propped shear wall is aunique steel and concretebracing system for ret - PDF document

propped shear wall is aunique steel and concretebracing system for retrofitseismic strengthening of existingbuildings that combine frictionsteel braces and concrete shearwalls. This system creates a high-tem that is less expensive and lessarchitecturally intrusive than ei-ther steel braced frames or con-crete shear walls acting alone. Thesystem consists of a tall slenderconcrete shear wall “propped”onal steel braces. During largeprops, along with flexural yieldingat the base of the shear wall, pro-vide seismic energy dissipatingTipping Mar has used this strat-egy in the seismic retrofit of twentystructures in the earthquake-proneSan Francisco, CA, bay area. In thisarticle, the most recent proppedshear wall projects will demon-strate how this lateral strengthen-The I. Magnin Building, Oak-The most recent building retro-fitted with propped shear walls isBroadway, Oakland, CA. Con-structed in 1930, the building is atown Oakland, sporting a greenfaçade. The structure, a flexibleand tough four-story riveted steelModern SteelConstruction / January 2001 By John Wolfe, S.E., David Mar, S.E., and Steve Tipping, S.E. Combining Steel Braces and Concrete Shear Walls for Seismic Strengthening of Existing Buildings The I. Magnin Building, Oakland,CA. Rendering by Thai Nguyen. Seismic rehabs are tough. Existing construc-tion, the architect’s program and/or the devel-oper’s drive to lower costs often restrainengineers. Traditional solutions include con-crete shear walls and steel braced frames—sys-architectural program—and moment frames—a system too flexible to protect existing façades.To this end, propped shear walls can often sat-isfy disparate project demands and budget con- Propped Shear Walls frame measuring approximately100’ by 113’ in plan, contains abasement and large penthouse. Ex-isting floors are reinforced concreteslabs spanning between steeland columns are encased in con-crete fireproofing. The steelcolumns are founded on concretePlan Torsion, Soft Story and SoftThe building’s geometry andsite geology pose special seismicalong the street sides and solid con-crete infill walls between the steelframes on the back two propertylines. As demonstrated in the after-math of many earthquakes aroundor rotation, the front street façades,and especially the outermost street-side corner, can sustain extensiveCompounding the torsion prob-“soft” first story, 24’ tall in contrastrather than being distributed moreevenly over the height of the build-Finally, the site has special geo-ers are relatively soft clays thatground shaking, and the buildingis located only three miles from theHayward fault. The Retrofit Criteriathis was considered a voluntaryseismic retrofit, and the owner hadsome latitude in selecting the retro-fit design criteria. Like many retro-fits of existing buildings in the SanFrancisco bay area, lateral designforces for this project were set at75% of the 1994 UBC lateral forcelevels. This reduction in designforce level increases the chance ofexceedance from 10% in fifty yearsfor modern code to 20% in fiftyyears for this retrofit (see FEMA273 for more information). January 2001 / Modern SteelConstruction Deformed shape of propped shear wall. torsional irregularity. The Propped Shear Wall Solu-The developer, SRM Associates,decided to seismically strengthenthe building as part of the renova-bleTwist. Propped shear wallsprovided the least architecturallyintrusive solution, allowing anopen office environment at theupper floors and open retail spaces,required by the city of Oakland, onthe first floor. The exposedW14x211 props provide reassuringmuscular bracing and an interest-ing architectural feature to the of-fice and retail spaces. To fight both the plan torsionand soft story problems, thepropped shear walls are located 12’away from the windows on boththe street sides of the building.Each vertical concrete shear wall,three feet wide and 12’ long, ex-ing. The steel props intersect thewalls at the top floor. Each wall andpair of props are anchored in athree-foot diameter by 50’ deepSeismic forces are gathered fromof the concrete slabs with epoxy-grouted bolts. The angles areter A36 rods through holes coredthrough the webs of the existingThe prop’s connections act asenergy-absorbing dampers underlarge earthquakes. The friction con-nection is made up of bolted spliceplates that clamp onto the prop.The prop has long slotted holes andbrass shims that are inserted be-tween the prop and splice plates.The brass shims, with their pre-dictable friction coefficient, alongwith a preset bolt clamping force,give the damper slip force. The de-sign of the prop connections arebased on experimental research byPopov, Grigorian and Yang of theUniversity of CAat Berkeleyplates are ” thick Bethlehem SteelV-Star plates with a Charpy V-0 ºF. The bolts are pre-tensioned to66 kips, based on a coefficient ofabove code design force levels. Theslip and ductile plate yieldingoccur well before non-ductile boltshear or W14 fracture. The toughplate material ensures that theThe