atc 40 vol. 2

"

Seismic evaluation and retrofit. of concrete buildings Volume 2-Appendices

,'. 4.183~ G8 ~

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Applied Technology Counr:;;i

CALIFORf\IIt1. SEISMIC SAFETY COMMISSION Proposition 122 Seisn lie Retrofit Practice8 impr

I-:';':~ ::,;~:: ;:;"1 \ /"U" I 1 ~llivel'5ity of Roorkee I I I \ I \ Call No ........... .. .. \

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unci} ) is a non-:d in 1971 ineers Asso-1 Board of ppointed by , the Struc-I, the Western ,ssociations, oed with the }irector

In practitio-design spe-

.1quake) in vely using lentifies and

_ s consensus opinions on structural engineering issues in a nonpro-prietary fonnal. ATC thereby fulfills a unique role in funded infonnation transfer.

Project management and administration are carried out by a full-time Executive Director and support staff. Project work is conducted by a wide range of highly qualified consulting professionals, thus incor-porating the experience of many individuals from academia, research, and professional practice who would not be available from any single organization. Funding for ATC projects is obtained from govern-ment agencies and from the private sector in the form of tax-deductible contributions.

,

1996-1997 Board of Direetors

John C. Theiss, President C. Mark Saunders, Vice President Bijan Mohraz, Secretaryffreasurer Edwin T. Huston, Past President Arthur N. L. Chiu John M. Coil Edwin T. Dean Robert G. Dean Douglas A. Foutch James R. Libby Kenneth A. Luttrell Andrew T. Merovich Scott A. Stedman Jonathan G. Shipp S:;harles Thornton

California Seismic Safety Commission The California Seismic Safety Commission consists of fifteen members appointed by the Governor and two members representing the State Senate and State Assembly. Disciplines represented on the Commis-sion include seismology, engineering, geology, fire protection, emergency services, public utilities, insur-ance, social services, local government, building code enforcement, planning and architecture.

As a nonpartisan, single-purpose body, the mission of the Commission is to improve the well being of the people of California through cost-effective measures that lower earthquake risks to life and property. It sponsors legislation and advocates building code changes to improve buildings and other facilities, provides a forum for representatives of all public and private interests and academic disciplines related to earthquakes, and publishes reports, policy recommen-dations, and guides to improve public safety in earth-quakes.

It works toward long-term improvements in all areas affecting seismic safety by: encouraging and assisting local governments, state agencies, and businesses to implement mitigation measures to make sure that they will be able to operate after earthquakes; establishing priorities for action to reduce earthquake risks; identi-fying needs for earthquake education, research, and legislation; and reviewing emergency response, re-covery, and reconstruction efforts after damaging earthquakes so that lessons learned can be applied to future earthquakes.

Current (1996) Commission Members Lloyd S. Cluff, Chairman James E. Slosson, Vice Chairman Alfred E. Alquist. State Senator Dominic L. Cortese. State Assemblyman Hal Bernson Jerry C. Chang Robert Downer Frederick M. Herman Jeffrey Johnson Corliss Lee Gary L. McGavin Daniel Shapiro Lowell E. Shields Patricia Snyder Keither M. Wheeler H. Robert Wirtz

Disclaimer While the information presented in this report is believed to be correct, the Applied Technology Council and the California Seismic Safety Commission assume no responsibility for its accuracy or for the opinions expressed herein. The material presented in this publication should not be used or relied upon for any specific application without competent examination and verification of its accu-racy, suitability, and applicability by qualified professionals. Users of information from this publi-cation aSSume all liability arising from such use.

Cover IllustratIOn: SLate Office Bldg, 12'h and N St.. Sacramento. CA, provided by Chris Arnold. b

lmissio: n consists morand ! and State Cornmis-,ogy. fire ities, insUl ,ilding cod

~ mission c ng of the ! measures Jerty. It . g code cilities. public anI related to recommer

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logy lracyor ,sed or its accu-; publi-

ATC-GO Seismic Evaluation and Retrofit

of Concrete Buildings volume 2-Appendices

~~D TECHN~LOGY COUNCIL 555 Twin Dolphin Drive, Suite 550

Redwood City, California 94065

Funded by

SEISMIC SAFETY COMMISSION State of California

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

PRINCIPAL INVESTIGATOR Craig D. Comartin

CO-PRINCIPAL INVESTIGATOR PROJECT DIRECTOR Richard W. Niewiarowski

SENIOR ADVISOR Christopher Rojahn

Report No. SSC 96-01 November 1996

preface Proposition 122 passed by California's voters in 1990, created the Earthquake Safety and Public Buildings Rehabilitation Fund of 1990, sup-ported by a $300 million general obligation bond program for the seismic retrofit of state and local government buildings. As a part of the program, Proposition 122 authorizes the California Seismic Safety Commission (CSSC) to use up to 1 % of the proceeds of the bonds, or approximately $3 million, to carry out a range of activities that will capitalize on the seismic retrofit experience in the private sector to im-prove seismic retrofit practices for government buildings. The purpose of California's Proposi-tion 122 research and development program is to develop state-of-the-practice recommenda-tions to address current needs for seismic retro-fit provisions and seismic risk decision tools. It is focused specifically on vulnerable concrete structures consistent with the types of concrete buildings that make up a significant portion of California's state and local government invento-ries.

[n 1994, as part of the Proposition 122 Seismic Retrofit Practices Improvement Program, the Commission awarded the Applied Technology Council (ATC) a contract to develop a recom-mended methodology and commentary for the eismic evaluation and retrofit of existing con-

~rete buildings (Product 1.2). In 1995 the :::ommission awarded a second, related contract :0 ATC to expand the Product 1.2 effort to in-:lude effects of foundations on the seismic per-'ormance of existing concrete buildings Product 1.3). The results of the two projects lave been combined and are presented in this \TC-40 Report (also known as SSC-96-01). rwo other reports recently published by the :a1ifornia Seismic Safety Commission, the 'rovisional Commentary for Seismic Retrofit 1994) and the Review of Seismic Research Re-'ults on Existing Buildings (1994), are Products .. 1 and 3.1 of the Proposition 122 Program, re-.pectively. These two previous reports provide he primary basis for the development of the ecommended methodology and commentary :ontained in this document.

This document is organized into two volumes. Volume One contains the main body of the evaluation and retrofit methodology, presented in 13 chapters, with a glossary and a list of ref-erences. This volume contains all of the parts of the document required for the evaluation and retrofit of buildings. Volume Two consists of Appendices containing supporting materials re-lated to the methodology: four example building case study reports, a cost effectiveness study related to the four building studies, and a review of research on the effects of foundation condi-tions on the seismic performance of concrete buildings.

This report was prepared under the direction of A TC Senior Consultant Craig Comartin, who served as Principal Investigator, and Richard W. Niewiarowski, who served as Co-Principal In-vestigator and Project Director. Fred Turner served as CSSC Project Manager. Overview and guidance were provided by the Proposition 122 Oversight Panel consisting of Frederick M. Herman (Chair), Richard Conrad, Ross Cran-mer, Wilfred Iwan, Roy Johnston, Frank McClure, Gary McGavin, Joel McRonald, Jo-seph P. Nicoletti, Stanley Scott, and Lowell Shields. The Product 1.2 methodology and commentary were prepared by Sigmund A. Freeman, Ronald O. Hamburger, William T . Holmes, Charles Kircher, Jack P. Moehle, Thomas A. Sabol, and Nabih Youssef (Product 1.2 Senior Advisory Panel). The Product 1.3 Geotechnical/Structural Working Group con-sisted of Sunil Gupta, Geoffrey Martin, Mar-shall Lew, and Lelio Mejia. William T. Hol-mes, Y oshi Moriwaki, Maurice Power and Nabili Youssef served on the Product 1.3 Senior Advisory Panel. Gregory P. Luth and Tom H. Hale, respectively, served as the Quality Assur-ance Consultant and the Cost Effectiveness Study Consultant. Wendy Rule served as Tech-nical Editor, and Gail Hynes Shea served as Publications Consultant.

