, nearly full-scale testswith somewhat different base shear capacities.

Admittedly, the SPSW

strength would have to beincreased to make a directcomparison; however, it isclear that the SPSW hassuperior ductility, retain-ing a larger percentage of

its peak strength to significantly larger drifts. Temaximum drift of nearly 5% for the SPSW ismore than could be expected for many momentframe systems as well.

Recent Advances in SPSWs

Recently, important advances in SPSW designand behavior have been made and are describedbelow. Note that many other considerations fordesign, including the use of reduced beam sectionbeam-to-column connections, perforated webplates, and horizontal struts for tall first stories,

were described along with general SPSW deignmethods in AISC Design Guide 20(Sabelli andBruneau 2006).

Tension Field Action & Web Plate Modeling

Experimental and computational studies on inelas-tic tension field action have recently demonstratedthat the angle of inclination of the web plate ten-sion field changes as the web plate undergoes plasticstrain. Web plate yielding and significant plasticstrain is expected in design level earthquake events

when maximum demands will be imposed on thesurrounding beams and columns. Terefore, itmakes sense that numerical models used for designof SPSWs should use the angle of inclination afteryielding. Equation F5-4 in ANSI/AISC 341-10

was derived using elastic strain energy and is agood approximation of the inclination angle afterweb plate elastic buckling but prior to yielding.Figure 2shows the migration of the inclinationangle with increasing story drift from experi-ments and numerical simulation where the angleapproaches 45. Tis result has been supported byother tests and analyses (Webster et al., 2014) anda 45 angle is proposed for seismic design.

Figure 1. Large-scale SPSW and SCBF test specimens andhysteretic behavior. Courtesy of K.C. sai and NEESHub.

Figure 2. Web plate inelastic tension field action test setup and angle of inclination migration with increasing drift.

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Similar tests and analyses on web plateswithin a pin-connected boundary framehave improved the understanding of theinelastic cyclic response of web plates. Tisunderstanding has led to the developmentof a phenomenological material model thatcan represent the complex web plate behavioras shown in Figure 3 (Webster 2013). Tismaterial model can be used in simple stripmodels of SPSWs and will soon be avail-

able as a material option in OpenSEES. Tisadvance provides an efficient way to modelthe nonlinear behavior of SPSWs.

Improved Efficiency inSPSW Column Design

One of the critical factors limiting the imple-mentation of SPSWs is the large column sizesrequired to resist the combined axial andflexural demands from overturning, frameaction and web plate forces. Recent researchby Li et al. (2014a and 2014b) has developedrecommendations for design that allow the

formation of the column plastic hinges, notat the base as previously recommended, butat a height of to of the story heightabove the base where the moment is typicallymaximum in the compression column. Tisreduces flexural demands significantly anddoes not impact performance of the systemas long as the column does not form a plastichinge at the top of the first story. In full-scaletwo-story tests, Li et al. (2014b) found thata 20% reduction in column weight could beachieved with no impact on performance.

Coupled SPSWs

Coupled SPSWs offer designers the flexibilityto use SPSW systems in cores of taller build-ings but there has been little guidance ondesign methods, steel coupling beam detail-ing, and general behavior until recently.Borello and Fahnestock (2013) describedesign concepts for coupled SPSWs, recom-mend target values for the degree of coupling(ratio of the overturning moment resistedby the individual walls to the total overturn-ing moment), demonstrate significant steel

weight savings when two individual walls arecoupled and show, through nonlinear analysis,that they have excellent seismic performance.Recently completed large-scale tests on twocoupled steel plate shear wall systems as shownin Figure 4 confirmed the numerical analysisresults and validity of the design procedure(Borello 2014).

Self-Centering SPSWs

Minimizing residual drift and ensuringsimple post-earthquake repair strategies isa current focus of much earthquake engi-neering research. In this context, recentresearch has implemented self-centeringsteel moment frame technology in steelplate shear walls. Te system, illustrated inFigure 5, utilizes post-tensioned beam-to-column connections to provide recenteringafter earthquakes and web plate tension fieldaction to provide stiffness and energy dissipa-tion. Recent research on these systems hasincluded the development of performance-

based design recommendations (Clayton etal., 2012), large-scale subassemblage tests toexplore design parameters (Clayton et al.,2013), shake table tests on systems with dif-ferent connections to demonstrate system

performance (Dowden and Bruneau, 2014),and two-story full-scale proof-of-concept testsas in Figure 5(Clayton et al., 2014).

