Modern Steel Construction / November 1998

REMINISCENT OF AN ANCIENTMAYAN TEMPLE, THE NEWNATIONAL HEADQUARTERSfor The Money Store, in WestSacramento, CA, is one of thefirst new buildings in the UnitedStates to use seismic dampers tocontrol a buildings response dur-ing a seismic event. The 11-storypyramid-shaped steel momentframe building occupies approxi-mately 450,000-sq. ft., includinga partial basement and exteriordeck areas.

The ground floor footprint is300 by 300. There is a 10 set-back at each story, and theuppermost floor footprint is 90by 120. The typical story heightis 14, with the exception of theheight between the first and sec-ond level, which is 16. The totalheight of the structure is 156.The typical floor system consistsof structural steel beams (includ-ing W30x116, 124, 132, andW33x141) with 3" lightweight,cast-in-place concrete over 3" x20 ga. metal deck. The struc-tures lateral system consists of

Fluid viscousdampers aredesigned tocontrol this

complexbuildings

response duringa sesmic eventBy H. Kit Miyamoto, S.E.

and Roger E. Scholl

elastic steel moment frames withfluid viscous dampers (FVDs).The typical column sizes wereW14x370 and W14x398. Thestructure is supported onapproximately 580 12"-squareprestressed, precast piles,embedded approximately 75 intothe ground. The geotechnicalengineers, Wallace-Kuhl, deter-mined that a site coefficient ofS2 was appropriate for this stiff,sandy soil site.

DESIGN CRITERIAEarthquake performance, cost

effectiveness and architecturalrequirements were the primaryconsiderations in designing thisbuilding. Additionally, the ownerwanted to minimize disruption ofbusiness operations after amajor seismic event.

Faced with these require-ments, the use of the currentUniform Building Code (ICBO,1994) was insufficient, since itdid not address the performanceobjectives in quantifiable ways.Furthermore, its design philoso-

phy relies heavily on plastichinge formations within thestructural elements to absorbthe seismic energy. This increas-es the uncertainty of the struc-tures non-linear behavior.Therefore, the design team andthe owner agreed that all struc-tural members and connectionsshall remain below yield levelsfor the Design Basis Earthquake(DBE). The maximum inter-storydrift ratio shall be less than0.005 at the DBE to protect non-structural elements. An unre-duced Time History Analysisshall be utilized to study theactual behavior of the structureduring the DBE. The DesignBasis Earthquake is defined as aseismic event with a 10% proba-bility of occurrence in a 50-yearduration. This event is consis-tent with the Blue Book ofStructural Engineers Associationof California.

SITE-SPECIFIC ANALYSISThe geotechnical consultant

performed a site-specific groundmotion study for this site.Historically, the most significantground shaking activity occurredduring the Vacavill-Wintersevent (Richter magnitude 6.75)in 1892, which had an epicenterapproximately 22 miles west ofthe site. Deterministic and prob-abilistic analysis were performedto estimate the Peak GroundAcceleration (PGA). These analy-sis revealed that a 0.17-g DBErock site acceleration wouldoccur from an event of 6.5 mag-nitude on the Dunnigan Hillsfault. Since the CaliforniaDivision of Mines and Geologyhas estimated the maximumcredible earthquake (MCE) forthis fault to have a magnitude of6.5 to 6.75, the DBE and MCEare approximately equal in mag-nitude.

Actual California earthquaketimes histories were utilized todevelop site specific responsespectra. These ground accelera-tions were selected based onevents 15 to 25 miles from therecording station, at an alluviumunderlain station, and from an

event with a similar fault mech-anism (thrust faults). Eachorthogonal pair of time historieswere scaled to reflect a PGA, andeach of the six time historieswere then processed using thecomputer program SHAKE. TheSHAKE program computes theresponse of horizontally layeredsoil deposits subjected to verti-cally propagating shear waves.The analysis revealed a 0.38gPGA for the site-specificresponse spectrum. Finally, thesynthesized time history was cal-culated from this response spec-trum for design purposes.

LATERAL FORCE RESISTINGSYSTEM

Numerous lateral systemswere considered to satisfy thedesign requirements set forth bythe owner and the design team.

Conventional shear wall, as wellas eccentric and concentricbraced frames were rejectedbecause they caused two prob-lems: the extent of the structuraland non-structural damage aftera seismic event was potentiallygreat, since stiffening the build-ing caused the natural period ofthe structure to shift to the highacceleration spectra range, andthe amount and location of shearwalls and braces would interferewith tenant improvements. Baseisolation was also rejected as apotential solution because: theadditional cost was greater than5% of the total construction cost,and the effectiveness of the isola-tion was found to be not substan-tial for this long period building.

