Kajima - CUREe Research Project Design of High-Rise Reinforced Concrete Building LM -4 Dr. Norio Inoue Dr. Norio Suzuki Mr. Hiroshi Morikawa Mr. Masaaki Okano Mr. Takaharu Fukuda Mr. Hitoshi Hatamoto Mr. Motomi Takahashi Mr. Satoshi Orui Mr. Makoto Maruta Prof. Vitelmo V. Bertero Prof. Helmut Krawinkler Prof. James C. Anderson Prof. Graham H. Powell Prof. Filip C. Filippou Mr. Hatem Y. Goucha Mr. Pasan Seneviratna Mr. Mohsen Rahnama Mr. Amador Teran-Gjlm ore Mr. Raul D. Bertero CUREe
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Kajima - CUREe Research Project
Design of High-Rise Reinforced Concrete Building
LM
-4
Dr. Norio Inoue
Dr. Norio Suzuki
Mr. Hiroshi Morikawa
Mr. Masaaki Okano
Mr. Takaharu Fukuda
Mr. Hitoshi Hatamoto
Mr. Motomi Takahashi
Mr. Satoshi Orui
Mr. Makoto Maruta
Prof. Vitelmo V. Bertero
Prof. Helmut Krawinkler
Prof. James C. Anderson Prof. Graham H. Powell
Prof. Filip C. Filippou
Mr. Hatem Y. Goucha
Mr. Pasan Seneviratna Mr. Mohsen Rahnama
Mr. Amador Teran-Gjlm ore Mr. Raul D. Bertero
CUREe
Kajima Corporation CUREe;
California Universities for Research
in Earthquake Engineering
* Kajima Institute of Construction Technology * The California Institute of Technology
* Information Processing Center * Stanford University
* Structual Department, Architectual * The University of California, Berkeley
Design Division * The University of California, Davis * Civil Engineering Design Division * The University of California, Irvine * Kobori Research Complex * The University of California, Los Angeles
* The University of California, San Diego
* The University of SOuthern California
SUMMARY REPORT
CUREe—Kajima Research Project University of California, Berkeley
High-Rise Reinforced Concrete Buildings
Vitelmo Bertero, Team Leader
OBJECTIVES
The ultimate goal was to advance, through evaluation of present knowledge and of the results of new coordinated research, the states of the art and the practice in the earthquake-resistant design of high-rise reinforced concrete (RC) buildings. To achieve this goal, particular emphasis was placed on:
identification of RC buildings systems which are particularly suitable for high-rise con-struction in seismic regions with soft-soil sites;
evaluation of seismic response characteristics of these systems, considering linear and non-linear responses;
development of new computer programs for computer analysis of 3-D nonlinear responses;
evaluation of recent developments in energy dissipation mechanisms for possible integra-tion into these systems;
use of high strength concrete (500 to 670 kg/cm2); and
00 -
• development of improved methods of preliminaiy design to enhance seismic performance of these systems.
MAJOR ACCOMPLISHMENTS
To achieve the above objectives, a series of studies were planned, grouped under five main tasks. The major accomplishments in each of these five tasks are the following.
Review of the State of the Art and the State of the Practice
The maln results of this review are described in two reports.
Structural Systems. In spite of the fact that the use of dual structure systems Consisting of shear walls (particularly coupled walls) and special moment-resisting space frames (SMRSF) and other systems, such as outrigger structures and tubes, are very promising for earthquake resistant design, up to now most of the tall RC buildings in zones of high seismic risk have been built using just SMRSFs. The main reason for this is that the practicing engineers have more confidence in their ability to predict the behavior of SMRSF than for other systems and relatively reliable com-puter programs are available to analyze the linear and nonlinear responses of SHRSF buildings. This points out the need for investigating the nonlinear seismic response behavior of the other sys-tems such as tube (framed and braced), outrigger, and dual (SMRSF and shear walls).
Foundation. The use of encasement walls with drilled piles (as used by Kajima in the S-K building) is technically the most effective type of foundation for tall buildings on soft soils, how-ever, its use is expensive. U.S. seismic codes do not address the design of foundations in ade-quate detail and as the foundation may constitute a weak link in the entire soil-foundation-super-structure-and-nonstructural components systems, better code regulations for earthquake-resistant design are needed. The practice of Kajima of including the soil and foundation mechanical char-acteristics in the model of the tall building at the start of the design should be adopted in the U.S.
Review of Seismic Performance of High-Rise RC Buildings
The reviews conducted are described in three separate reports, which concentrate on the response of this type of building in Japan, the U.S., Mexico and Chile. Site soil effects, which proved to have a predominant effect on the performance of structures located on soft soil, need to be accounted for more realistically in earthquake resistant design than at present. Soil-structure interaction is not always beneficial in reducing response demands, and therefore needs to be con-sidered in the design process. More reliable methods for estimating damping of soil materials and radiation damping are needed. There is a need for improved methods to estimate the effective structural period (7), and the predominant period of the soil. Site spectra for earthquake resistant design should account for the uncertainties in computing the T and Tg. Initial tilting, due to dif-ferential settlement, when an earthquake occurs has been shown to be detrimental for seismic response of the building. Overstrength with respect to the ultimate (maximum) strength expected from seismic code requirements depends upon the T (height) and the design method that is used. The taller the building, the smaller the overstrength. Design methods based on elastic force distri-bution result in larger overstrength than those based on plastic redistribution. The lowest over-strength results from use of optimal plastic methods.
Dynamic Response Analysis of Promising Systems
This task covered two main groups of studies. One is the development of computer pro-grams for nonlinear dynamic analysis of RC buildings. User guides for the DRAIN-2DX, DRAIN-3DX and DRAIN-31) BUILDING have been released. This, together with the develop-ment of a "Nonlinear Analysis Method by the Fiber Model for Applications to RC Columns, Beams and Beam-Column Subassemblages," conducted and reported by the Kajima Team, will enable conducting more reliable analyses of the real nonlinear responses of RC structures, and will
therefore improve earthquake resistant design of new RC structures and the vulnerability assessment of existing RC buildings. The second main group of studies were conducted to evalu-ate the effectiveness of present U.S. seismic code design procedures, which were applied to different structural systems; SMRSF, Framed Tube and Braced Tube. Two reports discussed in detail the results obtained for SMRSF. Another report in prepaation describes the results obtained for tube systems. In all these designs, problems in complying with serviceability requirements have been detected. Tall buildings designed according to U.S. seismic code regula-tions while they can be safe are not serviceable. Analysis of seismic response demands using the static push-over technique can detect the weaknesses of the design but cannot give reliable esti-mations of several important response parameters. In this type of analysis, P—E effects are usually overestimated (however time-history analyses show that these effects are very sensitive to the dynamic characteristics of the earthquake ground motions), the maximum strength is underesti-mated, and the inelastic deformations are underestimated. Time-history analysis of the response of buildings using available nonlinear 2-13 computer pmgrams (such as DRAIN-2DX), although giving more reliable predictions of the demands than the static push-over analysis, still has serious limitations, such as that it cannot give estimations of the effects of bi-directional earthquake ground motions and of torsion. Therefore there is a need for a reliable and effective 3-D non-linear computer program. Although Kajima has a 3-13 program based on concentrated plasticity, it uses supercomputers and is expensive.
