Earthquake-Resistant Design with Rubber

133 Pages·1993·4.287 MB·English

by  James Marshall Kelly PhD, BSc, MSc (auth.)

Earthquake-Resistant Design with Rubber
James M. Kelly
Earthquake-Resistant
Design with Rubber
With 43 Figures
Springer-Verlag
London Berlin Heidelberg New York
Paris Tokyo Hong Kong
Barcelona Budapest
James Marshall Kelly, PhD, BSe, MSe
Department of Civil Engineering, University of California,
Berkeley, California 94720, USA
Cover illustrations: Ch. 1, Fig. 2. Cross-section of the Foothill Communities Law and
Justice Center showing isolators in sub-basement. Ch. 1, Fig. 3. Location offaulting system
and site of the Foothill Communities Law and Justice Center. Ch. 8, Fig. 2. Internal forces
and external loads on buckled bearing.
ISBN-13: 978-1-4471-3361-2 e-1SBN-13: 978-1-4471-3359-9
001: 10.1007/978-1-4471-3359-9
British Library Cataloguing in Publication Data
Kelly, James Marshall
Earthquake-Resistant Design with Rubber
I. Title
624.1
Library of Congress Cataloging-in-Publication Data
Kelly, James M.
Earthquake-resistant design with rubber / James M. Kelly.
p. em.
Includes bibliographical references and index.
: $105.00 (est.)
1. Earthquake-resistant design. 2. Rubber. I. Title.
TA658.44.K4S 1993 92-43292
693.8'S2-dc20 CIP
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© Springer-Verlag London Limited 1993
Softcover reprint of the hardcover I st edition 1993
The publisher makes no representation, express or implied, with regard to the accuracy of
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Printed at the Alden Press, Oxford
69/3830-543210 Printed on acid-free paper
Contents
Preface vii
1. Isolation for Earthquake Resistance 1
2. Vibration Isolation ......................... 11
Introduction ............................ 11
Theory of Vibration Isolation .................. 12
Frictional Vibration Isolators .................. 18
3. Seismic Isolation .......................... 23
Linear Theory of Base Isolation ................. 23
4. Extension of Theory to Bulldings 37
5. Code Requirements for Isolated Buildings ............ 49
Introduction ............................ 49
1986 SEAONC Tentative Provisions .............. 50
1991 UBC .............................. 52
6. Coupled Lateral-Torsional Response of Base-Isolated
Buildings ............................... 57
7. Behavior of Multilayer Bearings Under Compression and
Bending ............................... 69
Shear Stresses Due to Compression ............... 75
Tilting Stiffness ofa Single Pad .................. 77
Pure Compression of Single Pads with Large Shape Factors. 79
Compression Stiffness for Circular Pads with Large Shape
Factors .... . . . . . . . . . . . . . . . . . . . . . . . . . . .. 81
Compression Stiffness for Square Bearings with Large Shape
Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 83
Tilting Stiffness of Single Pads with Large Shape Factors 84
vi Contents
8. BuckUng Behavior of Elastomeric Bearings .......... . 89
Influence of Vertical Load on Horizontal Stiffness ...... . 94
A Simple Mechanical Model for Bearing Buckling ..... . 95
Post-Buckling Behavior ..................... . 99
Influence of Compressive Load on Damping Properties of
Bearing .............................. . 101
Roll-out Stability ......................... . 103
9. Design Process for Multilayer Elastomeric Bearings ..... . 107
Preliminary Bearing Design Process .............. . 107
Recent Experimental Studies on Elastomeric Performance .. 109
Compact Design Bearings .................... . 116
Afterword ............................... . 125
Shake Table Tests of Base-Isolated Models .......... . 125
Influence of Base Isolation on Secondary Systems and
Equipment ............................ . 126
Torsional and Rocking Response in Base-Isolated Structures 127
Bearing Mechanics ........................ . 127
Bearing Testing .......................... . 128
Application of Isolation to Nuclear Facilities ........ . 128
Combined Isolation and Active Control ........... . 128
Reviews 129
Index 131
Preface
My involvement in the use of natural rubber as a method for the protec
tion of buildings against earthquake attack began in 1976. At that time,
I was working on the development of energy-dissipating devices for the
same purpose and had developed and tested a device that was even
tually used in a stepping-bridge structure, this being a form of partial
isolation. It became clear to me that in order to use these energy devices
for the earthquake protection of buildings, it would be best to combine
them with an isolation system which would give them the large displace
ments needed to develop sufficient hysteresis. At this appropriate point
in time, I was approached by Dr. C.J. Derham, then of the Malaysian
Rubber Producers' Research Association (MRPRA), who asked if I was
interested in looking at the possibility of conducting shaking table tests
at the Earthquake Simulator Laboratory to see to what extent natural
rubber bearings could be used to protect buildings from earthquakes.
