Seismic isolation bearing having a tension damping device

The present invention is directed to a seismic isolation bearing having an ultimate translational distance and capacity for supporting an upper structure on a lower structure, comprising a load bearing core for absorbing forces which causes a horizontal sliding movement between the upper structure and the lower structure, an upper load bearing plate positioned on top of the uppermost load bearing body and a lower load bearing plate positioned beneath the lowermost load bearing body. The bearing further includes a tension damping device comprising a matrix material and a reinforcement material disposed therewithin and having an activation point and range. Each tension damping device is adapted to be connected between the upper load bearing plate and the lower load bearing plate for damping and stiffening the sliding movement between the upper and lower structures.

FIELD OF THE INVENTION 
The present invention relates to a seismic isolation bearing and more 
particularly to an improved seismic isolation bearing comprising a damping 
and stiffening tension device for use on structures which are subjected to 
seismic forces. 
BACKGROUND OF THE INVENTION 
Translation type isolation bearings, or bearings that are relatively rigid 
in one direction and relatively flexible in the two orthogonal directions, 
consist of at least a core of horizontally sliding or translating surfaces 
or materials that reduce the transmitted vibration energy. At the same 
time, the core is also capable of carrying the vertical gravity loads of 
the structure. 
In the case of seismic isolation bearings, movement at the base of the 
bearing is associated with earthquake ground motion. The maximum possible 
translation that might be imposed on the isolation devices is not well 
understood because there is disagreement on the maximum potential ground 
motions. It is believed that the worst case ground motion would be 
adjacent to the terminating end of a slipping fault line. But incomplete 
understanding of the potential effects of geologic factors, such as fault 
size and type, as well as limited localized information, such as fault 
locations or soft soil conditions, reduce the accuracy of maximum bearing 
displacement estimates at a specific site. Therefore, it is considered 
prudent to guard against unexpected bearing failure, caused by 
unexpectedly high bearing displacements, that may lead to sudden loss of 
structure support. 
At present, control of maximum bearing displacements is accomplished by 
utilizing a variety of means, including: (1) designing overly stiff 
bearings; (2) relying on increasing elastomer stiffness with strain; (3) 
adding hydraulic type dampers or pistons in parallel with the bearings; 
(4) providing metal chains, cables or rods to stop the bearings; (5) 
providing auxiliary friction sliding devices that provide increasing 
friction resistance with bearing translation; and, (6) providing 
stabilizing columns that "catch" the building if the bearings should fail. 
Representative bearings having maximum bearing displacement control include 
U.S. Pat. Nos. 4,910,930 (Way) and 5,014,474 (Fyfe et al.). Way discloses 
a seismic isolation structure comprised of a high damping elastomeric 
bearing and a restraint means mounted between a building's footings and 
support columns. This restraint means is comprised of a curled steel rod 
located outside the bearing core. Fyfe discloses an apparatus having a low 
friction elastomeric load bearing pad disposed between an upper and lower 
load bearing plate and a freely disposed restraining means, such as a 
steel cable or chain, in an axial bore through the center of an 
elastomeric bearing and attached to the upper and lower load bearing 
plates. 
Although capable of handling unexpected inputs, conventional displacement 
controlled bearings do not exhibit an optimized design capable of handling 
expected inputs. In the case of a displacement controlled seismic 
isolation bearing, an optimum bearing design would entail one in which 
structure accelerations are reduced as much as possible for typical, 
expected ground motions ranging in scale from Richter Magnitude 5 to 7, 
and failure is prevented in extreme, unexpected ground motions, or those 
greater than Richter Magnitude 8. 
Bearings having displacement control, to date, are deficient in their 
design and function for a number of reasons, including: (1) overly stiff 
bearing cores reduce isolation effectiveness in the expected input range; 
(2) elastomer stiffness increase with strain is too slow to prevent 
displacement bearing failure; (3) hydraulic or similar viscous systems are 
expensive and overly stiffen the bearing, reducing its effectiveness; (4) 
friction sliding devices tend to be unreliable because they depend on 
long-term, consistent stick-slip action at the sliding surfaces and are 
sensitive to normal or vertical force which varies in a dynamic and 
complicated way; and (5) steel cables, chains, springs or rods generate a 
sudden impact load on the structure when they become taut and typically do 
not return to their initial configuration after becoming taut. 