Richard McCarthy CSSC Executive Director

Christopher Rojalm ATC Executive Director & ATC-40 Senior Advisor

III

Oversight Panel for proposition 122 Seismic Retrofit Practices

Improvement program Richard Conrad Frederick M. Herman, Chair

Seismic Safety Commission Local Government/Building Official

Building Standards Commis-sion

Ross Cranmer Building Official Structural Engineer

Dr. Wilfred Iwan Mechanical Engineer

Roy Johnston Structural Engineer

Joel McRonald

Frank McClure Structural Engineer

Gary McGavin Division of the State Architect Joseph P. Nicoletti Structural Engineer

Seismic Safety Commission Architect

Stanley Scott Research Political Scientist

Lowell E. Shields Seismic Safety Commission Mechanical Engineer

Seismic Safety commission Staff

Iv

Richard McCarthy Executive Director

Karen Cogan Deborah Penny Carmen Marquez

Fred Turner Project Manager

Chris Lindstrom Ed Hensley Teri DeVriend Kathy Goodell

S

c

V Ii

s 1

~ 1

c

IS Product 1.2 Senior Advisory Panel

Sigmund A. Freeman Wiss. Janney. Elstner & Associates

Charles Kircher Charles Kircher & Assocates

Ronald O. Hamburger EQE International

Jack Moehle Earthquake Engineering Research Center

Nabih F . Youssef Nabih Youssef & Associates

William T. Holmes Rutherford & Chekene

Thomas A. Sabol Engelkirk & Sabol

Product 1.3 Senior Advisory Panel William T. Holmes

mission Rutherford & Chekene Maurice Power Geomatrix Consultants. Inc.

Yoshi Moriwaki Woodward-Clyde Consultants

Nabih F. Youssef Nabih Youssef & Associates

Product 1.3 Geotechnical/structural working Group Sunil Gupta Q Tech Consultants

Marshall Lew Law/Crandall. Inc.

Quality Assurance Consultant Jregory P. Luth 'Jregory P. Luth & Associates

:ost Effectiveness study Consultant rom H. Hale fimmy R. Yee Consulting Engineers

Geoffrey R. Martin University of Southern California

Lelio Mejia Woodward-Clyde Consultants

Technical Editor Wendy Rule Richmond. CA

Publications Consultant Gail Hynes Shea Albany. CA

v

p

Seismic Evaluation and Retrofit of Concrete Buildings

products 1.2 and 1.3 of the proposition 122 seismic Retrofit Practices Improvement Program

Table of Contents Volume 1

Preface ................................................................................................... iii Glossary ................................................................................................. xi Executive Summary ................................................................................... xv Chapter 1 Introduction ........................................................................... \-1

1.1 Purpose ........................................................................ \-\ 1.2 Scope .......................................................................... 1-2 1.3 Organization and Contents ................................................. 1-5

Chapter 2 Overview .............................................................................. 2-1 2.1 Introduction ............................................................ : ..... 2-\ 2.2 Changes in Perspective ..................................................... 2-3 2.3 Getting Started ............................................................... 2-6 2.4 Basic Evaluation and Retrofit Strategy ................................. 2-11 2.5 Evaluation and Retrofit Concept ........................................ 2-14 2.6 Final Design and Construction .......................................... 2-19

Chapter 3 Performance Objectives ............................................................. 3-1 3.1 Introduction .................................................................. 3-1 3.2 Performance Levels ......................................................... 3-\ 3.3 Earthquake Ground Motion ................................................ 3-8 3.4 Performance Objectives .................................................... 3-9 3.5 Assignment of Performance Objectives ................................ 3-12

Chapter 4 Seismic Hazard ...................................................................... .4-1 4.1 Scope ......................................................................... .4-1 4.2 Earthquake Ground Shaking Hazard Levels ............................ .4-1 4.3 Ground Failure .............................................................. .4-2 4.4 Primary Ground Shaking Criteria ........................................ .4-5 4.5 Specification of Supplementary Criteria ............................... 4-12

Chapter 5 Determination of Deficiencies ..................................................... 5-1 5.1 Introduction .................................................................. 5-1

able Of contents vii

SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS

5.2 Description: Typical Layouts and Details ............................... 5-1 5.3 Seismic Performance ....................................................... 5-5 5.4 Data Collection ............................................................ 5-12 5.5 Review of Seismic Hazard ............................................... 5-17 5.6 Identification of Potential Deficiencies ................................ 5-18 5.7 Preliminary Evaluation of Anticipated Seismic Performance ...... 5-20 5.8 Preliminary Evaluation Conclusions and Recommendations ....... 5-21

Chapter 6 Retrofit Strategies .................................................................... 6-1 6.1 Introduction ................................................... ~ .............. 6-1 6.2 Alternative Retrofit Strategies ............................................. 6-4 6.3 Design Constraints and Considerations ................................ 6-24 6.4 Strategy Selection ......................................................... 6-27 \ 6.5 Preliminary Design ....................................................... 6-30

Chapter 7 Quality Assurance Procedures ..................................................... 7-1 7.1 General. ....................................................................... 7~1 7.2 Peer Review .................................................................. 7-2 7.3 Plan Check ................................................................... 7-8 7.4 Construction Quality Assurance ........................................ 7-10

Chapter 8 Nonlinear Static Analysis Procedures ............................................ 8-1 8.1 Introduction .................................................................. 8-1 8.2 Methods to Perform Simplified Nonlinear Analysis ................... 8-3 8.3 Illustrative Example ....................................................... 8-34 8.4 Other Analysis Methods .................................................. 8-54 8.5 Basics of Structural Dynamics .......................................... 8-57

Chapter 9 Modeling Rules ....................................................................... 9-1 9.1 General ......................................................................... 9-1 9.2 Loads ............. : ............................................................ 9-2 9.3 Global Building Considerations ........................................... 9-4 9.4 Element Models ............................................................. 9-7 9.5 Component Models ....................................................... 9-19 9.6 Notations .................................................................... 9-46

Chapter 10 Foundation Effects ................................................................. 10-1 10.1 General. . .. .... . . . . . . . .. .. .. . . .. . . . . .. . . . .. .. . .. . . . .. . . .. . . .. . . . . . .. . . . . . .. .. 10-1 10.2 Foundation System and Global Structural Model .................... 10-2 10.3 Foundation Elements ..................................................... 10-7 10.4 Properties of Geotechnical Components .............................. 10-12 10.5 Characterization of Site Soils ........................................... 10-20 10.6 Response Limits and Acceptability Criteria .......................... 10-28 10.7 Modifications to Foundation Systems ................................. 10-29

Chapter 11 Response Limits .................................................................... 11-1 11.1 General. ..................................................................... 11-1 11.2 Descriptive Limits of Expected Performance ......................... 11-2 11.3 Global Building Acceptability Limits ........... '" .................... 11-2 11.4 Element and Component Acceptability Limits ........................ 11-5

Chapter 12 Nonstructural Components ....................................................... 12-1

viii Table of Contents T

~S SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS

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

Chapter 13 Conclusions and Future Directions .............................................. 13-1 13.1 Introduction ................................................................ 13-1 13.2 Additional Data ............................................................ 13-1 13.3 Potential BenefIts .......................................................... 13-4 13.4 Major Challenges .......................................................... 13-5 13.5 Recommended Action Plan .............................................. 13-6

References ........................................................................................... 14-1

volume 2-Appendlces Appendix A Escondido Village Midrise, Stanford, California .............................. A-I Appendix B Barrington Medical Center, Los Angeles, California ......................... B-1 Appendix C Administration Building, California State University at Northridge,

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

I Of contenl'able Of contents Ix

f SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS

APpendix A Example Building Study Escondido village Mldrlse stanford, California prepared by

EQE International 44 Montgomery Street, Suite 3200 San Francisco, California 94104

'1Il1endlx A, Escondido village Mldrlse A-'


Table of Contents 1. Introduction ................................................................................................. A-5

1.1 Purpose ...................................................................................... A-5 1.2 Scope of Example Building Study ....................................................... A-5 1.3 Summary of Findings ..................................................................... A-5

2. Building and Site Description ............................................................................ A-7 2.1 General ...................................................................................... A-7 2.2 Structural Systems and Members ....................................................... A-8 2.3 Soil and Seismicity ........................................................................ A-9 2.4 Building Performance During the Lorna Prieta Earthquake ........................ A-9

3. Preliminary Evaluation ................................................................................... A-9 3.1 Summary .................................................................................... A-9 3.2 FEMA-178 Evaluation Statements ..................................................... A-II 3.3 Elastic Analysis ........................................................................... A-I4

4. Evaluation by Product 1.2 Methodology .............................................................. A-IS 4.1 Introduction ................................................................................ A-IS 4.2 Analysis Methodology ................................................................... A-IS 4.3 Structure ryIodeling ....................................................................... A-IS 4.4 Pushover Analysis ........................................................................ A-22 4.5 Performance Point. ....................................................................... A-27 4.6 Performance Assessment ................................................................ A-31

5. Conceptual Retrofit Designs .......................................................... , ................. A-33 5.1 Performance Objectives ................................................................. A-33 5.2 Retrofit Strategies ........................................................................ A-33 5.3 Retrofit Systems .......................................................................... A-34

6. Assessment of the Product 1.2 Methodology ......................................................... A-36 6.1 Damage Prediction ....................................................................... A-36 6.2 Comparison with Preliminary Evaluation Findings ................................. A-36 6.3 Comparison with Inelastic Time-History Analysis .................................. A-37 6.4 Conclusions ................................................................................ A-37

7. Foundation Analysis ...................................................................................... A-38 7.1 Introduction ................................................................................ A-38 7.2 Varying Soil Parameters ................................................................. A-38 7.3 Comparisons with Inelastic Time-History Analysis ................................. A-42 7.4 Conclusions ................................................................................ A-43

8. References ................................................................................................. A-43

I!Ipendlx A. Escondido Village Mldrlse

---


APpendix A Example Building Study Escondido village Midrise stanford, California 1. Introduction 1.1 purpose

The purpose of this example building study is to illustrate and evaluate the techniques outlined in products 1.2 and 1.3 of Proposition 122 as a tool for the evaluation and retrofit of existing concrete buildings. Titled Seismic Evaluation and Retrofit of Existing Concrete Buildings, Volume 1, the document is referred to herein as the . Methodology.