effectiveness of the friction-damped propped shear wall solu-the existing structure’s characteris-tics and vulnerabilities. TippingMar’s strategy implements a sys-tem that works with the existingstructure to simultaneously correctstrengths and protect the architec-ground shaking. The propped walls are initiallyrigid up to moderate levels of shak-ing, because the friction spliceshold without slipping and protectthe brittle façade. For rare and in-tense ground shaking, the frictionhinges at its base and rocks as arigid element. This adds strength,stiffness and damping while resist-ing the dominant torsion mode andeliminating the soft story. The plas-forces a tilting mode, since the wall“mode shaping” creates a uniformdistribution of drift over the heightof the building, preventing softstory and higher mode effects thatcan cause destructive concentra- Slotted bolted prop connection. The new and the old: props appear alongside a1930s vintage riveted steel transfer girder. works well with the existing flexi-uting rotation demands over all thebeam-column joints in the build-the propped shear wall system candemand and building capacity aregraph of force versus displace-ment. Adescending curve repre-demand, with demand forces gen-erally decreasing with increasingdeformations. Hysteretic dampingby the prop friction connectionsgreatly reduces the inelastic de-mand curve. The retrofitted build-region,” flattens out over the “pri-mary performance region” andmaintains lateral resistance overthe large-deformation “fail-safe”region. The pushover curve of asuccessful seismic design crossescurve, indicating that the structurestill has some reserve strength andBroadway retrofit are shown at left.In small earthquakes and low seis-mic forces, the prop connections donot slip, the system is very stiff andsteeply. This provides good ser-viceability, protecting non-struc-windows and façades from dam-age. Under large earthquakes,seismic forces will exceed the codeelastic design forces, and the slot-ted bolted connections will slip. Asforth over the internal brass shims,tremendous amounts of seismic en-ergy dissipates in nearly rectangu-lar hysteresis loops. When the propconcrete shear wall will begin tosipate large amounts of seismic en-ergy. The pushover curve flattensout at bolt slip and wall yieldingand extends far enough to cross theends of the props may butt to-end of their travel. Forces on thethe notch-tough V-Star splice platesbegin to yield and stretch plasti-cally. Under this stretching, thesplice plates will strain harden, andforces will increase until eventuallythe compression prop buckles.Even after the compression propbuckles, the splice plates in the ten-sion prop will continue to stretch.steeply at the point of buckling and January 2001 / Modern SteelConstruction Inelastic demand and pushover curves for the 2001 Broadway retrofit. Section at prop splice. likely event that under enormousW14 eventually fractures, the no-longer-propped cantilevered shearwall has more than sufficient baseresist P-delta effects under largeseismic drifts, and thereby preventPropped Shear Walls as Archi-tectural FeaturesThe propped shear walls pro-vide distinct architectural advan-tages. Kevin Ames, projectmanager for the developer, SRMAssociates, notes: “In our renova-tion work, we make every effort tonot only create the most viablealso preserve as much of the exist-propped shear wall solutionMagnin Building in terms of visu-ally preserving its classic art decofaçade, while retaining the maxi-mum level of ‘openness’ for the in-Because the diagonal steel propsresist forces only during earth-quakes, they do not need to be fire-proofed and can be concealed inproject architect, Eric Ibsen ofIbsen-Senty Architecture, took fulladvantage of the propped shearwalls: “We recognized the proppedcrete elements in what is princi-pally an open office space. By usingthem as an architectural feature,they became a catalyst aroundwhich we designed the feature con-ference rooms at each floor.”Fabrication, Erection, BudgetSteel erection was straightfor-ward and proceeded on schedule.Project manager Bruce Cox notes:“The erection went smoothlyunder a very aggressive schedule.While the prop-wall work pointhad a construction tolerance ofalso an interesting project for us,since brass shims, Belleville wash-ers and V-Star plate are new ingre-Total structural cost of this retro-fit in the San Francisco bay area’soverheated construction marketwas about $1,735,000, or roughly$27.50 per sq. ft. The structuralabout $664,000. Ed Todd, vice pres-ident of Westfour Corporation (thetwo main challenges for this proj-ect were “the constant fight forPropped shear walls provide anattractive, effective seismic retrofitStructurally, the system providesgood initial lateral stiffness, excel-lent ductility and high damping.Architecturally, the props andoriginal construction of renovatedJohn Wolfe, S.E., David Mar, S.E.,and Steve Tipping, S.E. are all struc-tural engineers with Tipping Mar &Associates in Berkeley, CA. Structural Engineers:Tipping Mar + Associates,Berkeley, CAArchitect:Emeryville, CAETABS