ATC-40 Seismic Evaluation and Retrofit of Concrete Buildings

ATe E!1

Seismic evaluation and retrofitof concrete buildingsVolume 1

aTe Applied Technology Council", ...

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._~ .~ CALIFORNIA SEISMIC SAFETY COMMISSIONo .

Proposition 122 Seismic Retrofit Practices Improvement Program

Report sse 96-01.,,

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ATC-4D

Seismic Evaluation and Retrofitof Concrete Buildings

Volume 1

by

APPLIED TECHNOLOGY COUNCIL555. Twin Dolphin Drive, Suite 550

Redwood City, California 94065

Funded by

SEISMIC SAFETY COMMISSIONState of California

Products 1.2 and 1.3 of the Proposition 122Seismic Retrofit Practices Improvement Program

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PrefaceProposition 122 passed by California's voters in1990, created the Earthquake Safety and PublicBuildings Rehabilitation Fund of 1990, sup-ported by a $300 million general obligationbond program for the seismic retrofit of state _and local government buildings. As a part ofthe program, Proposition 122 authorizes theCalifornia Seismic Safety Commission (CSSC).to use up to 1% of the proceeds of the bonds, orapproximately $3 million, to carry out a rangeof activities that will capitalize on the seismicretrofit experience in the private sector to im-prove seismic retrofit practices for governmentbuildings. The purpose of California's Proposi-tion 122 research and development program isto develop state-of-the-practice recommenda-tions to address current needs for seismic retro-fit provisions and seismic risk decision tools. Itis focused specifically on vulnerable concretestructures consistent with the types of concretebuildings that make up a significant portion ofCalifornia's state and local government invento-ries.

In 1994, as part of the Proposition 122 SeismicRetrofit Practices Improvement Program, theCommission awarded the Applied TechnologyCouncil (ATC) a contract to develop a recom-mended methodology and commentary for theseismic evaluation and retrofit of existing con-crete buildings (product 1.2). In 1995theCommission awarded a second, related contractto ATC to expand the Product 1.2 effort to in-clude effects of foundations on the seismic per-formance of existing concrete buildings(Product 1.3). The results of the two projectshave been combined and are presented in thisATC-40 Report (also known as SSC-96-01).

TV/o other reports recently published by theCalifornia Seismic Safety Commission, theProvisional Commentary for Seismic Retrofit(1994) and the Review ofSeismic Research Re-sults on Existing Buildings (1994), are Products1.1 and 3.1 of the Proposition 122 Program, re-spectively. These two previous reports providethe primary basis for the development of therecommended methodology and commentarycontained in this document.

. This document is organized into two volumes.Volume One contains the main body of theevaluation and retrofit methodology, presentedin 13 chapters, with a glossary and a list of ref-

_erences, J1d~ volume contains all ofthe parts ofthe document required for the' evaluation andretrofit of btrildings. Volume Two consists ofAppendices containing supporting materials re-lated to the methodology: four example buildingcase study reports, a cost effectiveness studyrelated to the four building studies, and a reviewof research on the effects of foundation condi-tions on the seismic performance of concretebuildings.

This report was prepared under the direction ofATe Senior Consultant Craig Comartin, whoserved as Principal Investigator, and Richard W.Niewiarowski, who served as Co-Principal In-vestigator and Project Director. Fred Turnerserved as esse Project Manager. Overviewand guidance were provided by the Proposition122 Oversight Panel consisting of Frederick M.Herman (Chair), Richard Conrad, Ross Cran-mer, Wilfred Iwan, Roy Johnston, FrankMcClure, Gary McGavin, Joel McRonald, J0-seph P. Nicoletti, Stanley Scott, and LowellShields. The Product 1.2 methodology andcommentary were prepared by Sigmund A.Freeman, Ronald o. Hamburger, William T.Holmes, Charles Kircher, Jack P: Moehle,Thomas A. Sabol, and Nabib. Youssef (Product1.2 Senior Advisory Panel). The Product 1.3GeotechnicallStructural Working Group con-sisted of Sunil Gupta, Geoffrey Martin, Mar-shall Lew, and Lelio Mejia. William T.Hol-mes, Yoshi Moriwaki, Maurice Power andNabih Youssef served on the Product ~.3 SeniorAdvisory Panel. Gregory P. Luth and Tom H.Hale, respectively, served as the Quality .A,..SSUf-ance Consultant and the Cost EffectivenessStudy Consultant. Wendy Rule served as Tech-nical Editor, and Gail Hynes Shea served asPublications Consultant.

Richard McCarthyCSSC Executive Director

Christopher RojahnATC Executive Director & ATC-40" SeniorAdvisor

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.oversight. Panei far._..-----~Propositla~-,-2-2--5eismic .Re~roFrt -pracflces-------- --._- .--' ....

Improvement program

Frederick M. Herman, ChairSeismic Safety Commission \Local Government/BuildingOfficial

Dr. Wilfred IwanMechanical Engineer

Gary McGavinSeismic Safety CommissionArchitect

Stanley ScottResearch Political Scientist -e :

Richard ConradBuilding Standards Commis-sion

Roy JohnstonStructural Engineer

Joel McRonaldDivision of the State Architect

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Ross CranmerBuilding OfficialStructural Engineer

Frank McClureStructural Engineer

Joseph P. NicolettiStructural Engineer

Lowell E: ShieldsSeismic Safety CommissionMechanical Engineer

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S~ismi~ Safety Commission Staff

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Richard McCarthyExecutive Director

Karen CoganDeborah PennyCarmen Marquez

Fred TurnerProject Manager

Chris LindstromEd HensleyTen DeVriendKathy Goodell

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product 1.2" Senior" Advisory PanelSigmund A. FreemanWiss, Janney, Elstner & Asso-ciates

Charles KircherCharles Kircher & Associates

Ronald O. HamburgerEQE International

Jack MoehleEarthquake Engineering Re-search Center

Nabih F. YoussefNabih Youssef & Associates

William T. HolmesRutherford & Chekene

Thomas A. SabolEngelkirk & Sabol

Product 1.3 Senior Advisory PanelWilliam T. HolmesRutherford & Chekene

Yoshi Mori wakiWoodward-Clyde Consultants

Maurice PowerGeomatrix Consultants, Inc.

Nabih F . YoussefNabih Youssef & Associates

Product 1.3 Geotechnical/Structural working GroupSunil GuptaEQ Tech Consultants

Marshall LewLaw/Crandall, Inc.

Quality Assurance ConsultantGregory P. LuthGregory P. Luth & Associates

Cost Effectiveness study consuttarrtTom H. HaleJimmy R. Yee Consulting Engineers

Geoffrey R. MartinUniversity of Southern Californ.ia

Lelio MejiaWoodward-Clyde Consultants

Technical EditorWendy RuleRichmond, CA

PUblications consultantGail Hynes SheaAlbany, CA

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Seismic Evaluation and Retrofit of Concrete Buildings

Products 1.2 and 1.3 of the proposition 122Seismic Retrofit Practices Improvement Program

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Volume 1Preface iiiGlossary " xiExecutive Summary xvChapter 1 Introduction 1-1

1. 1 Purpose 1-11.2 Scope 1-21.3 Organization and Contents 1-5

Chapter 2 Overview ~ 2-12.1 Introduction 2-12.2 Changes in Perspective 2-32.3 Getting Started ' 2-62.4 Basic Evaluation and Retrofit Strategy 2-112.5 Evaluation and Retrofit Concept 2-142.6 Final Design and Construction 2-19

Chapter 3 Performance Objectives 3-13 -.1 Introduction 3-13.2 Performance Levels 3-13.3 Earthquake Ground Motion 3-83.4 Performance Objectives 3-93.5 Assignment of Performance Objectives 3-12

Chapter 4 Seismic Hazard 4-14.1 Scope 4-14.2 Earthquake Ground Shaking Hazard Levels 4-14.3 Ground Failure 4-24.4 Primary Ground Shaking Criteria 4-54.5 Specification of Supplementary Criteria 4-12

