*EAI 3 09 imp - ENEAold.enea.it/produzione_scientifica/pdf_EAI/2009/3/... · earthquake safety and...

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sicurezza post-sismanelle scelte diadeguamentostrutturale pre-sismaLinee guida e applicazioni

Paolo Bazzurro*Karl Telleen**Joe Maffei**Jay Yin**C. Allin Cornell***

* AIR Worldwide Co., San Francisco, CA, USA ** Rutherford & Chekene Consulting Engineers,

San Francisco, CA, USA*** Stanford University, Stanford, CA, USA

(Professor Cornell was instrumental in envisioningthe Advanced Seismic Assessment Method. To thesadness of all of us who worked with him, hepassed away in December 2007)

The Role of Post-earthquake Structural Safetyin Pre-earthquake Retrof it Decision:

Guidelines and ApplicationsCritical structures, such as hospitals, police stations, local administrative office buildings, and critical lifeline faci-lities, are expected to be operational immediately after earthquakes. Any rational decision about whether these struc-tures are strong enough to meet this goal or whether pre-emptive retrofitting is needed cannot be made without anexplicit consideration of post-earthquake safety and functionality with respect to aftershocks. Advanced SeismicAssessment Guidelines offer improvement over previous methods for seismic evaluation of buildings where post-earthquake safety and usability is a concern. This new method allows engineers to evaluate the likelihood that astructure may have restricted access or no access after an earthquake. The building performance is measured

in terms of the post-earthquake occupancy classifications "Green Tag," "Yellow Tag," and "Red Tag," definingthese performance levels quantitatively, based on the structure's remaining capacity to withstand aftershocks.

These color-coded placards that constitute an established practice in US could be replaced by the stan-dard results of inspections (A to E) performed by the Italian Dept. of Civil Protection after an event.

The article also shows some applications of these Guidelines to buildings of the largest utility com-pany in California, Pacific Gas & Electric Company (PG&E).

Le strutture strategiche, quali ospedali,caserme, uffici pubblici e infrastrutture,devono rimanere operativi ancheimmediatamente dopo un eventosismico. Qualsiasi giudizio sulla capacitàdi soddisfare tale obiettivo, o sullanecessità di intervenire preventivamente,deve basarsi sulla valutazione dellasicurezza e dell’agibilità di tali strutturedopo un evento sismico. Le AdvancedSeismic Assessment Guidelinescostituiscono un miglioramento deiprecedenti metodi di valutazione nei casiin cui la sicurezza e la fruibilità post-sisma sono la preoccupazione principale.Il metodo proposto consente ai tecnici divalutare la probabilità che possa esserenecessario limitare, o proibire, l’accesso auna struttura a seguito di un eventosismico. Il comportamento di un edificioè misurato in termini di agibilità post-sisma, sulla scala a tre colori “Verde”,“Giallo” e “Rosso”, che definiscono i

ENERGIA, AMBIENTE E INNOVAZIONE 3/2009 117

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Guidelines. The researchers worked withpracticing structural engineers, who testedthe procedure on several example structu-res [R&C and SGH 2004, Maffei et al. 2006]. The Guidelines were developed for the sei-smic assessment of utility buildings, butthey are applicable to most types of struc-tures. Rutherford & Chekene (R&C) has ap-plied the method to a range of differentbuilding types in PG&E’s utility network. Asubset of these applications will be discus-sed later in this article.

Features of the Advanced SeismicAssessment Guidelines andRelationship to ATC 20The main features of the method include:• Green Tag (G), Yellow Tag (Y), and Red

Tag (R) as quantifiable performance le-vels. The Guidelines provide a frameworkfor rationally make the decison reardingpost-earthquake structure’s accessibilityand functionality;

• explicit use of probabilistic concepts. Themethod incorporates estimates of uncer-tainty regarding how a building will re-spond to ground motions, as well as un-certainty about structural capacity in theintact and damaged conditions;

• structural analysis of the “Intact” structu-re and the “Damaged” structure in se-veral damaged conditions. Whereas mo-st seismic assessment procedures analyzea building only in its intact (pre-earth-quake) condition, the Advanced SeismicAssessment method also includes analy-sis of the damaged (post-earthquake)


Introduction

Methods for the seismic assessment of exi-sting buildings have been in developmentfor a number of years. In the United Sta-tes, milestones in this development includethe ATC-14 report Evaluating the SeismicResistance of Existing Buildings [ATC 1987],the FEMA 273 document NEHRP Guideli-nes for the Seismic Rehabilitation of Buil-dings [ATC 1997], and the recent standardASCE/SEI 41-06 Seismic Rehabilitation ofExisting Buildings [ASCE 2007]. Outside theUnited States, notable publications on sei-smic assessment methods include the ap-proach by Park [1997], Seismic Assessmentand Retrofit of Reinforced Concrete Buil-dings from the Féderation Internationaledu Béton [FIB 2003], and Assessment andImprovement of the Structural Performan-ce of Buildings in Earthquakes from theNew Zealand Society for Earthquake En-gineering [NZSEE 2006]. Advanced SeismicAssessment Guidelines, described in thispaper, offer new features and advantagescompared to previous methods.Researchers at Stanford University, withfunding from the Pacific Earthquake En-gineering Research (PEER) Lifelines Pro-gram, have developed Advanced SeismicAssessment Guidelines [Bazzurro et al.,2006], which offer a methodology forquantifying expected structural performan-ce in terms of the post-earthquake occu-pancy classifications “Green Tag,” “YellowTag,” and “Red Tag.” These occupancy clas-sifications are called “limit states” in the

