Istanbul Bridge Conference August 11-13, 2014 Istanbul, Turkey SEISMIC ISOLATION CHARACTERISTICS OF BALL RUBBER BEARINGS C. Özkaya 1 ABSTRACT A new type of seismic isolation bearing, ball rubber bearing, has been extensively tested under combined axial load and cyclic lateral loads. Small size steel balls placed in the central hole of a reinforced elastomeric bearing can provide remarkable amount of energy dissipation during cyclic loads. Energy dissipation is mainly developed due to friction and sliding of steel balls during earthquake induced lateral displacement. Performance at cold climate conditions has also been investigated throughout the experimental program. Effect of size of central hole, diameter of steel balls, vertical load level and cross-sectional area of the bearings are documented in identifying the seismic isolation characteristics of the bearing. It has been concluded that the ball rubber bearings can be used as a seismic isolation system in the future. The bearings do not heat up under lateral cyclic loadings and their seismic performance does not degrade after multiple cycles of lateral loads. Keywords: seismic isolation; elastomeric bearing; steel ball; energy dissipation; friction 1 PhD, M.S.C.E., Middle East Technical University, Ankara-Turkey Özkaya C., Seismic Isolation Characteristics of Ball Rubber Bearings. Proceedings of the Istanbul Bridge Conference, 2014.
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Istanbul Bridge Conference August 11-13, 2014
Istanbul, Turkey
SEISMIC ISOLATION
CHARACTERISTICS OF BALL RUBBER
BEARINGS
C. Özkaya 1
ABSTRACT
A new type of seismic isolation bearing, ball rubber bearing, has been extensively tested
under combined axial load and cyclic lateral loads. Small size steel balls placed in the central
hole of a reinforced elastomeric bearing can provide remarkable amount of energy dissipation
during cyclic loads. Energy dissipation is mainly developed due to friction and sliding of
steel balls during earthquake induced lateral displacement. Performance at cold climate
conditions has also been investigated throughout the experimental program. Effect of size of
central hole, diameter of steel balls, vertical load level and cross-sectional area of the
bearings are documented in identifying the seismic isolation characteristics of the bearing. It
has been concluded that the ball rubber bearings can be used as a seismic isolation system in
the future. The bearings do not heat up under lateral cyclic loadings and their seismic
performance does not degrade after multiple cycles of lateral loads.
Keywords: seismic isolation; elastomeric bearing; steel ball; energy dissipation; friction
1 PhD, M.S.C.E., Middle East Technical University, Ankara-Turkey
Özkaya C., Seismic Isolation Characteristics of Ball Rubber Bearings. Proceedings of the Istanbul Bridge
Conference, 2014.
Istanbul Bridge Conference August 11-13, 2014
Istanbul, Turkey
Seismic Isolation Characteristics of Ball Rubber Bearings
C. Özkaya 1
ABSTRACT A new type of seismic isolation bearing, ball rubber bearings, has been extensively tested under combined axial
load and cyclic lateral loads. Small size steel balls placed in the central hole of a reinforced elastomeric bearing
can provide a remarkable amount of energy dissipation during cyclic loads. Energy dissipation is mainly
developed due to the friction and sliding of steel balls during earthquake induced lateral displacement.
Performance at cold climate conditions has also been investigated throughout the experimental program. Effect
of size of central hole, diameter of steel balls, vertical load level and cross-sectional area of the bearings are
documented in identifying the seismic isolation characteristics of the bearing. It has been concluded that the
ball rubber bearings can be used as a seismic isolation system in the future. The bearings do not heat up under
lateral cyclic loadings and their seismic performance does not degrade after multiple cycles of lateral loads.
Keywords: seismic isolation; elastomeric bearing; steel ball; energy dissipation; friction
Introduction
Over the last couple of decades, use of seismic isolation technology has become very popular
in new design or seismic retrofit of important structures such as bridges, viaducts, buildings,
nuclear power plants, museums and historical structures [1, 2]. In general, application of
seismic isolation system to structures results in shift in seismic response of structure in such a
way that expected seismic damage is minimized or eliminated. Use of such a system in
various structures is very important to maintain the rescue operations in the aftermath of an
earthquake. Over the years different types of seismic isolation systems are invented and used
for the purposes described above.
