Design of Cotton Duck Bridge Bearing Pads

Bearings are used to support loads and accommodate movements in bridges. Cotton duck bearing pads (CDP) offer a versatile and economical solution for bearing design. The CDPs have closely spaced layers of elastomer and fabric which result in large compressive strength and strain capacities. However, the engineering response of CDP has not been well understood and CDP design provisions are incomplete because of this limited knowledge. To better establish the engineering response and develop improved design provisions, an extensive experimental research study was conducted. The experimental program modeled loads and deformations induced on a bridge bearing and included both static and dynamic loading regimes in compression, shear, and rotation. The primary study parameters included pad geometry, pad manufacturer, and stress and strain levels. The results indicate that CDPs have significant compressive stress and deformation capacities. Using the experimental results, design limits to control pad damage and quality assurance provisions are proposed to ensure adequate service during the lifetime of the bridge. Delamination of the top pad layers occurs after many cycles of repeated load or deformation and limits on the maximum stress, stress range, and uplift are proposed to limit this type of damage. Diagonal fracture occurs when a CDP is subjected to large strains. Strict maximum strain limits are proposed to prevent this failure mode. Finally, quality control provisions are proposed to ensure adequate engineering performance of the pads.