1.2 scope of Example Building study This study presents the evaluation and

conceptual retrofit design of a concrete building located on the Stanford University campus, following the recommendations of the Methodology. This study was performed ;oincident with the various draft stages of :ievelopment of the Methodology and feedback from this study was used to affect final nodifications of the Methodology. Our scope neluded: Preliminary evaluation (Section 3 of this

report) Modeling, analysis, and assessment by

nonlinear pushover analysis (Section 4) Conceptual retrofit (Section 5) Assessment of the Methodology (Section 6) Foundation analysis (Section 7)

1.3 summary of Findings The tools currently available to the structural

ngineer for seismic evaluation and retrofit of

Ippendlx A, Escondido Village Mldrlse

existing concrete structures are essentially limited to the building codes for new construction and the FEMA-178 document. In comparison with these existing tools, the Methodology appears to represent a significant enhancement in the state of practice. Based on the Escondido Village Midrise (EVM) case study, the Methodology appears to provide a realistic and conservative, if not completely accurate, approach to seismic evaluation of complex reinforced concrete structures yet also permi ts the engineer to develop retrofit strategies that are significantly more cost effective than were traditionally utilized in the past.

FEMA-178 evaluations of the EVM buildings indicate an inability to satisfy the life safety performance level for the design earthquake, due to a lateral force resisting system comprised of discontinuous shear walls, with inadequate shear capacity. Prior to development of the Methodology, the standard approach for mitigation of such deficiencies would have been the introduction of an extensive number of supplemental shear walls to the structure. This would have great architectural and economic impact on the building. In comparison, the Methodology identified that the existing walls essentially provide adequate drift control for the structure, but that several other vulnerabilities related to shear capacity of the lower story columns and punching shear capacity of the floor slabs exist. Retrofit of these vulnerabilities, which were not specifically identified by the FEMA-178 approach, was found to be possible with much less architectural impact on the buildings and at

A-5


Table 1.$1. ComparIson Of flOOf DlsPlacemen.;;;ts~ __

significantly reduced cost compared to alternative approaches suggested by the FEMA-178 evaluation. These retrofit modifications have actually been constructed, within a time period of approximately 3 months and while the buildings remained nearly completely occupied.

Compared to existing approaches, the Methodology does require more complex and time consuming work on the part of the structural designer. However, the additional level of effort required is well within the capability of the average practicing engineer in California, who has the familiarity with the basic concepts of structural dynamics and inelastic behavior of structures that is essential to being able to design effective seismic resistant systems, either for new or existing buildings. In the case of the EVM buildings, the additional effort and cost invested in the evaluation and analysis of the structure resulted in a very substantial reduction in retrofit construction costs, and consequently in overall project costs.

Notwithstanding the above, it can not be overemphasized that this Methodology does not provide an "exact" tool for the seismic evaluation of structures, and that in fact, such an "exact" tool does not exist within our current technological capabilities. In the EVM case study, target displacements were determined by two alternative methods encompassed by the Methodology, the Displacement Coefficient Method and the

A-a

Capacity-Spectrum approach; as well as by two other approaches that are commonly cited in the literature - the so called "Equal Displacement Approximation" and non-linear response history analysis, in which the average result for 20 different response histories is shown. Table 1.3-1 indicates the range of computed roof displacement obtained by these alternatives methods, and also provides a normalized index that consists of the ratio of the displacement computed by each method to the maximum displacement predicted by the nonlinear response history analyses.

As can be seen by evaluating the data contained in Table 1.3-1, the various approaches for estimating the maximum roof displacement produced in the building vary by as much as + 25 %, to 35 %. The method with the largest variation, and the least conservative estimate, is actually the use of the average of the series of non-linear response history analyses. The two methods contained in the methodology; the displacement coefficient approach and capacity spectrum approach, produce the most conservative estimates. This apparent conservatism would appear to be the result of the way in which the various approaches treat the pinched hysteretic response. The equal displacement rule and response history analyses both neglect the effects of hysteretic pinching. Both the displacement coefficient and capacity spectrum techniques account for this effect. Although the research

Appendix A, EscondidO Village Midrise

C il b \I

A~

- SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS

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Figure 2.1,1. Typical Floor Plan

)redicted 1:community is currently divided with regard to the importance of pinched hysteresis to overall

Ita building response, it would seem pruden~ given the pproaches wide range of variation in the response history Icement analyses to take the conservative approach as has lch as been done by the methodology. Such conservatism ,argest is further warranted, given that our ability to timate, is Iccurately estimate the ground motions that a !ries of milding will be subjected to is quite limited. 'he two As noted earlier, although the Methodology ; the Ippears to provide conservative estimates of capacity lUilding response, compared to other approaches, ;onservativetrofit designs developed using the Methodology would ICtually appear to be quite cost effective and hich the :conomical relative to the designs commonly ysteretic Iroduced in the past using more traditional and pproaches. the effects cement niques esearch

lIIage Mldr!bpendlx A. Escondido Village Mldrlse

2. Building and Site Description

2.1 Ceneral The Escondido Village Midrise buildings are a

set of five, similar, reinforced concrete shear wall structures. The buildings were constructed in two phases. The first phase, designed in 1961, consisted of three structurally identical buildings -Abrams, Barnes, and Hulme. The second phase, designed in 1964, consists of Hoskins ll?d . McFarland, which are also structurally Identical to each other. The two phases of construction were designed by the same designers and have nearly identical floor plans. The primary difference between the two phases is in the layout of basement areas. The buildings have overall plan dimensions of

65 feet by 109 feet, and are approximately rectangular in plan (Figure 2.1-1). They are

A7


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Figure 2.2,1. Typical Floor Slab constructfon

arranged in random directions on the Stanford campus, but are all located at the northeast corner of the site, near EI Camino Real and Stanford Avenue.

Each building is 8 stories tall, with a mechanical equipment penthouse and a full basement. The typical story height is 9'-1" (the basement story height is 12'-7"). The basements are only partially embedded within the ground, with the first floor located about 4 feet above adjacent grade. 2.2 structural systems and Members

Gravity IDad-reslstlng system 12" one-way concrete core slabs (7" diameter

hollow cores spaced 9" apart) carry floor loads to walls and columns (Figure 2.21)

Strips of slabs aligned with column lines are solid and provide a beam-like element at the columns

10" concrete walls at stairs, elevators, and perimeter of typical floors

12" concrete walls at basement 15x24 interior concrete columns, 15x22

re-entrant corner columns, i Ix II balcony columns

A-a

Figure 2.2-2. Typical Connection Of Floor to wall

Continuous strip footings support walls Isolated spread footings support columns

LaterallDad-reslstlng system Load-path: rigid slabs, through shear walls, to

foundation

Materials Per original design drawings, specified 28-day

concrete strength: 3000 psi for slabs, beams, and walls; 3750 psi for columns

Per test program conducted in 1989, tested concrete strength: 2470 psi for slabs

Concrete strength used in analysis: 2470 psi for slabs, beams, walls; 3000 psi for columns

Specified steel reinforcing: "intermediate" (40 ksi) grade for slabs, beams, and walls; "hard" (60 ksi) grade for columns Concrete shear walls are typically reinforced

with two curtains of reinforcing steel. Vertical steel is lap spliced at each floor level. Floor slabs are doweled to the wall, as indicated in Figure 2.2-2.

Above the first floor level, the walls are of uniform layout in all of the buildings, as shown in Figure 2.1-1. There is a substantially larger

Appendix A, Escondida Village Midrlse

;S SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS

valls lumns

umber of walls in the basements of the buildings ~an there is in the upper stories, and the two phases of construction have slightly different arrangements of basement walls. Figure 2.2-3 shows the arrangement of typical basement walls in the first increment of buildings.