SUMMARY OF THE INVENTION 
In view of the aforesaid drawbacks of the conventional displacement 
controlled seismic isolation bearings, it is an object of the present 
invention to provide an improved seismic isolation bearing capable of 
being optimized for expected inputs, while at the same time remaining 
protected from failure caused by the extreme inputs. This effect is 
achieved through employing a tension damping device that gradually imparts 
an increased stiffness and damping to the bearing assembly when required, 
but otherwise does not affect the operational characteristics of the 
bearing. 
Accordingly, the present invention is directed to a seismic isolation 
bearing having an ultimate translational length and capacity for 
supporting an upper structure on a lower structure, comprising a load 
bearing core for absorbing forces which causes a horizontal sliding 
movement between the upper structure and the lower structure, an upper 
load bearing plate positioned on top of the uppermost load bearing body 
and a lower load bearing plate positioned beneath the lowermost load 
bearing body. The bearing further includes a tension damping device 
(referred to herein as a TDD) comprised of a matrix material and 
reinforcing material disposed therewithin and having an activation point 
and range. Each tension damping device is adapted to be connected between 
the upper load bearing plate and the lower load bearing plate for damping 
and stiffening the sliding movement between the upper and lower 
structures. 
The TDD employed herein exhibits the following characteristics; high 
strength, adjustable activation points and ranges, and a gradual and 
smoothly increasing stiffness and energy damping. Furthermore, the TDD 
controls the maximum displacement but does not affect actual bearing 
behavior prior to becoming active, thus allowing the bearing to be 
optimized for expected inputs while the gradual stiffening and damping 
effect may be relied on to prevent bearing failure in unexpected events. 
An example of such bearings are seismic isolation bearings which are 
preferably optimized for seismic ground motions less than approximately a 
Richter Magnitude of 7 to 8, but which may be subjected to larger ground 
motions in some circumstances, i.e. those seismic motions adjacent to 
fault lines. 
Generally, employing tension damping devices as the means of displacement 
control in the inventive seismic isolation bearings provides the following 
advantages over employing the aforementioned conventional displacement 
control means: 1) the TDD gradually stiffens and thereby avoids 
transmitting sudden impacts as may occur with cable, rod or chains; 2) the 
TDD provides damping energy losses; 3) the TDD has a recoverable and 
repeatable stiffening factor; 4) the TDD exhibits an adjustable activation 
point and range; 4) the TDD behavior is more predictable because it is 
based on the simple operational principal of axial force; 5) the TDD is 
more easily tested, installed, serviced and detached.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 generally illustrates a seismic isolation bearing embodying the 
present invention, having an ultimate translational distance and capacity 
for supporting an upper structure on a lower structure (both not shown). 
Bearing 10 comprises a load bearing core 11 for absorbing forces which 
causes a horizontal sliding movement between the upper structure and the 
lower structure. Structure as referred to herein includes buildings, 
bridges and equipment as well as other structures and their components, 
which are subject to seismic vibrations or the like. Bearing 10 further 
includes an upper load bearing plate 12 positioned on the top surface of 
the load bearing core and a lower load bearing plate 13 positioned on the 
bottom surface of the load bearing core. Lastly, bearing 10 includes at 
least two, tension damping devices (TDD) 14 comprised of a matrix material 
and reinforcing material disposed therewithin and having an activation 
point and range. Each TDD is connected to, and between, upper load bearing 
body 12 and lower load bearing body 13 by a suitable manner. In the 
embodiment illustrated therein TTD's exhibit an arcuate shape. 
In general, the TDD's may vary in length, diameter, composition, number, 
and in their attachment to the bearing core in order to optimize the 
behavior of a particular isolation bearing. 
Specifically, any TDD should exhibit a length whereby the activation of the 
TDD (its activation point) occurs at some point prior to the bearing core 
reaching it's ultimate translation capacity at which point the bearing 
core begins to fail or become unstable due to either tearing or buckling. 