Conclusions and

Future Challenges

SPSWs remain an under-utilized lateral forceresisting system despite their excellent seismicperformance. Some of the recent advances

described here are helping to solve this problemand also push the technology further. A betterunderstanding of inelastic web plate behavioris making the system more efficient to designand analyze, alternative approaches for columndesign have resulted in reduced required sizes,advances in coupled steel plate shear wall designnow provide a solution for building cores, andsystems with self-centering provide a solutionfor applications where minimizing post-earthquake downtime is critical. Additionalinnovation is necessary from practicing engi-neers, fabricators and erectors to develop

fabrication and construction techniques thathelp improve efficiency. Such advances wouldincrease SPSW implementation which, con-sidering its excellent performance, should bea priority for the industry.

Figure 3. New web plate material models for truss elements in strip modeling of SPSW. Figure 4. Coupled SPSW test at the University ofIllinois. Courtesy of Daniel Borello.

Figure 5. Self-centering SPSW schematic and full-scale test specimen.

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References

Borello, D. (2014). Experimental and Analytical Study of Steel Plate Shear Walls withCoupling, Ph.D. Dissertation, University of Illinois at Urbana-Champaign.

Borello, D. and Fahnestock, L. (2013). Seismic Design and Analysis of Steel PlateShear Walls with Coupling.Journal of Structural Engineering, ASCE, Vol. 139,SPECIAL ISSUE: NEES 2: Advances in Earthquake Engineering, 12631273.http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0000576

Clayton, P.M., Winkley, . Berman, J.W., Lowes, L.N. (2012) Experimental Investigation

of Self-Centering Steel Plate Shear WallsJournal of Structural Engineering, ASCE, Vol.138, No. 7, pp. 952-960. http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0000531

Clayton, P.M., Berman, J.W., and Lowes, L.N. (2013). Subassembly esting andModeling of Self-Centering Steel Plate Shear Walls. Engineering Structures, Vol. 56, pp.1848-1857. http://dx.doi.org/10.1016/j.engstruct.2013.06.030

Clayton, P.M., Li, C.H., Dowden, D.M., Berman, J., sai, K.C., Lowes, L.M., Bruneau,M., (2014).Advances in Self-Centering Steel Plate Shear Wall esting and DesignNEESResearch,10thNational Conference on Earthquake Engineering, Anchorage, Alaska, July2014.

Dowden, D.M., Bruneau, M., (2014). Cyclic and Dynamic esting of Self-centering SteelPlate Shear Walls, 10thNational Conference on Earthquake Engineering, Anchorage,

Alaska, July 2014.Li, C.H, sai, K.C., and Lee, H.C. (2014a). Seismic design and testing of the bottom

vertical boundary elements in steel plate shear walls. Part 1: design methodology.Earthquake Engineering and Structural Dynamics, http://dx.doi.org/10.1002/eqe.2443

Li, C.H, sai, K.C., and Lee, H.C. (2014b)Seismic design and testing of the bottomvertical boundary elements in steel plate shear walls. Part 2: experimental studies.Earthquake Engineering and Structural Dynamics, http://dx.doi.org/10.1002/eqe.2442

Roeder, C.W., Lehman, D.E., Clark, K., Powell, J., Yoo, J.H., sai, K.C., Lin, C.H.,and Wei, C.Y. (2011) Influence of gusset plate connections and braces on the seismicperformance of X-braced frames. Earthquake Engineering and Structural Dynamics; Vol.40, pp. 355374.

Sabelli, R., Bruneau, M., (2007). Steel Plate Shear Walls (AISC Design Guide 20), AmericanInstitute of Steel Construction, Chicago, Illinois, 144 p.

Webster D.J., Berman J.W., and Lowes L.N. (2014). Experimental Investigationof SPSW Web Plate Stress Field Development and Vertical BoundaryElement DemandJournal of Structural Engineering, ASCE, Vol. 140, No 6.http://dx.doi.org/10.1061/(ASCE)ST.1943-541X.0000989

Webster D.J. (2013). Te Inelastic Seismic Response of Steel Plate Shear Wall Web Plates andTeir Interaction with the Vertical Boundary Members. Ph.D. Dissertation, University

Washington.