Chapter 5 Determination of Deficiencies 5-15.1 Introduction 5-1

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Chapter 6

Chapter 7

Chapter 8

Chapter 9

Chapter 10

Chapter 11

Chapter 12

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5.2 Description: Typical Layouts and Details 5-15~3 Seismic Performance "'_'" .~ .~ ._.~._.,._._._.. ~ .._.. _.~~.-_- _.._._.5",5--- ._- _.. _.- ._~-:- -- -5.4 Data Collection .i ~ ~: ~ : 5-12 . ~:5.5 Review of Seismic Hazard 5-175.6 Identification of Potential Deficiencies 5-185.7 Preliminary Evaluation of Anticipated Seismic Performance 5-205.8 Preliminary Evaluation Conclusions and Recommendations 5-21Retrofit Strategies 6-16.1 Introduction 6-16.2 Alternative Retrofit Strategies 6-46.3 Design Constraints and Considerations 6-246.4 Strategy Selection ; : 6-276.5 Preliminary Design 6-30Quality,Assurance Procedures 7-17.1 General 7-17.2 Peer Review 7-27.3' Plan Check 7-8-7.4 Construction Quality Assurance 7-10Nonlinear Static Analysis Procedures 8-18.1 Introduction 8-18.2 Methodsto P~rf0fI!l Simplified Nonlinear Analysis 8-38.3 Illustrative Example 8-348.4 Other Analysis Methods ' 8-548.5 Basics of Structural Dynamics 8-57Modeling Rules 9-19'.1 General 9-19.2 Loads 9-29.3 Global Building Considerations 9-49.4 Element Models : 9-79.5 Component Models 9-199.6 Notations ; 9-46Foundation Effects 10-110.1 General 10-110.2 Foundation System and Global Structural Model 10-210.3 Foundation Elements 10-710.4 Properties of Geotechnical Components . ~ ........................... 10-1210.5 Characterization of Site Soils 10-2010.6 Response Limits and Acceptability Criteria.. r 10-2810.7 Modifications to Foundation Systems 10-29Response Limits 11-111.1 General 11-111.2 Descriptive Limits of Expected Performance 11-211.3 Global Building Acceptability Limits 11-211.4 Element and Component Acceptability Limits 11-5Nonstructural Components 12-1

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12.1 Introduction 12-1. 12.2. Acceptability Criteria 12-1

Chapter 13 Conclusions and Future Directions 13-113.1 Introduction .. " " 13-113.2 Additional Data 13-113.3 Potential Benefits 13-413.4 Major Challenges 13-513.5 Recommended Action Plan 13-6

References 14-1

Volume 2-AppendicesAppendix A Escondido Village Midrise, Stanford, California : A-IAppendix B Barrington Medical Center, Los Angeles, California B-1Appendix C Administration Building, California State University at Northridge,

Northridge, California C-lAppendix D Holiday Inn, Van Nuys, California D-1Appendix E Cost Effectiveness Study. _ " '" E-1Appendix F Supplemental Information on Foundation Effects F-lAppendix G Applied Technology Council Projects and Report Information G-l

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GlossaryAcceptability (response) limits: Refers to

specific limiting values for thedeformations and loadings, fordeformation-controlled and force-controlled components respectively, whichconstitute criteria for acceptable seismicperformance. .

Brittle: see nonductile.

Capacity: The expected ultimate strength (inflexure, shear, or axial loading) of astructural component excluding thereduction (

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Degradation: Refers to the loss of strength that a.component or structure may suffer whensubjected to more than one cycle ofdeformation beyond its elastic limit.Degrading components are generallyreferred to as being force-controlled,brittle, or nonductile. Some or all of theirflexural, shear or axial loading must beredistributed to other, more ductile,components in the structural system.

Demand: A representation of the earthquakeground motion or shaking that the buildingis subjected to. hi nonlinear .static analysisprocedures, demand is represented by anestimation of the- displacements ordeformations that the structure is expectedto undergo. This is in contrast toconventional, linear elastic analysisprocedures in which demand is representedby prescribed lateral forces applied to thestructure.

Demand spectrum: The reduced responsespectrum used to represent the earthquakeground motion in the capacity spectrummethod.

Displacement-based: Refers to analysisprocedures, such as the nonlinear staticanalysis procedures recommended in thismethodology, whose basis lies inestimating the realistic, and generallyinelastic, lateral displacements ordeformations expected due to actualearthquake, ground motion. Componentforces are then determined based on thedeformations.

Displacement coefficient Method: A nonlinearstatic analysis procedure that provides anumerical process for' estimating thedisplacement demand on the structure, byusing a bilinear representation of thecapacity curve and a series of modificationfactors, or coefficients, to calculate a

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target displacement. The point on thecapacity curve at the target.displacementisthe equivalent of the performance point. in'the capacity spectrum method.

Ductile: see ductility.

Ductility: The ability of a structural component,element, or system to undergo both largedeformations andlor several cycles ofdeformations"beyond its yield point orelastic limit and maintain its strengthwithout significant degradation or abruptfailure. These elements, only experience areduction in effective stiffness afteryielding and are generally referred to asbeing deformation controlled or ductile.

Ductility demand: Refers tothe extent ofdeformation (rotation or displacement)beyond the elastic limit, expressednumerically as 'the .ratio of the maximumdeformation tothe yield deformation.

Elastic (linear) behavior: Refers to the firstsegment of the bi-linear load-deformationrelationship plot of a component, element,or structure, between the unloadedcondition and the elastic limit or yieldpoint. This segment is a straight linewhose slope represents the initial elasticstiffness of the component.

Elastic limit: See yield point.

Elastic response spectrum: The 5% dampedresponse spectrum for the (each) seismichazard level of interest, representing themaximum response of the structure, interms of spectral acceleration Sa, at anytime during an earthquake as a function ofperiod of vibration; T.

Elements: Major horizontal or vertical portionsof the building's structural systems that actto resist lateral force'S or support.vertical

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gravity loads such as frames, shear walls,frame-walls; diaphragms, and.foundations ..Elements are composed of components.

Force-controlled: Refers to components,elements, actions, or systems which arenot permitted to exceed their elastic limits.This category of elements, generallyreferred to as brittle or nonductile,experiences significant degradation afteronly limited post-yield deformation,

Nonductile: Refers to a component or behaviorthat is not ductile and is generally subjectto strength degradation beyond the elasticlimit. These components are generallyforce-controlled.

Nonlinear static (analysis) procedure: Thegeneric name for the group of simplifiednonlinear analysis methods central to thismethodology characterized by: use of astatic pushover analysis to create acapacity curve representing the structure'savailable lateral force resistance, arepresentation of the actual displacementdemand on the structure due to a specifiedlevel of seismic hazard, and verification ofacceptable performance by a comparisonof the two.

Performance-based: Refers to a methodology inwhich structural criteria are expressed interms of achieving a performanceobjective. This is contrasted to aconventional method in which structuralcriteria are defined by limits on memberforces resulting from a prescribed level ofapplied shear force.

Performance level: A limiting damage state orcondition described by the physicaldamage within the building, the threat tolife safety of the building's occupants dueto the damage, and the post-earthquakeserviceability of the building. A building

Glossary

performance level is the combination of astructural, performance. level and anonstructural performance level.

Performance objective: A desired level ofseismic performance of the building(performance level), generally describedby specifying the maximum allowable (oracceptable) structural and nonstructuraldamage, for a specified level of seismichazard.

Performance point: The intersection of thecapacity spectrum with the appropriatedemand spectrum ill the capacity spectrummethod (the displacement at theperformance point is equivalent to thetarget displacement in the coefficientmethod).

ap, dp: coordinates of the performance point onthe capacity spectrum,

api, dpi: coordinates of successive iterations (i = 1,2, etc.) of the performance point,

ay, dy: coordinates of the effective yield point onthe capacity spectrum.

Primary elements: Refers to those structuralcomponents or elements that provide asignificant portion of the structure's lateralforce' resisting stiffness and strength at theperformance point. These are the elementsthat are needed to resist lateral loads afterseveral cycles of inelastic response to theearthquake ground motion.

Pushover curve: see capacity curve.

Pushover analysis: An incremental static analysisused to determine the force-displacementrelationship, or the capacity curve, for astructure or structural element. Theanalysis involves applying horizontalloads, in a prescribed pattern, to a

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computer model of the structure,incrementally; i.e .- "pushing." the -structure; and plotting the total appliedshear force and associated lateraldisplacement at each increment, until thestructure reaches a limit state or collapsecondition.

Retrofit strategy: The basic overall approachadopted to improve the probable seismicperformance of the building or tootherwise reduce the existing risk to anacceptable level.

Retrofit system: The specific method used toimplement the overall retrofit strategy.

Secant (effective) stiffness. The slope of astraight line drawn from the origin of thecapacity curve for abuilding (or other'structural element) to a point on the curveat a displacement "d", beyond the elasticlimit, represents the secant or effectivestiffness of the structure (or.element) whendeformed by an earthquake to thatdisplacement. The secant stiffness willalways be less than the elastic stiffness ofthe structure.

Secondary elements: Refers to those structuralcomponents or elements that are not, orare not needed to be, primary elements ofthe lateral load resisting system. However.secondary elements may be needed tosupport vertical gravity loads and mayresist some lateral loads.

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Seismic hazard: The level of ground motion or.shaking at the- site for- a-given-earthquake.Three standard levels of seismic hazard arespecified in the methodology;

Serviceability Earthquake (SE); 50%chance of being exceeded in 50 years,

Design Earthquake (DE); 10% chance ofbeing excee_ded in 50 years,

Maximum Earthquake (ME); 5% chanceof being exceeded in 50 years.

Strength: See capacity.

Target displacement; In the displacementcoefficient method. the target displacementis the equivalent of the performance pointin the capacity spectrum method. Thetarget displacement is calculated by use ofa series of coefficients.

