International Concrete Abstracts Portal

Elastomeric bearings have long been used for bridge structures to provide thermal movements and girder rotations under service loads and in building structures to isolate them from ambient vibrations. By increasing the total rubber thickness and by inserting a lead plug for energy dissipation, the function of the bearing may be extended to provide seismic isolation of bridges. Seismic isolation is a design concept that reduces earthquake forces by factors of up to 10, thus greatly enhancing the seismic resistance of the supported structure. This paper provides design guidelines to enable lead-rubber bearings to safely and efficiently act as seismic isolators and reviews the mechanical properties of lead-filled elastomeric bearings. Factors affecting the elastic and postelastic properties are listed, and other design-related parameters are described. A set of design procedures to incorporate the lead-rubber bearings for bridge structures is presented. These procedures include the determination of plan size to support gravity loads, the rubber thickness to provide the required flexibility, and size of the lead core to provide energy dissipation. The procedures are applicable for areas of low and high-seismic risk. The application of the procedures is illustrated by use on a seven-span bridge.

PTFE is now the most commonly used sliding material used in bridge bearings throughout the world. However, achieving the full advantages of its use over more traditional materials requires careful detailed design. The design and manufacture of PTFE bridge bearings has become a specialty field. Some critical features in the design of PTFE bridge bearings are described. Experimental work on actual structures is described, as well as the calculations made of the life of PTFE surfaces in bearings and their rotational characteristics. Suggestions for further investigations are made.

The selection of bearings for a structure depends on consideration of a number of factors by designers. The success or failure of the entire structure may depend on the performance of these devices. There are a number of bearing types available on the market. In some situations, only one type of bearing may be appropriate for the particular type of structure, while in other situations the designer has to select from the variety of bearings available. The precise function of bearings will obviously vary between structures and even from one end to another end within a given structure. The bearing selection criteria described in this paper is dedicated to assisting the designer when selecting an appropriate and efficient bearing, to perform all structural functions with the least maintenance during the life of the bridge. Experience has shown that with a careful study of the functions, limitations, and initial and maintenance costs of the bearings, the designer can achieve the most desirable bearing.

The importance of effectively sealed expansion joints in all types of structures has been recognized for a long time. Bridges, buildings, dams, water/sewage treatment facilities, etc., require weatherproofing expansion joints. This task has become even more difficult, particularly in buildings, with the incorporation of seismic considerations and the increased awareness of "life safety" with regard to pyrogenics. In the event of fire, the expansion joints are primary candidates for the rapid spread of fire and toxic/noxious fumes. Protection is of particular importance in hospital/medical facilities and high-rise structures. The need not only to confine the fire to the point of origin, but to inhibit the spread of fumes, is most crucial. It is estimated that 80 to 90 percent of fire victims succumb to inhalation of smoke/noxious/toxic gases well before the flames reach them. In those cases where the heat and flames reach the victims, the first signs of incapacitation result from toxic fumes. The major North American manufacturers of expansion joint sealing systems are developing composite fire resistive/weatherproof expansion joint sealing systems incorporating a noxious fume barrier that will resist the impact of the "hose stream" from fire-fighting equipment.

The state of the art in the design of pot bearings and, in particular, the relationship between the stress condition in the pot and the adjacent structure are overviewed. The difficulties encountered in performing precise calculations are discussed. Safe and simple design criteria are obtained through the analysis of the ultimate state. The most recent research programs on bearing plates, the factors relevant to the stress condition under the distribution plate, and deformations are reported. Formulas for the design of the pot and the distribution plate are given. The transmission of horizontal forces through the bearing is analyzed. The necessary formulas are presented as well as current values for friction between various materials and the strength of typical shear connectors in use. A bibliography covering many aspects of the theoretical and practical research performed on this subject concludes the paper.