Paolo Bazzurro, Karl Telleen, Joe Maffei, Jay Yin, C. Allin Cornellriflettore

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livelli di comportamento, sulla base della capacità residua della struttura di resisterea scosse successive. In Italia i tre colori, che negli Stati Uniti costituiscono una praticaconsolidata, potrebbero essere sostituiti dagli esiti usuali delle ispezioni (dalla A allaE) eseguite dal Dipartimento della Protezione Civile a seguito dell’evento.Nell’articolo sono anche mostrate alcune applicazioni delle Guidelines agli edificidella più importante compagnia fornitrice di gas e di energia della California, laPacific Gas & Electric Company (PG&E)


mine whether the building should be po-sted Green or Red. In practice, the YellowTag has been used to permit occupancyby a limited number of people and/or fora limited period of time. For example, su-ch a posting might allow property ownersto retrieve possessions from their residen-ces, or it might permit emergency person-nel to briefly access switching equipmentin utility buildings.Post-earthquake occupancy status is typi-cally determined after an earthquake byan inspector performing a visual screeningof the damage and assigning a Green, Yel-low, or Red Tag to the building to indicatefull access, limited access, or no access per-mitted. For different building types, ATC-20indicates particular damage observationsthat can be associated with each tag co-lor. In qualitative terms, ATC-20 states thatto receive a Green Tag, “the structure mu-st be capable of withstanding at least a re-petition of the event that caused the ini-tial damage without collapse” (p. 26).The Advanced Seismic Assessment Guideli-nes are not a replacement for ATC-20’s ra-pid post-earthquake evaluation procedu-res. Instead, the Advanced Seismic Asses-sment method defines quantitative perfor-mance criteria for use in (pre-earthquake)seismic assessments so that they can be re-lated to the ATC-20 framework. Expectedpost-earthquake tagging classifications areused because they are more specific andmore quantifiable alternatives to traditio-nal performance classifications such as “Li-fe Safety”. The method leads to retrofitrecommendations that are directly relatedto building accessibility. Also, for buildingsthat have been assessed according to theAdvanced Seismic Assessment Guidelinesprior to an earthquake, descriptions of ex-pected visible damage, of the type thatcan be drawn from the method, will assistpost-earthquake inspectors in making ra-tional tagging decisions when an earth-quake does occur.

structure to assess the structure’s remai-ning capacity to withstand aftershocks;

• emphasis on identifying the structure’sgoverning mechanism of nonlinear re-sponse, that is which structural elementswill yield first in an earthquake, and howdoes the post-yield behavior of these ele-ments (ductility and strength gain vs.strength degradation) affect the overallstrength and stability of the building?

• the effect of residual drift (horizontal di-splacement that remains after an earth-quake) on the capacity of the damagedstructure to resist aftershocks.

Some features listed above draw on con-cepts from the FEMA 306/307/308 docu-ments Evaluation of Earthquake DamagedConcrete and Masonry Wall Buildings [ATC1999]. The FEMA 306 approach includesan emphasis on identifying the governingmechanism of building response and thebehavior mode for critical members of astructure, and it considers how earthquakedamage sustained by structural compo-nents affects the future earthquake-resi-sting capacity of a structure.The seismic performance levels “GreenTag,” “Yellow Tag,” and “Red Tag” rela-te to the established practice of post-earthquake occupancy classification, or“tagging,” described in ATC 20 Procedu-res for Post-earthquake Safety Evaluationof Buildings [ATC 1989]. ATC-20 formali-zed a systematic method of posting pla-cards on buildings immediately followingan earthquake to indicate whether or noteach building is safe for occupancy. Theposting of a Red Tag on a building meansthat occupancy is not permitted becauseit would be unsafe to be inside the buil-ding in case of an aftershock. The postingof a Green Tag means that occupancy ispermitted because the building’s earth-quake-resisting capacity has not been si-gnificantly compromised. The posting ofa Yellow Tag, per ATC-20, means thatfurther investigation is necessary to deter-

Il ruolo della sicurezza post-sisma nelle scelte di adeguamento strutturale pre-sisma: linee guida e applicazioni

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Sa(T1) needed to induce a certain level ofdrift in the original building. This step wasmade practical by using a tool calledSPO2IDA (Vamvatsikos and Cornell, 2002).This spreadsheet provides an estimate notonly of the Sa(T1) vs. δ curve (Vamvatsikosand Cornell, 2001) for the building but al-so of its record-to-record variabilitywithout the need of running any nonli-near dynamic analysis of the real structu-re. For most low- and mid-rise buildings,where higher mode effects typically donot significantly influence nonlinear dy-namic behavior, the SPO2IDA programprovides a consistent and practical meanfor obtaining a Sa(T1) vs. δ curve for thebuilding within an acceptable accuracy.