It is at this activation point when the TDD begins providing its 
characteristic stiffening and damping. 
The thickness and composition of a TDD are selected depending on the 
required strength and stiffness for the desired bearing application. For 
higher strength TDD's a thicker TDD, with an increased amount of, or with 
a stronger, reinforcing material is selected. For stiffer TDD's, a stiffer 
matrix or reinforcing material is used and/or the wrapping angle of the 
reinforcing material is changed. 
The matrix material employed in the TDD may include any natural and or 
synthetic rubbers which are capable of being cured or vulcanized utilizing 
the combination of heat and suitable vulcanizing agents and which exhibit 
characteristic properties which will make it suitable for use as a tension 
damping device in an isolation bearing application. 
Representative matrix or elastomeric materials include common rubbers such 
as ethylene-propylene rubber, nitrile rubber, butyl rubber, halogenated 
butyl rubber, chloroprene rubber, natural rubber, isoprene rubber, 
styrene-butadiene rubber, butadiene rubber, acrylic rubber, ethylene-vinyl 
acetate rubber and polyurethane rubber, special rubber such as epoxied 
natural, fluororubber, ethylene-acrylic rubber, polyester elastomer, 
epichlorohydrine rubber, and chlorinated polyethylene. These rubber 
materials may be used alone or in combination with one another. In 
addition, they may be incorporated with additives such as thermoplastic 
and thermosetting resins, filler, tackifier, slip agent, antioxidant, 
antiozident, plasticizer, softening agent, low molecular weight polymer, 
fire retardant and oil which are commonly used for plastic processing to 
impart desired hardness, loss characteristics, and durability according to 
the object of use. 
Generally, the reinforcing material utilized can be any material which 
exhibits at least an elastic modulus and strength of at least 5 times 
greater than that exhibited by the surrounding matrix material. Metal 
reinforcing materials include ferrous and non ferrous metals and alloys 
thereof such as iron, copper, aluminum, brass, tin and similar metals. 
Preferably, a high strength, cold drawn steel may used for the reinforcing 
material because of its characteristic high strength and stiffness. 
Non-metal reinforcing materials may include, kevlar, nylon, fiberglass, 
polypropylene, orlon or rayon. 
FIG. 2 illustrates one embodiment of the bearing 10 comprising a metal 
elastomer laminate core 11 exhibiting alternately layered metal 15 and 
elastomer 16 substrates having a layer of adhesive 17 disposed 
therebetween. Tension damping device 14 is adapted to be attached to the 
upper and lower load bearing plates. The metal substrates preferably 
exhibit a uniform thickness of between about 0.02 to 0.5 inches, while the 
elastomer substrates preferably exhibit a thickness between 0.05 to 2.0 
inches thick. Any room-temperature curing adhesive exhibiting the 
following requisite properties will be suitable--a thixotropic, trowelable 
viscous paste with a working life of at least about 60 minutes, an 
elongation at failure of approximately 2-15%, a tensile strength of at 
least 3,000 psi and an adhesive strength of at least 1000 psi. In other 
words, many commercially available structural epoxy adhesives may be 
suitable. 
Referring now to FIG. 3 illustrated therein is one embodiment of attaching 
the tension damping device; the TDD 14 is attached to an upper pivot 18 
and a lower bearing pivot 19 which are attached directly to the upper load 
bearing body 12 and the lower load bearing body 13, respectively. 
Referring now to FIG. 4 illustrated therein is a more detailed embodiment 
of another manner of attachment. Generally, the TDD is attached via upper 
20 and lower joints 21 that are free to pivot about at least two axes. A 
metal fitting device 22 is attached to each end of the TDD. Metal fitting 
devices 22 include external collars 23 which fit tightly around and is 
crimped to the each end of the TDD 14. Additionally, the metal fittings 
include stems 24 which extend past the end of the element, typically 
several inches. The actual joints 20 and 21 comprise a bored hole in the 
stems 24, as well as each of the upper 18 and lower 19 pivots; each hole 
adapted to receive a cross pin bolt 24a which pivotably secures the TDD 14 
in place. Lastly, the upper 18 and lower 19 pivots are aligned vertically 
one above the other and are secured or anchored within the upper 12 and 
lower 13 load bearing plates, respectively, in a suitable manner, e.g., 
through the use of a threaded rod 25. 