Yield (effective yield) point: The point along thecapacity spectrum where the ultimatecapacity is reached and the initial linearelastic force-deformation relationship endsand effective stiffness begins to decrease.For larger elements or entire structuralsystems composed of many components,the effective yield point (on the bi-linearrepresentation of the capacity spectrum)represents the point at which a' sufficientnumber of individual components orelements have yielded and the globalstructure begins to experience inelasticdeformation.

Glossary

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SEISMiC eVALUATION .AND RETROFIT OF CONCRETE BUU..DINGS

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Executive- Summary

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II Existing concretebuildings pose a greatchanenge in California

Concrete is popular as a building material inCalifornia. For the most part, it serves itsfunctions well; however concrete is inherentlybrittle and performs poorly during earthquakes ifnot reinforced properly. The San FernandoEarthquake of 1971 dramatically demonstrated thischaracteristic. Shortly thereafter, code writersrevised the design 'provisions for new concretebuildings to provide adequate ductility to resiststrong ground shaking. There remain, nonetheless,millions of square feet of nonductile concretebuildings in California.

The consequences of neglecting this generalrisk are inevitably catastrophic for some individualbuildings. The collapse of a single building has thepotential for more loss of life than any othercatastrophe in California since 1906. The potentialdefects in these buildings are often not readilyapparent. Condemnation of all to mandatoryretrofit is an unacceptable economic burden.Unfortunately, procedures to identify and toretrofit efficiently those that are vulnerable tocollapse have not been available. As.a part of itsmandate under the California Earthquake HazardsReduction Act of 1986, the Seismic SafetyCommission is moving aggressively to meet thisneed by helping to develop standards for theevaluation and retrofit of existing concretebuildings with this document, Seismic Evaluationand Retrofit of Concrete Buildings (Product1.2/1.3). It contains the combined results of twocontracts with the Applied Technology Council(Product 1.2 for the development of an analytical

Executive summary

methodology and Product. 1.3 for the inclusion offoundation effects).

The challenge spansabroad spectrum from highlytechnical engineering detailsto general issues of publicpolicy

This document has a dual focus. On atecbnicallevel, engineers will find systematicguidance on how to investigate concrete buildingssubject to seismic shaking. Depending on thespecific characteristics of a particular building,they may select from an array of alternatives.These technical procedures are not alone sufficientfor effective evaluation and retrofit. Owners,architects, building officials, and others mustmake critical decisions based on technicalinformation coming from the engineers.Conversely, policy and management issues affectthe course of the technical analysis. Therecommended approach advocates a broad contextfor the process to expand the perspectives of allinvolved.

II Multipie performanceobjectives are the contextfor defining and managingseismic risk

In TurningLoss to Gain (CSSC 1995) itsreport to Governor Wilson on the NorthridgeEarthquake, the Seismic Safety Commissionidentifies a fundamental drawback of the seismicprovisions of current building codes. The seismic

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SeISM!C EVALUAT!ON AND RETROFIT OF CONCRETE sun.DINGS

performance that can be expected from a buildingdesigned.in accordance with the code is notexplicit. The implication is that buildings will notcollapse in large earthquakes. Owners rarelyrecognize that this goal allows for substantialdamage contributing to the potential for largecapital losses and business interruption. In spite ofsignificant improvements in codes afterearthquakes inthe past, their traditional approachis not conducive to effective overall managementof seismic' risks in California. This is particularlytrue of existing buildings fdr' which codes for newbuildings are effectively meaningless when itcomes to seismic performance, The Commissionconcludes that multiple performance objectives arerequired to define alternatives and quantify' ,acceptable risks.

A seismic performance objective has twoessential parts-e-a damage state and' a level ofhazard. "Life safety" and "immediate occupancy"are descriptors of damage states that do notconstitute performance objectives until they areassociated with a specific level of seismic' hazard.The hazard might be an earthquake (M7.0 on theHayward Fault adjacent to a site) or a probabilityof an intensity of ground shaking (10% chance ofbeing exceeded in 50 years). Defined in this way.a performance objective represents a specific risk.Using the new analysis procedures in thisdocument as a technical tool, it is possible toinvestigate buildings for multiple performanceobjecti:v-es...-'IlJis-appr.oach~prmlides..~ilding

owners and others a framework for informedjudgments on the acceptability of various risks andthe benefits of mitigative action in light of theassociated costs.

III NeW' structural an~lysis, ~ ...procedures give engineers amare realistic pic;ture ofbuilding performance. duringearthquakes

Traditional retrofit design techniques assumethat buildings respond elastically to 'earthquakes,

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In reality, large earthquakes can severely damage- buildings causing inelastic behaviorthat dissipates--~net~.!The ~Silmption thatbuildings remain .blastic simplifies the engineer's work but obscuresa basic understanding of actual performance. Theuse of traditional procedures for existing buildingscan lead to erroneous conclusions on deficienciesand unnecessarily high retrofit costs. Moredisturbingly, they can miss important defects insome buildings. Foundations are a good example.Traditional analyses normally assume thatbuildings are rigid at their base, which can lead tothe prediction of high forces implying extensiveretrofitting measures for walls and floors. It alsocan underestimate the structural displacements thatcontrol damage to other parts of the structure,such as columns. In reality, foundations often arequite flexible. Rocking or yielding of thesupporting soil material might reduce forces andthe need to retrofit the shear walls. The foundationmovements. however, also lead to largerdisplacements which may. imply potential collapseof columns.

Relatively new analysis procedures descr;i.bedin this document help describe the inelasticbehavior of the structural components of abuilding. These techniques can estimate moreaccurately the actual behavior of a building duringa specific ground motion. The document providesextensive guidance on the use ofthese proceduresincluding properties for concrete components anddetailed information to incorporate foundationeffects. Using this information. the engineerformulates a component model of the building.structure. The. analysis procedure tells how toidentify which part of the building will. fail first.As the load and displacement increase, otherelements begin to yield and deform inelastically.The resulting graphic "curve" is an easy-to-visualize representation of the capacity of thebuilding: Several alternative techniques allow thedemand from a specific earthquake or intensity ofground shaking to be correlated with the capacitycurve to generate a point on the curve wherecapacity and demand are equal. This "performance

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point II is an estimate of the actual displacement of. the .building. for the-specified ground.motion, ......Using thisperformance point, the engineer cancharacterize the associated damage state for thestructure and compare it with the desired .performance objective. This allows the engineer topinpoint deficiencies in each building part andaddress them directly with retrofit measures onlywhere necessary. In short, the procedure gives theengineer a better understanding of the seismicperformance characteristics of the building andresults in a more effective andcost-efficientretrofit.

II The new techn~lagiesrequire extensiveengineering jud.gment

A large team of earthquake engineeringexperts compiled and generated the information inthis document. A panel of respected leaders in thefield periodically reviewed the development asrepresentatives of the Seismic Safety Commission.Practitioners from throughout California voicedtheir opinions at a series of"workshops on thedocument: There is a consensus that the technicalprocedures ate complex. There are several sourcesand implications of this complexity.The nature ofthe inelastic analysis itself requires a basicunderstanding of the principles of structuraldynamics arid mechanics of materials. The scopeof the analysis typically requires computer-aidedsolutions. While most competent engineers withseismic design experience in California are capableof dealing with these issues, traditional designprocedures commonly used in current practice donot demand that they do. Unfortunately, in thecompetitive .design environment, most uninformedowners are not yet willing to pay larger fees forthe more time-consuming approach. Although thebenefits to owners in reduced construction costs,more reliable building performance, and reducedcosts to repair damage due to future earthquakescan justify the higher fees in many cases, this hasnot yet been widely comniunicated. In the future,'better communication and changes in the

Executive summary

marketplace for engineering services could resolve. this aspect of complexity. . -..- _ _..

The document provides guidance applicable toall concrete buildings. Within a general frameworkfor evaluation and retrofit, new procedures forinelastic analysis are alternatives to.simpler .traditional methods for detailed analysis of some,but not all, buildings. The dividing line betweenbuildings that can benefit from inelastic analysisand those that will not can b~ subtle, however...Every building has its own characteristics and .often only experienced engineers can decide whentraditional design methods are adequate. Thisnecessity of experience and judgment on the part.of the engineer. extends beyond the selection ofappropriate analysis techniques. The. new inelasticprocedures require many decisions 0ll componentproperties and modeling techniques that involveconsiderable. judgment. :Tl1e interpretation ofresults must carefully include consideration ofinherent uncertainties' and the limitations. of basicassumptions. Qualifying experience and judgmentis not the exclusive domain of a select fewengineers or firms. No one is capable of infallibleprediction of the seismic performance of concretebuildings. The solution to this unavoidable .complexity is to eliminate complete 'reliance on-thejudgment of a single engineer and, instead, rely onconstructive and cooperative peer reviewprocesses. The Seismic Safety Commission, inTurning Loss to Gain, advocates such a change inthe California Building Code to requireindependent peer review of complex buildings.