Step 2: Definition of damage statesat roof drifts where significantlosses in seismic-force-resistingcapacity occur (figure 1) An inspection of the nonlinear dynamicanalyses results or of the pushover curveidentifies typical patterns of damage thatare reached for given levels of roof drift.These damage patterns are called “dama-ge states” in the Guidelines. A damage sta-te is a condition that a building could beleft in after a (mainshock) earthquake. Forexample, a building with concrete wallscould have minimal cracks, it could havesmall cracks, or it could have large cracksand buckled reinforcing bars; presumablythe earthquake-resisting capacity of thedamaged walls is reduced from what itwas before the first earthquake. For prac-tical purposes, the identification of two tofour representative damage states is typi-cally sufficient. Damage states should bechosen to capture the range of damageseverity (or roof drifts) that will affect struc-tural performance in aftershocks. In cer-tain cases, note that the residual drift af-ter the mainshock can also play a signifi-cant role in assessing the reduced capacityof the damaged buildings. In those cases


Basic stepos of the AdvancedSeismic Assessment Method

The basic conceptual steps in the Advan-ced Seismic Assessment process are as fol-lows (for more details refer to Bazzurro etal., 2006; and Maffei, et al., 2006).

Step 1: Relationship betweenground motion intensity and ameasure of structural response forthe intact structure (figure 1)This step can be done by building a com-puter model of the structure capable ofcapturing its nonlinear behavior andsubjecting it to a large suite of groundmotion records including some strongenough to cause collapse. The “strength”of each record can be gauged, for exam-ple, by the spectral acceleration, Sa(T1), atthe fundamental initial period of vibra-tion, T1, of the intact building, while theperformance of the building can be re-presented in terms of an appropriate re-sponse measure, such as roof drift, δ. Therelationship between, say, Sa(T1) and δ canbe established via regression analysis. Ifthe structure is strong and ductile, thenrecords may need to be scaled so thatthey are sufficiently strong to collapse thestructure. In this alternative case that in-volves record scaling, the procedure isknown as Incremental Dynamic Analysis,IDA (Vamvatsikos and Cornell, 2001). The-se two alternative procedures that usenonlinear dynamic analysis are, however,beyond what most practicing engineersare prepared to routinely do today. The-refore the guidelines followed an alter-native route to produce the relationshipbetween Sa(T1) and δ that involves a)performing a pseudo-static (pushover)analysis of the building and b) use a di-stillation of myriads of analyses of nonli-near single-degree-of-freedom oscillatorswith the same force-deformation back-bone curve of the building to infer the


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and Maffei et al. [2008] offer recommen-dations for modeling strength degrada-tion and accounting for residual drift.Although impractical, as in the case of theintact building, the ultimate capacity ofthe damaged building in each damage sta-te can be more accurately estimated usinga suite of nonlinear dynamic analysesperformed on a computer model of thebuilding rather than the simplified methodsuggested above. In the damaged buildingcase, the residual capacity can be estima-ted via back-to-back nonlinear dynamicanalyses that mimic the sequence main-shock-aftershock affecting the building.The mainshock will be appropriately sca-led in such a way that the building will ex-perience the desired damage state and theaftershock will be appropriately scaled insuch a way that brings the damaged buil-ding to the verge of collapse (figure 3).This exercise was originally devised by Lu-co et al. [2004] and then followed by R&Cand SGH [2004].

Bazzurro et al. [2006] and Maffei et al.[2008] provide guidance for estimating re-sidual drift and reducing a building’s earth-quake-resisting (displacement) capacity ac-cordingly.

Step 3: Estimation of the ResidualCapacity of the Damaged Structure(figure 2)This is achieved using similar methods tothose in Steps 1 and 2 although out of theresulting Sa(T1) vs. δ curves only the right-most part that relate to the ultimate ca-pacity is retained. The strength and stiff-ness degradation of damaged elementswill cause the damaged structure to haveproperties that are different from those ofthe intact structure and these should beaccounted for in the structural analysis mo-del. The procedure is repeated for each da-mage state defined in Step 2. If significantresidual drift is expected, the deformationcapacity of the damaged structure shouldbe appropriately reduced. Luco et al. [2004]


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Figure 1Steps 1 and 2: Nonlinear Analysis of the Intact Structure. A. Nonlinear static force-displacement response (pu-shover curve) for the intact structure, with damage states (DS1, DS2, DS3, and DS4) defined at roof drifts wheresignificant losses in seismic-force-resisting capacity occur. B. Sa(T1) vs. δ curve for the intact structure, obtained usingthe four-segment simplification of the static response from (A) and the spreadsheet tool SPO2IDA. SPO2IDA (orregression, which was not used in this case) accounts for variability in nonlinear dynamic response given Sa(T1).For example, the IDA-84% curve indicates 84% confidence that the corresponding roof drift will not be excee-ded for a given level of Sa(T1). In the elastic range, the Sa(T1) vs. δ curves are the same as the static response; at grea-ter roof drifts, the larger variability in nonlinear dynamic response make the IDA curves spread apartSource: Pacific Earthquake Engineering Research Center (PEER)

