Q.: What do you think of elastomeric bridge pads, used to allow movement of prestressed concrete girders upon their supports? Do they really deform enough to allow the needed movement and do they last as long as machined steel bearing surfaces lubricated with graphite?

A.: As far as we know, the elastomeric pads seem to be actually durable and reliable. A difficulty with steel bearings is that sooner or later, for one reason or another, the bearings stop moving at all and the result is that the ends of the girders crack. Steel bearings were used on all the original bridges of the Illinois Tollway, built mostly in 1957 and 1958. Subsequently, as these bearings began to fail they were replaced by elastomeric pads, which fortunately are cheaper. Gustaferro, Hillier and Janney have reported ("Performance of Prestressed Concrete on the Illinois Tollway After 25 Years of Service," PCI Journal, January/February 1983, pages 50 to 67) that the elastomeric bearing pads seem to have completely solved the problem of so-called frozen bearings.