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MODERN STEEL CONSTRUCTION april 2011

NEaRLy aLL bUILDINgS aND STRUCTURES today aredesigned to conorm to the prescriptive strength, detailingand deection limitations specifed in the applicable build-ing code and its reerenced industry standards including theAISC Specifcation (AISC 360), Seismic Provisions(AISC 341),and Prequalifed Connections(AISC 358). These requirementsare intended to provide structures an ability to meet certain

perormance objectives such as resisting likely loading with-out ailure, and normal loading without occupant discom-ort. But in most cases, the ability o the structure to actuallyprovide this perormance is never evaluated.

Perormance-based design is an alternative approach,specifcally permitted under Section 104 o theInternationalBuilding Code, which permits building ofcials to approve anydesign or means o construction on the basis o satisactoryevidence that the completed construction will be capable oproviding equivalent protection to the public as designs thatconorm to the codes prescriptive requirements. This articleprovides a brie overview o perormance-based designsdevelopment history, recent advances in perormance-based

earthquake engineering and some recent applications o thetechnique to building design.

a brief HistorPerormance-based approaches have been permitted by

nearly every U.S. building code in the past 100 years, primar-ily because, 100 years ago, this was the only means availableto allow new technological approaches entry into practice.The prescriptive requirements o early building codes werebased on the observed perormance o real buildings. Whenbuilding ofcials and engineers noted that wood rame struc-tures in dense urban areas lead to requent conagrations, thecodes banned combustible construction in urban settings and

required the use o noncombustible or protected construc-tion. Similarly, in 1933, Caliornia engineers recognized thatunreinorced masonry buildings had collapsed in nearly everyearthquake over the past 100 years and wrote requirementsinto the building code prohibiting such construction wherestrong earthquakes could be anticipated, a requirement thatremains in the code to this day. Perormance-based design

gave building ofcials the ability to approve designs that hadnot been tested by time and real events based on submittalo evidence that the design would perorm adequately. Thisapproach was used to introduce such innovations as rein-orced concrete, welding, high-strength (Grade 50) steel andother technologies common in todays construction.

During the 1970s and 1980s, engineers in the WesternU.S. began to adopt perormance-based design approaches orseismic design, both or new buildings and existing structures.Initially, these eorts were driven by the observation that dur-ing the 1971 San Fernando earthquake, several hospitals andemergency response acilities did not perorm well (see Figure1), creating the demand that important buildings be designed,

not only to protect lie saety, but also to enable continued post-earthquake occupancy and unction. This prompted engineersto adopt judgmentally enhanced versions o the code require-ments or the design o important structures. Later, in the1980s, ollowing a series o Caliornia earthquakes that seemedto occur on an almost annual basis, building owners began torequest that engineers evaluate their existing buildings andupgrade them to achieve various perormance criteria rangingrom protection o lie saety, to post-earthquake unctional-ity, to limiting probable repair costs to specifed percentageso building replacement cost. This created a problem or engi-neers who had no tools, other than their proessional judgment,to determine criteria or these designs.

An explanation o specifc perormance criteria and how the process works.

By ronald o. HamBurger, S.e., p.e., and JoHn d. Hooper, S.e., p.e.

Performnce-bsed Seismic Desin

NASCC: The SteelCONFeReNCe

Ronald O. Hamburger, S.E., P.E.,is senior principal with SimpsonGumpertz & Heger Inc., San Fran-cisco. John D. Hooper, S.E., P.E., is

principal and director o earthwuakeengineering with Magnusson Kle-mencic Associates, Seattle. Both are

AISC Proessional Members.

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april 2011 MODERN STEEL CONSTRUCTION

In the mid-1990s, the Federal EmergencyManagement Agency responded to this needby unding a joint association o the AppliedTechnology Council, the Building SeismicSaety Council and American Society oCivil Engineers to develop consensus guide-lines or seismic rehabilitation o existing

buildings. Initially published as the FEMA273 report, the resulting eort underliesthe present ASCE 31 Seismic Evaluationand ASCE 41 Seismic Rehabilitation stan-dards. Both implement perormance-basedapproaches to evaluation and design, andtogether, orm the core technology underly-ing present-generation perormance-baseddesign procedures, both or seismic engi-neering and also orce-protection design.The standards defne a series o standardperormance levels or structural and non-structural components, illustrated in Figure

2. These range rom Operational, a peror-mance state in which ater a design event, thebuilding and its contents are undamaged, toCollapse Prevention, a state o extreme dam-age to structural and nonstructural systems,just short o collapse. Figure 3 illustrates thebasic ASCE 41 design process.

The process begins with a group ostakeholders including the building owner,building ofcial and engineer jointly select-ing one or more project-specifc peror-mance objectives as the design basis. Eachperormance objective is a statement o the

acceptable building perormance given thatthe structure experiences a particular inten-sity o earthquake motion. Many building

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MODERN STEEL CONSTRUCTION april 2011

method contained in the building code, (2) alinear dynamic procedure, that uses responsespectrum analysis, (3) a nonlinear static (push-over) procedure, and (4) a nonlinear dynamic(response history) procedure. In the nonlinearprocedures, analysis is used to predict peakinelastic deormations on ductile elements andpeak orces on non-ductile elements. Theseare compared against acceptable values odeormation and strength that depend on theelement type (e.g. brace, moment connection)and the material properties and detailing. Forlinear procedures, elastic demand-to-capacityratios are computed as the ratio o strengthdemand to element capacity. These are takenas surrogates or ductility demand and com-pared against acceptable values, similar to, butmore conservative than, those contained in thestandard or use with nonlinear procedures.