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Step 4: Definition of OccupancyLimit States

Original GuidelinesThe objective of this step is to apply a spe-cified criterion that states whether thestructure in a given damage state afterthe mainshock should be assigned a G, a Y,or a R tag, i.e., one of the three occupancylimit states considered here. This step iscentral to the Advanced Seismic Asses-sment method because it relates theperformance of the building in an earth-quake (main shock) to the safety of occu-pants following the earthquake (definedin terms of collapse potential in the eventof an aftershock). The specific criteria thatdefine the assignment of an occupancy li-mit state to a damage state are a matterof policy rather than engineering. The ori-ginal Guidelines define the rules of thisassignment based on the likelihood thataftershocks will exceed the (reduced) ca-pacity of the damaged structure after amainshock and compares it with the like-lihood of collapse of the structure in its in-



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Figure 2Step 3: Nonlinear Analysis of the Damaged Structure shown here, again, as a coupling of static pushover andSPO2IDA rather than nonlinear dynamic analysis of the building followed by statistical regression. A. Nonlinearstatic pushover curve for damaged structure in Damage State DS3 - B. Sa(T1) vs. δ curve for DS3, obtained using thefour-segment simplification of the static response from (a) and the spreadsheet tool SPO2IDA. (Similar resultsshould be created for damage states DS2 and DS4; though they are not shown here.)Source: Pacific Earthquake Engineering Research Center (PEER)

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Figure 3Nonlinear response history for a selected main shock tocause damage state DS4, followed by an incrementally sca-led aftershock. (Similarly, response histories must also berun for the intact structure and for damage states DS2 andDS3; though for brevity, they are not shown here.) [Figu-re reprinted from R&C and SGH, 2004.]Source: Pacific Earthquake Engineering Research Center (PEER)

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Figure 4Step 4: A. Graphical interpretation of the recommended tagging criteria in the original Guidelines - B.Average relationship for loss of ground motion capacity and rate of increase in mean annual frequencyof exceedance of ground motion for coastal California sitesSource: Pacific Earthquake Engineering Research Center (PEER)

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tact conditions. More precisely the propo-sed criteria for tagging a damaged buil-ding are expressed in terms of:• P0, the building-site-specific mean an-

nual frequency (MAF) of exceedance ofthe ground motion corresponding to theultimate capacity of the building in itsintact conditions. P0, which refers to thepre-earthquake conditions, can be ob-tained using conventional ProbabilisticSeismic Hazard Analysis (PSHA) codes ordirectly from the USGS website (http://eqhazmaps.usgs.gov/) or INGV Site(http://zonesismiche.mi.ingv.it/) for a se-lected set of oscillator periods and firm-soil to soft-rock conditions;

• P, is the building-site-specific MAF of theaftershock ground motion level corre-sponding to the residual capacity of thebuilding in a given damage state;

• estimates of the ultimate capacity forthe intact building and of the residualcapacities of the damaged building forall the considered damage states.

These tagging criteria are summarized infigure 4. The figure has two scales for the

ordinates, the percentage of loss in ultima-te capacity and the ratio of P/P0 that measu-res the increase in frequency of exceedingthe residual capacity of the damaged buil-ding. The relationship between the twoscales (see Panel b) has been tuned for coa-stal California sites, for which the absolutevalue of the (log-log) slope of an averageground motion hazard curve in the sur-roundings of 10–3 annual frequency of ex-ceedance is about three. The hazard curveslope, which depends on the magnitudedistribution and on the rate of decay of sei-smic waves with distance, is lower in Ea-stern United States and in Italy, for exam-ple. Hence, caution should be exercisedwhen using this chart in other areas of theworld. The abscissa refers to the pre-earth-quake conditions only; it is simply the long-term MAF of exceedance of the intact buil-ding ground motion capacity. Before explaining the basis for the defini-tion of the tagging “fields” in figure 4, letus describe how the tagging criteria can beused. Any building is identified by a particu-lar value of P0 that can be computed du-

mewhat arbitrarily at constant values ofcapacity loss of 2%, 20%, and 40%.“Weak”, under-designed buildings thatare potentially unsafe (i.e., larger P0) evenin pre-earthquake condition would not betagged Y or R and possibly, at a later stage,retrofitted unless some identifiable physi-cal damage occurred in the building. Tag-ging Y or R an undamaged building wouldbe difficult to accept by owner and occu-pants. A hardly detectable capacity loss ofonly 2% encourages Y and R tagging andit is meant here to simply serve as a trig-ger for action for such buildings. The linesat 20% and 40%, that increase the Y andR tagging areas of “strong” (smaller P0)buildings, have been dictated by a diffe-rent concept. Results from dynamic analy-ses have shown, in general, that a ratherwidespread damage in the building is nee-ded before the capacity drops by theseamounts. In cases of widespread damagethe assessment of the true building capa-city is more uncertain and it is conservativeto force some restriction of occupancy un-til further more detailed analyses areperformed.Finally, note that P is, strictly speaking, atime-varying quantity that decreases withtime elapsed from the mainshock and, the-refore, it is better computed using an af-tershock PSHA (APSHA) approach (Wiemer[2000], Yeo and Cornell, 2008). However,software for performing APSHA is not yetwidely used. For the sake of simplicity, thecriteria in figure 4 assumed that pre-earth-quake and post-earthquake hazard do notdiffer. This assumption is removed in thestudy by Yeo and Cornell [2005] where thetagging criteria accounts for the increasedprobability of collapse from pre- to post-earthquake conditions due both to decrea-sed capacity and to increased (aftershock)seismicity.