Feasibility of the Use of Energy Dissipation Devices and High-Strength Concrete
Due to the lack of necessary funds and time, only preliminary studies have been conducted. Preliminary redesign of the S-K building using high-strength concrete (500 to 670 kg/cm2) has been completed and the results are encouraging. The use of high-strength concrete results in reduction of 12% in the total weight of concrete without any significant increase in the deforma-tion demands. A report describing the design of the S-K building using high-strength concrete has been prepared. The use of passive control of the seismic response of SMRSF buildings has been considered through preliminary analysis. The results of this analysis are encouraging, particularly when energy dissipation devices are used. The use of additional viscous dampers over the use of devices based on yielding of metals or friction is favored.
Development of Preliminary Design Methods
Two reports discuss the development of a "Conceptual Methodology for Numerical Earthquake Resistant Design," Results from the analyses of the linear and nonlinear responses of one 30-story building (of similar layout to the S-K building) designed according to this new con-ceptual methodology to service and safety levels of design earthquake ground motions are very encouraging. The overall conceptual approach to earthquake resistant design consists of two main parts. The first part covers guidelines for conceptual design of the entire facility system including building configuration, structural layout, structural system, structural material, nonstruc-tural elements and their materials, as well as type of foundation. The second part is the concep-tual methodology for numerical earthquake resistant design of the entire building system. It complies with the worldwide accepted philosophy of earthquake-resistant construction and is based on fundamental principles of structural dynamics, mechanical behavior of the entire building
system (soil under and surrounding the building, foundation, superstructure, nonstructuraj com-ponents and contents) and comprehensive design (which is a probabilistic approach to design which includes not only the reliability of the design at each limit state, but also the cost/benefit analysis of reaching each of these limit states. The main advantage of this conceptual methodol-ogy is that notwithstanding the great uncertainties in the quantification of some of the concepts involved in its codification, such numerical quantification can be improved without changing the format of the codified methodology as new and more reliable data are acquired.
IMPL&CT OF THE RESEARCH RESULTS
It is well recognized that one of the most effective ways to improve seismic risk reduction in urban areas is through improving the earthquake resistant design of new facilities and the upgrad-ing (rçtrofitting) of existing hazardous constructions. In the past two decades there has been an increaM in the construction of high-rise RC buildings in regions of high seismic risk. There is a lack of reliable data on the performance of these tall RC buildings, particularly those built on soft soils, and there is doubt about the adequacy of the methods that have been used and are presently used in their design. This has been dramatically illustrated by the failure of many tall buildings during the 1985 Mexico earthquake.
The methods of analysis and particularly the conceptual design methodology in this research project will lead to better earthquake resistant design of tall buildings, as well as improved reliabil-ity of the vulnerability assessment of existing buildings which is so much in need for deciding the demolition or upgrading of these buildings.
RECOMMENDATIONS REGARDING ADDiTIONAL RESEARCH
Study of the seismic behavior of dual systems, tubes (framed and braced), an outrigger structural system, particularly their nonlinear response (yielding mechanisms)
Search for more effective (technical and economic) foundations (design and construction)
Development of more reliable methodology to estimate the effect of soil-structure interac-tion and the damping values for soil materials and radiation
Improve methods for estimating the period of RC structures considering the effects on the stiffness of cracking, aging, etc.
Develop more efficient 3-D computer programs
Investigate through integrated analytical and experimental studies the use of energy dis-sipation devices to control the seismic response of tall RC (as well as steel) buildings
Apply conceptual methodology to the design of typical buildings and then to analyze their response tocritical earthquake ground motions
Based on the results obtained in the application of the conceptual methodology, develop seismic code regulations that are simple, practical and reliable.
CUREe-KAJIMA RESEARCH PROJECT
REVIEW OF THE STATE OF THE ART AND THE STATE OF THE PRACTICE OF
EARTHQUAKE-RESISTANF DESIGN OF HIGH-RISE REINFORCED CONCRETE
BUILDINGS ON SOFT SOILS IN HIGH SEISMIC RISK AREAS
Hatem Y. Goucha and Vitelmo V. Bertero
Earthquake Engineering Research Center
University of California at Berkeley
July 15, 1991 - January 14, 1993
ABSTRACT
The objectives of this study are the following: First, to review the different super-
structure and foundation systems that have been used or could be considered for high-rise
reinforced concrete buildings on soft soils; then, to evaluate the limitations and advantages
of each system in resisting earthquake effects; and finally, to investigate analysis methods
and design procedures that have been used or proposed recently. Different definitions can
be found for tall buildings, but it was found that the slenderness ratio is key to determining
whether a building should be considered a high-rise building. A slenderness ratio greater
than 4 in a building will generally cause problems that are specific to high-rise structures.
Ten structural systems have been examined: moment resistant space frames; single
Finally, after discussion of the seismic behavior of existing tall reinforced concrete
buildings and analysis of the advantages and limitations of possible structural systems
keeping in mind the state-of-the-art in analysis methods, some recommendations on
promising systems are made. Reinforced concrete MRSFs have been built in seismic regions
for up to 37 stories (precast MRSF recently built in Tokyo, Japan), and have performed well
when subjected to ground motion. The methods of analysis for 3-D linear systems are very
reliable and widely used. However, non-linear analysis of reinforced concrete frames can
only be used reliably with 2-D programs. Shear walls are believed to be adequate systems
for buildings with less than 30 stories, as proved by existing structures, and are believed to
have good capabilities for ultimate-level seismic response as well. However, more
sophisticated and "modelling-intensive" computer analysis is required. For heights greater
than 40 stories, outrigger structures, tubes, tube-in-tube bundled tubes and braced tubes need
to be used for deflection limitation. No explicit cut-off heights can be drawn for any one of
the five systems, but they each have advantages and disadvantages. The non-linear behavior
under safety earthquake ground motions (some features of which are mentioned in the study)
needs to be examined in a case-by-case manner.