Ii Effective and efficientseismic evaluation andretrofit of concrete buildingsdemand fundamentalchanges

The need for technical peer review is only oneof the changes to conventional planning and designprocesses. The design engineers themselves facethe challenge to develop and maintain theirtechnical skills beyond those that they currently

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use in practice. Architects must recognize theimpact ofseismic risk on-building. function and-the .importance of nonstructural damage to buildingperformance. Building officials are accustomed todesigns that can be easily checked againstprescriptive codes and standards. They mustexpand or supplement their own skills andimplement procedures to monitor performance-based designs. As important as these changes fordesign and buildingprofessionals are, they alonewill do little without the demand and support ofbuilding owners for change.

The perspective of building owners is the keyto progress. If a buildingmeetsthe code underwhich it was built and there is no legalrequirement to retrofit it, owners generally havebeen satisfied. Few understand the risks theyactually face. Performance-based evaluation ofbuildings can give them a picture of howearthquakes impact their businesses andinvestments, They can then begin to makeinformed decisions to manage and reduce risks in acost-efficient way. The most basic change thatowners will face is the realization that they are thedecision maker. Engineers can advise them on .relative risks, but acceptability rests with theowner. This concept runs counter to the prevailingattitude that it is the design professional whodecides on acceptable risk.

II Product '1.2/'1.3 initiatesthe tffii'isitiGn with a step In .the right direction

The new technical analysis procedures,coupled with performance-based evaluation anddesign concepts, have great promise. Realistically, .their implementation and complete development'will take some time. The' realization of the full .potential of the new approaches demands technicalinformation and data not currently available.Significant changes to business as usual arerequired on the part of all involved in theevaluation and retrofit process. There are severalimportant strategies that can enhance futureprogress.

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California is not acting alone in pursuing.effecti-ve-evaluation-and-retrofit-methodologies.-In _...fact, many of the individuals responsible forthisdocument are also involved on the federalinitiative to develop national guidelines for theseismic rehabilitation of buildings. They initiallycapitalized on the federal effort by using it as aSpringboard Jar further development. In return,key enhancements from Product 1.211.3 have beenfunneled back into the federal document. There areat least two desirable outcomes of this synergism.Product 1.211.3 uses concepts and languagecompatible with the federal guidelines. Thisconsistency will raise..thecomfort level of ~.involved and accelerate the implementation of theprocedures. The federal government, through theFederal Emergency Management Agency and theNational Science Foundation, hasplans to continuethe development of performance-based designaggressively. The benefit for existing concretebuildings will include iJ:!l:proved information on theinelastic properties of both structural andnonstructural components,

The interest on the part of structural engineersin inelastic analysis procedures is very high. TheStructural Engineers Association of NorthernCalifornia recently sponsored a seminar on thesubjectand had to turn people away. Futuresessions are planned and interest probably willspread through the larger state organization.Focused sessions are required for geotechnical andstructural engineers. The importance of foundationeffects on the seismic performance of somebuildings requires. greater communication andcooperation. Training sessions are also essentialfor building officials throughout California. Thisdocument is a natural curriculum for these effortsand the Seismic Safety Commission encourages itsuse.

A concerted effort to educate building .ownerson effective seismic risk management is essential.First of all, the benefits of the new proceduresneed to be documented with extended examplebuilding studies. The proposed procedures havebeen used successfully by others andtheir stories

Executive Summary

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need to be told. Side by side comparison of theresults of the.proposed procedures with. those .oftraditional methods including retrofit costs wouldquantify the differences. State agencies are anatural starting point for workshops and seminarsaimed at the management level. These could beexpanded to the private sector throughorganizations such as the Building Owners andManagers Association. This initiative to engagebuilding owners has not yet been implemented inany effective program.

Finally, this document needs to be continuallyupdated to reflect advancement in the state of theart and the valuable lessons from practicalapplication. A repository of information should be

Executive summary

established to allow users to submit suggestionsand.share experiences on evaluation and retrofitprojects.

The Seismic Safety Commission is confidentthat California can meet the challenge of concretebuildings with improved understanding andinformation. Product 1.211.3 provides the basisfor improved understanding of the actual behaviorof structures for realistic earthquakes and forinformed management of seismic risks. Withcontinued vigilant effort on the part of designprofessionals, building officials, and owners toenhance the process, the risks to safety andeconomy posed by earthquakes can be steadilyreduced to acceptable levels.

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Chapter 1

Introduction

Owner

AUdience Interest spectrum

Architect Bldg. Official Engineer AnalYst

1.1 Purpose

1.1.1 ceneralA major portion of state and local government

buildings in California are cast-in-place concretestructures designed and constructed before the midto late 1970s. The seismic performance of theseolder buildings has been observed to be relativelypoor compared to the performance of modem, post1970s concrete buildings. Accordingly, a growingnumber of these buildings have been evaluated andretrofit in recent years and many more will beretrofit in the near future.

Very little has been, or is currently, availablein the way of guidelines for use in the retrofit ofexisting concrete buildings. Therefore, most of theretrofit design and construction to date has beenbased on the use of the simple equivalent lateralforce analysis procedures prescribed in buildingcodes for the design of new buildings. Theseprocedures do not directly address the actualforces induced in buildings by earthquake groundmotions. More importantly, since buildings willrespond to the earthquake ground motions in aninelastic manner, the linear elastic equivalent"lateral force procedures do not provide a directmethod to determine the resulting maximumdisplacements.

Given these shortcomings of the simpleprocedures, the concern has arisen that present

Chapter 1, Introduction

approaches to retrofit may not deliver appropriateor cos~-effective designs. Unrealistic or inadequateassessment of buildings may not identify the truefailure modes, leading to unsafe retrofit designs,or may produce overly conservative retrofitswhere none is needed to meet the Owner'sperformance objective, leading to unnecessarilycostly retrofit designs. Therefore, moresophisticated "methods that consider both the actualloading and inelastic responses buildingsexperience in large earthquakes are needed.

The primary purpose of this document is toprovide an analysis and design methodology andsupporting commentary for use in the seismicevaluation and retrofit of existing state and localgovernment concrete buildings in California. Thismethodology is intended to serve as the basis forthe future development of building code provisionsand standards and to provide guidelines for interimuse until the more formal provisions are available.

It is expected that this document will be usedby both retrofit design professionals performingseismic evaluations and retrofit designs andgovernment agency personnel and policy makerscharged with implementing retrofit programs.Portions of the document will be of interest toothers, such as building owners and architects,involved in various aspects of building retrofitprojects. However, the engineering expertise of adesign professional, in particular the expertise of astructural engineer experienced in building retrofit

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design, is a prerequisite for appropriate use of theanalytical.procedures at the core of thismethodology.

1.1.2 proposition 122 seismic Retrofitpractices Improvement Program

Passed by California's voters, Proposition 122created the Earthquake Safety and Public BuildingsRehabilitarionPund of 1990, supported by a $300million general obligation bond program for theseismic retrofit of state and local governmentbuildings.

As a part of the program, Proposition 122authorizes the Seismic Safety Commission to useup to 1% of the proceeds of the bonds issued andsold, or approximately Ss million,' to carry out arange of activities that will capitalize on theseismic retrofit experience in the private sector toimprove seismic retrofit practices for governmentbuildings.

The overall purpose of California'sProposition 122 research and developmentprogram is to develop state-of-the-practicerecommendations and methods to address currentneeds for uniform seismic retrofit provisions andseismic risk decision tools. It is focusedspecifically on vulnerable concrete structuresconsistent with the types of concrete buildings thatmake up a significant portion of California's stateand local government inventories.

The two primary goals of the commission'sSeismic Retrofit Practices Improvement Programare:

.. To achieve cost-effective expenditure of stateand local government funds allocated for theseismic retrofit of government buildings

.. To obtainseismic retrofit designs thatconsistently and reliably achieve their intendedseismic performance objectives

A 1991 Commission report titled Breaking thePattern (CSS'C 1991a) outlines four products to bedeveloped over the multiyear program:

12

.. Product 1: Provisions and commentary for thedesign of seismic retrofits-for existing .... --- ... :-~government buildings

.. Product-Ze Risk-management-tools for use. in ;seismic retrofit decision making by facilityowners and managers

.. Product 3: Short-term research projects tosupport the first two activities

.. Product 4:.Information transfer activities toinform government officials, facility ownersand managers, and design professionals aboutthe other products

This document reports the results of twoseparate but related projects conducted as part ofthe commission's Proposition 122 Seismic RetrofitPractices Improvement Program: Product 1.2,Development of a Recommended Methodology forthe Seismic Evaluation and Retrofit of ExistingConcrete BUildings and Product .1.3, Effects ofFoundations on the Seismic Performance ofExisting Concrete Buildings.