Modified GuidelinesThe modified Guidelines (figure 5) follow amore conventional approach in line with


ring “peace” time before any earthquakehas occurred. A larger value of P0 may im-ply that the building is relatively “weak”or that it is located in an area of compara-tively high seismicity, or both. The oppositeis true for lower values of P0. How the tagcolor changes with capacity loss can befound by searching on a vertical line at thatspecific value of P0. A building whose P0 isequal, for example, to 3 x 10–4 needs to bedamaged severely enough to lose about5% of its initial capacity before it is taggedY and about 30% before it is tagged R. Ifthe intact building had been weaker or in aharsher seismic environment such that itsvalue of P0 were equal, for example, to 1x 10–3, then a nominal loss of lateral capa-city of only 2% or larger would cause thebuilding to be R-tagged. No Y tag could beassigned in this case, either G or R.If the tagging criteria were simply basedon maximum acceptable collapse risk ofthe partially damaged building, then theG, Y, and R tag areas would be obliquebands delimited by straight lines of con-stant P values. For these guidelines P va-lues were selected to be equal to 2% in 50years (mean return period, MRP, of 2,475years) and 5% in 50 years (MRP of 975years) for the boundaries between G and Ytags and between Y and R tags, respecti-vely. These values may appear too restricti-ve when compared to building code requi-rements for new buildings that prescribelife safety as performance objective for a10% in 50 years (i.e., MRP of 475 years)ground motion level. Low values were se-lected to implicitly and partially accountfor the increased aftershock hazard thatthe damaged building is subject to whenthe inspection may take place perhaps oneor two days after the earthquake. Thesevalues, which represent quantitative mea-sures of acceptable risk, should be modi-fied according to the building importan-ce and severity of failure consequences. The diagonal bands in figure 4, however,are delimited by horizontal lines drawn so-


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uses the loss of capacity for each damagestate coupled with (aftershock) hazard con-siderations and criteria on allowable riskto assign each damage state to a limit sta-te that restricts occupancy after the main-shock (i.e., Y or R). These considerationscan guide the decision on whether the re-trofit of a given building at a given siteprior to the mainshock is advisable.Let’s first apply the criteria from the origi-nal Guidelines to the example case in figu-res 1 and 2. Step 3 tells us that the residualspectral acceleration capacity of the dama-ged structure in damage states DS3 andDS4 would be about 20% and 40% lowerthan the original capacity of the intactstructure, respectively (see figure 5 wherethe capacity values are plotted). If theanalyses in Step 1 and a PSHA for the gi-ven site showed that this structure had a

ATC-20. The limit state “Onset of Y Tag” isdefined as having 84% confidence of no col-lapse in an aftershock ground motion equalto the main shock ground motion. (The ca-pacities extracted from the 84% Sa(T1) vs.δ curves in Step 3 are used.) “Onset of RTag” indicates 50% confidence (i.e., the ca-pacities from the 50% Sa(T1) vs. δ curves areused). Based on these boundaries, betterthan 84% confidence represents G Tagperformance, between 50% and 84% con-fidence is Y Tag, and less than 50% confi-dence is R Tag. See Maffei et al. [2008] forfurther discussion of these tagging criteria,their development, and adaptability.

Step 5: Pre-earthquake RetrofittingDecision based on Post-earthquakeFunctionalityThe tagging procedure presented above


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Figure 5Step 4: Graphical interpretation of the tagging criteria in the Modified Guidelines.For a given spectral acceleration demand, the building's expected seismic perfor-mance level (G, Y, or R Tags) is based on the capacity of the damaged structure (af-ter an earthquake) to survive an aftershock without collapse. Point A marks the li-mit state “Onset of Y Tag”; point B marks “Onset of R Tag.” Intersection with thehorizontal axis marks “Collapse” in the main shockSource: Pacific Earthquake Engineering Research Center (PEER)

gure 5 are compared with the values ofSa(T1) from the uniform hazard spectra atdifferent probability of exceedance in 50years for the building site (figure 6). If thelikelihood that the Spectral accelerationvalues corresponding to the onset of Y andR states are deemed acceptable, then noaction is required, otherwise the buildingshould be retrofitted.Finally, note that the decision above hasbeen simplified to assume that if the valueof the mainshock Sa(T1) that corresponds(according to whichever of the two crite-ria above is used) to the onset of Y or R isobserved at the building site, then the DSthat grants a Y or a R tag is reached withcertainty. Of course, it is possible that thatthe damage state at the onset of Y or R isreached at a lower or higher value of Sa(T1).Accounting for this uncertainty requiresthe development of a fragility curve, who-se details are beyond the scope of this ar-ticle. Interested readers should refer to Baz-zurro et al. [2006] and Maffei et al. [2006].