111
REVIEW OF SEISMIC PERFORMANCE OF TALL REINFORCED CONCRETE BUILDINGS
Report on Task 2 of the CUREe/Kajima Project on
DESIGN OF HIGH-RISE REINFORCED CONCRETE BUILDINGS ON SOFT SOILS
by
Pasan Seneviratna
Helmut Krawinkler
and
and the Kajima Project Research Team
A CUREe-Kajima Research Report
February 1993
1. INTRODUCTION
During the last two decades taller and more slender reinforced concrete buildings are
being designed and constructed in areas of high seismic risk. Developments in high strength
concrete and concrete pumping technology has contributed to this popularity of reinforced
concrete for high-rise buildings. Current seismic code regulations, however, do not adequately
address specific problems encountered in seismic resistant design and construction of tall
concrete buildings. The goals of the research project that led to the writing of this report is to
advance the state-of-the-art and the-state-of-the-practice in seismic design of high-rise
reinforced concrete buildings. The main objectives are
Identification of reinforced concrete building systems particularly suitable for
high-rise construction at soft soil sites.
Evaluation of the seismic response characteristics of these systems
considering both linear and non-linear response.
Development of improved methods of preliminary design that will enhance
the seismic performance of these systems.
As part of this study a review has been carried out on the performance of reinforced
concrete building structures during past earthquakes. The objectives are to evaluate and
document the observed performance of various reinforced concrete tall building systems and to
summarize the salient findings in a format that can serve as a background for the research work
to be performed in this project. Emphasis is placed on instrumented buildings for which
ground motion and building response records are available and have been evaluated in past studies.
1971 San Fernando, 1979 Imperial Valley, 1984 Morgan Hill, 1987 Whittier Narrows,
and 1989 Loma Prieta earthquakes are used in this study. Research results which have been
obtained from shaking table tests of structural systems suitable for tall buildings are also
incorporated wherever pertinent.
This report summarizes the findings from a review of reports and papers that discuss
the performance of R/C buildings in the following U.S. earthquakes: 1971 San Fernando,
1979 Imperial Valley, 1984 Morgan Hill, 1987 Whittier Narrows, and 1989 Loma Prieta. The
data from the U.S earthquakes is supplemented with data from several R/C frame buildings in
Japan for which vibration test results, earthquake response measurements, and analytical
predictions were provided by members of the Kajima research team. Pertinent data of the
reviewed U.S. and Japanese buildings are summarized in Table 1(a) and Table 1(b),
respectively.. Qualitative and quantitative information extracted from this review and related to
1
dynamic characteristics of buildings, structural and non-structural damage, site soil effects, and
soil-structure interaction is presented in the Appendices to this report.
The performance review focused on the dynamic characteristics of buildings, soil-
struèture interaction effects, and site soil effects. In the assessment of performance it must be
considered that none of the reviewed earthquakes was very severe and that only few of the R/C
buildings included in this review exhibited noticeable structural damage. But all earthquakes
were of sufficient severity to cause various degrees of cracking in structural elements. Moment
resisting frame as well as shear wall buildings from 5 to 30 stories high were included in the
performance review.
2
BUILDING 8 Of LEN. .1 LOAD RESISTING SYSi TYPE TYPE YEAR OF CSMIP EARThQUAKE EN. [NSTRUMENI STRUCTURE REFERENCE VERTICAL LAT ERAL DESCRIPTION WIDTh OF OF DESIGN [USGS] SUBJECTED CENT. LOCATIONS PERFORMANCE
LONGiTUDINAL TRANSVERSE STOR. (ft) SOIL FOUNDATION Stn# 3D DIST.
CanuryClty 18 105 RCSlab/Beam/Wafl DuctlleRCMom. DuctlleRCMwn. 1967 San Fernando Building suataInednostnrctur- Berteroet.al. 1988
Medical Plaza 105 Floor Resisting Concrete Resisting Concrete 1971 al damage and Insignificant I
Frame Frame - non-structural damage
Certified Life 14 180 R.C. Flat Slab on R.C. Shear R.C. Shear Loose Fill on Raymond Piles 1966 San Fernando 17ml Ground FIr, Hairline Cracka on Shear Conrad Aasocltea
Building 120 Columns & Walls Walls Walls FInn Under- Both Vert. & EQuake 11 6th FIr & Walls Observed. 1973
Building 82 Concrete Frame Walls Mt Lewis '86 2nd, 5th Wall bases In E-W diem Sederit Ct.al. 1991
San j - - - Iowa Prleta '89 33km Roof
HiltonHotel 13 249 RCSIab/Beam RCMomwit RCMoment 1968 San Fernando Building sustalnednostructur- Berlaoetal. 1988
Ventura Blvd. 165 Floor Resisting Resisting 1971 ii damage and Insignificant
Frame Frame - - non-structural damage
Holiday Inn 7 160 R.C. Flat Slab on PcrlmeterM.R.P. Perimeter M.R.F. Recent Alluvi. Can In situ 1965 San Fernando IlmI Ground Fir., Extensive non-structural Blume 1973
LcsAngelea I I SlabFramea jSlibFrames I dyallis) lion - Damage
Table 1(a) Summary of Instrumented Buildings - U.S. Buildings
BUILDING #01 LEN.& WAD RESISTING SYSTEM TYPE TYPE YEAR OF CSMIP EARThQUAKE EPL INSTRUMENT STRUCFURE REFERENCE VERTICAL LAT ERAL DESCRIPTION WIDTh OF OF DESIGN [USGS] . SUBJECTED CF. LOCATIONS PERFORMANCE
LONGITUDINAL TRANSVERSE STOR. (ft) SOIL FOUNDATION Stn# TO DIST.
Sears Warehouse 5 360 Rat Slab on Perimeter Perimeter Not Given Spread 1970 24463 WhittIer 14km Basement No atrueteral or ncat-gnactural Filhippou 1990
Los Angeles 280 Ciroular Columns Moment Resist Moment Resist. Footings (1970 Narrows 87 243rel damage reported.
Cone. Frame Conc. Frame ?..A.Code) I - - Roof
Sheraton-UnIversal 20 184 RCSIab/BeamFlr. Moment Resist Moment Resist. Beddedsoft SpresdFootlngs 1967 24464 SanFernan.11 19m1 Basement, Minornon.strueturaldamage Blumel9l3
Hotel 58 LWt R.C. used Cone. Frame Cone. Frame Sandstone/ B. C. - 6ksf (1966 WhinIer 28km 3rd .9th • Significant Torsional Filuippou 1990
lslmtan Airport 14111 200 RCflat Slab on RC Moment RC Moment RCCalssms 1967 San Fernando Building suatalnednoatructur. Bestaroetal. 1988
Center 200 Frame Resisting Frame Resisting Prime & Footings 1971 at damage and insignificant
nn.structoraldamage
UnlonBank 13/2 193 RC.Slab/Beam Moment Resist. Moment Resist Ailuviam Pile Foundation 1964 24322 SanFernandoll I7mi Basement San FerstEquake..Sw.& 8lume 1973
Building 75 Floor on Columns R.C. Frame R.C. Frame WhIttier 17 38km Ground Fir. on-struc. damage. Whiflier U. Y. 1991
Vslley Pesbitar1an 7//I 841 L.WL Concrete Slab RC Moment RC Moment Pile Foundation 1968 San Fernando Building sustained no stiuctur- Beticro et al. 1988
Hospital dii. on Steel Deck supp- Resisting Frame & Resisting Frame & 1971 al damage and Insignificant
Van Owen - - ,ited on RC Frame Partial. Rigid Core Partial. Rigid Core - on-structwal damage
Warehouse 14 217 RC Flat Slab Moment Resist Moment Resist. 100 ft. deep RC Footings on 1925 24236 WhittIer 25km Free Reid Soil Sna1czure intairtlon Fenves Ct. aL 1989
Building 51 Cone. Frame Cone. Frame Sandy-Clay Raymond Piles Narrows 17 Basan.,Bth observed In longitidInal dim.