In recent years, the ASCE 41 procedures

have become an accepted method not onlyor seismic retroft o existing buildings, butalso or the seismic design o new buildings,including very tall structures. Recently, thePacifc Earthquake Engineering ResearchCenter (PEER) developed perormance-based seismic design criteria or tall build-ings that signifcantly extend and improvethe ASCE 41 procedures, but employ thesame basic technologies and principals. ThePEER methodology, which can be down-loaded rom http://peer.berkeley.edu,requires the use o response spectrum analy-

sis and near-elastic perormance or service-level earthquake shaking, having a 43-yearreturn period, and nonlinear response his-tory analysis or Maximum Consideredearthquake shaking, with a perormancegoal o substantial margin against collapse.

The Next genertion

In 2001, FEMA unded the Applied Tech-nology Council to begin development onext-generation perormance-based seismicdesign criteria. The resulting FEMA P-58document is scheduled or publication in

early 2012. Rather than using standard per-ormance levels to characterize perormance,the P-58 methodology directly uses the prob-ability o incurring casualties, repair costs andrepair time as measures o perormance.

Because the prediction o earthquake per-ormance includes many uncertainties, asso-ciated with prediction o the actual intensityand character o ground motion, the numbero people and contents present in the buildingat the time o the earthquake, the strength andconstruction quality o the building and the

inaccuracy o our analytical techniques, the

ofcials have accepted a pair o standard-ized perormance objectives, designatedby ASCE 41 as the Basic Saety Objective,as being equivalent to the perormanceintended by the building code or Occu-pancy Category I and II structures.

The Basic Saety Objective consists oCollapse Prevention perormance or Maxi-mum Considered Earthquake shaking andLie Saety perormance or Design Earth-quake shaking, both as defned in the ASCE7 standard. Despite this common acceptance,

the original developers o the FEMA 273report envisaged the Basic Saety Objectiveas being slightly inerior to the perormanceobjectives inherent in the building code, butwhich represented a practical equivalent oruse in upgrade o existing buildings.

Perormance verifcation consists o theuse o analysis to demonstrate that the build-ing is capable o meeting the desired peror-mance objectives. ASCE 41 includes ouranalysis types: (1) a linear static procedure, thatis comparable to the equivalent lateral orce


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april 2011 MODERN STEEL CONSTRUCTION

The PEMC Acute Care Tower projectencompasses more than 700,000 sq. t overits 12 stories. Design began the spring o2006 and fnal occupancy is scheduled orJune 2011. The U-shaped plan requiredcareul placement o the BRBF. At the sub-terranean levels, the BRBF are transerredto concrete shear walls.

The BRBF was analyzed to confrm thatit was capable o providing Immediate Occu-pancy Perormance or Design Earthquakeshaking. A non-linear response history pro-cedure was implemented to reevaluate key

member sizes, such as columns and oundationcaissons, and to confrm the fnal system wascapable o delivering immediate occupancyperormance. The resulting analysis indicatedthat the prescriptive essential acility BRBFcode design could be reduced by approxi-mately 200 tons and still meet the intendedimmediate occupancy perormance goal.

This article is the basis o a presentation the authorswill make at NASCC: The Steel Conerence, May11-14 in Pittsburgh. Learn more about The SteelConerence atwww.aisc.org/nascc.

methodology expresses perormance proba-bilistically in the orm o perormance curves.Illustrated in Figure 4, perormance curvesindicate the probability that a perormancemeasure, such as repair cost, will exceed di-erent amounts. The P-58 methodology willpermit perormance assessments or a single,user-defned shaking intensity, defned by

a response spectrum; a user-defned earth-quake scenario, characterized by a magnitudeand distance rom the site; or on a time-basis,considering all earthquakes that may occur,the probability o their occurrence, and theprobable intensity o shaking given that theyoccur. The P-58 methodology will be pro-vided with companion sotware that can per-orm the necessary probabilistic calculations,will produce the perormance curves and alsowill indicate the sources o loss.

Exmples of Recent Use

Two recent projects, the Mineta San JoseAirport Terminal B and Concourse and theProvidence Everett Medical Center (PEMC)Acute Care Tower, used ASCE 41-basedperormance levels outlined previously intheir design. Both buildings were initiallydesigned, and the seismic elements sized,based on the prescriptive building code andAISC standard requirements. The San JoseAirport used a steel, special truss momentrame (STMF) as its primary seismic orce-resisting system while the 12-story PEMCAcute Care Tower consists o steel buckling-

restrained braced rames (BFBF).The Mineta San Jose International Air-

port Terminal B and Concourse extendsmore than 2,100 t in length and reaches 55t in height. The project encompasses morethan 600,000 sq. t and was constructedover a nearly eight-year time rame.

The STMF was analyzed to confrm thatit was capable o providing Lie Saety Peror-mance or the Design Earthquake shaking. Anon-linear static procedure was implementedin confrming the as-designed system wascapable o delivering lie-sae perormance.

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