2.3 5011 and seismicity The Escondido Village is underlain be

approximately 200 feet of alluvial soils over Franciscan formation bedrock. As reported in various project geotechnical reports''J .. " the alluvial soils are generally dense interbedded layers of clayey sands, sandy clays, sands, and gravels. Woodward-Clyde Consultants" developed estimates of the force-deformation relationships for shallow spread foundations, like those for the Escondido Village Midrise buildings, founded on these soils. These force-deformation properties, that were evaluated at loading rates similar to those expected during an earthquake, are presented

II in Figure 2-3.1. :ar wa s, I As indicated in the figure, an effective

subgrade modulus of 800/B tons/ft2/ft is estimated. Initial stiffness of footings founded on this material is estimated as being 4K,A, where A is the area of

ified 28-da the footing and J(, is the subgrade modulus. )s, beams, Ultimate permissible bearing pressures are

estimated by Woodward-Clyde as being on the 9, tested order of 15 tons/ft'. It is projected that the )s foundation conditions could vary from 2/3 to . 2470 . 150 percent of the stiffness projected in the figure. : I pSI The Escondido Village Midrise buildings are orcouIDIlJl h S ~ dU" . ocated on t e tan.or mverslty campus In nediate" (4northern California. The western border of the alls; "hardcampus along Junipero Serra Boulevard is

approximately 4.0 miles northeast of the reinforcedmid-peninsula segment of the San Andreas fault, Vertical and the eastern border along El Camino Real is Floor slababout 5.5 miles northeast of the fault. n

lls are of 2.4 Building Performance During the

Lama Prieta Earthquake as shown i The Escondido Village Midrise buildings were larger jamaged during the October 17, 1989 Lorna Prieta

lIIage Mldril'ppendlx A. Escondido Village Mldrlse

Earthquake. This included moderate but widespread cracking of the cast-in-place concrete walls, including both shear cracking in classic diagonal "x" patterns, flexural cracking consisting of cracks that were approximately horizontal near the bases of the walls, and horizontal cracking along the construction joints present at floor levels. The walls around the stair towers experienced the heaviest damage. Most damage to the walls was repaired shortly after the earthquake with the injection of epoxy grout.

3. Preliminary Evaluation 3.1 summary

As recommended in Chapter 5 of the Methodology, a preliminary seismic evaluation of the Escondido Village Midrise buildings was conducted using the procedures contained in FEMA-178" to determine if nonlinear analysis is warranted. The FEMA-178 evaluation procedure was developed with national consensus of the engineering community and is intended to serve as a preliminary screening tool to determine if a building is a potential unacceptable risk to life. The procedure contains a series of checklists, organized by model building type, that guide the evaluator through examination of important structural features of the building, relative to earthquake performance. In some cases, rapid approximate calculations of capacity are performed. The premise of the procedure is that most building failures in earthquakes can be traced to a relatively limited number of critical flaws, that the checklists are designed to specifically explore. Failure of a building to pass the screening test of the checklist does not necessarily indicate that a life safety hazard exists. It is expected that some buildings that fail the checklist screening can be demonstrated to be adequate to a substantial life safety performance objective upon more detailed evaluation.

A9

A-10


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--

... :-,' .. ..... _ ... ~_l _______ _

12' 12' 12' 12' '2' 12'

Figure 2.2/1. Basement Floor Plan

16 B _ feet

I'

12 i2' .[ 10 ! 8 6

Note - "B" is the footing width

" c , c

.I! 2

0 0 O.OS 0.10 O.IS

Foundation DisD]acement rfeetl Figure 2./11. $011 ForceDeformatlon Relationships

. )1:

.. Jolm

10'-7~

Appendix A. Escondido Village Mldrlse

f ,

,

~---------------------------------------SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS

-

G) G) G) 0 (0 (0 0 0 G) GD , W23

1 , J. _J I"'"W20 I - ... - 1 -~1~r~1 ,

- -

W26

>< 10'7" -. i5

SEISMIC EVALUATION ANIi RETROFIT OF CONCRETE BUILDINGS

8tbfloor

7tbfloor

6th floor

sthnoor

4tbfloor

3rdfloor

2ndfl"",

Buemcnt

-i!-- TypicallrUllverK wall (Walll3t and 41)

Typical comer wall

~-n' .... 'tinu"" at lhear 41

n... .......

Figure S.22. Discontinuity at Transverse Shear Walls (Walls S1 and 41J

however, there are local discontinuities in some of the vertical elements of the lateral force resisting system. These are located at stairways #1 and #2 and at the primary shear walls along lines 1 and 10, designated as W31 and W41, respectively in Figure 3.2-1. Figure 3.2-2 presents an elevation of walls 31 and 41, indicating the discontinuity condition that occurs at the first floor level in these walls. The effect of this discontinuity is to create a severe condition for the boundary elements of these walls.

Figure 3.2-3 presents partial plans of stairway #1 at the basement, first floor and typical floors. The primary lateral load resisting components of this stairwell core are designated as walls "a", "b", "en, "d" and "e". Wall "a" is offset below the second floor and walls "CO, "d" and "e" have large door openings in the mid length of each wall at the basement level. The discontinuity of wall "a" is not believed to represent a severe problem because the return walls "c" and "d", that serve as the flanges of wall "a" under flexural behavior, are continuously connected to the wall above, and are continuous themselves through the zone of

A-12

discontinuity. Therefore, it is believed that an adequate load path exists across this discontinuity. The openings in walls "c", "d", and We" are not considered significant because there are an extensive number of additional shear walls present in the basement, and the portions of these walls that are removed are not critical to the flexural behavior of this element.

Figure 3.2-4 is a plan of stair way #2. Primary walls resisting lateral load are indicated as walls "f", "gH, "h", "iH and "j". Wall "f", along column line 6 is discontinuous at the first story, where it is replaced by a column at grid coordinate D-6. This represents both a shear and flexural discontinuity, but is primarily a concern because of the flexural condition. The column at D-6 and boundary element at B-6 must resist all of the overturning demands delivered by wall "f" above. Vertical Discontinuities. As described above,

under "Weak Stories", there are three conditions of vertical discontinuity - the transverse walls (W31 and W41), stairway #1, and stairway #2.

Deterioration of Concrete. Many of the floor slabs have horizontal cracks present. These cracks appear to be a result of drying shrinkage of concrete that was cast too wet.

Concrete Wall Cracks. The buildings experienced significant cracking in the Lorna Prieta Earthquake of 1989. Nearly all such cracks have been repaired with epoxy injection, except at the basement, where some cracks with widths as much as 4mm width were observed. It is not believed that these cracks are detrimental to the building's future behavior, however.

Complete Frames. The concrete shear walls resist a significant portion of the building's total weight.

Shear Walls Shearing Stress Check. The maximum

calculated stress in the walls, when the

Appendix A. Escondido Village Midrise

App.

, ~ ---------------------------------------------------------

it an mtinuity, are not In Is presen ! walls !xural

t. Primar is walls' ong t story, coordinru ' !xural because D-6 and

of the ;'f" above Jed above 'ee . the :airway #1

Jf the flrnJ t. These 19 too wet.

ngs the Lorna all such ,xy Nhere som n width lat these ing's futu~

;hear walls ,uilding's

,imum


,"', Wall"b" ,i' /-', " (~; ',;!) ',~; Wall"b" + \!: Wall"b" C~) . (~) f,- :,~,' Wall Wall

i,~)' (~l Wall "a" ~ Wall "a" T~!:' 1., Basement Aoor

Figure ~,2-~. Partial Floor Plans at stairway NO. 1

~ (7 (~ (if All .". ~. I

Wall Wall Wall Wall Wall all all Wall 'r 'g' 'h' "g" "h' . "f' g' "h'


of 50 percent of the longitudinal reinforcing provided. (This was an assumption made during the initial evaluation, see Section 4.3.4 for additional discussion.)

Conrmement Reinforcing. Ties provided at boundary elements of shear walls are #3 at 12 inch spacing. However, ties are provided with 1350 hooks, so that confinement could be considered of intermediate quality.

Reinforcing Steel. The typical reinforcing pattern for walls provides a ratio of 0.0023 times the gross cross sectional area.

Reinforcing at Openings. Trim bars are typically provided at openings, however, these are not confined with special ties.

Diaphragms Plan Irregularities. Re-entrant corners occur

at column locations B-3, B-7, G-4, and G-8. Special chord bars have not been provided in these areas. However, the distribution of shear walls throughout the building is such that diaphragin flexural demands are moderate and slab reinforcing is generally adequate to handle corner stresses.

Transfer to Shear Walls. Dowels provided between the floor slabs and walls are not adequately embedded to fully develop their yield strength. Consequently, the connection of diaphragms to walls cannot develop the diaphragm strength. In addition, most walls do not extend the full length of the diaphragm, and collector reinforcing has not been provided to drag diaphragm loads into the walls.

Vertlcalcor.nponents Shear Wall Boundary Columns. As

previously described, the lap splice of wall boundary reinforcing is not adequate to develop the reinforcing strength.

A'4

capacity of Foundations Overturning. The ratio of the effective

horizontal dimension, at the foundation level of the seismic force resisting system, to the building height exceeds 1.4Av. Neglecting near-source effects, A. for the Stanford campus is 0.4 and the ratio is 0.56. In the transverse direction, the ratio of foundation width to building height is 0.38.