Example applications

The co-authors at R&C have applied themodified version of the Advanced SeismicAssessment method to perform seismicevaluations of several buildings in PG&E’sutility network (for further details see Tel-leen et al., 2009). The buildings includeelectrical substations, a parking facility, andcommand centers (figure 7). The buildingsrange from one to eight stories, with origi-nal construction ranging from 1908 to the1990s. The structural systems include steelmoment frames, concrete walls, and con-crete walls with steel frames.For each building, the seismic assessmentand design for seismic retrofitting (if ne-cessary) focus on these key objectives.

Ensure adequate lateral displacement capa-city of the gravity-load-resisting system,particularly columns. Gravity-load-resisting


P0 of less than 2 x 10–3, then figure 4 wouldsuggest that such loss of capacity wouldput the DS3 and DS4 at the onset of Y andR limit states, respectively. The Sa(T1) vs. δrelationship defined in Step 1 defines thevalues of the mainshock Sa(T1) that are ex-pected to bring the intact structure in tho-se two damage states (e.g., cause a valueof δ that correspond to DS3 and DS4) and,therefore, cause a Y or a R occupancy re-striction after the mainshock. If the MAFat the building site of those values of themainshock Sa(T1) that correspond to DS3and DS4 are unacceptably high, then thebuilding should be retrofitted. (Note thatif the capacity loss values correspondingto defined damage states do not coincidewith the onset of Y and R limit states as inthe example above, then some interpola-tion is needed. The details are omitted he-re for brevity but can be found in Bazzur-ro et al. 2006.)The application of the modified Guideli-nes is more straightforward. The values ofthe Onset of Y and R limit states from fi-


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Figure 6Step 5: One method for comparing earthquake hazard tostructural performance: uniform hazard spectra at diffe-rent probability of exceedance in 50 years overlaid to ex-pected main-shock spectral acceleration to cause each li-mit state (the ordinates of points A and B in figure 5) in-dicated at the building’s fundamental period of vibrationSource: Pacific Earthquake Engineering Research Center (PEER)

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than they are in flexure, or if walls are notcontinuous to foundations. To ensure adesirable mechanism of response, we firstidentify the governing response of the exi-sting structure. If the existing structure’sresponse is inadequate to achieve the de-sired seismic performance for the building,we provide retrofit items that change thegoverning response to a more desirablemechanism (for example, providing addi-tional shear strength to concrete walls sothat they will yield in flexure in a majorearthquake and will not exceed theirshear strength capacity).Both identification of the existing struc-ture’s response and design of seismic re-trofit items are based on the principles ofcapacity design [Paulay and Priestley 1992].For evaluation of existing structures, thismeans determining the expected strengthsof structural components and then perfor-ming a plastic analysis, a nonlinear analy-sis, or a comparison of relative strengthsin order to establish which component ac-tions (e.g. flexure in walls, shear in walls,flexure in columns, flexure in beams, etc.)reach their capacities under the expectedpattern of seismic lateral forces [Maffei2000]. For designing retrofit items, this

elements must be able to deform laterallywith the building while supporting verti-cal loads. The lateral displacement capa-city of concrete columns, for example, canbe limited by inadequate shear reinforce-ment ties. If lateral deformations are ex-pected to be larger than the deformationcapacity of existing columns, we can in-crease the columns’ deformation capacity(such as by fiber wrapping), or we can pro-vide an alternate path for gravity loads (su-ch as supplemental columns placed adja-cent to the vulnerable columns).

Ensure that the structure’s governing me-chanism of nonlinear seismic response isdesirable. For all but the simplest buil-dings, there are several potential waysthat the building could respond to astrong earthquake, and the seismic asses-sment should consider each of these po-tential mechanisms. For example, in a mo-ment frame building, an undesirable“story mechanism” (a concentration of la-teral deformations in a single story) isusually more likely if beams are strongcompared to columns. In a building withconcrete walls, a story mechanism is usual-ly more likely if walls are weaker in shear


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Figure 7PG&E buildings that were evaluated and, if needed, retrofitted using the Advanced Seismic As-sessment methodSource: Rutherford & Chekene Consulting Engineers