Two other reports recently published by theCalifornia Seismic Safety Commission, theProvisional Commentary for Seismic Retrofit(CSSC 1994a) and the Review of Seismic ResearchResults on Existing Buildings (eSSe 1994b), areProducts 1.1 and 3.1 of the program, respectively.These two previous reports provide the primarybasis for the development of the recommendedmethodology and commentary contained in thisdocument:

...2 Scope

1.2.1 GeneralThis document provides a comprehensive,

technically sound recommended methodology andsupporting commentary for. the seismic evaluationand retrofit design of existing concrete buildings.Althouzh it is not intended for the design of newo .buildings, the analytical procedures are applicable.The document applies to the overall structuralsystem and its elements (concrete fraines, shear


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SEISMIC eVALUATION AND RETROFIT OF CONCRETE BUU"DINGS

walls, diaphragms, foundations) and components(stiffness" strength, and.deformability, of.columns..beams, walls, slabs, and joints). Consideration ofnonstructural systems and components is also'included in this document.

The methodology is performance based: theevaluation and retrofit design criteria areexpressed as performance objectives, which definedesired levels of seismic performance when thebuilding is subjected to specified levels of seismicground motion. Acceptable performance ismeasured by the level of structuraland/ornonstructural damage expected from theearthquake shaking. Damage is expressed in termsof post yield, inelastic deformation limits forvarious structural components and elements foundin concrete buildings. The analytical procedureincorporated in the methodology accounts forpostelastic deformations of the,structure by usingsimplified nonlinear static analysis methods. .

This type of performance-based methodologyfor evaluation and retrofit design represents afundamental change for the structural engineeringprofession. This type of analytical procedure ismore complex than traditional force-based}prescriptive procedures such as those embodied inbuilding codes for the design of new buildings.Although the use of simplifiednonlinear staticanalysis procedures and their application toevaluation and retrofit design of existing buildingshas grown over the past 15 to 20 years,widespread acceptance of these methods by theprofession will come only through a considerableinformation transfer and learning process. Fullacceptance will be achieved only when the abilityof this method to identify potential structuraldeficiencies and to produce economical retrofitdesigns better than conventional practice has beendemonstrated.

1.2.2 uncertainty and ReliabilityUncertainty is a condition associated with

essentially all aspects of.earthquake related scienceand engineering and of the evaluation and retrofit,of existing buildings. The principle sources of

Chapter 1, tntroductlon

uncertainty lie in the characterization of seismicground.shaking.. the.determination of materialsproperties and of existing structural andgeotechnical component capacities, and theassignment of the acceptance limits on structuralbehavior. These uncertainties, for the most partstemming from the lack of and!or the imperfectreliability of the specific supporting data available,affect all analytical methods and proceduresapplied to the challenge' of seismic evaluation andretrofit.

The performance-based methodology presentedin this document cannot and does not eliminatethese uncertainties. However, through the use ofsimplified nonlinear static analysis, it provides amore sophisticated arid direct approach to addressthe uncertainties than'do traditional linear analysisprocedures. By explicit consideration of the post-yield behavior of individual structural components,estimation of the degradation of member stiffness 'and strength, and representation of foundationeffects, the methodology provides a more. realistic,generally conservative, estimate or approximationof the actual deformations which. will occur in the .building in response to.seismic ground motion. Asa result, it is a useful and reliable design tool forassessment of expected building behavior andverification of proposed retrofit designs.

1.2.3 Procedure for Evaluation andRetrofit Design

The methodology is presented in the form of astep-by-step procedure for both evaluation andretrofit of existing buildings. The procedurerecognizes, however, that some steps may be de-emphasized or performed in a different order on acase-by-case basis.

The primary components of the methodologyused in various steps of the evaluation and retrofitprocedure include:

+ Definitions of seismic performance levels andseismic demand criteria for establishingseismic performance objectives

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+ Guidance for the review of existing conditions,preliminary determinationof deficiencies,' - -.- .formulation of a retrofit strategy, and forestablishing an appropriate quality assuranceprogram

+ Analytical methods or techniques for detailedinvestigations to assess seismic capacity andexpected seismic performance of existingbuildings and for verification of retrofitperformance .

+ Materials characteristics rules and assumptionsfor use in modeling, assignment of capacities,and .assessment of acceptable performance

The owner's orbuilding code official'sselection of the performance objective that shouldbe achieved by a building retrofit is beyond thescope of this document. This includes theidentification of the level of.seismichazard thatshould be combined with the selected performancelevel. Once those decisionshave.been made,however, and a performance objective has thusbeen established, this methodology- providesguidelines to meet that objective. Compliance withthe procedures and requirements of this documentwill be deemed adequatefor these purposes.However, due to the uncertainties noted in Section1.2.2, the seismic performance incorporated intothe performance objective is not guaranteed.

1.2.4 Building Types .Two specific types of older, cast-in-place

concrete buildings which were designed andconstructed prior to the late 1970s, when ductiledetailing requirements were first incorporated intobuilding standards, and which are common toCalifornia state and local government buildinginventories, will be the focus of the methodology:

Concrete frame buildings, generallyconstructed from the 1940s to the mid-1970s

Concrete.frame buildings with concrete walls,generally constructed from the early 1900s tothe mid-1970s

1-4

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1.2.4.1 Materials, components, and---- --~ -.--.Elements- ... -----" -."'-" .. - --- -.-

Modeling rules and acceptance limits areprovided for a variety of reinforced cast-in-placeconcrete elements and components found in thetwo building types, including beam-columnframes; slab-column frames; solid, coupled, andperforated shear walls; concrete diaphragms; andfoundations. Unreinforced masonry infill andprecast concrete components are not considered inthis document. These rues, assumptions, andlimits are included for existing, non-complyingelements and components, and for new,complying, elements and components used inretrofits.

1.2.4.2 FoundationSoilEFFectsThe methodology includes guidelines for the

consideration of foundation-soil effects. Detailedmodeling rules and acceptance limits for varioustypes. of foundations and foundation-structurecombinations in various soil conditions areincluded in this document.

1.2.4.3 DiaphragmsThe methodology includes detailed guidelines

for modeling rules and acceptance limits forconcrete slab diaphragms, which may beconsidered to be rigid. Although generalguidelines and commentary for the considerationof flexible diaphragms are included, the provisionof detailed rules and assumptions for flexiblediaphragms is not included in this document.

1.2.5 Alternative Analytical MethodsA variety of alternative analytical methods,

using either simple (linear, static) procedures,approximate inelastic (simplified nonlinear static)capacity procedures, or complex inelastic(nonlinear time history) procedures, are availablefor use within the overall evaluation and retrofitmethodology. The type of analytical approachdescribed in this document is simplified nonlinearstatic analysis. Several methods of performingnonlinear static analyses are presented, and the

Chapter 1, tntrcetuctlon

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capacity spectrum method is emphasized. Otheranalytical methods.are.also, noted,and,discussion.is- ., .provided to assist the retrofit professional in theselection of an analytical procedure appropriate foruse in the detailed analysis of a particular building.

1..3 Organization andContents

This document is organized into two volumes.Volume One contains the main body of theevaluation and retrofit methodology, presented in13 chapters, with a glossary and a list ofreferences. This volume contains all the parts ofthe document required for application and use ofthe methodology for evaluation and retrofit of abuilding. Volume Two consists of variousAppendices containing supporting materials relatedto the methodology; four example building casestudy reports, a cost effectiveness study reportrelated to the four building studies, and a reviewof research on foundation effects on the seismicperformance of concrete buildings.

1.3.1 Volume One Chapter summariesThe methodology has been organized into 13

chapters. The first seven address the more generaland conceptual aspects of the methodology, whichwill be of interest to the broader range of theexpected audience of building owners and agencyrepresentatives, architects, and building officials,as well as structural engineers and analysts. Thenext five chapters, 8 through 12, address the moretechnical and analytical aspects of themethodology, expected to be of primary interestonly to the structural engineer/analystmembers ofthe audience. The last chapter, 13, providessummary concluding remarks which are of interestto the broader audience..The title page of eachchapter contains an audience spectrum bar to assistthe reader in assessing the appropriate level ofinterest.

1.:5.1.1 Chapter 1: mtroauctionChapter I provides a statement of the purpose

and scope of this document, followed by a brief


description of the content of each of the chaptersand supporting .study. reports .__._. .... - ..... _..... .

1.3. 1.2 Chapter 2: OverviewChapter 2 presents an overview of the general

evaluation and retrofit methodology. The broadaudience this document is intended to address isdiscussed and an Audience Interest Spectrum isprovided to assist individuals in identifying whichportions may be of interest or appropriate to them.Then, following a logical sequence of stepsthrough the evaluation andretrofit design process,this chapter also serves as a road map for use ofthe document, with references to the appropriatechapters and sections at each step.