The advantage of this type of pivoting joint is that it allows initial 
single curvature geometry for easy installation of the TDD, as well as 
providing for a smooth transition from an initial arcuate shape to 
straight line as the element becomes activated as illustrated by FIG. 5. 
Furthermore, kinks in the TDD are avoided and the force applied to the TDD 
and the fitting is a simple axial force. 
Referring again to FIG. 5 and to FIG. 5A, activation of any TDD occurs when 
its pivot-to-pivot length becomes equal to the hypotenuse of a triangle L 
defined by the pivot-to-pivot height H and predetermined activation point 
or distance length D. In other words, an activation distance which is less 
than the ultimate translation capacity of the core is first chosen. For a 
given pivot-to-pivot height H, the required pivot-to-pivot length of the 
tension damping device is then calculated using the triangular 
relationship described above. 
In one embodiment, the TDD is a cylindrical body; more preferably, a hollow 
cylindrically body. The cylinder should exhibit an external diameter 
between about 0.5 inches and 4.0 inches and an inner diameter between 
about 0.25 inches and 3.5 inches. Referring now to FIG. 6 illustrated 
therein is a partial view of a seismic isolation bearing 10 having 
attached thereto another embodiment of a hollow cylindrical body shaped 
TDD. TDD 14 is comprised of two hollow cylindrical matrix material bodies 
26 and 27 having different diameters and a reinforcing material 28 
disposed between an inner circumferential surface 29 of the larger hollow 
cylinder 26 and the outer circumferential portion 30 of the smaller hollow 
cylinder 27; see also FIG. 7. 
The advantage of disposing the reinforcing material between the matrix 
material cylinders is that it provides protection for the reinforcing 
material, as well as providing for a maximum bonding surface area between 
the reinforcing material and the surrounding matrix material. Multiple 
layers of reinforcing and matrix material are contemplated and may be 
included within the cylinder geometry. 
In another embodiment the reinforcing material is disposed on an inner 
circumferential surface of the cylindrical body with optionally, the 
hollow center being filled with additional energy damping material. 
Alternatively, the reinforcing material may also be disposed on the 
exterior of the matrix material cylinder. 
The reinforcing material may take the form of spirals or braids of 
individual wires which are embedded in and bonded to the matrix material 
such that they at least partially move with the matrix material when the 
TDD is stretched. 
The reinforcing braids or spirals are comprised preferably of metal wire 
between 0.004 and 0.06 inches diameter, and more preferably high-strength, 
cold drawn wire between 0.01 and 0.02 inches diameter, as well as, brass 
coated wires because of their superior elastomer bond characteristics. 
In another embodiment of the present invention, the tension damping device 
may be pre-tensioned during installation so as to exert an initial 
compressive force on the bearing. 
Illustrated in FIGS. 8 and 8a are two further embodiments of the seismic 
isolation bearing of the present invention; see also FIG. 9 which is an 
elevational view of the bearing embodiment of FIG. 8. The bearing 10, in 
either embodiment, is similar to that bearing in FIG. 1 and utilizes the 
universal joint attachment 20 embodiment detailed in FIG. 4, except that 
the bearings comprise a plurality of symmetrically arranged tension 
damping devices 14 each comprising a pair of cylindrical bodies attached 
to opposing sides of the upper 18 and lower 19 bearing pivots. Hence, like 
parts for FIG. 8 and 8a are identified with the same reference numerals 
used for the parts of the bearings detailed in FIGS. 1 and 4. The strength 
and stiffness of the actual bearing may be increased by increasing the 
number of TDD's. Although, the arrangement of TDD's is preferably 
symmetric, it may be asymmetric in some cases. If the pivots at each end 
of the TDD are offset horizontally, as illustrated in FIG. 8a, the bearing 
will then exhibit varying stiffening and damping properties depending on 
the direction of translation. 