.os Angeles - - Vp2400ft - - 12th,Roof I
Wilshire 16//I 166 PSC Slab onRC RCMoment RC Moment Cast-in-sItu 1969 San Fernando Buildingsustsinednostructur. Berteroetal. 1988
CuaistlanTowem 60 Frame Resisting Frame Resisting Frame Pile Foundation 1971 al damage and Insignificant
on-struetwil damage
WilahireCoraredo 13111 239 RC Slab on RCMontcstt RC Moment RC Pile 1970 San Fernando Buildlngaustainednoatnicsur. Berteroetal. 1988
Wilshire 142 Frame Resisting Frame Resisting Frame Foundation 1971 al damagearef insignificant
non-structural damage
Table 1(a) Summary of Instrumented Buildings - U.S. Buildings.(Contd.)
BUILDING 4 Of LEN. .1 LOAD RESISTING SYSTEM TYPE TYPE YEAR OF CSMIP EARThQUAXE EPL INSMUMENI STRUCTURE REFERENCE
DESCRIPTION WIDTh OF OF DESIGN [USGS SUBJECFED CENT. LOCATIONS PERFORMANCE VERTICAL LAT ERAL STOR. (ft) SOIL FOUNDATION 5us$ TO DISt LONGiTUDINAL TRANSVERSE
Holidaylms 7 160 R.C. Flat Slab on PerimeterM.R.F. PerlmcterM.R.F. RecentAlluvi- Cast Insitu 1965 San Fernando 26m1 Ground Fir., SurealmilartoVanNuya Blume 1973
Los Angeles - - $lab Frames 'iab Frames Sandt B.C.- 12hf - - Roof
Office Building 6 2DO R.C. Slab/Beam Moment Resist Moment Resist IndIvidual 1977 58490 Loma Prleta 89 811cm Ground Fir. No souctural or nas-struct. Celebi Ct. al.
San Bruno 90 Floor on Columns R.C. Frame R.C. Frame Spread FootIngs 2 nd,S th & damage was observed.
Building 64 WaIl System WaIls Walls Coner. Piles Mt. Lewls86 6th,Roof predlctedbyUBC.88 Sedarat et. al. 1991
San Jose - - underwalls I - LomaThicta'89 133km . Moehiectal. 1990
Table 1(a) Summary of Instrumented Buildings - U.S. Buildings (Contd.)
BUILDING DESCRIPTION
#01
STOR.
LEN..1 WIDTH
(It)
LOAD RESISTING SYSTEM TYPE OF
SOIL
TYPE OF
FOUNDATION
EARTHQUAKES SUBIECFED
TO
EN. CENT.
INSTRUMENT LOCATIONS
DIS'I'.
STRUCfURE PERFORMANCE
REFERENCE
VERTICAL LAT ERAL LONGITUDINAL TRANSVERSE
Shinamachi 18 80 R.C. Slab Beam R.C. Moment R.C. Moment Clay I Pile Ize-Peninsula Coastal '74 150 kin Under Pile Fdn. No structural or Non-structural Kajima
Apartment 45 Floor Resisting Resisting SandyGravet Foundation lbaragi-kenCoastall4 190krn Base.,9th& damage reported. Team
S-K Building 30 99 R.C. Slab Beam R.C. Moment R.C. Moment Soft B.Wall under perip- Chiba To-Ho Oki '87 63km But, of pile Fdn. No structural or Non-structural Kajima
99 Floor Resisting Resisting Soil feral columns. Piles TokyoTo-Bu'88 13km 1 st Floor & damage reported. Team
Frame Frame -mder interior column - Roof
Sun-City 25 138 R.C. Slab Beam R.C. Moment R.C. Moment Not Pile Chiba-ken-Central '80 50km 1 St Floor, No structural or Non-structural Kajima
0-Tower 93 Floor Resisting Resisting Known Foundation II th Floor & damage reported. Team
Frame Frame Roof
Tohoku 9 123
_________
Conerete Encased Steel Structure Not Pile Miyagi-ken Old '70 115km 1 at Floor & No structural or Non-structural Kajima
University 57 Known Foundation Miyagi-ken Oki 18 128km 9th Floor damage reported. Team
Building - - ________________ Miyagi-ken Oki '78 1211m
Table 1(b) Summary of Instrumented Buildings - Japanese Buildings
CUREe-KAJIMA RESEARCH PROJECT
NON-LINEAR BEHAVIOR OF A TUBULAR STRUCTURE
Hatem Y. Goucha and Vitelmo V. Bertero
Earthquake Engineering Research Center
University of California at Berkeley
July 15, 1991 - January 14, 1993
ACKNOWLEDGEMES
The work presented herein is a follow-up on the report on Task 1 of the CUREe-Kajima research project on the design of high-rise buildings. The previous report is entitled, "Review of the State of the Art and the State of the Practice of Earthquake Engineering Design of High-Rise Reinforced Concrete Buildings on Soft Soils in Regions of High Seismic Risk."
I wish to thank Professor Vitelmo V. Bertero for his constant support and advice. I would like also to th ik Amador Terán and Raul Bertero for the support they provided, as well for some very fruitrul conversations on the subject of this report.
Finally I wish to thank my family in Tunisia, who have continuously supported me and encouraged me throughout the length of the research project.
11
1 INTRODUCTION
1.1 STATEMENT OF THE PROBLEM
Since it was first introduced by F.R. Khan [1974], the concept of tubular structures has been
used mainly in regions of low seismic risk or regions of no seismic risk at all. Recently, taller
and taller buildings are being designed in high seismic risk regions. In some cases, conventional
structural systems such as space frames or shear walls can not be used due to their insufficient
lateral stiffness and development of significant tensile forces in the columns at high slenderness
ratios. The use of tubular structures, because of the optimal location (at the perimeter) of the
lateral structural system offers a possibility for a better solution. In the elastic range, a tube can
provide a system stiff enough to satisfy serviceability constraints otherwise harder to achieve with
the more conventional MRSF structural systems (e.g., a 30-story RC MRSF has a fundamental
period of vibration of 2.5 secs., while a 30-story RC bundled tube has a period of 0.90 secs. The
overall higher stiffness is obtained by approaching as closely as possible the cantilever behavior
of a perfect tube: the columns are closely spaced and the spandrel girders are very deep, and
therefore very stiff, resulting in what can be considered a perforated tube. However, the post-
yielding range is not well understood. In this study, which is a continuation of a previous study
by the authors [Goucha and Bertero, 1992], an attempt is made to have a better understanding
of the behavior of tubular structure in the non-linear (inelastic) range when subjected to severe
seismic loading. Halabieh [1990] concentrated mostly on the method of analysis, and proved that
the 2D simplification he used (similar to the one used in this paper) gave good results. He
obtained a plastic hinge distribution during the static pushover that suggests a soft-story
mechanism at the 21st story. No specific conclusion was made regarding the collapse mechanism.
