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Conferences & Symposia : Housner Symposium


The CUREE Symposium
in Honor of George Housner


Speaker Abstracts
FRIDAY - OCTOBER 27, 1995

Welcome From Caltech
Paul C. Jennings
Professor of Civil Engineering and Applied Mechanics
California Institute of Technology


THE HOUSNER YEARS I
Session Chair: Donald E. Hudson (Caltech)


The Housner Years: Introductory Remarks
Donald E. Hudson
Professor Emeritus
Department of Mechanical Engineering and Applied Mechanics
California Institute of Technology

In this presentation, an overview is given of the early career of George Housner in earthquake engineering, starting with his graduate student years, by one who shared these student experiences. The situation at Caltech when Housner began his studies is reviewed, and it is shown that his Ph.D. thesis on “An Investigation of the Effects of Earthquakes on Buildings” was able to combine in a very effective way much of the meager information on the subject which was then available. Housner was able in that 1941 thesis to arrive at a number of far-reaching conclusions about ground motion that have withstood the test of time. The thesis also included a complete mathematical analysis of the response of shear and bending structures with a flexible first story to earthquake excitations, and a study of damping in structures caused by the propagation of elastic waves in the earth produced by foundation movements. An aspect of Housner’s early career which is perhaps not well known was his early efforts at developing and deploying strong motion instrumentation. These efforts resulted in the first commercially available strong motion accelerograph, and in the installation of these devices at sites which produced such key accelerograms as the Parkfield array accelerograms, and the Pacoima Dam record during the San Fernando earthquake.


The Early Years of Earthquake Engineering
Ray W. Clough
Nishkian Professor of Structural Engineering, Emeritus
University of California at Berkeley

It may be argued that earthquake engineering began with the strong-motion instrumentation program of the U.S. Coast and Geodetic Survey. Certainly, effective earthquake-resistant design was dependent on having reliable information about the seismic input to which structures might be subjected. Thus, it is reasonable to designate the Advisory Committee on Engineering Seismology (ACES)—which advised the USCGS on the requirements for instruments to record earthquake motions and on the most appropriate places to locate these instruments—as the group which initiated the earthquake engineering field.

In this paper the successive stages of development of earthquake engineering will be described—from its initiation by the ACES, through the formation of the EERI and subsequently the IAEE, which was responsible for holding the sequence of World Conferences on Earthquake Engineering at four year intervals for the past 40 years.


Structural Engineering: Exciting Past – Demanding Future
Roy Johnston
Consulting Structural Engineer
Brandow & Johnston Associates

The practice of structural engineering has had an exciting past and will have a demanding future. This is particularly true for the seismic design of tall buildings. Through the years, there have been changes in the perception of the magnitude of the earthquake risk and in engineering analysis and procedures, coping with what may be called an acceptable risk. The process includes the representation of the ground motion, a conceptual and schematic plan, mathematical analysis and detailed design, and the observation and verification of the construction.


Inelastic Behaviour of Ductile Structures in Aseismic Design
Navin C. Nigam
Vice-Chancellor and Professor
Goa University, India

In the first World Conference on Earthquake Engineering held in 1956, George Housner introduced the concept of inelastic design based on energy input to a structure during an earthquake and its capacity to dissipate energy while undergoing inelastic deformations. Later, in a seminal paper (ASCE, 85EM4, 1959), Housner explained the anomalies in the response of structures during past earthquakes, and the significance of inelastic deformations in the aseismic design of ductile structures. Housner’s pioneering work, the work of his students and other researchers, has formed the basis for the evolution of the present earthquake resistant design philosophy, namely, elastic behavior for frequent small earthquakes, ‘limited’ inelastic deformation for moderate earthquakes and ‘large plastic deformation without collapse’ for infrequent large earthquakes. Recent developments and the present status of research on inelastic behavior of structures is reviewed.


George Housner and the International Association for Earthquake Engineering
Thomas Paulay
Emeritus Professor of Civil Engineering
University of Canterbury
Christchurch, New Zealand

President of the International Association
for Earthquake Engineering

A brief record of the role of George Housner in the founding and expansion of the International Association for Earthquake Engineering over close to four decades is presented. A light-hearted description of a unique seismic response control system, a subject known to be of considerable appeal to George Housner, conceived at the Swiss Federal Institute of Technology in Zurich and studied further at the University of Canterbury with respect to its potential for application to reinforced concrete buildings, will be illustrated.