Very soon after this meeting, we were able to do such a test using a 20-ton
model and hand-made isolators. The eady tests were very promising.
Accordingly, a further set of tests was done with a more realistic five
storey model weighing 40 tons with bearings that were commercially
made. In both of the test series, the isolators were used both alone and
with a number of different types of energy-dissipating devices to en
hance damping. Some of these devices were hydraulic, some frictional
and others were based on the elastic-plastic behavior of lead or mild
steel. The test results showed that when additional damping devices
were added to the isolation system, the increased damping did not al
ways lead to decreased response of the models, but induced accelera
tions in higher modes of the structures. It became clear that the best
way to increase damping is to provide it in the rubber compound itself
and that a high level of damping is unnecessary and can be detrimental.
This text will concentrate on isolation systems that use a damped
natural rubber, although the results would apply to other types of
elastomers such as Neoprene and EPDM. The main source for the de
velopment of the natural rubber compounds has been MRPRA.
The isolators are themselves a source of fascinating problems in solid
mechanics and in this text it will be possible to touch on only a few of
viii Preface
them. Much of the original work on the mechanics of isolators was done
at MRPRA under the leadership of Dr. A.G. Thomas. Over the past
fifteen years, many graduate students at the Earthquake Engineering
Research Center (EERC) have worked with me on the mechanics of
isolation bearings, the dynamics of isolated structures and the design of
these systems. Their research has been both theoretical and experimen
tal and has been instrumental in making the approach acceptable to the
structural engineering profession.
While the text covers in detail only natural rubber isolation systems,
it should not be inferred that these are the only types of isolators used.
In fact, most isolation systems use rubber bearings as only part of the
system, combining them with steel bars, lead plugs, or other types of
damping devices. It is my opinion that systems using these additional
elements will eventually become obsolete and the standard isolation
system will be one with only rubber bearings. However, the field is
undergoing rapid changes at the present time and other systems may
emerge.
This text has been written for the structural engineer with a back
ground in structural dynamics and an interest in structural mechanics. I
have included material from structural design codes that the structural
engineer must follow in designing an isolated building, but have not
included any material from the code requirements for rubber bearings
used in bridges. It is my opinion that the bridge bearing codes have little
application to seismic isolation since bridge bearings have a completely
different role to play. Many of the bridge bearing requirements are un
necessary for isolation bearings and if applied would not permit the
engineer to make use of the special characteristics of the elastomer that
make seismic isolation so effective.
Much of the analysis in the text may be applied to other types of
isolation systems and the code requirements covered here apply to all
systems. If this text succeeds in making this new, low-cost and effective
method of providing superior earthquake protection to hospitals and
other critical structures, and additionally, to schools and housing in the
developing world, it will have served its purpose.
Chapter 1
Isolation for Earthquake Resistance
The idea that a building can be protected from the damaging effects of an earth
quake by using some type of support that uncouples it from the ground is an
appealing one, and many mechanisms to produce this result have been proposed
during the last hundred years. Many of these utilized rollers or layers of sand or
similar materials that would allow a building to slide. Some examples of these have
been built. A building in Savastopol, Ukraine and a five-storey school in Mexico
have been built on rollers and there is at least one building in China with a sand
layer between the foundation and the building, specifically intended to let it slide in
an earthquake.