of reinforced concrete slabs, beams, co-lumns, and spread footings. A concretepier-and-spandrel system around the buil-ding perimeter provides the majority ofthe building’s strength and stiffness to re-sist lateral seismic forces. The building is L-shaped in plan, with two expansion jointsat each floor level.The building serves as a parking andmaintenance facility for PG&E’s fleet oftrucks, and PG&E’s seismic performancegoals for the building focus on enablingtrucks to safely exit the garage followingan earthquake so that technicians maydrive to sites within the utility networkand perform post-earthquake repairs. Thislevel of post-earthquake occupancy re-quires G or Y Tag performance, and theseismic retrofit is designed to achieve thisperformance should a ground motionwith 10% probability of exceedance in 50years be experienced at the site.To achieve PG&E’s desired seismic perfor-mance, the primary retrofit objectives forthe structure are to increase the lateraldisplacement capacity of gravity-loadcarrying elements, and to change the go-verning mechanism of lateral deforma-tion from one that concentrates seismicdisplacements in a single story to one thatdistributes displacements over multiplestories (figure 9). Whereas the concentrated displacementmechanism tends to exhibit strength de-gradation caused by shear failure in exi-sting wall piers, the distributed displace-ment mechanism exhibits post-yieldstrength gain as more structural elements,including spandrels, columns, and beams,contribute to the structure’s overall late-ral force resistance.The retrofit design includes the followingitems:• Fiber jackets at interior columns that

enable the columns to maintain gravityload-carrying capacity when subjectedto lateral displacements in an earth-quake.


means designating the desired compo-nent actions that will experience nonli-near behavior (usually material yielding),and providing all other actions with suf-ficient strength to ensure essentially ela-stic behavior (little to no yielding) whenthe designated yielding actions reach theircapacities. For both evaluation and retro-fit, we consider the post-yield behavior ofall elements expected to reach their yieldstrength, in order to estimate the overallbuilding response.

Provide ductile detailing and adequateconnections. To ensure desirable post-yieldbehavior and avoid strength degradation,we provide special structural details atmembers that we expect to yield in amajor earthquake. Ductile detailing means,for example, providing closely-spaced ho-rizontal (shear) reinforcement in concretewalls and columns to promote flexure-go-verned behavior and to restrain bucklingof vertical reinforcement under cyclic loa-ding. To ensure a complete force path forcarrying earthquake forces to the building’sfoundations, we check connectionsbetween elements of the seismic-force-re-sisting system in the existing structure, aswell as in the seismic retrofit design. If con-nections represent weak links in the sei-smic force path, we provide strengthenedconnections and/or ensure that yieldingconnections have ductile detailing.Providing the type of ductile behavior de-scribed above tends to make structures re-silient to earthquakes, such that they cansustain some damage in a large main shockand still provide adequate safety in theevent of an aftershock. The followingexamples describe seismic assessment andretrofit projects for selected buildings infigure 7.

San Francisco Central Services GarageThis is a three-story 140,000-square-footbuilding built in the early 1930s (figure 8).The gravity-load-resisting system consists


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• Steel supplemental columns at existingperimeter wall piers and pilasters. Thenew steel columns provide gravity load-carrying capacity in the event that anearthquake causes lateral displacementsand shear cracking in the existing peri-meter wall piers and pilasters.

• Strengthened concrete wall piers thatprevent concentration of lateral defor-mation in a single story and increase thebuilding’s overall strength and stiffness.

• Steel ledger angles at existing slab ex-pansion joints that increase lateral di-splacement capacity to prevent unsea-ting of slabs and beams in an earth-quake.

Construction of this retrofit work on theCentral Services Garage began in 2009.

Vaca-Dixon SubstationThis is an 18,000-square-foot 1922 buildinglocated alongside Interstate 80 near Vaca-ville, California (figure 10). It consists of atall one-story “crane bay” portion and ashorter two-story “switch house” portion.The superstructure consists of a steel gra-vity frame with concrete walls. In 1924, aone-story addition was added to the swit-ch house, and a concrete wall was demoli-shed at the Ground Floor, where the ad-dition connects to the original building.Most of the original electrical equipment

Figure 8San Francisco Central Ser-vices Garage. A. West ele-vation - B. First Floor inte-riorSource: Rutherford & ChekeneConsulting Engineers

A B

Figure 9Retrofit objective: Change the governing mechanism of lateral deformation from one that concentra-tes seismic displacements in a single story to one that distributes displacements over multiple storiesSource: Rutherford & Chekene Consulting Engineers

tion of the telecommunications room lo-cated at the Ground Floor in this area.The retrofit design includes a steel buck-ling-restrained brace in the crane bay por-tion of the building, located inline withthe existing discontinuous wall (figure 10b).Whereas conventional steel braces havegreater strength in tension than in com-pression, a buckling-restrained brace re-sponds approximately the same in com-pression as in tension. This symmetric beha-vior is desirable in the single-brace confi-guration shown here because a conven-tional brace would tend to “ratchet” thebuilding toward the compression (buck-ling) direction in a strong earthquake.A new steel collector transfers lateral sei-smic forces from the discontinuous wall


has been replaced with outdoor equip-ment in the adjacent yard, but the switchhouse still includes a telecommunicationsroom, offices, and some control equip-ment. PG&E’s seismic performance goal forthis building is to achieve Y Tag or G Tagperformance so that technicians may sa-fely access telecommunications equipmentif necessary following an earthquake.To achieve this performance, the structu-ral seismic retrofit objective is to mitigatethe discontinuous concrete wall createdduring construction of the 1924 buildingaddition (figure 11). While a straight-forward structural solution would be toadd concrete walls or steel braces directlybelow the discontinuous wall, constructionof such a scheme would disrupt the func-