THE HOUSNER YEARS II
Session Chair: John F. Hall
Associate Professor of Civil Engineering
California Institute of Technology


Cable-Supported Bridges and Earthquakes:
Recent Developments Toward the 21st Century
Ahmed M. Abdel-Ghaffar
Professor, Department of Civil Engineering
University of Southern California

Recent developments and general characteristics of seismic behavior of modern cable-stayed bridges and conventional suspension bridges are presented. Large-scale recently completed or under-construction projects of these long-span bridges are introduced. Parameters affecting their free-vibration and seismic response are discussed. Among these factors are: three-dimensionality, of both the earthquake ground motions at the supports and the structural modeling; nonlinearities; cable-vibration; structural connections; energy absorption (damping augmentation) devices at critical connections; the spatial variation of ground motion; and finally the feasibility and effectiveness of structural control with its passive, hybrid, and active categorization. Examples from Japan, Europe and the U.S. are presented. The American example is the well-instrumented, well-studied and well-shaken (by two moderate earthquakes) suspension bridge (the Vincent Thomas Bridge at Los Angeles harbor which was strongly shaken by the October 1, 1987 ( ML = 5.9) Whittier, California earthquake and more recently by the January 17, 1994 (ML = 6.8) Northridge earthquake. Recommendations are made for strengthening and retrofitting of existing bridges; moreover, seismic counter measure devices of some international bridges are shown.


The Caltrans Years
James E. Roberts
Chief Structures Engineer
California Department of Transportation

George Housner has had a profound influence on the California Department of Transportation since the San Fernando Earthquake of February 9, 1971. He was already known as a leader in earthquake engineering, and his influence on revised seismic design practice and specifications for bridges helped launch a revolutionary change in bridge seismic design. The first use of improved seismic analysis and design for a major Caltrans bridge was on the high-level crossing of the New Melones Reservoir on State Highway 49. Caltrans designers used “Housner’s Response Spectum” in the dynamic analysis of that bridge in early 1971.

The most important contributions made to bridge engineering and to Caltrans occured after the disastrous October 17, 1989 Loma Prieta Earthquake. In early November, 1989, Governor George Deukmejian appointed Dr. Housner as Chairman of the “Governor’s Board of Inquiry” to investigate the causes of bridge failures and recommend corrective actions to insure future public safety. The Board of Inquiry issued its final report with the warning title, “Competing Against Time” on June 1, 1990. Since then, Caltrans staff engineers, consulting firms, independent peer review teams, and university researchers have cooperated in an unprecedented program of bridge seismic retrofit strengthening to meet the challenge presented in that report. The State Legislature, the California Transportation Commission, and Department of Transportation Management designated this program the number one priority for budget allocation. Dr. Housner agreed to Chair the Caltrans Seismic Advisory Board, and it has been an invaluable asset in reviewing our criteria, design specifications, and design procedures. In many instances, the Advisory Board has positively influenced management decisions to support a strong research program to support seismic design and retrofit, through its recommendations to the Caltrans Director. Dr. Housner recently resigned from the Advisory Board after serving for five years as Chairman.

The success of the Bridge Seismic Retrofit program and the success of future seismic design for California bridges is based, to a large degree, on the accelerated and “problem-focused” seismic research program. That program has been supported at a level more than ten times the pre-Loma Prieta level of support for all bridge research. We have been able to sustain the necessary high level of research support over the past five years, and we have a commitment for that level of support for the foreseeable future.


Earthquake Engineering for California’s State Water Project
and Safety of Dams
Vernon H. Persson
Chief, California Dept. of Water Resources
Division of Safety of Dams

The Department of Water Resources designed and constructed the State Water Project, a major civil engineering work which transfers water from Northern to Southern California, a distance of over 600 miles. Those in charge had the foresight to employ advisors with special expertise. Dr. George Housner served as a consultant to DWR on the Earthquake Analyses Board from its inception in 1962. He and his colleagues provided earthquake engineering oversight to DWR staff during design and construction of the three billion dollar project. Dr. Housner played a major role in reviewing criteria and facility performance after the 1971 San Fernando earthquake and the 1975 Oroville earthquake. DWR’s Division of Safety of Dams has a separate regulatory function for providing supervision to California dam owners and their engineers with regard to safe dams. For many years, Division management has used the EAB to advise them on especially difficult projects, the seismic review of Auburn Dam being the most prominent. Dr. Housner and his long time colleagues Dr. Allen and Dr. Bolt served on the EAB from 1962 to 1994 at which time Dr. Housner stepped down from the Board after 32 years of exemplary service to DWR. This presentation will expand on the many important roles he held and the effect of his sound advice on water facilities throughout the State.


GROUND MOTION
Session Chair: Hiroo Kanamori
John E. and Hazel S. Smits Professor of Geophysics
California Institute of Technology


Seismic Hazard In Southern California
Clarence Allen
Professor of Geology and Geophysics, Emeritus
California Institute of Technology

During the George Housner years, publicized estimates of seismic hazard in Southern California have varied markedly. Statements by eminent scientists have ranged from “the region is one where long accumulations of strain are probably prevented by frequent, slight releases” to “in southern California there will be a severe shock which is more likely to come in three years than in ten.” George Housner himself has often joked about the diverse and confusing meanings of “active fault” as promulgated by various investigators, but he has actively encouraged geological and geophysical seismic hazard studies, particularly those of practical use to engineers.