These are examples of an earthquake-resistant design strategy that is referred to
as base isolation or seismic isolation, which is now becoming quite widely accepted
in earthquake-prone regions of the world. There are recent examples of base-isolated
construction in New Zealand, the USA, Italy and, in what is by far the most wide
spread use of the approach, Japan.
The ideas behind the concept of base isolation are quite simple. There are two
basic types of isolation system. The system which has been most widely adopted
in recent years is typified by the use of elastomeric bearings, the elastomer being
predominantly natural rubber. This system works by decoupling the building or
structure from the horizontal components of the earthquake ground motion by
interposing a layer with low horizontal stiffness between the structure and the foun
dation. This layer gives the structure a fundamental frequency that is much lower
than both its fixed-base frequency and the predominant frequencies of the ground
motion. The first dynamic mode of the isolated structure involves deformation only
in the isolation system, the structure above being, to all intents and purposes, rigid.
The higher modes, which produce deformation in the structure, are orthogonal to
the first mode and consequently also to the ground motion. These higher modes do
not participate in the motion so that if there is high energy in the ground motion at
these higher frequencies this energy cannot be transmitted into the structure. The
isolation effect in this type of system is produced not by absorbing the earthquake
energy but by deflecting it through the dynamics of the system. It is worth noting
that this type of isolation system works when the system is linear and even when
undamped. A certain level of damping, however, is beneficial to suppress any poss
ible resonance at the isolation frequency.
2 Isolation for Earthquake Resistance
The first use of rubber for earthquake protection was in an elementary school at
Skopje, in the former Yugoslavia. The building is a three-storey concrete structure
resting on large blocks of natural rubber and was completed in 1969. In contrast
with more recently developed rubber bearings, these blocks are completely unrein
forced so that the weight of the building causes them to bulge sideways. The vertical
stiffness of the system is about the same as the horizontal stiffness so that the build
ing will bounce and rock backwards and forwards in an earthquake. These bearings
were designed at a time when the technology for reinforcing rubber blocks with steel
plates, as in bridge bearings, was not highly developed nor widely known, and it is
unlikely that this approach will be used again.
Almost all recent examples of isolated buildings use multilayer laminated rubber
bearings with steel reinforcing layers as the load-carrying component of the system.
These are very stiff in the vertical direction owing to the presence of the reinforcing
steel plates, but are soft horizontally to produce the isolation effect. They are easy to
manufacture, have no moving parts, are unaffected by time, and are very resistant to
environmental hazards.
Many isolation systems, particularly those in New Zealand and Japan, use a com
bination of natural rubber bearings with low internal damping and some form of
mechanical damper. These have included hydraulic dampers, steel bars, steel coils,
or lead plugs within the isolator. Every type of damper requires mechanical connec
tors and routine maintenance. In addition, the yielding of the metallic dampers
introduces a non-linearity into the response that complicates the analysis of the
dynamic response of the isolated building and reduces the degree of isolation by
causing response in the higher modes that would be unaffected in a linear system.
The ideal method to include damping in an isolation system is to incorporate it in
the elastomer itself and this has been done and used in a few recently completed
buildings in the USA, Japan and Italy. The simplicity of the approach is such that
its use can be expected to spread rapidly. The emphasis here will be on the analysis
and design of isolation systems that use this approach, and other systems that use
mechanical dampers or sliding systems will not be treated in detail.
The second type of isolation system is that using sliders. The basis of this ap
proach is that a low level of friction will limit the transfer of shear across the isola
tion interface. The lower the coefficient of friction, the less shear is transmitted. This
is the earliest and simplest of all the proposed systems but it is not without its draw
backs. To sustain wind load and unnecessary slip under small earthquakes or other
disturbances, a fairly high value of the frictional coefficient is needed. Many fric
tional surfaces have sliding characteristics that are sensitive to pressure and to the
relative velocity of slip, and the fact that the slip process is intrinsically non-linear
means that a proper dynamic analysis must also be non-linear. A further problem is
that the sudden change in the stiffness of the overall structure when slipping or
sticking occurs has the effect of generating high-frequency vibrations in the struc
ture, vibrations at frequencies which may not even be present in the ground motion.