1.2 OBJECTIVE
The main objective of this work is to achieve a better understanding of how RC tubular
structures can dissipate large amounts of the EQ energy input by studying their non-linear
behavior of reinforced concrete tubular structures through mathematical modelling and non-linear
analysis. The analysis includes static pushover and a dynamic time history ground motion
analysis of a 2D model.
To achieve the main objective, it was necessary to accomplish the following secondary
1
objectives: first, the development of a reliable 2D model of the tubular structure, and second, the
establishment .f the importance of including the internal gravity frames in the modelling process
of a tube building for the purposes of design. This last objective was motivated by the fact that
previous works on the subject by Anderson [1975], Grossman [1986] and Halabieh [1990] did
lateral load analytical studies on tube buildings without including the internal gravity frames.
Anderson and Halabieh considered ground motions, and Grossman only-considered wind (due
to low seismicity at the site).
1.3 SCOPE
In order to achieve the above-mentioned objectives, the following steps were taken:
Identification of an existing reinforced concrete tubular structure.
Creation of a 2D model of the structure for use in the 2D non-linear program DRAIN-2DX
[Allahabadi,1988]. Use of a 2D program was selected for two reasons: first, the available 3D
non-linear programs are still inadequate for use with large structures having many degrees of
freedom (59 bays, 11 stories) due to the still inadequate and slow hardware freely available.
Second, the beam column elements available in FACTS [SSD, 1987] and the ANSR series
[Mondkar, 1975; Mondkar, 1979; Oughourlian, 1982] have element libraries more geared
toward steel elements than reinforced concrete elements.
Creation of a 3D model of the structure for use in the 3D linear program SAP90
[Wilson, 1992].
Static analysis of the 3D model. Two load patterns were used: LOAD 1 is an inverted triangle
shape; LOAD 2 is a rectangular shape. LOAD 1 is the expected lateral load distribution
caused by ground motion for the case of a building that stays elastic and behaves in a first
mode of vibration approaching a straight line. LOAD 2 is the expected lateral load distribution
if a soft story develops in the first story.
Determination of whether the internal frames in the model can be neglected or not by
comparing the results from the model with internal frames with the results from the model
without internal frames. One criterion is the following: is it always conservative to ignore the
contribution of the internal frames?
Checking the accuracy of the 2D model in the elastic range by comparing the results with the
3D analysis.
2
Non-linear static monotonic analysis (static pushover) of the 2D model.
Linear elastic static analysis on the 31) model to obtain moment demand and supply ratios.
Determination of the reliability of the 2D model by comparing the sequence of plastic hinge
formation with respect to the stress ratios obtained in the 3D elastic analysis.
Dynamic linear elastic analysis on the 3D model to obtain simultaneous stress ratios at the
critical time during the ground motion.
Dynamic non-linear analysis of the 2D model.
Evaluation and comparison of the results of the analyses, drawing the appropriate conclusions
from the non-linear behavior.
Removal of the weakness of the selected tubular structure and repeat the non-linear analysis.
This is done by having the closely spaced columns in the perimeter extend to the foundation.
Design of an ideal tubular structure for a 30-story RC building and repetition of the non-linear
analysis to study its non-linear behavior.
3
EFFECTS OF SOFT SOILIS AND HYSTERESIS MODELS ON SEISMIC DESIGN SPECTRA
Report on Task 5 of the CUREe/Kajima Project on
DESIGN OF HIGH-RISE REINFORCED CONCRETE BUILDINGS ON SOFT SOILS
by
Mohsen Rahnama
and
- Helmut Krawinkler
A CUREe-Kajima Research Report
February 1993
ABSTRACT
The main objective of this research is to develop procedures that will permit an
explicit incorporation of the effects of deterioration of structural properties and site surface
geology on the seismic demands imposed on structures by strong ground motions. This
implies consideration of both the influence of stiffness degradation and strength
deterioration of structures and the effects of source-site distance and site soil conditions on
those demand parameters that can be used directly for design of structures. Thus, the
research combines ground motion and structure response issues, with an emphasis on
response parameters that incorporate both relevant ground motion as well as structural response characteristics.
The seismic behavior of a structure subjected to a strong earthquake needs to be
evaluated with due consideration to the fact that the mechanical properties of the structure
(i.e., stiffness, strength) are being modified during each inelastic excursions. In order to
consider this effect on the seismic demands, a general hysteresis model is developed.
During dynamic loading the dissipated hysteretic energy of SDOF systems is describe to define the deterioration of strength and stiffness by means of a deterioration parameters.
This deterioration parameter is also used to develop a damage index for damage evaluation of SDOF structures. In order to investigate the effects of deterioration of structural properties on seismic demands, a parametric study on SDOF systems was conducted using different hysteresis models. The seismic demands are evaluated statistically for a specified
target ductility ratio and the results are presented in terms of the spectral ratio between the
deteriorated structures and the non-deteriorated systems.
The research related to the study of soft soil effects on seismic demands is carried out
in two parts. In the first part advantage is taken of the extensive set of ground motions
recorded during the Loma Prieta earthquake and the availability of data on local soil
conditions at recording stations. These data sets are utilized to improve the basic
understanding of the phenomena involved, identify the most relevant parameters, develop
analytical models, and calibrate these models. In the second part of the study simplified
models of soil columns are employed for an extensive parameter study on the effects of site
soil conditions on seismic demands. The properties of the soil columns are varied to cover soil periods ranging from 0.5 to 4.0 seconds. The study has shown that the soft soil effects cannot and should not be lumped into a constant soil factor. Rather, it appears to be
Abstract
feasible to relate the strength demand for a soft soil motion to that of the motion in the
underlying rock by a modification factor which is a function of soil period and ductility
demand of the system. It was found that the amplification in the vicinity of the soil period
is approximately 5 for elastic strength demands and between 3 and 4 for inelastic strength
demands. This amplification of strength demands is approximately the same for soil
columns of all periods and for near-source and far-source motion, whereas the
amplification of PGA is strongly period dependent on soil period and source-to-site distance.