3.3 Elastic Analysis Elastic analysis is the conventional method of

evaluating the seismic demands on elements of a structure used in both design of new structures and detailed evaluation of existing structures. For this project, a dynamic response spectrum method analysis was performed. Using the ETABS21 software package, a three-dimensional computer model was constructed and analyzed. The resulting displacements are a reasonable estimate of those that the real structure would see, if it remained elastic. Forces calculated for individual elements by this technique are also a reasonable estimate of the maximum demands on these elements if the structure were to remain elastic.

The primary benefits of the elastic analysis is that it provides a rapid method of determining the strength of the building relative to current code requirements, the distribution and locations of large strength demands on the structure, and the overall level of lateral displacement the building would experience in the design earthquake. Buildings with limited displacement demands, well distributed elastic strength demands, and relatively moderate conditions of strength deficiency relative to current code can generally be judged to provide acceptable performance.

On the basis of the elastic analysis, using cracked section properties and accounting for elastic flexibility of the foundation system, the Escondido Village Midrise buildings are demonstrated to have strength in the longitudinal direction, comparable to that required by the current UBC. Strength in the transverse direction,

Appendix A. Escondido Village Midrise

he thl bu thl on m:

an re

4.

Iy m bl M is d) In fo pI

e~ W su d( m m of stl ei m bI di th di

th su Sl in n( df

~ --------------------------------------------------------------------------------SEISMIC EVALUATION ANO RETROFIT OF CONCRETE BUILDINGS

--------------------------------------------

e

1 level .0 the :ting d the

lation

ethod .of

however, is substantially less than that required by the current UBC. The effective periods of the building are 0.83 and 0.71 seconds respectively in the transverse and longitudinal directions. Based on the elastic response spectrum analysis, maximum elastic interstory drifts of l. 2 percent and I percent are estimated in these directions respectively.

4. Evaluation by Product 1.2 Methodology

nts of a 4.1 Introduction :tures ali For this

:thod IS" ,mputer e resultin )f those nained

~lements stimate (j .s if the

A series of simplified inelastic analyses of the type known as static pushovers were performed to more accurately evaluate the behavior of the buildings in response to strong ground motion. Most design of buildings for earthquake resistance is based on an elastic analysis of the building's dynamic response to the expected ground motion. In such analyses, it is assumed that the amount of force induced in an element is directly proportional to the amount of deformation it experiences in response to the ground motion . While all buildings behave in this manner when

nalysis is subjected to low levels of loading, most structures nining the do not have adequate strength to respond in this nt code manner when subjected to intense levels of ground .ons of motion. In reality, when subjected to such levels , and the of ground motion, individual elements of the building structures will be stressed to a point at which they Ike. either yield - that is continue to deform while nands, we maintaining a relatively constant stress state, or ,d relativel break. Following such yielding or breaking, the ncy relatil distribution of both deformations and stresses I to provid throughout the structure can be significantly

different than predicted by an elastic analysis. . using Elastic design and analysis procedures, such as ing for those contained in FEMA-178 incorporate [em, the substantial factors of safety in the permissible re stress states and configuration limits they specify, mgitudinal in recognition of the fact that the elastic analysis is by the not accurately predicting the distribution of ;e directiordemands at high load levels. Inelastic analyses,

'lIIage Mld~pendlx A, Escondido Village Mldrlse

such as that outlined in the Methodology allow for more accurate prediction of the demands on individual elements of the building and therefore permit lower factors of safety to be used in evaluating the adequacy of specific structural components. Many buildings that appear to be highly deficient when evaluated by elastic analysis methods can be demonstrated to be only modestly deficient, or perhaps completely adequate, when evaluated to these more accurate approaches.

4.2 Analysis Methodology The static pushover technique is one of the

simpler types of inelastic analyses. Essentially it consists of a series of elastic analyses of successive models of the building that have been progressively modified to represent the stiffness of the structure at a given stage of lateral deformation. In other words, as structural components yield, the stiffness of the structure is reduced to reflect that yielding.

For the example building study, tl1e following basic steps were implemented based on the Metl1odology: Structure modeling (Section 4.3 of this report) Pushover analysis (Section 4.4) Performance point (Section 4.5) Performance assessment (Section 4.6)

4.3

4.$.1 Structure Modeling

Software Limitations The static pushover analyses of tl1e Escondido

Village Midrise buildings were performed using DRAIN-2DX software. As with any software package, limitations can significantly affect the nature of the analysis. Some of the limitations imposed by the DRAIN-2DX software include: No Inelastic Panel Elements. Walls subject to

potential flexural and shear yielding were modeled as column elements. See Section 4.3.4.

A15


W23

Stair #1 W20 W16

I I>"?J W31 W41 I WIO WI7

Stalr#Z

WI3

FIgure 4.$-1. wall Element Numben

No Degrading Elements. All yielding elements maintain their strength, but the Methodology requires degrading for elements with high ductility demands. See Section 4.4.2.

Two-Dimensional Modeling. The program allows for twodimensional modeling only; resulting in the loss of torsional effects.

No Graphics or Post-Processing. This limits the efficiency of the analysis.

4.$.2 Materials The same material properties used in the

elastic analysis (see Section 2.2) were used for the nonlinear static analysis: Existing Concrete Strength. 2470 psi for

slabs, beams, walls; 3000 psi for columns Existing Steel Reinforcing Strength. 40 ksi

for slabs, beams, walls; 60 ksi for columns

4.$.$ structural systems DRAIN 2DX is capable only of analyzing two

dimensional structures. Therefore, independent analyses of the building response were performed for the longitudinal and transverse building axes, using different models. Figure 4.3-1 is a typical floor plan for the building, indicating the numbering scheme used for various walls

A-'.

contained in the buildings. Figure 4.3-2 schematically represents the model developed for the longitudinal axis of the building.

The principal disadvantage of using two-dimensional models to represent the building is that torsional effects are lost, as are the combination of effects from simultaneous loading in different directions. The elastic analysis, previously performed, demonstrated that the building is torsionally quite regular. Therefore, it was not felt necessary to model its torsional response characteristics. Modeling of the effects of combined response in two directions on those elements of the lateral system which participate in both directions could not be captured. In addition to the inability of the two dimensional approach to capture this behavior, it was not possible to develop constitutive models (force-deformation curves) for the infinite number of combinations of loadings about the two axes of these walls that are possible.

As seen in Figure 4.3-2, the longitudinal model essentially consisted of 7 stick type sub-models interconnected at each floor level by rigid translational links. Each stick represents one or more vertical elements of the lateral force resisting system. Individual sticks were provided to represent each of the major shear wall

Appendix A. Escondido Village Mldrlse

co cal sti, W the reI f01 co

an 26 we sui ch the #2

re~

reI e1c fie in be

JUilding

loading s,

the efore, it Lal effects d lose cipate in lddition )roach to to lation ations of. ; that are

nal

:vel by :ents one~ ,rce

rovided

- SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS --------------------------------------------------

Ui:," W13 ;- W23 . _ .. .

flexural elements

shear elements

WLO , W20

WL7 W26 ~~-l!~ ~-

~, '-1

"

J -' W70', ,

~;,' -, p, 1c"

Frame Roof

8th

7th

6th

5th

4th

3rd

2nd

1st

- Foundation soil springs

Figure 4.~2. Non-linear Model-Longitudinal Direction

configurations contained in the buildings, categorized by response direction. Thus, a single stick was provided to model both walls W13 and W23, the long rectangular walls along the sides of the building (Figure 4.3-1). Since the stick represented two identical walls, the force-deformation relationship for the stick consisted of a composite of both walls stiffness and capacity characteristics. Walls 10, 17, 20, and 26, all of which are identical "L" shaped walls, were combined into two different stick sub-models, each sub-model representing the characteristics of these walls when pushed in either the positive or negative direction. Stair #1, Stair #2, and the elevator core (W70) each have unique response characteristics and were provided with separate models.

As shown in Figure 4.3-2, each of the sticks representing the shear walls is comprised of two elements at each story. One element represents the flexural behavior of the walls and is infinitely rigid in shear. The second element represents the shear behavior of the walls and is infinitely rigid in

flexure. Each stick is also provided with a rigid beam a,t its base, supported by a series of inelastic soil springs. The soil springs are preloaded with the calculated dead load soil pressure under the foundation and are set with compressive spring rates. The springs have null tensile stiffness.

Stair #2 has the additional complication of the vertical irregularity at the first story, previously described in Section 3. This was modeled by using altered flexural stiffness properties for this wall at the first story.

