Performance of Thermal Expansion Bridge Bearings under Seismic Loading

Elastomeric bridge bearings have a history of use in the USA of close to 50 years. They are capable of carrying large vertical loads and allowing horizontal movement with low lateral force due to the extreme flexibility of the elastomeric material. They require little maintenance, have no moving parts, have good longevity and are very economical. The elastomer in bridge bearings is usually either polyisoprene (Natural Rubber) or polychloroprene (Neoprene). Thermal expansion bearings are designed for a variety of deformations produced by thermal expansion of the bridge superstructure, shrinkage due to aging or prestress, misalignment or beam rotation through a process of shear deformation, the material being essentially incompressible. Under currently accepted design practice, the maximum shear strain developed in the elastomer due to all sources of deformation is not more than about 50%. On the other hand, seismic isolation bearings for bridges and buildings are designed for much larger shear strains, and it is known that under shear loading the shear strain at failure can exceed 400%. This raises the question that bridge-bearing specifications may be excessively conservative.

It is the purpose of this research to examine the behavior of bridge bearings by conducting a series of experiments. The experimental program used the CALTRANS Seismic Response Modification Device (SRMD) Testing System at the University of California at San Diego. The three types covered by the test program are

1) Steel-Reinforced Elastomeric Bearings,
2) PTFE-Elastomeric Bearings, and
3) PTFE-Spherical Bearings.

The test program includes wide-range dynamic tests at velocities and displacements that could be experienced by these bearings but for which they have not been designed.

Application of Elastomeric Bridge Bearings as Low-Cost Seismic Isolators in Highly-Seismic Developing Countries

The recent earthquakes in India, Turkey and South America have once more emphasized the fact that the major loss of life in earthquakes happens when the event occurs in developing countries. Even during relatively moderate earthquakes in areas with poor housing, many people are killed by the collapse of brittle heavy unreinforced masonry or poorly constructed concrete buildings. Modern structural control technologies such as active control or energy dissipation devices can do little to alleviate this, but it is possible that seismic isolation could be adopted to improve the seismic resistance of poor housing and other buildings such as schools and hospitals in developing countries.

The intention of this research is to provide a low-cost lightweight isolation system for housing and public buildings in developing countries.

The theoretical basis of seismic isolation shows that the reduction of seismic loading produced by the isolation system depends primarily on the ratio of the isolation period to the fixed-base period. Since the fixed-base period of a masonry block or brick building may be of the order of 1/10 sec, an isolation period of 1 sec or longer would provide a significant reduction in the seismic loads on the building and would not require a large isolation displacement. For example, the current UBC code for seismic isolation has a formula for minimum isolator displacement which, for a 1.5 second system, would be around 15cm (6in.).

The problem with adopting seismic isolation in developing countries is that conventional isolators are large, heavy, and expensive. Each isolator can weight one ton or more and cost as much as $10,000. To extend this valuable earthquake-resistant strategy to housing and commercial buildings, it is necessary to reduce the weight and cost of the isolators. The primary weight in an isolator is due to the steel reinforcing plates, which are used to provide the vertical stiffness of the rubber-steel composite element. A typical rubber isolator has two thick end-plates (25mm) and 20 thin reinforcing plates (3mm each). The high cost of producing the isolators results from the labor involved in preparing the steel plates and laying-up of the rubber sheets and steel plates for vulcanization bonding in a mold. The steel plates are cut, sand-blasted, acid-cleaned, and then coated with bonding compound. Next, the compounded rubber sheets with the interleaved steel plates are put into a mold and heated under pressure for several hours to complete the manufacturing process. The manufacturing process for conventional isolators has to be done very carefully because the testing requirements in the current codes for seismic isolation require that the isolators be tested prior to use for very extreme loading conditions. The bond between the rubber and the steel reinforcement and between the rubber and the end plates must be very good for the bearing to survive these tests. The effect of a large shear displacement of the isolator is to generate an unbalanced moment which must be equilibrated by tensile stresses. The compression load is carried through the overlap region between top and bottom surfaces and the unbalanced moment is carried by tension stresses in the regions outside the overlap.

The purpose of this research is to suggest that both the weight and the cost of isolators can be reduced by using thinner steel reinforcing plates, no end-plates and no bonding to the support surfaces. Since the demands on the bonds are reduced, a simpler and less expensive manufacturing process can be used. Thermal expansion elastomeric bridge bearing feature all these characteristics.

Thermal expansion bridge bearings in contrast to seismic bearings are much less expensive. The in-service demands on these bearings are, of course, much lower, but the tests I conducted showed that even if displacements of seismic-demand magnitude are applied to them they can deform without damage. The primary reason for this is the fact that the top and bottom surfaces can roll off the support surfaces and no tension stresses are produced. The unbalanced moments are resisted by the vertical load through offset of the force resultants on the top and bottom surfaces.

The bearings as tested in this test series survived very large shear strains comparable to those expected of conventional seismic isolators under seismic loading. However their cost is in the hundreds of dollars as compared to the cost of conventional isolators in the thousands of dollars, and could therefore be used as low-cost seismic isolators in highly-seismic developing countries.