Referring now to FIG. 10, the tension damping device may, alternatively, be 
pivotably attached directly to the structure base 41 at one end, and 
directly to a point on the structure 42 at the other, separate and apart 
from the bearing core 11. All other operational characteristics of the 
tension damping assembly are the same as previously discussed, therefore, 
like parts as detailed above are identified with the same reference 
numerals used for the parts of the bearings and the TDD. 
The function of the any TDD is controlled by following eight desirable 
operating characteristics: 
Firstly, the ultimate strength or maximum force in the element before 
failure may be adapted to suit a wide variety of bearings and preferably 
ranges between 500 to 50,000 pounds. 
Second, the maximum elongation in the element at the maximum force, i.e., 
the maximum elongation, may also be adapted to suit a wide variety of 
bearings; preferably in the range of 10 to 20 percent. 
Third, a variable activation point as detailed above and referred to in 
relation to FIG. 5. Prior to reaching the activation point the TDD's 
extend and rotate without contributing any appreciable restraining force 
on the bearings. For example, if the elements are installed without any 
curvature or, in other words, as straight vertical elements, the TDD arc 
length is the height of the bearing, and they become active as soon the 
bearing is subject to any shear displacement. The initial activation 
distance in this case is 0. If the tension damping devices are installed 
as semi-circular arcs, the TDD's arc length is about 3 times the 
pivot-pivot height H and the activation distance D is about 2.8 times the 
pivot-to-pivot height. Distance may also be expressed as a percent of 
bearing elastomer height and is then referred to as strain. 
Fourth, the activation range or the distance between the activation point 
and the point at which the TDD itself has reached its ultimate elongation. 
The active range is related to the maximum elongation and bearing height 
and may vary between 10 and 200 percent of the bearing shear strain 
Fifth, a smoothly and continuously characteristic, after engagement, as 
detailed in FIG. 11. The TDD in general shows three different regions of 
behavior: (1) relatively flexible response to loading 35, followed by; (2) 
a relatively stiff response 36; and thereafter, (3) an unloading response 
similar to the load response but offset. The area between the offset 
curves is damping energy. The amount of damping energy per cycle may be 
expressed as an equivalent amount of viscous damping and as a percent of 
critical damping. Damping values vary between 5 and 50 percent. Based on 
the ultimate elongation, the rate of stiffness change may be represented 
by a stiffening ratio, defined as the change in force during the last half 
of element elongation, divided by the change in force during the first 
half of the element elongation. The stiffening ratio may vary between 2 
and 50. 
Sixth, the tension damping devices have a substantially recoverable 
operational range, defined by recovery within a time span of one minute. 
However, elongation of the elements past a certain point will result in 
some unrecoverable deformations. The ratio of TDD length increase at the 
maximum elastic elongation, divided by the original unstretched length is 
referred to as the elastic elongation. These values may be in the range of 
20 to 150 percent. 
Seventh, the elongation characteristics of the TDD rely in part on 
contractions around the hollow or filled core. As the cylinder elongates 
its diameter decreases. The contraction ratio is the ratio of the change 
in the outside diameter of the TDD at ultimate elongation, compared to the 
unactivated outside TDD diameter, divided by the unactivated outside TDD 
diameter. Contraction ratios may vary between about 0.1 and 0.8 
Lastly, as the tension damping devices are stretched and relaxed, their 
internal forces are time and path dependent. 
The inventive bearing described herein having as its means for displacement 
control the TDD, improves on previous displacement controlled bearings as 
follows: 1) the TDD gradually stiffens thereby avoiding transmitting 
sudden impacts as may occur with cable, rod or chain means; 2) the TDD 
provides damping energy losses; 3) the TDD has a recoverable and 
repeatable stiffening factor; 4) the TDD has an adjustable activation 
point and range; 5) the TDD behavior is more predictable because it is 
based on the simple operational principal of axial force; 6) the TDD may 
be easily tested, installed, serviced and detached. 
The suitability of the present invention is hereinafter illustrated by way 
of an Example. However, the present invention is not restricted to this 
example and the following non-limiting example is presented to more fully 
illustrate the invention. 