A horizontal linear translational spring is attached to the model at the level of the first floor. This spring represents the shear stiffness of the numerous additional concrete wails present in the basement story of the buildings (Figure 2.2-3). The value of this spring was calculated as the difference in stiffness of the basement story of the building in the linear elastic ETABS model, and the DRAIN model constructed without this spring. A final sub-model stick was provided to represent the stiffness of the concrete frame (beams and floor slabs) and the smaller walls within the

Ie Mldrl' AIIpendlx A. Escondido Village Mldrlse A-17


Roof

8th

7th

6th

5th

4th

3rd

2nd

1st

- Foundation soil springs

Figure 4.88. Nonlinear Model Transverse DirectIon

building that do not contribute significantly to the lateral load resistance. The initial stiffness of this stick was chosen such that the initial stiffness of the entire inodel matched that of the elastic analysis model. Based on evaluation of the elastic analysis results, a lateral deformation was selected for each story at which flexural yielding of the frame would commence. This information was than used to construct an elastic-purely plastic representation of the frame stiffness at each story.

The model for the transverse building response was constructed in a similar manner to that for the longitudinal direction. A schematic diagram for that model is presented as Figure 4.3-3. An important difference between the two models is the way in which the discontinuity at the base of the main transverse walls was handled in the transverse model. This problem was previously discussed in Section 3.2 and illustrated in Figure 3.2-2. At this discontinuity, the wall boundary elements are the only continuous components. The behavior at this discontinuity

A-'8

was modeled by running two columns, each representing the boundary element properties of the wall, through the basement and first stories of the building. These boundary element columns were linked together by a rigid beam at the underside of the second story. The rigid beam was provided at the underside of the second story since the first story wall would not be completely effective due to the discontinuity below. To illustrate, Figure 4.3-4 shows an assumed effective axial zone of the first story wall panel relative to the door and louvre openings at the basement level. Because of the modeling of this discontinuity, the transverse model was judged to be slightly more flexible than the real structure, but of adequate accuracy to investigate the concentrations of demands likely to occur in the real structure at this area of discontinuity.

It should be noted that coupling beams between the main transverse walls and comer walls were not modeled. From the elastic analysis, it was

Appendix A, Escondido Village Midrlse

fo an Cc co re

4.

cb us pe m se sp df to se bf w aI Vl c( fr Pl c(

,

--.... --- SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS --.... -------------------------------------------------------------------------------------

effective '. " area of wall -t------H~Jl...

i, / \ : 2nd floor

1st floor

Foundation

louvre opening door opening

transverse wall

comer wall, typo

basement wall

vertical shear crack forms at wall, typo

Figure 4.S4. Effective Axial zone at Discontinuous Transverse wall

i1 .es of lries of mns

,am was:

Iry since, y o effective' tive to ent

dged to. cture,

in the

s bet'lVeri. s were was

found that these beams would be highly stressed and would fail early during a severe earthquake. Consequently, these beams were judged to contribute minimally to the building's lateral-load resistance.

4.So4 structural Elements and components

Wall Element Flexural Properties. Flexural characteristics of wall elements were determined using the software package BIAXI7. This software permits the development of non-linear moment-curvature relationships for concrete sections of arbitrary cross section, subjected to specified axial load. The program was actually developed for use in analyzing columns as opposed to walls and incorporates the Euler assumption that sections that are plane prior to initiation of bending remain plane after bending. Since the walls in the Escondido Village. Midrise buildings are quite slender, this assumption is thought to be valid. The program has several concrete compressive behavior models programmed into it, from which the user may choose. These include parabolic stress-strain distributions for both confined and unconfined models.

Ie Hllrlrl,;' Appendix A, Escondido village Mldrlse

For this project, moment-curvature curves were generated based on an unconfined model with an ultimate compressive strength (f' c) of 2470 psi, matching the findings from previous testing conducted at the buildings. The comer L-shaped walls, elevator C-shaped walls, and stairwell walls were each modeled as complete walls with entire flanges assumed effective. All concrete was assumed to be unconfined.

As previously described in Section 3.2, the splices of boundary reinforcing for the shear wailS are inadequate to develop the tensile strength of the bars. General notes on the original construction documents indicate that lap splices in continuous bars should be staggered. It was judged, therefore, that 50 percent of the longitudinal boundary bars would be fully effective in tension at any horizontal section through the walls. Therefore, in the BIAX models, only 50 percent of the boundary steel was incorporated. Assumed strength of the steel is 40 ksi, based on the notes contained in the drawings.

It should be noted that there was some uncertainty with regard to these assumptions. Lap splice details for chord reinforcing in walls are not

A-1.


300000

_ 200000 "L.~!" ~ ;! 150000

Looooo '0000

-+-Iongitudinal Mlls ~ comer .:lis 1-. -M- comer 'Mll1-X .....,&- elevator -4-ltair#l +:1 -+-Itair 4H -x -B-Itair #2 +x ___ .tair #2 -x

o~--~-----+----~----~----+---~-----+----~ O.OOBofOO 2.00E-04 4.00&.04 6.00E-04 8.00B-04 1.00E03 1.20E'()3 1.40E-03 1.60E03

Curl'8ture [radlaa5llncb}

Figure 4.11-5. longitudinal Moment-curvature Relations lOr First Floor wails

specifically shown on the drawings, while column splices are. In details for column reinforcing splices, all of the bars in a column are lap spliced just above each floor level. There was some possibility that the boundary steel for the shear walls was spliced in a similar manner. This would result in lower flexural capacity for these walls. There was also some uncertainty with regard to the strength of the reinforcing used for the boundary elements of walls. The general notes on the . construction drawings indicate that Intermediate Grade steel, with a yield strength of 40 ksi, was to be used for all reinforcing except longitudinal column bars, where Hard Grade steel, with a yield strength of 60 ksi was specified. It was possible that the Hard Grade steel was also used for the boundary elements of walls. In such a case, the lap splices provided for the bars would be even less adequate.

After the completion of our analyses, it was subsequently learned, through x-ray photography and minor destructive testing, that the boundary element reinforcing had lap splices just above the floor level. Chemical and tensile testing also confirmed the reinforcing to be Intermediate Grade steel.

It should also be noted that BIAX tends to under-estimate the flexural stiffness of elements

A-20

with minimal steel reinforcing. Professor Jack Moehle at the University of California, Berkeley, recommended that the initial effective stiffness of the wall elements be one-half of the gross sectional value. Based on the moment-curvature relations from BIAX, the initial effective stiffnesses were generally on the order of 25 percent of the gross sectional value.

Figures 4.3-5 and 4.3-6 present the moment curvature relationships for each of the major walls of the building, acting respectively in the longitudinal and transverse directions of the building. These curves are based on the assumption of 40 ksi boundary steel with staggered lap splices. The curves were computed for the dead load axial stress condition at the base of the walls. They have been terminated at peak concrete compressive strains of 0.005, as suggested in the Commentary of Section 9.5.4.2 of.the Methodology.

Examination of the curves for the longitudinal direction (Figure 4.3-5) indicates that the primary lateral load resistance for the structures in this direction is provided by the main longitudinal walls (W13 and W23, Figure 4.3-1), and the walls around stairways # 1 and #2 and at the elevator core. The wall at stairway #2 has substantially greater strength and deformation capacity in the

Appendix A, Escondido Village Mldrlse

+ di: de pr rei 60

pr m:

the co

ea

on

stl is

the \ir ac

Ie' 10 w, ca

as:

fu 51 afl

----- -------------------------------------------------------------------------------------

ack :keley, less of ;ectional .tions were

gross

)ment or walls

Ie

;taggered : the l of the concreW :I in the

gitudinal primary

1 this !inal the walls

,vator ltially r in the

SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS ------------------------------------------------------------------------------------

400000

350000

300000 t~:..:-a-a-~ d2S0000~ 1-~ 200000 1l ~ 150000 :Ii

.......... transverse walls -+- comer "'03.1.15 +y ",","*,,-comer walls-y -6-eievator +y -.-elevator-y -+- stair #1 -y -+- stair -#1 +y -B-nair-#2+y _5tair#2 -y

o~----~----~------+_----~----~------+_----~ O.OOE..oo 2.00E-04 4.()(JE.Q4 6.00E-04 8.00E-Q4 l.OOE-03 l.20E03 1.40EQ3

Curwture [radlansllncb.]

Figure 4.~6. Transverse Momentcurvature Relations for First Floor Walls

+ X direction than in the -X because of the discontinuity in the first story, previously described. The two main walls (W13 and W23) provide more than 50 percent of the total lateral resistance in the + X direction and more than 60 percent in the -X direction.

In the transverse direction (Figure 4.3-6) the primary lateral resistance is provided by the two main walls (W31 and W41, Figure 4.3-1) and by the walls around the two stair wells. The configuration of the stairwell walls is such that each stairwell has substantially more resistance in one direction than the other. Stairwell # 1 is strongest in the + Y direction, while Stairwell #2 is strongest in the -Y direction.