In actuality, there has been a relatively steady progress in our understanding of the earthquake potential in Southern California, starting with studies of the 1906 earthquake, which recognized that its culprit, the San Andreas Fault, also extended through Southern California. Geodetic observations over the years, although sometimes misinterpreted, have been particularly important in establishing the rates of tectonic deformation and in identifying the principal seismic source areas. If one had to name the four most significant advances in seismic hazard assessment in Southern California during the Housner years, I would judge them to be (1) the development and use of paleoseismic techniques in identifying the times and natures of prehistoric earthquakes and in determining long-term fault slip rates, (2) the innovative use of seismological data, particularly from a new generation of wide-band instruments, in understanding the physics of the fault-rupture process, (3) the recognition of blind thrusts as significant seismic sources, and (4), the use of a new generation of geodetic instruments, the Global Positioning System (GPS), in quickly and cheaply delineating the ongoing crustal deformations that cause earthquakes.

Our greatest challenges at the present time are twofold: In a specific sense, it is critical to understand the seismogenic potential of the numerous blind thrusts that underlie many urbanized parts of Southern California. And in a more general sense, we must work to better quantify seismic hazard for effective and realistic use by engineers, planners, and public officials who must balance the costs, benefits, and priorities of earthquake mitigation.


Near-Source Ground Motions from Large Earthquakes
Thomas Heaton
Faculty Associate in Geophysics
California Institute of Technology

Ground shaking is often quite violent close to large earthquakes; of more than 30 recordings at distances of less than 5 km from earthquakes with magnitudes of 6.5 or greater, the median peak acceleration and velocity are 0.85 g and 105 cm/sec, respectively. Unfortunately, near-source ground motions have not yet been recorded for really large earthquakes (M>7.5); the 1992 M 7.3 Landers earthquake and the 1978 M 7.4 Tabas, Iran, earthquake are the largest earthquakes for which near-source records are available. The Lucerne recording of the Landers earthquake was taken at a distance of 1 km from the fault rupture and the peak acceleration, velocity, and displacement were 0.90 g, 142 cm/sec, and 255 cm. The Tabas recording was at a distance of 3km from the rupture and the peak acceleration and velocity were 0.92g and 125 cm/sec.

Near-source peak displacements are often comparable to the slip that occurs on faults. As earthquake magnitude grows, the average fault slip grows and so does peak displacement. It is very difficult to put an upper limit on displacement; slip on the San Andreas fault reached as much as 10m in the 1857 earthquake. Fortunately, it seems unlikely that peak acceleration continues to grow in the same way that displacement does. Nevertheless, it seems likely that the peak accelerations in very large earthquakes will be at least as large as the already strong values recorded in smaller earthquakes, but they will last longer. Near-source peak velocities for very large earthquakes will almost certainly exceed 100cm/sec, but it is not known just how large they can become for a really large earthquake.


Are Earthquake Ground Motions Affected by the Local Site Conditions?
I.M. Idriss
Professor, Department of Civil Engineering
University of California, Davis

The damage resulting from earthquakes may be influenced in a number of ways by the local site conditions in the affected area. Where the damage is related to a gross instability of the soil, resulting in permanent movements of the ground surface, association of the damage with the local site conditions is readily apparent. The influence local site conditions may exert on the characteristics of earthquake ground motions, however, is not as apparent. Examination of motions recorded during earthquakes over the past 30 years indicates that local site conditions can have varying effects on these motions. The earthquake ground motions recorded during numerous earthquakes (such as Parkfield, San Fernando, Imperial Valley, Coalinga, Palm Springs, Whittier, Loma Prieta, Landers, Big Bear and Northridge) are used to assess these effects on accelerations, velocities, displacements, and spectral ordinates.

The results of these assessments will by summarized at this symposium.


SIMULATION OF STRUCTURAL BEHAVIOR
Session Chair: Joseph Penzien
Professor of Civil Engineering, Emeritus
University of California at Berkeley


Linear and Nonlinear Seismic Analysis of Three Dimensional Structural Systems
Edward L. Wilson
T.Y. and Margaret Lin Professor in
Engineering, Emeritus
University of California at Berkeley

The development, during the past forty years, of computational methods for the simulation of the dynamic behavior of complex structural systems will be summarized. In the early days of earthquake engineering, the Rayleigh-Ritz method of dynamic analysis was used extensively to calculate approximate solutions. With the development of high-speed computers, the use of exact eigenvectors replaced the use of Ritz vectors as the basis for seismic analysis. It will be illustrated that Load-Dependent Ritz vectors can be used effectively in modern seismic analysis of both linear and nonlinear structures. The new modified Ritz method produces more accurate results, with less computational effort, than the use of the exact eigenvectors. In addition, the importance of creating realistic computer models and the need for experimental verification will be emphasized.


