Predicting the Service Life of Sliding Surfaces in Bridge Bearings

Abstract

Bearings are important bridge elements that must be considered when evaluating potential ways to extend overall bridge service life (SL). Many bearing types, including cotton duck pads and high-load multirotational bearings, use sliding surfaces to accommodate bridge movements. Elastomeric bearings have improved durability but with certain combinations of load and movement, the capacity of elastomeric pads to accommodate the required translation can be exceeded. Additional movement capacity can be provided by means of sliding surfaces. Currently, polytetrafluorethylene (PTFE) is the material used for sliding surfaces. However, PTFE wears under service conditions, primarily under fast rates of movement owing to truck load. This paper provides a summary of recommended design steps that could be used to quantitatively predict the SL of sliding surfaces. The method incorporates a process that estimates and compares demand (load) with supply (resistance). The demand is the imposed pressure and horizontal movement on the sliding surface that causes wear, and is primarily the result of applied thermal and traffic loads and the rate of movement. The supply is the sliding material thickness and its capacity to undergo horizontal movements and subsequent wear before failure, and is a function of the sliding material properties. The supply is expressed in terms of a deterioration model based on test data. Finally, by comparing the demand to supply, one could determine whether a given sliding surface could provide the desired SL without failure. Deterioration models are developed for commonly used sliding surfaces such as PTFE, as well as higher-performing sliding surface materials, such as glass-filled reinforced PTFE. These models are based on the results of previous work and new experimental tests, and are used as part of the overall process to predict SL. The recommended design steps are general in nature and could be used for various sliding surface types. An example based on the analysis of a prototype bridge is provided to illustrate the process.