Manual calculations of the shear capacity of the walls indicated that they are, in general, limited by the shear friction capacity of the walls across the construction joints present at each floor level. Typically, this capacity is approximately 10 percent less than the nominal capacity of the walls derived using UBC formulas without capacity reduction factors (cp). It was arbitrarily assumed that a 114 inch displacement is required to fully mobilize the shear friction strength. A

. 5 percent strain hardening factor was permitted after attainment of the 114 inch initial slip.

It should also be noted that by using BIAX, the modeling rules for flexural properties in shear walls presented in Chapter 9 of the Methodology were ignored. This is allowed per Commentary in Section 9.5.1 of the Methodology.

Wall Element Shear Properties. As noted above, each wall element in the DRAIN model was built with two elements - a flexural element and a shear element. Shear properties used in the DRAIN model were computed based on the shear capacity of the wall as calculated per ACI 318. Although shear friction capacities per ACI 318 were typically less than wall shear capacities, shear walls generally do not fail at their construction joints when sufficient dead loads are applied to the walls. Strain-hardening was not included in the modeling of these shear elements. The inclusion of strain-hardening would have slightly increased the overall shear capacity of the building, but not the deformation capacity. By not including strain-hardening, we could more easily account for shear degradation in the wall elements. (See Section 4.4.2 for discussion on shear degradation.)

Foundation Rotational Stiffness. Non-linear springs were used to model the rotational stiffness of foundations beneath the major shear walls. Initially, the stiffness assumptions provided by

Ige Mldrll', Appel1C1lx A, Escondido Village MIClrise A-21


DdI ..........

, Linear !tOil tprinp ~;=Jm.. .. "", ...... on LoDgIh

FIgure 4.$-7. DetermlnatiDfl OF Effective Foundation length

Woodward-Clyde were used as the basis of the model. Finite vertical translational soil springs were incorporated into the model. Each spring represented the stiffness of a length of the wall foundation, equivalent to its width. Beams, with springs representing the foundation stiffness were provided beyond the width of the shear walls, with linear elastic properties corresponding to a section comprised of the basement walls, the strip foundations and a portion of the first floor slab. The effective width of the first floor slab was taken based on the limitations for flange widths in "T" beams contained in the ACI code.

The length of the foundation systems effective in resisting shear wall overturning was taken based on independent, beam-on-elastic foundation type analyses. These analyses are schematically represented in Figure 4.3-7. The effective foundation length was taken as the point at which foundation uplift was produced beyond the compressive side of the shear wall.

In performing visual surveys of the buildings, it was noted that some of the basement walls have vertical cracks through them. It was surmised that these cracks may be the result of shear failures, induced in the walls by the Lorna Prieta Earthquake, as they attempted to spread overturning demands from the shear walls.

A-22

Therefore, the computed shear capacity of these basement walls was programmed into the DRAIN model to simulate this failure mode.

As determined by the DRAIN analyses, initial inelastic behavior of the structure was dominated by foundation rotation and liftoff effects. It was expressed by Stanford Facilities Management that the stiffness suggested by Woodward-Clyde for the soil springs appeared to be significantly larger than revealed by previous plate load test data for various locations on the campus. Therefore, a series of sensitivity analyses were performed in which the spring stiffness and ultimate capacities of the soil compression springs were evaluated for 150 percent, 67 percent and 25 percent respectively of the values suggested by Woodward-Clyde. It was found that these assumptions had negligible effect on the overall behavior of the model. The predominant factor in the inelastic behavior of the foundation, as predicted by the model is the liftoff of the foundation on the tension side. This appeared to be independent of the compression spring stiffness assumed. The total effect on structural elements of the model, for the various assumed soil stiffness properties, was a change in demands of approximately 2 percent. Therefore, it was concluded that the structure's behavior is insensitive to the spring stiffness of the soils beneath the foundations, but is quite sensitive to the ability of the foundations to rock about their bases.

4.4

4.4.1

pushover Analysis

Deriving and Applying Pushover FOl'Ces

Per Section 8.4 of the Methodology, the Escondido Village Midrise buildings were evaluated based on a Level 3 pushover analysis. Level 3 is prescribed as the basic level of analysis for the Methodology. Lateral forces are applied in proportion to the product of story masses and first mode shape of the elastic model of the structure.


T d

cl 4 tl d P CI b

~ tl d u b a: fl IT

tJ n d p fc sl P b c c d TI C SI o

TI [

s p c c

-- ~-~==~-----------------------SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS

--

~se AIN

nitial ated vas t that for the

~r than

a I in :ities

~ed for

:rall ;tor in

ed to be ness

Jents of Jness

Is ive to : their

This distribution is obtained from the initial dynamic DRAIN-2DX model in each direction.

Although the fundamental mode shape will change due to changing stiffnesses (see Section 4.4.2 below), the initial load distribution was used throughout the analyses for each respective direction. Changing the load distribution would probably yield a slightly more accurate pushover curve. However, the overall effect on the building behavior and evaluation results was judged to be negligible.

The effects of higher modes on the structure were also ignored in this study. Evaluation of these higher modes may result in additional damage to the building that was not discovered by using just the fundamental mode. However, because the building is relatively regular in plan and stiffness (with the exception of the basement floor), higher mode effects are anticipated to be minimal.

4.4.2 Model Degradation Element properties can be characterized by a

typical elastic-plastic force-deformation relationship with strength degradation at high ductility demands as shown in Figure 4.4-1. As previously indicated, the flexural force-deformation relationships for the concrete shear walls were obtained using the software program BIAX. For walls exhibiting ductile behavior with strain-hardening, force-deformation curves were terminated at a peak concrete compressive strain of 0.005. For walls with degrading strength at large rotations prior to reaching a concrete compressive strain of 0.005,

Shover, curves were terminated at the point of initial : strength degradation. As a result, we constructed

Our own component force-deformation relationships that were implemented into the

alysis. , DRAIN-2DX models. analysis( Unfortunately, DRAIN-2DX does not have

he

.pplied in; strength degradation capabilities built into the and first; program. Consequently, the continuous pushover

ructure.! curves shown in Figures 4.4-5 and 4.4-6 were } constructed from a series of incremental pushover

ge Mldrt!'; Appendix A. Escondido Village Mldrlse

~Yield

r residual strength Deformation

FIgure 4.4,1. TYpical Force-Deformation Relationship for Model Elements

curves. Each increment was defined at the displacement that a critical element reached its degradation point. The degraded element would be replaced by a similar, weaker element (with a new yield strength that was 20 percent of the original yield strength per Methodology Table 9-10). With this new element, the pushover analysis would then be started again and continue until the next critical element reached its degradation point.

In addition, it should be noted that not only was the strength of the degraded element reduced to 20 percent of the initial undegraded element, but the degraded stiffness was also similarly reduced to 20 percent of the initial. The Methodology provides no quantitative guidance with respect to post-yield shear stiffness, axial strength, axial stiffness, or degradation rate as a function of ductility demand.

In the case of our building models. consideration of a ductile model with no strength degradation would have overestimated the maximum pushover base shear by less than 10 percent. Although this is not significant, the implementation of a degraded model, per requirements of the Methodology, would more accurately determine a building's seismic behavior. In some buildings, the effect of degradation may be significant.

A-25


2!iOO r hingefi ~tion at floor

-7 2000

~ / \

, .. / "- - hmge fonnation at basement waDs /

o o , 10 20

bor DI.pJ.cement. d [Incher]

Figure 11.11,2. LDngltudlnal PushDver Curve fDr EXlst/ng structure

2000

lr ;inge !ormati n'l I floor beams I

'00

ex I ~ I / \ ~ hinse fomation ~lstnoor I and buem:nt w: i ,

1/ I I

1>00

I ~ ,

1000

!

o I o 10 I' 20 25

Roor DllpI.CUltDt. d [IDdlel]

Figure II.IIS, Transverse PushDver curve fDr EXisting structure

4.4.S Pushover Force-Displacement Curve

Figures 4.4-2 and 4.4-3 show the pushover curves for the existing (unstrengthened) Escondido Village Midrise buildings, when pushed in the longitudinal and transverse directions, respectively. As can be seen, the first critical events consist of hinging of floor beams throughout the frame. This is considered

A-24

potentially life threatening because of the lack of adequate development of the bottom reinforcing of the beams through the beam column joint. Hinging of the beams - first in positive flexure and on the return cycle in negative flexure - will result in formation of a vertical crack through the beam column joint. Following such behavior the floor systems would rely on the catenary behavior of the


to su th pt

sh re re.

sh ro

po ob ca

tho co

a I ex to be, di!

r "

----- ------------------------------------------------------------------------------------

SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS ----- ------------------------------------------------------------------------------------

lack of orcing oI . Hinginl' Ion the Iltin beam e floor 'ior of tIt,

r

hinge fonnatio at

I floor beams -~

. ~ ./ \ / Y{J

2500

2000

'01

/ \ compression failure at "toe" ~ I ~ 1500 :;. ofbaserrent orner walls / Lhing fonnation at compression failure ~#2 J bas, frent walls baserrent stairs #1 a

t! j ~" 1000

! 500

o o 5 10 15 20

Roof Displacement, d [inches]

Fll/Ure 4.44. Longitudinal Pushover curve For strengthened Building

top reinforcing steel in the beams for vertical support. However, because there are no stirrups in the beams, there is potential for this top steel to pull free of the slabs, resulting in floor collapse.