The system responds by transforming low-frequency energy in the ground motion
into high-frequency energy in the structure.
Another problem in using sliders, and only sliders, in an isolation system is that
there is no effective restoring force, and thus the code requirements for the displace
ment will become extremely large. Since this displacement can be in any horizontal
direction, the diameter of the bearing plates and the support system must be very
large; in addition, the superstructure components bearing on the isolators must be
designed for the large moments caused by these large displacements.
Isolation for Earthquake Resistance 3
It is possible to introduce a restoring force capability in several ways. The sliding
bearings can be combined with elastomeric bearings. This combination of sliders
and elastomers was originally proposed by the author in 1978 [1] as a way to make
use of the best features of both types of isolator. The use of sliders gives a system
with a long period, and the rubber bearings control the displacement by providing
a centering section; additionally, they control torsion and if the displacements ex
ceed the design level they will produce a stiffening action. A system of this type is
used in the seismic rehabilitation of a University of Nevada building, the Mackay
School of Mines, in Reno, Nevada [2]. The retrofit of this building is due to be
completed in 1992.
At the present time, retrofit projects constitute a large proportion of the base
isolation projects that are under design or are being proposed in California, and
sliding systems have been proposed for several of these. The use of base isolation for
retrofit generally involves a brittle and weak structure, for example an unreinforced
masonry building or a reinforced-concrete building of early design not including the
type of detailing of the reinforcement that will ensure ductile performance. Base
isolation will lower the force demand on the structural system and impart a certain
degree of energy absorption to the structure. However, if a sliding system is used for
retrofit it is absolutely essential that the force which causes the slider to break be
predictable. If the break -away force increases over years of quiescence, the possibil
ity exists that the structure could be damaged before the isolation system begins to
move. If in an earthquake a weak, brittle structural system begins to deteriorate
above the isolation system, it may never be able to produce the necessary force to
cause the isolation system to start to slide, and the building will act as if it were
unisolated; the mitigating effects of the isolation system will not be achieved, thus
negating the whole point of the retrofit.
Up to 1992, there were at least three retrofit projects that were completed or
under construction: two using rubber bearings with lead plugs and, as mentioned
above, one using the combined high-damping rubber and slider system. Several
other large retrofit projects in the northern California area were in the design phase,
including Oakland City Hall, Hayward City Hall and San Francisco City Hall. Two
US Government buildings in San Francisco are to be retrofitted with base isolation,
and at least one office building of the State of California.
The first base-isolated building to be built in the USA was a legal services center
for the County of San Bernardino, the Foothill Communities Law and Justice Cen
ter (FCLJC). It is located in the city of Rancho Cucamonga, roughly 96 km (60
miles) east of downtown Los Angeles. In addition to being the first base-isolated
building in the USA, it is also the first in the world to use bearings made with
high-damping natural rubber. The building is large (Fig. 1.1), approximately 15300
m2 (170000 ft2), and had a construction cost of 38 million US dollars. It is four
storeys high with a full basement. The isolators are in a special sub-basement (Fig.
1.2) so from the isolation point of view it is five-storeys high. It was designed with
rubber isolators at the request of the County of San Bernardino, as the site of the
building (Fig. 1.3) is only 20 km from the San Andreas fault which is capable of
generating very large earthquakes on its southern branch. This fault runs through
the county and, as a result, the county has had for many years one of the most
thorough earthquake preparedness programs in the USA. The design of the building
was based on an earthquake of a Richter magnitude 8.3. Four high-damping natural
rubber compounds were developed by the Malaysian Rubber Producers' Research
Association (MRPRA) of the UK for this building and are used in 98 isolators [3].