Abstract
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CUREe-KAJIMA RESEARCH PROJECT
SEISMIC PERFORMANCE OF A 30-STORY BUILDING
LOCATED ON SOFT SOIL AND DESIGNED ACCORDING TO UBC 1991
Amador Terán-Gilmore and Vitelmo V. Bertero
Earthquake Engineering Research Center
University of California at Berkeley
July 15, 1991 - January 14, 1993
ABSTRACT
Researchers and practitioners interested in the seismic design of buildings have expressed
serious concerns about the reliability and rationality of present building code methodology for
the Earthquake-Resistant Design (EQRD) of tall buildings, particularly those to be built on deep
soft soil. Therefore it was decided to conduct the studies reported herein, which have the
following main objective: to assess the adequacy of the U.S. Uniform Building Code (UBC)
methodology for tall buildings on soft soil.
To achieve the above main objective, it was decided to design a tall building on soft soil
using the UBC methodology, and then to analyze its performance under the critical Earthquake
Ground Motions (EQGMs) that could occur at the building site. First, a 30-story RC building
with identical configuration, structural system and structural layout as those of a building that
has been designed and built in Japan was selected. The 30-story building, assumed to be
located in the San Francisco Bay Area, is constituted by a three-dimensional assemblage of
Reinforced Concrete (RC) special moment-resisting frames. The following studies that were
conducted are reported herein.
In Chapter 2, a description of the function, configuration, structural system, and structural
layout of the selected building is given. In this chapter, some of the general requirements for
the design of the 30-story building according to UBC are presented, together with the methods
used for the preliminary design of the 30-story building.
A detailed discussion and interpretation of some UBC design guidelines is presented in
Chapter 3. In this chapter, the design procedure for obtaining the final design from the
preliminary design is presented, as well as a summary of the member sizes and reinforcement
for the final design of the 30-story building. Some considerations about the way acóidental
torsion is accounted for in the UBC specifications are presented.
In Chapter 4, the studies conducted on the behavior of the designed 30-story building when
subjected to different levels of EQGM was studied. A summary of the results obtained from
the several analyses that were carried out on this building are presented: elastic response
spectra analyses (RSA) and elastic time-history analyses (THA) for service and safety limit
states; nonlinear pushover analyses, and nonlinear THA for service and safety. It should be
noted that the nonlinear THA were limited to plane (2D) models. The results obtained from the
elastic and nonlinear analyses, which include floor displacements, Interstory Drift Indices (IDI),
maximum and cumulative plastic rotations at the ends of the members (O and O), global 1
displacement ductility demands (Ii), story displacement ducthhity demands (t), local
rotational ductility demands (ib), and finally, an estimate of local damage index to the
members of the building (DM1), provide information on which the assessment of the adequacy
of UBC specifications and design methodology in the design of the 30-story building can be
based for the service and safety limit states. In Chapter 4, only the performance of the 30-story
building is discussed; while the discussion of the possible reasons for its behavior are
presented in Chapter 5.
Chapter 5 provides information that complements that presented in Chapter 4. Additional
analyses of the 30-story building were carried out to complete the information on which the
assessment of the behavior of the building can be based: elastic analyses were carried out
to assess the adequacy of a 2D model of the building has for predicting the global three-
dimensional (3D) behavior of the building; an extra nonlinear 2D ThA was carried out to
assess the effect of P-is on the dynamic nonlinear behavior of the building; and finally, the
nonlinear response of single-degree-of-freedom systems (SDOFS) is computed to study the
reliability of estimating nonlinear response from the linear elastic response of systems
subjected to EQGMs recorded in soft soils. Once the information regarding the behavior of the
building is completed, an assesment of the seismic performance of the building is made.
Finally, Chapter 6 presents a comparison of the seismic performance of three 30-story
buildings with exactiy the same structural layout, but designed according to three different
design philosophies: American practice, Japanese practice, and a newly developed Conceptual
Design Methodology. Based on the results obtained from this comparison and from those
discussed in Chapters 3 to 5, conclusions are drawn and recommendations are formulated for
improving present EQRD Code procedures and for the needed research to achieve such
improvements.
CUREe-KAJIMA RESEARCH PROJECT
TALL BUILDING BEHAVIOR
DURING THE 3 MARCH 1985 CHILEAN EARTHQUAKE
Raul D. Bertero and Vitelmo V. Bertero
Earthquake Engineering Research Center
University of California at Berkeley
July 15, 1991 - January 14, 1993
1
1. INTRODUCTION
On 3 March, 1985, a major earthquake of magnitude Ms=7.8 occurred off the coast of central Chile. The affected area included the most populated area of Chile and the cities of Santiago and Viña del Mar, where many tall buildings exist. The Chilean earthquake of 3 March, 1985, was particularly significant in that the ground motion was well recorded, was of strong motion and long duration, and modem structures performed extremely well (Wyllie 1986). Although the design forces for buildings in Chile are comparable to those in the U.S., the Chilean engineers developed a structural design philosophy that is unlike those in the U.S., Japan, or other regions of high seismicity. Most moderate-rise residential buildings in Chile rely on reinforced concrete structural walls to resist lateral and gravity loading. In general, the walls have moderate amounts of reinforcement without special detailing for producing a ductile structure (Wood 1991).
The main objective of this report is to review and evaluate the performance of tall buildings during the 3 March, 1985, Chilean earthquake in order to improve the seismic design of high-rise reinforced concrete buildings, particularly when they have to be built on soft soil.
This report is divided into seven chapters. The Introduction describes the general goal of the report as well as the partial objectives of each chapter.
The second chapter, Seismic Characteristics of the Region, has two objectives: a) to evaluate the return period of a ground motion with the severity of the 3 March 1985 earthquake at Vifla del Mar, intending to classify it as a serviceability, damageability, or survivability ground motion for the city; and b) to evaluate ground motion characteristics (peak instrumental values. Arias intensity, linear elastic response spectra, effective peak acceleration and velocity. Housner intensity, and strong motion duration) and to compare their severity with those corresponding to El Centro, which is traditionally considered as a severe ground motion.
The third chapter, Structural Engineering Practice in Chile, studies different aspects of Chilean practice that affect the design and behavior of tall buildings in Vina del Mar. Seismic code, design concepts, construction materials with nominal and real properties, structural and nonstrüctural details are summarized with a view to emphasizing the most peculiar aspects of Chilean practice.
The fourth chapter, Tall Buildings in Viña del Mar, studies specific aspects of the tall buildings in Viña del Mar: configuration, structural lay-out, structural systems and foundations. Also summarized are: the tall building inventory; analysis of its periods and damping, periods comparison with Mexico, Japan and US; and geotechnical characteristics of soil in Viña del Mar (soil profile, standard penetration tests, depth of the soil deposit to bedrock and the characteristic period site).
The fifth chapter, Effects of the 3 March 1985 Earthquake in Viña del Mar, has three objectives.
I a-
2
To evaluate the accuracy of the recorded ground motion in Viña del Mar as input for the studied tall buildings.
