Thomas Telford's saddle bearings for Menai Suspension Bridge

摘要

The bearings of Menai Suspension Bridge represent a significant development in the understanding of structures and of the mechanism of suspension bridges. Thomas Telford's interest in suspension bridges began with studies for Runcorn in 1814, but at a time when little was known. Although a number of suspension bridges were built prior and were contemporary with Menai and Conwy, few survive and most were of largely intuitive design construction. Telford's research identified that it was necessary for the catenary to transmit load into the back spans and that the load in the catenary would vary depending on the live load on the suspended span. If the saddle were fixed, then the loads would be transmitted into the masonry. It was also acknowledged that changes in temperature would also lead to changes in load. To overcome this problem a means was required that would allow the saddle to move freely. The solution was the use of a series of roller bearings located beneath the chain saddles and fixed to the top of the pyramids, Telford's description for the towers. This paper discusses the design of these bearings, how they were incorporated into the catenary and the towers, reports of the bearings in service and the bearing components that survive in the care of Menai Bridge Community Heritage Trust and displayed in the Menai Heritage exhibition in the Thomas Telford Centre.