In addition to the hinging of floor beams, shear failure of first floor columns occurs at relatively small roof displacements. This also results in significant collapse hazard.

Because the beam hinging and the column shear failure mechanisms form at relatively small roof displacements (3.5" to 4.0"), a performance point as defined by the Methodology cannot be obtained since the demand spectrum and the capacity spectrum do not intersect. This indicates that the structures, as they are, present significant collapse hazards when subjected to the demands of a large magnitude earthquake. Furthermore, the existing structure does not present a good example to evaluate the procedures of the Methodology because of the high collapse potential at small displacements. Consequently, for the purposes of


this example building study, it is more instrumental to follow the Methodology using the life-safety retrofit concept.

To create a more stable structure and allow the pushover analysis a chance to develop some ductility, the problems of the hinging beams and shear critical columns were initially addressed. The retrofit concept is discussed in Section 5 of this report. For the purpose of continuing our discussion of the pushover curve, assume that the hinging of floor beams and the shear failure of first floor and basement columns are adequately addressed with structural upgrades.

Figures 4.4-4 and 4.4-5 present the pushover curves for the strengthened building. Significant events in the progressive lateral response of the building are annotated on the figures, and more fully described in Tables 4.4-1 and 4.4-2. Critical events listed in the tables are indicated in italics.

A-25


2000

J500

i i JOOO II>

! sao

a

A-2G

hinge fonmtion at rocking oH undation at

floor bearna 7 stair II 1 V- /'ompreSSion til \ -----

ure at -0; ""l"""'" Ie . un" W~ \ v:r mpression failure at b~ement transverse" aDs co~res ion failure at base_ t elevator core

/ '-- shear failure of .. emont and 1st floor inteno coluIIIlS 1/ ~ hin e fonnation at bas e lOnt and 1st floor walls o 5 JO J5 20 25

Roof Dlaplac:ement, d [lnche.]

Figure 4.45. Transverse pushDver curve IDr strengthened Building

11 compression failure at "toe" of basement corner 12.84 walls


4

4

SI te E S

A

r ~ -------------------------------------------------------------------------------------

SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS ---- ------------------------------------------------------------------------

I

4.5

4.5.1

Table 4.4,2 Transverse Pushover Events ... /toOl p/sP!acement

.. . I . (inCheS)< .. .... ........... .

. EVent


,.,r--------------,---------------,--------------,

rr 1'''1/

'.00 ~_-------_+----------__+_--_----___i ,

0.30 ,-----

:; 020

" SpeClnll DI.plaeemeat. Sd (ID~UJ

Figure 4.S1. Longitudinal capacity Spectrum

----I I'" ! /

'.00 ,

" " SptdJ1ll DI.pI.ceraut. Sd (ladle.]

Figure 4.S2. TranSverse capacity spectrum

"

_--1 I

I , I ! ! , ! i

I "

Table 4.S1. conversion Of v and d,.., to So and Sd for Longitudinal Direction

A 1633 2.07 0.138 1.449 0.653 0.211 1.43 0.83 B Q98 1756 3.08 0.148 1.449 0.653 0.227 2.13

2011 11.37 0.170 1.449 0.653 0.260 7.85 2052 12.84 0.173 1.449 0.653 0.265 8.86 o 1.85 2011 18.08 0.170 1.449 0.653 0.260 12.48 E 2.H

A-28 Appendix A. Escondido Village Mldrlse

-

as afi ef

4. TI pa M bt

sp

f ---- ------------------------------------------------------------------------

---


Table 4.5,2. conversIon 01 V and d, .. , to Sa and Sd lor Transverse DIrection

point " V'klpil d""" fln.1 .,j,/W A 1137 1.80 0.096 8 1258 2.29 0.106 C 1354 3.28 0.114 0 1478 6.10 0.125 E 1790 14.10 0.151 F 1824 17.02 0.154 G 1839 23.11 0.155

assumed to remain constant throughout our analysis, the participation factor (PFroof) and the effective mass coefficient (CXm) remain constant. 4.5.S Demand Spectrum The 5 percent damped spectrum is derived from parameters described in Chapter 4 of the Methodology. For the Escondido Village Midrise buildings, the following parameters were used:

Soil Profile Type = D for stiff soil (Methodology Table 4-3)

Seismic Zone, Z = 0.4 for seismic zone 4 (Methodology Table 4-4)

PIT"",

1.451 1.451 1.451 1.451 1.451 1.451 1.451

Near Source Factor, N = 1.18 for seismic source type A, linearly interpolated between 5 and 10 kIn (Methodology Table 4-5)

Seismic CoeffIcient, CA = 0.47 for shaking intensity larger than 0.4 (Methodology Table 4-7)

Seismic Coefficient, Cv = 0.76 for shaking intensity larger than 0.4 (Methodology Table 4-8) Based on the capacity spectra, the demand

spectra can be reduced with the modification

" 0/"'< 1"'5.'91'" Sdfln.1 T,seCI ' 0.671 0.143 1.24 0.94 0.671 0.158 1.58 1.01 0.671 0.170 2.26 1.17 0.671 0.186 4.20 1.52 0.671 0.225 9.72 2.10 0.671 0.229 11.73 2.29 0.671 0.231 15.93 2.65

factors SR. and SR, as calculated by the following relations (see Chapter 8 of the Methodology): SR, = --- 3.21- 0.681n 'P" pi + 5 1 ( [63.7 K(a d ' - d a) ])

2.12 ap;dp;

1 ( [63.7 K(a,d p; - d,a p.) ]) SR, = - 2.31- O.4l1n + 5 1.65 ap;dp;

By guessing the maximum displacement of the capacity spectrum, the values of dp; and ap; (based on the capacity spectrum) can be calculated. These, in turn, effect the values of SR, and SR,. Through an iterative process of adjusting the value of dp; until the capacity spectra intersects the demand spectra at dp, a performance point can be determined. Figure 4.5-3 shows the relationship between api, ay, dp; and dy.

For the longitudinal direction, the total spectral roof displacement at the performance point was Sdmox=9.51" (see Figure 4.5-4), which corresponds to a total roof displacement of d.,.,= 13.8". For the transverse direction, the total spectral roof displacement at the performance point was Sdmox= 11.2" (see Figure 4.5-5), which corresponds to a total roof displacement of d.,.,=16.2".

'Idrlse i Appendix A, Escondido Village Mid rise A-29

A-SO


I I actual

api ,. r pushover curve

. _. _. _. _ . J. _. _. _. _. _. _.:-................. ................ .. .. .. .. .. .. ..

ay - _._._.-

dy

idealized pushover curve

Displacement dpi

Figure 4.5-$. Idealized Bilinear RepresentatIon of MDt/al Pushover curve

1.0

0.0

0.'

~ 0.1 .ll r 0.' -< 0.4 i 0.'

0.2

0.1

0.0 0 2 4 , 7 o 10 11 12 13 14

"

Figure 4.5-4. Demand vs. capacity Spectra Showing performance point for longitudinal Direction

Appendix A. Escondido Village Midrlse

4.

de pc de of fo CL bl

s~ 10 in

4.

P( ac Ie C( M C( PI PI

4

N s:

--

Ildrlse

~--------------------------------------------SEISMIC EVALUATION AND RETROFIT OF CONCRETE BUILDINGS ~--------------------------------------------

1.0

0.' ---7--_____

0.8 ,

~ 0.7 i dpi=3.0" ~

I 0.' 0.' < 0.'

i

~ 0.3 l ~ 0.2

0.1 m Sd=l1 "

0.0 0 4 ,. 6 1 8 9 10 II. 12 13 14 IS

Spectul D11p1aeement,Sd [Inches]

Figure 4.55. Demand VS. capacity spectra showing performance point for Transverse Direction

4.5.4 Performance point The intersection point of the capacity and

demand spectra is the performance point. This point represents the expected level of seismic demand on the structure. The spectral coordinates of the performance point can be converted back to force-displacement coordinates on the capacity curve. For the Escondido Village Midrise buildings, the performance point occurs at a base shear of 2010 kips and 13.8 inches in the longitudinal direction, and 1750 kips and 16.2 inches in the transverse direction.

4.6 Performance Assessment Component deformations at the performance

point displacements must be checked against acceptable limits. The acceptable deformation levels for various structural elements and components are presented in Chapter II of the Methodology. In

atc 40 vol. 2

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