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Previous work on experimental and theoretical
studies on fiber-reinforced bearings has shown the
feasibility of using them as lightweight low-cost elastomeric
isolators for application to housing, schools and other public
buildings in highly seismic areas of the developing world. The
theoretical analysis covered the mechanical characteristics of
these bearings where the reinforcing elements, normally steel
plates, are replaced by fiber reinforcement. The
fiber in the fiber-reinforced isolator, in contrast
to the steel in the conventional isolator (which is assumed to be
rigid both in extension and flexure), is assumed to be
flexible in extension, but completely without
flexural rigidity. This leads to an extension of the
theoretical analysis on which the design of steel-reinforced
isolators is based that accommodates the stretching of the
fiber-reinforcement. Several examples of isolators in the
form of long strips were tested at the Earthquake Engineering
Research Center Laboratory.
The theoretical analysis suggests, and the test
results confirmed, that it is possible to produce a
fiber-reinforced strip isolator that matches the behavior
of a steel-reinforced isolator. The fiber-reinforced
isolator is significantly lighter and can be made by a much
less labor-intensive manufacturing process. The advantage of the
strip isolator is that it can be easily used in buildings with
masonry walls.
The main difference between the behavior
of a fiber-reinforced and a steel-reinforced bearing is the
degree of run-in under vertical loading. In this context we mean
by run-in that a certain amount of vertical load must be applied
to the bearing before its vertical stiffness can be
developed.
The most likely source of the run-in is that
the fibers are initially not straight and as they have no
bending stiffness, the vertical stiffness cannot be
developed until they have been straightened by the action of the
applied vertical load. Straightening the fibers requires
them to push against the surrounding rubber. This causes an
increasing force in the fiber, and as it is straightening,
there will be a transition to the stretching of the fiber
and to the consequent stiffness of the composite system.
These bearings can be used in a wide range of applications in
addition to seismic protection of buildings including bridge
bearings and vibration isolation bearings, so there is a need to
be able to predict how much vertical load or vertical
displacement is needed before the full vertical stiffness
can be achieved. In this paper a theoretical analysis of the
effect has been developed in an attempt to formulate a
prediction for the transition from the initially bent to the
finally straight fiber.
The method takes the already formulated
analysis for the straight fiber and modifies it by
treating the fiber as a curved string on an elastic
foundation, adds to this an estimate of the subgrade reaction of
this foundation, and, using the basic equations of the
fiber-rubber composite, calculates the effective
compression modulus as a function of the vertical compression
strain or pressure.
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