EXAMPLE 
This example illustrates the application of composite restraints to 
elastomeric bearings for a building designed using guidelines of the 
Uniform Building Code 1994 edition 
Near-field seismic ground motions may contain a few large displacement 
pulses. For near-field type ground motion the objective is to insure that 
the isolation bearing continues to support the vertical load by preventing 
shear displacements greater than approximately 3/4 of the bearing 
diameter. However, for a given earthquake, these pulses may affect only a 
few percent of the total affected buildings. For more typical oscillating 
ground motion, which affects the majority of buildings affected by an 
earthquake and which is prevalent at distances greater than about 2 miles 
from the fault, the objective is to reduce structure accelerations as much 
as possible by providing the most flexible isolation bearing. An optimum 
isolation design as detailed below can be achieved by designing the 
bearing for maximum flexibility and using tension damping devices to limit 
the maximum displacement. 
A 2 foot diameter elastomeric bearing was designed for average expected 
column loads of 240 kips and an earthquake from a nearby fault expected to 
generate an seismic force greater than Richter Magnitude 8. The target 
period for the isolation system was 3 seconds and the maximum design 
displacement was 18 inches. Sixteen layers of 3/8 inch thick, 40 psi shear 
stiffness elastomer, bonded to 0.10 inch thick metal layers made up the 
bearing core with the desired vertical and horizontal stiffness. 
High pressure (10,000 psi) hydraulic hoses were utilized as the tension 
damping devices for this application. The composite tubes or cylinders 
were elastomer materials bonded, during elastomer cure, to embedded 
continuous metal wires arranged in 4 spiral layers; wire diameters were 
approximately 17 mils. 
Eight TDD's each comprising a pair of the aforementioned composite 
cylinders and having a combined ultimate strength of 180 kips, which, 
together with the bearing core combined to exhibit a lateral force of 50 
kips at eighteen inches displacement, provided a total lateral bearing 
resisting force of 220 kips or approximately 100% of the bearing vertical 
load. The arc length of the TDD was 10 inches and the total length of the 
tension damping assembly, including fittings, was 18 inches. The TDD 
activation range was between 14 and 18 inches or between 230 and 300 
percent shear strain. Other tension damping device characteristics 
included: 1) an ultimate elongation of approximately 40 percent; 2) a 
stiffening ratio of approximately 10; 3) a damping of approximately 20 
percent; 4) an elastic elongation of approximately 30 percent; and, 5) a 
contraction ratio of approximately 2. 
A metal fitting was attached to each end of the TDD consisting of a 
circular shaft that fit tightly into an annular space in the end of the 
cable, and projected several inches past the end of the TDD. An external 
collar was crimped over the outside of the TDD at each end. The elastomer 
was stripped from the wires within the fitting to strengthen the 
connection against pull-out. 
Referring now to FIG. 12, it is seen that a conventional bearing having no 
associated TDD and subjected to the shear force greater than 50 kips 
exhibited bearing translations past 18 inches which ultimately would lead 
to bearing instability. 
Referring now to FIG. 13, it is seen that the behavior of the bearing, with 
associated TDD's, outside of the activation range is unaffected. 
Specifically, a force of over 200 kips is required to advance this 
inventive bearing, as detailed in the Example above, past 18 inches. 
Furthermore, damping energy loss is increased. These data demonstrate that 
the TDD's are effective in optimizing the performance of seismic isolation 
bearings. 
It will be appreciated from the foregoing description that although the 
preceding detailed description of the invention is focused principally on 
such seismic metal/elastomer isolation bearings designed for use in high 
shear environments, it is contemplated that the description relates to all 
translating isolating bearings. As such, the description herein relates to 
equipment anti-vibration bearings, and impact reducing bearings, as well, 
e.g., expansion bearings for thermal movement in large structures, 
expansion bearings for isolation of structures from environmental 
vibrations other than earthquakes and equipment vibration isolation 
bearings. Lastly, although certain preferred embodiments have been shown 
and described in detail, it should be understood that various changes and 
modifications may be made without departing from the scope of the appended 
claims.