The seismic isolators can be categorized into rubber and sliding types. The most
common rubber isolators are; elastomeric bearings, lead rubber bearings and high damping
rubber bearings [3]. The most common sliding isolators are; those with a curved sliding
surface such as friction pendulum bearings and those with a flat-sliding surface such as
Eradiquake bearings [4].
1 PhD, M.S.C.E., Middle East Technical University, Ankara-Turkey
Özkaya C., Seismic Isolation Characteristics of Ball Rubber Bearings. Proceedings of the Istanbul Bridge
Conference, 2014.
Research Significance
Two popular seismic isolation systems utilizes different source of energy dissipation
mechanism; one being friction and the other one being material characteristic inherent
dissipation. At the meantime, seismic isolation systems are still questioned on cost, heat
generated during energy dissipation, durability and maintenance related issues. In this
research, a new type of rubber based seismic isolation bearing that combines material
inherent and friction based energy dissipation mechanisms in one single bearing is developed.
This new type of bearing maintains adequate energy dissipation capacity without significant
degradation, as the number of loading cycles increases.
Test Set-up
Test equipment has following properties: Load capacities of the vertical (C) and horizontal
jacks (D) are 3000 kN (674.4 kips) and 500 kN (112.4 kips), respectively [5]. Hydraulic jacks
in both directions are resistant to a pressure of 300 bars (30000 kN/m2 (4.35 ksi)). In the
vertical direction, total stroke is limited to 150 mm (5.91 inch) while in the horizontal
direction the limit is larger, 350 mm (13.78 inch). A horizontal load of 200 kN (44.96 kips)
is taken as the practical test limit due to resistance of connections.
Test Program
The maximum velocity attained in the test setup is 70 mm/sec (2.76 inch/sec). In tests, test
velocity, number of cycles and maximum horizontal displacement demand are input into the
computer program of test machine. The bearings are compressed to a certain level of vertical
load. Once these parameters are set and test is initiated, cycles succeed each other
automatically. Test can be stopped manually when an unexpected condition occurs. The test
data can be monitored during the test through a computerized data acquisition system.
In reversed horizontal loading tests, generally, cycles with constant displacement
amplitude are utilized. In addition to successive tests with different displacement amplitudes,
some incremental amplitude tests are performed using manual control to check the
performance of isolation system at different displacement amplitudes.
In general, eight cycles are applied to the test bearings in order to observe possible
degradations in response. Stability and survivability of the bearings are verified by using the
same set of bearings in multiple tests.
Test Bearings
Geometric details of test bearings are presented in Figure 1 for a test bearing with an internal
hole diameter of 100mm (3.94 inch). Different internal hole diameters have been investigated
to study the effects of variations in total volume of granular material and shape factor on
bearing response. The internal hole cap is designed to be thicker than the anchor plate of
elastomeric part to pre-compress the granular material when the cap is closed.
Fig. 1- Dimensions of the test bearings for D/d=3.0
Shape factor, a ratio of loaded area to bulge-free area, of the tested bearings are kept
intentionally low in the design stage to increase the ratio of the vertical compressive load
shared by the central core and hence, to benefit from the increase in friction during internal
sliding of the steel balls.
Bearings are designed with 1/3 scaled loads of a standard highway bridge with
simply supported precast prestressed I-girders having a length of 28.5 meters (93.5 ft).
Design of the bearing is performed based on the requirements of AASHTO specifications [6,
7].
Very low shape factors of unfilled elastomeric bearing (EB) result in significantly
high compressive strains under even at moderate levels of vertical compression. The unfilled
EB is not recommended to be used at high vertical loads, while the contribution of granular
material is expected to reduce the compressive stresses in the elastomeric part that can
improve the stability of the bearing.
Horizontal force-displacement curves of bi-linear isolation systems are as shown in