1.~ 1.:5 Chapter 5: Performance. Objectives

Chapter 3 presents a detailed discussion ofseismic performance objectives and how they areformed. Definitions, or damage state descriptionsconsistent with those included in otherperformance-based design documents, areprovided for six standard structural performancelevels: .

.. SP-l, Im.m.ediate Occupancy

.. SP-2, Damage Control

+ SP-3, Life Safety+ SPA, Limited Safety+ SP-5, Structural Stability

+ SP-6, Not Considered

and five standard nonstructural performancelevels:

.. NP-A, Operational

+ NP-B, Immediate Occupancy

+ NP-C, Life Safety

+' NP-D, Hazards Reduced

+ NP-E, Not ConsideredPerformance levels for a building are formed

by combining a structural and a nonstructuralperformance level to describe a complete building

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SEISMIC EVALUATION AND RETROFIT --bF CONCRETE 'BUILDINGS

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damage state. Performance objectives are thenformed.by combining a.desired,bnilding . _... _performance level with a given earthquake groundmotion. The chapter describes the process ofselecting appropriate performance objectives, andone standard performance objective, called theBasic Safety Objective, is defined.

1~5.1_4 Chapter 4: seismic HazardChapter 4 provides guidelines for quantifying

the seismic hazard atasite due to ground shakingfor three levels of earthquake hazard: theServiceability Earthquake, the Design Earthquake,and the Maximum Earthquake. Brief discussionsof seismic ground failure .hazards oilier thanshaking are also provided,

The chapter describes the primary groundshaking criteria, site geology and soilcharacteristics, site seismicity characteristics, andsite response spectra required for seismicevaluation and retrofit design of ~uildings. Forvarious combinations of site soil-profile types andsite seismic zone factors, site seismicitycoefficients are defined from which site responsespectra may be constructed for any site inCalifornia. The chapter also provides guidance andgeneral criteria for the use of acceleration timehistories and duration of ground shaking.

1.5.1.5 Chapter 5: Determination ofDeficiencies

Chapter 5 provides guidelines for a qualitative,preliminary evaluation of existing cast-in-placeconcrete frame and frame-wall buildings prior tothe performance of detailed or extensive analytical.work. A description of the common characteristicsof these types of construction is provided, alongwith a discussion of their past seismic performanceand typical deficiencies. Guidelines for collectionof as-built information, including physical testingof materials and exploration of existing conditions,appropriate to the level of detail of evaluation orretrofit studies are provided.

Assessment of the seismic -characteristics ofexisting buildings and determination of their

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seismic deficiencies and their-severity, based onthe collected.data.ris.discussed.. Also., . . -_ .considerations for establishing the extent offurther, more detailed analysis that may berequired to supplement the-available as-built data,and preliminary determination of the need forretrofit, are also discussed,' - ,~... . '

1.~ '1.6 Chapter 6: Retrofit strategiesChapter 6 provides. an overview of the process

of developing retrofit strategies (the basic ,approaches to improve the seismic performance ofbuildings) and preliminary retrofit designs forbuildings. Discussion of various alternative retrofitstrategies and the design constraints affectingretrofit strategy selection is provided. The processof selecting a retrofit strategy after an evaluationhas indicated the presence of unacceptable seismicdeficiencies and the decision to retrofit has been .madeis described. Considerations of alternativestrategies, evaluation of their applicability giventhe identified deficiencies, and selection of themost appropriate strategy in light of the existingdesign constraints are.discussed. Guidance forselection of an appropriate retrofit system toimplement the chosen strategy and fordevelopment of preliminary retrofit designs is alsoprovided.

1.~1.7 Chapter 7: oualityAssuranceProcedures

Chapter 7 provides guidelines for the variousquality control procedures that may be required toensure appropriate application of the methodology.Guidelines for peer review, plan check, andconstruction quality assurance procedures arepresented and discussed. Although comprehensiveprograms are presented, conditions for whichvarying levels of review may be appropriate,depending on the complexity of a particularbuilding, are discussed. Minimum requirementsfor field observation of the retrofit construction areprovided, as are guidelines for field verification,testing, and inspection. .

Chapter 1; Introduction

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SEI$MIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS

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1.5.1.8 Chapter S: Nonline.arstatic.Analysis. procedl!res-..- -- ---.._._-_..

Chapter 8 presents the generalized nonlinear .static analysis procedure characterized by use of astatic pushover analysis method to represent thestructure's lateral force resisting .capacity, arepresentation of the actual earthquakedisplacement. demand on the structure, andverification of acceptable performance by acomparison of the structure's available capacity tothe earthquake's demand. A detailed description ofeach of the three primary elements of the nonlinearstatic analysis procedure is presented: the step-by-step development of the capacity curve of astructure, the various alternative methods todetermine displacement demand by use of reduceddemand spectra or target displacementcoefficients, and the resulting identification of theperformance point or target displacement and the.subsequent .check for acceptable performance.

Additional considerations, including thedistinction between primary and secondarymembers, the effects of torsion, 'and the effects ofhigher modes, are discussed. 'An example isprovided to demonstrate the application of thenonlinear static procedure to abuilding.Alternative methods, including-linear elastic staticand dynamic methods, the secant stiffnessnonlinear static method, and nonlinear time historyanalysis methods, are discussed. The chaptercloses with a brief summary of the fundamentalstructuraldynamics basis of the nonlinear staticanalysis procedures. . .

1.5.1.9 Chapter 9: ModeiingRulesChapter 9 provides the guidelines, rules, and

assumptions required to develop the analyticalmodel of buildings as two- or.three-dimensionalsystems with nonlinear load-deformationproperties. The guidelines for modeling thestructural systems include application of loads;global building modeling considerations; materialsmodels; elements models (concrete frames,concrete walls, concrete diaphragms, and

Chapter 1r tntroductlon

foundations); and component models (columns,beams, joints,..wall~and slabs)..- ... - ...

1.:5.1.. 10 Chapter 10: Foundation EffectsChapter 10 provides guidelines for the

inclusion of foundation effects in the overallmethodology for evaluation and retrofit design ofexisting concrete buildings. Guidelines areprovided for modeling of geotechnical, componentsfor various types of shallow and deep foundationsystems. Discussion is provided on response limitsand accepta~~lity criteria...

1.5.1.. 11 Chapter 11:' Response limitsChapter 11 provides the':guidelines,

considerations, and assumptions required to assessthe acceptability of the seismic response of thevarious components and elements of the .structuraland nonstructural systems. Qualitative descriptionsof damage states are included for structural andnonstructural systems and quantitative limits areprovided for the structural systems. Strength limitsare provided for both ductile, deformation-controlled .components and brittle, force-controlledcomponents.. Component and element .deformability limits considering the ImmediateOccupancy, Life Safety, and Structural Stability

, performance levels are presented. .

1.5..1.12 Chapter 12: Nonstructuralcomponents

Chapter 12 describes the minimum acceptancecriteria that are expected to provide theOperational, Immediate Occupancy, Life Safety,and Hazards Reduced levels of performance fornonstructural systems and components. Acceptancecriteria consist of listings' of those systems andcomponents that should be investigated for eachperformance level.

1.3.1.15 Chapter 13: Conclusions andFuture Directions

Chapter 13 provides a detailed discussion ofthe various supplemental engineering studiesreported in the six Appendices in Volume 2 and asummary of the principal conclusions drawn from

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the development of this Product. The conclusionsare presented in a.discussion of the.potential., .-benefits of, and the challenges posed by, theanalysis and retrofit design methodology presentedin this document. Benefits are discussed in termsof the engineer's improved understanding ofseismic performance of buildings as well as theowner's enhanced options for implementingseismic retrofit goals in their buildings. Challengesare discussed in terms of both specific.technicalissues and broader practice issuesThe chapterconcludes with recommendations .,for future action,

. in terms of basic research to address the technicalchallenges and training and communicationprograms to address the practice issues.

1.3.2.1 Appendices A~D:EXample Building stuates

'These four Appendices cont:rln the reports ofengineering studies of four example buildings. Thestudies were performed primarily to test theproposed nonlinear static analysis methodology inthree ways by comparisons to actual observedearthquake-caused damage to the selectedbuildings, to the results of limited linear elasticanalyses, and to the results of limited time historyanalyses. In addition, these studies' alsodemonstrate the application of the proposedmethodology to actual buildings. The nonlinearstatic procedure was used to evaluate the expectedperformance of the unmodified buildings, and then,to develop retrofit concept designs to achieve oneor two specified levels of improved structuralperformance. The reports describe the results ofthe four studies, discuss observed strengths andlimitations of the methodology, and provide some

1.3.2 voiume Two Appendices'Summaries

valuable insights into the assumptions and. . . dgme d.. engmeermg.ju nts.ma e.--.-- _ .