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Figure 11Vaca-Dixon Substation, buildingsection showing seismic retrofititems. From Lines 1 to 4 is the"crane bay" portion of the buil-ding; Lines 4 to 8 is the "switchhouse" portion. A concrete wall(with window openings) existsat the Second Floor of the swit-ch house to resist lateral seismicforces from the Low Roof, butthere is no structural wall at thislocation at the Ground FloorSource: Rutherford & Chekene Con-sulting Engineers

Figure 10Vaca-Dixon Substation. A. Southelevation - B. Added buckling-restrained brace to mitigate di-scontinuous concrete wall.(See figure 11 for building sec-tion showing discontinuouswall)Source: Rutherford & Chekene Con-sulting Engineers


It includes a Basement, Ground Floor, LowRoof, Mezzanine Level, and High Roof. Thesuperstructure consists of a steel framewith concrete walls. Precast concrete pa-nels clad the building perimeter.The building houses transformers, circuitbreakers, switchgear, and control equip-ment. Transformers generate heat duringoperation, and the building is configuredto provide continuous airflow through thetransformer rooms to cool the transfor-mers. The seismic retrofit solution includesspecial considerations to avoid work

and the Second Floor diaphragm to thebuckling-restrained brace, and a new con-crete wall with tiedown anchors transferslateral and overturning forces to theground.Construction of the structural seismic re-trofit work described above for the Vaca-Dixon Substation was completed in 2008.

Larkin SubstationThe Larkin Substation in San Francisco is a75,000-square-foot building constructedin two phases during the 1960s (figure 12).


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Figure 12Larkin Substation. A. South ele-vation showing precast concre-te panel cladding - B. ControlroomSource: Rutherford & Chekene Con-sulting Engineers

A B

Figure 13Larkin Substation, building section showing seismic retrofit itemsSource: Rutherford & Chekene Consulting Engineers

fic buildings according to post-earthquakesafety classifications, such as “Green Tag”or “Yellow Tag”. For example, for an elec-trical substation, PG&E technicians shouldbe able to safely access switching equip-ment in the building to help bring powernetworks back online, in the event of outa-ges caused by an earthquake.To achieve PG&E’s performance goals foreach building, R&C performed seismic eva-luations using the Advanced Seismic Asses-sment method and designed seismic retro-fitting that focuses on key objectives:• ensure adequate lateral displacement

capacity of the gravity-load-resisting sy-stem;

• ensure that the governing mechanismof nonlinear seismic response is desira-ble;

• provide ductile detailing and adequateconnections.

In several cases, the method demonstra-ted that the desired performance can beachieved with a reduced scope of retrofit-ting compared to previous methods. Withthe recommended scope of seismic retrofit-ting, we expect these buildings to meet orexceed PG&E’s structural seismic perfor-mance goals. PG&E has completed retro-fit work for some of these buildings, andothers are in progress.

ACKNOWLEDGEMENTThe authors gratefully acknowledge the LifelinesProgram of the Pacific Earthquake EngineeringResearch Center, and Pacific Gas & Electric, forfunding the projects described in this paper. KentFerré of PG&E provided valuable input regardingthe practical applicability of the Advanced Sei-smic Assessment Guidelines, including selectingappropriate buildings for study. The authors al-so appreciate the close collaboration with Drs.Nico Luco and Chuck Menun, who also signifi-cantly contributed to the Guidelines. We grate-fully acknowledge the Kajima Corporation, whoprovided a two-year overseas study fellowship toMr. Yuki Nakayama. Finally we were truly bles-sed to work with Prof. Allin Cornell, who routi-nely saw through engineering problems fartherand clearer than anybody else.


around high-voltage transmission lines inthe transformer rooms, minimize interrup-tion of airflow during construction, andallow a path for removal and replacementof transformers in the future. PG&E’s sei-smic performance goal for this building isto achieve Y Tag or G Tag performance sothat technicians may safely access equip-ment in the building to help bring powernetworks back online, in the event of outa-ges caused by an earthquake. The primaryretrofit objectives for this building are toimprove connections of the building’s struc-tural elements and reduce earthquake de-mands at the floor and roof diaphragmsby providing a more direct seismic forcepath. The building section in figure 13shows the configuration of existing diaph-ragms and walls, along with seismic retro-fit items designed to tie these existingstructural elements together for resistingearthquake forces.This seismic retrofit work for the LarkinSubstation was completed in 2007.

Conclusion

The Advanced Seismic Assessment Guideli-nes provide a method for engineers toperform seismic evaluations of existingbuildings and quantify expected structu-ral performance in terms that are mea-ningful to building owners. Currently, themethod is most useful to knowledgeableengineers performing seismic evaluationsand retrofits of critical structures wherereal performance information is desired.With further consensus evaluation, the pro-cedure could be developed into a standardthat uses mandatory language. In such astandard, the mandatory language wouldbe of the type that would require accoun-ting for key variables affecting seismicperformance, but it would not include pre-scriptive requirements. In the example ap-plications presented here, PG&E, the lar-gest utility company in California, identi-fied seismic performance goals for speci-


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