To evaluate the damage potential of the ground motion in Villa del Mar, computing linear elastic and elasto-plastic strength demand, linear elastic and elasto-plastic displacement demand, cumulative ductility demand, and hysteretic energy demand spectra. Also, interstory drift, accelerations, and damage indices of buildings designed according to Chilean practice are evaluated and compared with observed structural and nonstructural damage of tall buildings. Using these data, an estimation of the average overstrength of tall buildings in Viña del Mar is obtained.
To evaluate the effect of the soil-structure interaction on the response of tall buildings in Viña del Mar.
In the sixth chapter, Evaluation of Four Buildings in Vina del Mar, studies of design. analysis, and performance during the 3 March 1985 earthquake [mainly based on a report by Wallace and Moehle (1989)] are presented for four buildings in Villa del Mar. Both damaged and undamaged buildings of similar construction were selected to determine the differences between the buildings, and to correlate the differences with observed damage.
Finally, in the last chapter, Implications of the March 3 1985 Chilean Earthquake on Tall Building Design, the lessons of the behavior of tall buildings during the 1985 Chilean earthquake are summarized and the main conclusions for tall building design are stressed.
TALL REINFORCED CONCRETE BUILDINGS:
CONCEPTUAL EARTHQUAKE-RESISTANT
DESIGN METHODOLOGY
by
R. D. Bertero and V. V. Bertero
Report to Sponsor: CUREe Kajima
Report No. UCB/EERC-92/16 Earthquake Engineering Research Center
College of Engineering University of California at Berkeley
December 1992
ABSTRACT
The ultimate goal of the studies reported herein is to formulate a methodology for preliminary Earthquake-Resistant Design (EQRD) of high-rise reinforced concrete buildings that is more reliable than those that are presently available. This improved methodology should be a conceptual one, i.e., one based on well-established fundamental principles of structural dynamics, mechanical behavior of actual buildings, and comprehensive design; it should also also be in compliance with the worldwide accepted EQRD philosophy. The studies undertaken and conducted to achieve this goal are presented and discussed in this report, which contains eight chapters.
The first chapter briefly states the need for formulating an overall rational conceptual methodology for EQRD and describes the main objectives of the report. The EQRD problem is summarized and a solution is presented. A flow chart summarizes the main steps and tasks to be conducted. This is followed by remarks clarifying that the conceptual methodology for numerical design presented herein should not be confused with what has been defined previously as "conceptual design." These are two different phases of the EQRD process. The "conceptual methodology" phase (numerical design) formulated herein should follow after the "conceptual design" phase of the overall EQRD and Earthquake-Resistant Construction (EQRC) has already been conducted.
Chapter 2 describes the acquisition and processing of the data needed to establish reliable design earthquakes, which is the first and perhaps the most important step in carrying out a conceptual preliminary EQRD. The different spectra needed to determine the critical earthquake ground motions (EQOMs) at service and safety levels are discussed, with emphasis on the advantages of introducing the concept of a Damage Index (DM) and the parameter 'y to define the critical EQGM at the safety level. The acquisition and processing of the data necessary for the establishment of the design EQs for a 30-story building to be built on a site with soft soil conditions are presented.
.Chapter 3 describes the different steps involved in the preliminary analysis of the preliminary EQRD procedure, and contains a flow chart summarizing these steps. It is noted that, although the main purpose of this preliminary analysis is analysis of the problems involved (i.e., what is given, what is known, and what is needed) in establishing the design criteria and finding the maximum fundamental period (T1), the seismic forces, and the critical load combinations that control the design, preliminary design of member sizes is in fact necessary in order to obtain from the displacement design equation the T1 that satisfies the design criteria to be used for estimating the seismic forces. After selecting the design criteria, which is based on satisfying the requirements of the two main limit states of serviceability and survival (safety) and the critical load combinations, the necessary design equations to obtain the maximum acceptable T1 for these two limit states are developed.
The theoretical aspects of the solution of the problems involved in the preliminary analysis step are discussed in detail in Chapter 4, where it is clearly shown that the design criteria adopted in Chapter 3 are in compliance with the world-wide accepted EQRD philosophy. The importance of simultaneously considering the Energy Input, E1, the Inelastic Design Response Spectra, IDRS (for strength and displacement), and the Energy
I
Dissipation, ED (which can be obtained through Damping Energy, E, and plastic Hysteretic Energy, EH), is discussed. In the studies reported herein, design for safety is based on developing large ED through structural damage, i.e., EH. Thus, the use of equivalent ductility ratios based on different damage models is emphasized. The Park-Ang model is then adopted, as is the use of the dimensionless parameter y introduced by Fajfar. This parameter permits the derivation of a simple equation for the equivalent ductility ratio where the reduction due to low cycle fatigue is controlled by two parameters, f3 and 'y; and by the permissible Damage Index, DM. The need to reduce the established equivalent Single-Degree-of-Freedom (SDOF) ductility ratio considering Multi-degree-of-Freedom (MDOF) effects is also discussed, and the use of a factor 12 is recommended. The potential sources of overstrength with respect to the strength used in the design observed in real structures are also discussed. Finally, the detrimental effects of torsion in the elastic and inelastic range of the behavior of the structure are analyzed, and simplified equations are developed to introduce these effects from the beginning of the design procedure. The formula that is presented for the reduction of the accidental eccentricity with increase in the number of stories of a building was derived based on a probabilistic analysis.
In Chapter 5, the theoretical aspects of the preliminary design step are discussed. A strong column-weak beam concept, together with a "capacity design" approach, is used. The need for amplification of column bending moment demands is discussed in detail, and the need to modify the axial forces according to the number of stories above the column under consideration is also discussed.
Application of the developed conceptual methodology to the preliminary EQRD of a building is discussed in Chapter 6. A 30-story RC building designed and built in Japan, whose structural system is a special moment-resisting space frame, was selected for the application of the developed methodology.
Chapter 7 describes the analyses that were conducted to evaluate the preliminary design of the 30-story RC building. These analyses were the following: a) linear elastic dynamic analysis (3-D linear elastic response spectrum using serviceability response spectra and 3-1) linear elastic time-history analysis for the EQGM component N90W recorded at SCT station during the 1985 Mexico EQ, but with a b) nonlinear static analysis using a 2-1) nonlinear "pushover" technique with a lateral force pattern obtained from the safety spectra modal analysis; and c) nonlinear dynamic analysis using as EQGM the N90W component recorded at SCT station during the 1985 Mexico EQ, but with a PGA y=0.30g.