1.5.2.2 AppendixE:CostEFFectiveness study

This section contains the report of a studyperformed to evaluate the cost-effectiveness andusability of the evaluation and retrofitmethodology. The approximate construction costof the various retrofit concept designs developed inthe four example building studies is estimated andthen compared to cost ranges from traditionalretrofit approaches andthe estimated cost ofdemolition and replacement. The cost effectivenessrelationships observed between the extent ofretrofit/improved seismic performance andconstruction costs is discussed.' In addition; the ~ease of use and consistency ofapplication of theproposed methodology, as demonstrated by thefour example building studies; is assessed anddiscussed.

1.5.2.3 Appendix F:Supplemental InFormation onFoundation EFFects

This section contains a report of a review ofresearch on the affects of foundations on theseismic performance of concrete, buildings.Presented in a format similar to the Review ofSeismic Research Results on Existing Buildings(CSSC 1994b), but in smaller scale, this reviewprovides an overview of the existing, pertinentresearch supporting Chapter 10 Foundation Effects'in Volume One of the document. The reportcontains discussions of past seismic performance;experimental and theoretical studies, and analysisand design issues for both shallow and deepfoundation systems, and acollection of briefreview summaries of selected published articles,papers, and reports.

Chapter 1 r uitroduction

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Chapter 2

Overview: 0. :.

AUdience Interest spectrum .

Owner Architect. Bldg. Offldal J;ngineer AnalY5t

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2.1 IntroductionThe seismic evaluation and retrofit of existing

concrete buildings pose a great challenge for theowners, architects, engineers, and buildingofficials of California. The risks, measured in bothlives and dollars, .are high. Equally high is theinevitable uncertainty of where, when, and howlarge future earthquakes will be. The inherentcomplexity of concrete buildings and of theirperformance during earthquakes compounds theuncertainty. Traditional design and analysisprocedures developed primarily for newconstruction are not wholly adequate tools formeeting this challenge'. '

This d~cument presents a general methodologydeveloped specifically to address' the seismicevaluation and retrofit. of concrete buildings in 'California. Promising new performance-basedtechnical procedures can provide engineers withvaluable insight about the actual performance ofbuildings during earthquakes. These and otherchanges from the status quo can greatly improve,the process. It is important to emphasize,however, that straightforward, simple solutionsthat will cost-effectively produce acceptableseismic performance for all buildings do not exist.Within the general methodology described below,there are branches and paths that engineers andowners can select on the basis of thecharacteristics of a given building, the desiredperformance, cost limitations, and other project-

Chapter 2, overview.

specific-factors. In the future, more direct anddefinitive processes may emerge. The procedurespresented here are a step in the right direction.

The intended audience for this documentincludes building owners, building officials,architects, engineers, and others who may have adirect or peripheral interest in the seismicevaluation and retrofit of cop-crete. buildings. Thisdocument is not a code, or even a comprehensiveguideline. In one sense it is a commentary with avery broad perspective. Current technologies aredeveloped and placed in context within the largerpicture. Guidance on the selection of alternativesis offered. The objective of the document extendsbeyond the general, however, to the pragmatic.Using it as a manual, qualified engineers can applythe general principles to the evaluation and retrofitof actual buildings,

Not every chapter of this document is meantfor detailed study by all readers. Within thegeneral methodology are some very technicalprocedures of interest only to structural engineers 'and analysts. The audience interest spectrum(Figure 2-1) is provided to assist the reader inassessing the appropriateness of each chapter to hisor her particular perspective. The audience interestspectrum bar for each chapter is also shown on therespective chapter's title page. Within eachchapter. key points and basic concepts arehighlighted in sidebars, figures, tables, andbulleted lists for the more casual reader.

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SEISMIC EVAUJATIO'N AND RETROFIT OF CO'NCRETE BUn,DINGS

F Foundation Supplement

1. Introduction

? overview

3. Performance Objectives

4. selsmle Haz~rd

~i.: ".1:- - ~

~ Deficiencies

6. Retrofit ~trategies

7. Quality Procedures

~Analytical Procedures

9. Modeling Rules

10. Foundation Effects

11. Response Limits

12. Nonstructural ~omponents

13. Conclusions

AD Example Building studies

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Primary interest; thorough reading recommended

seconoarv interest; general overview recommended

Limited interest; selective review recommended

Figure 2-1. AUdience Interest spectrum

Chapter 2, overview

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2.2.2 Architects. Most seismic retrofit work is conducted as partof a rehabilitation project of larger scope. In somecases this is because disabled access, fire and lifesafety, or other issues must be addressed at thesame time. Often, too, it is '~xpedient to modernizeand correct planning or programmatic problemswith buildings at the time that seismic deficienciesare corrected. For this reason, architects generallyhave had, and will continue to have, an importantrole in seismic retrofit work.

Performance-based design requires a change inperspective on the part of architects. It isimportant for architects to learn more of the details

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of ~e functional aspects of a}iCility so that theycan assist owners in making decisions aboutperformance goals. Fot example, certainequipment may be essential for the continuedoperation of a facility after earthquakes, whileother systems could sustain damage withoutseriously impeding building functions. In addition,some concrete buildings may be historicstructures. Historic preservation can often imposerestrictions on the 'type and extent of seismicretrofit measures that may be performed.

Table 2-1, adapted from Architectural Practiceand Earthquake Hazards (eSSe 1992), providesguidance for architects seeldng to improve theirpractice in seismic hazard mitigation.

2.2 Changes in perspective involved in predicting performance can affect theMeeting the challenge 'of concrete buildings---....c.._- _.._-. decision-making.process.-Owners. must understand

demands a change in perspective on the part of .' . ~at they are playing a role in balancing relativeeveryone involved .. The design and construction of nsks rather than transferring risk to designnew buildings follow a familiar pattern that has professionals or contractors. The process is aevolved over many years. Evaluation and retrofit change from the conventional; it requires thatare similar in some respects to new construction architects and engineers provide guidance forbut the differences greatly influence the ' building owners.effectiveness of the outcome. Just how should keyparticipants in the pro~ess expect seismic . .evaluation and retrofit to impact their . ';responsibilities?

2.2.1 Building Owners' :For new construction projects, building

owners rarely make explicit decisions with respectto design criteria. Most owners feel that .compliance with prevailing building codes andstandards is adequate for... theirpurposes. Fewrecognize that these ar~ prescriptive standards and,at best, merely imply an unspecified level of' .seismic performance. Owners rely on designprofessionals--architects and engineers--andbuilding officials to select, apply, and enforceappropriate design criteria for their projects. Asimilar situation applies to the retrofit and .rehabilitation of buildings; Th;~ptive codesin place today normally allow for repairs,additions, or alterations to buildings provided thatany new construction conforms to current coderequirements. Changes to the building must notdiminish its strength or, specifically, its ability toresist seismic forces. Building owners, again, haverelied on'design professionals and buildingofficials to work out the specifics of theserequirements for rehabilitation or retrofit projects.

Performance objectives form the basis of themethodology for evaluating and retrofittingconcrete buildings. Building owners must beinformed about the alternatives for each specificbuilding. Performance-based guidelines. givebuilding owners the flexibility to coordinateseismic performance goals with other goals for theuse of their facilities. The ~ereI:l.t uncertainty

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- ~ .SEISMIC EVALUATION AND REY'ROFIT dF' CONCRETE Bun.DINGS

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.' .. .Table 2-1... options..ForImpl"ovlngArchitectul"aJ.seismic-Design-practice--_.-------.- .---.,

.. participate in post earthquake site visits to examine damage and study patterns' .of building behavior.

.. Ensure tJ::l;3t conceptual andschematic designs are developed With joint.i:ltchiteetleifQineer participation. .

.. Develop formal architeettengineer interaction tecnntques todeat with basicseismic issues, such asa professional interaction guide for all critical aspects ofdesign (site characteristics, configuration, structural system and performance,and nonstructurat components).

.. seel< appropriate compensation for seismic design (based on defined scope ofwork and servlcesi,

.. Educate builderS on seismic design issues.Encourage owners to discussseismicdesign issueswith builders.

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2.2.3 Building 'OfficialsThe capabilities of building officials and

building departments throughout the state ofCalifornia vary widely. In urban jurisdictions,some state agencies have relatively, large buildingdepartments, which often include technical staff.Smaller cities and counties may rely on privatesector engineers and architects to check designdrawings as part of the permitting process/Theshift from prescriptive standards to those based onperformance is a major change for buildingofficials. Performance guidelines demand a lot ofjudgment on the part of design professionals, peerreviewers, and building officials.

There is no checklist with true-or-falseresponses in a performance-based design. Itrequires some flexibilityon .the part of buildingdepartments in their plan checking and reviewprocedures. Complicated evaluation and retrofitprojects for concrete buildings may ultimately beapproved by a general consensus among thedesigners, peerreviewers, and building. officials.This is quite different from normal buildingdepartment operations. Performance-based design,in general, and seismic evaluation and retrofit, inparticular, benefit greatly from the involvement ofbuilding officials early in the process and on acontinuing basis. A basic understanding and spiritof cooperation can be developed from thebeginning, and building officials can determine

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Seismic Design Approaches

Performance ."