Steel moment frames withFluid Viscous Dampers (FVDs)were elected as the best alterna-tive for the following reasons:

1. The additional cost for FVDswere less than 1% of the totalconstruction cost.

2. The natural period of thestructure was kept out of thehigh acceleration spectrarange, yet FVDs controlleddisplacement.

3. The choice of location formounting the FVD braces wasmore flexible than convention-al braces, since the force inFVDs is out phase with theframe forces.

4. Plastic hinge formation withinstructural elements and con-nections were prevented.

More than 500 weldedmoment connections are distrib-uted throughout this structurefor increased redundancy. Thedesign team and the owneragreed to reinforce the 6th and7th level connections with coverplates to provide ductility, whilethe other welded connectionsremained non-reinforced. Thisdecision was based on the follow-ing:1. The stress level of the connec-

tions are less than 50% ofyield, and the story driftration is limited to less than0.005 for the DBE/MCEevents. According to the latestresearch data, damage towelded connections would nottake place for this stress anddrift level.

2. The 6th and 7th level connec-tions reached 80% of yield atDBE/MCE events. The coverplates are provided extra safe-ty factor.

3. Based on the design teamsrecommendations, the ownerdecided that reinforcing allconnections would increasethe cost substantially withoutany benefit to the structuresbehavior and performanceduring the DBE/MCE events.

FLUID VISCOUS DAMPERSFluid Viscous Dampers were

selected over other dampingdevices for several reasons. SinceFVDs are velocity-dependentsystems, the forces are out ofphase with the axial loading ofthe columns, and the change inthe natural period of the build-

Modern Steel Construction / November 1998

ing is insignificant. Additionally,the long history of military appli-cation proved the systems relia-bility. The devices selected wereprovided by Taylor Devices Inc.from North Tonawanda, NY.Fluid Viscous Dampers operateon the principle of fluid flowingthrough orifices. A stainless steelpiston travels through chambersfilled with silicone oil, whichflows through an orificein/around the piston head.During a seismic event, the seis-mic energy is transformed intoheat, which dissipates into theatmosphere. The orifice con-struction utilized in FVDs is sim-ilar to that of classified applica-tions for the U.S. Armed Forces,and is considered state-of-the-art.

DESIGN PROCEDURESIn order to expedite the plan

approval process and determinea design starting point, steelmoment frames were designed toconform to ordinary steelmoment frames, as specified bythe 1994 UBC, discounting theeffect of FVDs.

The studies previously con-ducted have indicated that therequired frame member sizeswith 15-20% critical dampingwere approximately equal toUBC member sizes designed fora Rw=6 toRw=12. Code designcriteria is as follows:1. Seismic zone = 32. Rw = 63. Three dimensional dynamic

analysis4. Maximum allowable drift ratio

= 0.004Using the determined member

sizes, time history analysis wereperformed to study the effects ofFVDs. FVD design criteria is asfollows:1. Maximum allowable drift ratio

= 0.0052. All members, connections, and

foundations are to remainelastic using the followingLRFD strength factors: 1.2 DL+ 0.5 LL +/- 1.0 EQ 0.9 DL +/-1.0 EQ

3. The FVD capacity and its con-nections are designed for 1.5times the design force.

4. The allowable damper dis-placement is 2.0 times thedesign displacement.Two different mathematical

models of the building were con-structed to study the effect ofFVDs. One was a simple 2-dimensional stick model, and theother was a complex 3-dimen-sional finite element model.Time History analysis were per-formed using ETABS 6.04, whichutilizes the step-by-step linearacceleration method. All FVDswere modeled as discrete linkelements in order to study theinteraction between the momentframes and the FVDs.

The fundamental period of thebuilding in both directions was2.2 seconds. FVDs providedapproximately 15% of the criticaldamping at each story. A total of60 FVD assemblies (bays) with120 FVD units were distributedthroughout the stories. Thedamping constant for each FVDwas 40, 60, and 80 kipsecond/inch. The design forcewas 160 kip and 290 kips. Theexponential constant was set asa unity, which produced perfectlinear viscous behavior.

COST ANALYSISTotal structural construction

cost, including structural steel,FVDs, metal deck, concrete andfoundations was approximately$10.3 million, which equals $23per square foot. The above figuresatisfied the construction costrequirement, which was not toexceed that of a minimum code-conforming building.

In addition, the structure wassubjected to the following groundmotions:1. A synthesized time history

that matched the UBC Zone 4,S2, response spectra.

2. Recorded time histories fromthe 1994 Northbridge event.Using the previously

described design procedure,moment frames were redesignedto conform to special steelmoment frames of UBC Zone 4.FVDs provided approximately20% critical damping at eachstory.

H. Kit Miyamoto is presidentwith Marr Shaffer & Miyamotoin Sacramento, CA. Roger E.Scholl, now deceased, was for-merly president of CounterQuakeCorporation, Redwood City, CA.