The final chapter summarizes the main results obtained and observations made during the studies reported herein. Based on these results and observations, specific recommendations for improving the present EQRD of tall buildings are offered. Recommendations for needed research are also formulated. The chapter's concluding remarks point out that the main advantage of this proposed conceptual methodology is that, notwithstanding the great uncertainties in the quantification of some of the concepts involved in its codification, the numerical quantification of these concepts can be improved without changing the format of this codified methodology as new and more reliable data are acquired. It is also pointed out that it is not the intention of the authors
H
to propose that such a detailed methodology as has been presented be implemented in the code for all kinds of facilities. What has been proposed is that such a conceptual methodology for numerical EQRD be used for design of irregular buildings, tall buildings, and particularly buildings on abnormal sites. For short, regular buildings located on standard sites and using known, simple and reliable structural systems, this methodology can be used to formulate and calibrate seismic code regulations that are as simple or simpler, but still more reliable and transparent, than present ones.
Ill
ACKNOWLEDGEMENTS
The studies reported herein were conducted as Task #5, "Development of Preliminary Design Methods," of a research project entitled "Design of High-Rise Reinforced Concrete Buildings on Soft Soils." This project is part of a research program supported by a grant by Kajima Corporation and administered by CUREe, the California Universities for Research in Earthquake Engineering. The financial support from Kajima is gratefully acknowledged. Any opinions, discussions, fmdings, conclusions and recommendations are solely those of the authors and do not necessarily represent those of the sponsor.
The authors of this research report wish to thank Dr. Norio Inoue, Group Manager of the Kajima Research Team, and the members of his team for supplying all the information needed regarding the 30-story building used in this report, and for their valuable comments during the meetings that we had to discuss this research project.
The authors are also indebted to Profs. J. Anderson, P. Filippou, H. Krawinlder and G. Powell for the fruitful discussions held during the studies. Thanks are also extended to research assistants H. Goucha and A. Terán for their assistance during the project and to Brad Young for editing this report.
iv
PERFORMANCE OF TALL BUILDINGS DURING THE 1985 MEXICO EARTHQUAKES
by
Ainador Terán-Gi imore
Vitelmo V. Bertero
fr,
Report to CUREe-Kajima
Report No. UCB/EERC-92/17 Earthquake Engineering Research Center
College of Engineering University of California at Berkeley
December 1992
ABSTRACT
In the studies reported herein, the behavior of tall buildings during the 1985 Mexico earthquakes (EQ5) is investigated, with the main objectives of: (1) identifying and formulating seismic design guidelines for tall buildings built on soft soils, and (2) assessing which types of structural systems had acceptable performances.
To accomplish the above objectives, an extensive literature survey on the 1985 Mexico earthquakes has been carried out. The information gathered from the survey was analyzed, and the main results from such analyses are reported here in six chapters (Chapters 2-7).
Chapter 2 gives, firstly, a summarized overview of how the Earthquake Ground Motions (EQGM5) that were recorded in Mexico City were generated. Secondly, special attention is paid to the dynamic characteristics of the EQGM recorded at the SCT station. Linear Elastic and Inelastic Response Spectra (LERS and IRS, respectively) for strength and deformation are given, as well as the IRS for the Energy Input (E1) and the Energy Dissipation (ED). What can be called a nominal Drift Index [equal to tip displacement divided by the height of a Generalized Single-Degree-of-Freedom System (GSDOFS)}, or an average Interstory Drift Index (IDI) spectra, i.e., the average IDI vs. period, T, of the structure for the EQGM recorded at SCT, assuming a constant damping ratio, 4, and different values of the ductility ratio, t, have been plotted considering the relationships between the T and the height, H, (or number of stories, N) of typical Mexican, Japanese and U.S. moment-resisting frame buildings. From analyses of these spectra, it is concluded that Mexican waffle slab buildings having seven to 15 stories should be expected to suffer significant damage under the EQGMs recorded at SCT station. On the other hand, Japanese moment-resisting space frames subjected to the SCT recorded ground motions seem to limit the IDI adequately, and therefore to limit the damage. The advantages and disadvantages of present design practice for tall buildings in Japan are drawn from analyses of the Number of Equivalent Yielding Cycles at maximum j.t (NEYC ax), and the resulting Damage Index (DM) as defined by Park and Ang.
Chapter 3 deals with the statistics of damaged buildings in Mexico City during the 1985 EQs, providing an idea of the type of structural systems that were most vulnerable during the 1985 EQ5, as well as their dominant modes of failure.
The performance of tall buildings in Mexico City during the 1985 Mexican EQS is discussed in Chapter 4, where such performance is related back to the different seismic codes that were used in their design and construction. Special attention is paid to waffle slab framed buildings: results of case-history studies are presented and
1
discussed. This chapter ends with discussions of: (1) the different foundation systems used in Mexico City, and (2) the types and effects of nonstructural components.
Special aspects of the observed seismic behavior of tall buildings during the 1985 Mexican earthquakes are discussed in Chapter 5. Some of these aspects have been identified as direct consequences of the soft sites on which the buildings were built: these cases involve: (1) buildings that were already tilted before the EQs, and (2) soil-foundation flexibility effects on the seismic responses of tall RC buildings, tilted or not. Other special aspects discussed are the large number of observed collapses of upper stories, and the damage caused by pounding of adjacent buildings.
Chapter 6 is devoted to the discussion of what has been learned from the evaluation of the performance of buildings in Mexico City during the 1985 EQs. One of the main sources used for identifying the lessons learned was a review of the studies, discussions and comments that were carried out and offered as background after the EQs for the development and formulation of the new 1987 building code for the Federal District of Mexico. The main lessons learned are identified by comparisons between the 1976 and the 1987 building codes. The rationality of the changes introduced is discussed at the end of this chapter.
The main results obtained in the studies reported herein are summarized in Chapter 7, where general conclusions regarding the EQGM5, damage, Mexican RC building design practice, foundation systems used, special aspects, and the rationality and reliability of the new 1987 Mexican building code are offered. The important lessons learned from the studies conducted on single-degree-of-. freedom systems are also summarized. Finally, the special issues that need to be considered in the design of tall buildings on soft soils are identified, and recommendations for their proper seismic design, in the form of design guidelines, are offered.
ii
ACKNOWLEDGEMENTS
The studies reported herein have been conducted as Task #2, "Review of Seismic Performance of High-Rise Reinforced Concrete Buildings, of a research project entitled "Design of High-Rise Reinforced Concrete Buildings on Soft Soils." This project is part of a program of research which is financially supported by Kajima Corporation and administered by CUREe, the California Universities for Research in Earthquake Engineering. This financial support from Kajima is gratefully acknowledged. The findings, observations, conclusions and recommendations of this report are those of the authors alone.
The authors would like to express their gratitude to Professors J. Anderson, F. Filippou, H. Krawinkler and G. Powell for their valuable comments and fruitful discussions during the meetings that we had to discuss the studies reported herein.
Special thanks are due to Dr. Norio Inoue, Group Manager of the Kajima Team, and the members of his team for their valuable comments during the meetings that we had to discuss this work.
Thanks are also extended to Brad Young for editing this manuscript.