Fully applicable

Peer review"normally required

Relatively new concept

Supplemental enhancementto prescriptive

MUltiple

Safety/damage/downtime goalsfor specific seismic hazard

Basic Format

New Buildings

ReviewRequirements

Owner's Choices

Existing Buildings

Design Effort/Cost

Familiarity ofArchitects/Engineers

Routine

Traditional .,

Umited

Building codes;checklists

Directly applicable

Partially applicablebut limited-

Plan check normallysufficient

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SEISMIC EVALUATiDN AND RETROFIT OF' CONCRETE BUILDINGS

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2.3 Getting startedThe following sections of this chapter

constitute the general methodology for the seismicevaluation and retrofit of concrete buildings. Thischapter takes the reader step-by-step through theentire process, following the 12 steps indicated inthe "Step-by-Step" sidebar on the following page.Along the way, the purpose and use of each of thesubsequent chapters emerges. & with 'other designand analysis processes, the path to a solution is notalways direct. Reconsideration, changes, andrecycling through the steps are to be expected.

Initiate the PI'"cessA building .owner might choose to' evaluate and

possibly retrofit an existing concrete building for anumber of reasons. In the past, collapse preventionand the reduction of the risk to life safety foroccupants have been the primary goals; for most"Voluntary retrofits. In the future, life safety willremain the primary objective motivation forevaluation and retrofit. Increasingly, however,building owners recognize the benefits. of betterseismic performance in mitigating potential

, .economic. losses as well. This is particularly true-" .Jf.." '.- . :. f:... ._'"

'.. with respect to the.loss of income from a facilitythat fails to function after earthquakes.

Seismic issues are rarely the sole considerationfor the scope of a change or addition to an existingbuilding. Seismic performance evaluation andimprovement may be secondary considerations of a

concrete buildings make it vitally important thatexperienced engineers. actively participate. in themodeling and analysis process. Extensivecommunication with and guidance from those whocan'fnterpret overall structural behavior is requiredto avoid unrealistic .results, If the results do notmake sense, there is a good chance they are notright. Modem analysis techniques can onlyaugment the experience and intuition of a qualifiedengineer when it comes to understanding theseismic behavior 'of concrete buildings.

early on whether they will need the assistance of anoutside peer reviewer or whether they-might be.able..to handle some or all of the review with their ownstaff.

2.2.4 EngineersFor many years engineers have been using

unrealistic simplified static lateral force proceduresto design buildings to resist seismic forces anddisplacements. While traditional methods can resultin adequate designs, they obscure a basicunderstanding of actual structural behavior andperformance during earthquakes. Most engineers inCalifornia, particularly those experienced in seismicretrofit work, are capable of grasping the basicprinciples of the hew procedures'emerging forevaluation and retrofit. However, the majority arestill unfamiliar with these new methods. Theeffective use of the new procedures requires a basicunderstanding of structural dynamics, ductility, andinelastic behavior of structural materials. Sincethese procedures are relatively untested, peerreview becomes an important part of the process.Many individuals are understandably anxious whenothers review their work. For this reason, peerreview should happen early in the process with acooperative and collegial attitude on the part of allinvolved.. .

Performance-based design requires effectivecommunication among the engineer, the architect,and the building owner. The engineer mustcarefully explain the alternatives for performanceobjectives and the implications for costs. In manyinstances, engineers are accustomed to "staying inthe back room" and deferring to their architecturalcolleagues when it comes to communication withthe owner. This traditional arrangement impedesthe effectiveness of seismic evaluation and retrofitprojects.

Within many engineering offices, youngerengineers, more familiar with computer softwareand structural modeling techniques than theirmentors, do the detailed structural analyses. Thecomplexity and uncertainty inherent in the behaviorof many structural elements and components of

Chapter 2~ Overview

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SEISMIC EVALUATION AND RETROFIT Oi= CONCRETE- BUH..DI.NGS

------.,--Step by' Step ---- - ~ ..- _._-.'._ ....-....._.. -_.. __.- -- _.._...._..

S~Pertinent

IT] Initiate the process o ChaptersT Jurisdictional RequirementsR Architectural ChangesVoluntary UpgradeA

~Select Qualified Professionals 0T Structural Engineer

Architect

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~Establish Performance Objectives

CB0G Structural Stabilityy Limited Safetylife Safety

D:faControlImm . te Occupancy

[1]Review Building Conditions 0CDReview Drawings

Visual InspectionPrelininary Calculations

ffiJFormulate a Strategy

~Simplified ProceduresInelastic Capacity MethodsComplex Analyses

ffiJ Begin the Approval Process :DBuilding OfficialC Peer Review0 [I] Conduct Detailed Investigations 0:DSite AnalysisN Material Properties

Construction Details

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~Characterize Seismic Capacity CDE Modeling Rules

p Force and Displacement

T [[] Determine Seismic Demand 0Seismic Hazard

+Interdependence with CapacityTarget Displacement

[9 Verify Performance ~Global Response LimitsD

Component AcceptabilityConceptual Approval

E [] Prepare Construction Documents :DT Similarity to New ConstructionPlan CheckA Formof Construction Contract

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Monitor Construction Quality (2)Submittals, Tests, and Inspections

L Verification of Existing Conditions ,. ,.Construction Observation b Desi er

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best" to follow a selection procedu, (sin---f.oll.owina: .. , 0 .

1. Generate a list of potentially qual.. I. >candidates. This can be from past e. lerifand general familiarity on the part ot ).s\0\-or from references from others who ha 'ifsimilar projects. Public agencies' may dex lOprequest for qualifications announcement Wi .minimum criteria specified. \

2. Request written submittals of statements ofqualifications. The request should state thepreliminary scope of the project to the extent

'. possible.

3. Select several qualified candidates to submitformal proposals; In some cases's availabledrawings or other documents might beprovided to the proposers. Site visits are alsobeneficial.

4. Interview one or more of thri cmdidates on the. I basis of a review of the proposals. The

interview is an opportunity to imagine the.working relationship between an owner and tl

. potentialdesign team. Are the personalitiescompatible? Even if qualifications areexcellent, the relationship between the ownerand the consultants must also be -conducive tcsuccess.

5. Thoroughly check references on similarprojects. Ask references specific questionsabout the performance of the candidates andabout the results of the job:

6. Make a selection and negotiate acontract,Keep in mind that the scope may change onthe' evaluation and retrofit strategy isdeveloped in the initial stages'of the projecThis' selection process can be tailored to th

needs of individual projects. In most cases, atof design professionals led by either an architestructural engineer is sought In some cases, astructural engineer might fill the role withoutarchitectural assistance. An example 'would be

. _:,.~.}Vp"en. a; Pr.~limi:p.ary,',S~dYi to determine deficitand develop conceptual remedial structural

~j~lr~~6J:l~~ert;;;;un~n for any ,number .of reasons. Even if seismic performance...._:;+--improvement is the primary motivation, it is wiseto consider a broader potential scope at thebeginning ofthe project. Potential considerationsinclude the following:

Fire and life safety improvements

Hazardous material abatement

Disabled ~ccess improvement

Change in programmatic use

Functional improvements

BUildin~:~ysten;s improvements

Historic preservationSome ofthese are voluntary and may simply

make sense to include. Others may be required bylaw when changes are made to a building.Jurisdictional requirements vary, and it is prudentto make conservative assumptions early in theprocess. Theactual scope may not emerge untillater in the project, when more informationbecomes available. The expert advice of designprofessionals, including an architect and anengineer, is normally needed to finalize the scopeof a project. -Table 2-2 is a guideline to issuespertinent to the rehabilitation and retrofit processscope of work. ,

Slf!lect Quali#iedpJ:oFessiDntlls

Some owners have ongoing relationships withdesign professionals whom they know and trust.Others may never have dealt with architects orengineers. Public agencies are required to selectproject teams-according to prescribed procedures.In any event, the careful selection of qualifiedprofessional assistance is more important with theevaluation and retrofit of existing concretebuildings than with most other projects. This isbecause of the complexity of the building type, theuncertainties of earthquake technology, and the .lack of establishedprecedents indesign and"analysis methodologies. Therefore, it is usually

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SEISMIC EVALUATJON- AND -RETROFIT OF CONCRETE BUU.DINGS

Table 2-2. Seismic l!~sign (:hecklist fOFacilit;1t.e,l.;JrchitectiEngineer Interaction

structural stability

Limited safety {structural>

Hazards Reduced (non structurall

Life safetystructural

Nonstructural

Damagecontrol

Immediate tcontlnuern OccupancyContinued post earthquake function

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Near faultGround failure possibility [landslide,liquefaction, subsidence) -

Soft soil (long periods, amplification,duration)

vertical discontinuitysoft story

Setback

OffsetResistance elements

Plan discontinuity

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ATC-40 Seismic Evaluation and Retrofit of Concrete Buildings

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