The Role of Large-Scale Structural Testing in Earthquake Engineering
M.J.N. Priestley
Professor of Structural Engineering
Department of AMES
University of California, San Diego

Proof tests have always had a special place in structural design, and are routinely employed in Japan as an adjunct to the design process. The advantages of experimental research at as large a scale as possible would seem to be obvious, and following a decline in the perceived significance of experimental tasks in the 1960s and 70s, there is now a resurgence of interest in large-scale seismic testing in the USA, with many large structural testing laboratories being constructed in recent years.

The paper will examine this trend, which has applied not only to the research field, but also to proof testing for both new and existing designs. Reasons will be examined with reference to analytical developments which might have been expected to make structural testing less attractive. The paper will be illustrated by recent examples from seismic research, design, and assessment projects, and some of the efforts to improve load simulation will be emphasized.


DESIGN AND PERFORMANCE CRITERIA
Session Chair: James L. Beck
Associate Professor of Civil Engineering
California Institute of Technology


Ground Motion and Codes: On the Formulation and Calibration of
Site Response Spectra for Seismic Design in Mexico City
Luis Esteva
Institute of Engineering
National University of Mexico

A critical account is presented of the revision process of seismic design response spectra in Mexico City after the 1985 Michoacan earthquake. Two parallel lines of development are studied, within the probabilistic framework of structural reliability and seismic risk analysis: a) the formal mathematical approach, and b) the semi-empirical approach, in a great measure relying on expert opinions based on the observed behavior of a large number of structures in different parts of the city during the earthquake.

After revising the microzonation of the city, a set of design response spectra were proposed, consistent with those calculated from the ground motion records obtained in 1985 at different sites in the Valley of Mexico, on different soil conditions, as well as with some theoretical results produced by the simple 1d model of vertically traveling shear waves on stratified deposits.

Following the installation of strong ground motion recording instruments or arrays at a large number of sites in the Valley, a substantial number of records for a wide variety of soil conditions have been obtained during the last few years. Those records have been used to obtain transfer functions from Fourier amplitude spectra on firm ground in the Valley of Mexico to their counterparts on soft soil. In spite of the significant inconsistencies between observed and calculated ground motion on soft soil, in particular regarding the duration of the intense phase of the motion, linear response spectra derived from the recorded ground motions are not very different from those derived from theoretical models. With a few exceptions, the empirical transfer functions are not very sensitive to earthquake magnitude and source distance nor to the azimuth of the arriving waves. To what extent the differences in duration between observed and theoretically derived records affect the nonlinear response spectra has not been established yet.

The availability of sets of several records at many locations in the city, on different soil conditions, has stimulated interest in the development of criteria for determining site specific response spectra for seismic design, which make use of the empirical transfer functions mentioned above. This has created the need for criteria and methods to determine consistent-reliability design spectra, as well as to account for the uncertainties associated with the structural parameters. The discussion of this problem ends the presentation.


Performance Based Seismic Design
Jack Moehle
Professor, Civil Engineering Department,
University of California, Berkeley

Director, Earthquake Engineering
Research Center

Current seismic design codes and practices were written to achieve a loosely defined objective of providing life safety. While this objective appears to have been reasonably well achieved, two major shortcomings are recognized. The first shortcoming lies in the realization, made clear by recent earthquakes, that structures designed to provide basic life safety may suffer extensive structural and non-structural damage, often resulting in huge economic losses for the owner and the community. The second shortcoming is that our seismic design measures are very unevenly applied; some structures are subject to costly over-design, while others suffer large losses because designs are not properly related to performance needs and expectations. Recent years have witnessed a surge in organized efforts to develop performance based seismic design procedures, including ATC-32 for bridges, SEAOC Vision 2000 and FEMA/EERC Performance Based Design for new buildings, and ATC-33 and ATC-40 for existing hazardous buildings. Writing provisions is one thing. Writing provisions that are demonstrably effective and that will be used by the public is another. This paper investigates a range of issues surrounding the holy grail of performance based seismic design.


Seismic Risk Analysis and Seismic Risk Management:
Half a Century of Evolution
Haresh C. Shah
Obayashi Professor of Engineering
Department of Civil Engineering
Stanford University

In this presentation, we will review the evolution of philosophy and methods for assessing and managing seismic risk during the past fifty years. Major accomplishments of science and engineering toward understanding this problem during this period will be highlighted. Recent contributions from social sciences in understanding the complex interplay of nature, technology, socioeconomic development, and human activities toward increasing or mitigating seismic risk will be discussed. We will present the contributions of Professor Housner and his students in this discipline. Finally, some shortcomings of current work and future challenges in this field will be summarized.


[Day Two Abstracts]

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