Elastomeric Bearings

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Elastomeric Bearings: Plain and Reinforced



Plain, unreinforced elastomeric pads are used for short spans on which loads and movements can be accommodated by a single layer of elastomer. As vertical load and movement requirements increase, thin reinforcing plates are combined with multiple layers of elastomer to form a laminated reinforced elastomeric assembly
. Steel and fiberglass reinforcement layers have been used; however, fiberglass is weaker, more flexible, and does not bond as well to the elastomer as does steel reinforcement. As a result, the use of thin steel-plate reinforcement has become more common.


Neoprene is the most widely used elastomer, but some states also use natural rubber  particularly in colder climates, to meet AASHTO low temperature requirements. Natural rubber generally stiffens less than neoprene at low temperatures. Neoprene has greater resistance to ozone and a wide range of chemicals than natural rubber, making it more suitable for some harsh chemical environments.


Cotton Duck pads


Cotton Duck Pads Cotton duck bearing pads are another type of elastomeric bearings that are occasionally used in some states, typically for precast concrete I-girder bridges with span lengths up to the 150- to 180-ft range. Cotton duck pads (CDPs) are preformed elastomeric pads consisting of very thin layers of elastomer (less than 0.4 mm [1 /60 in.]) interlaid with cotton or polyester fabric. They are stiff and strong in compression, giving them much larger compressive load capacities than plain elastomeric pads; however, CDP shear deflection capability is very limited. The CDP bearings provide a high stiffness in the direction of applied compressive force and are helpful in limiting problems encountered during construction of heavy girders because of rotational instability, generally observed with other elastomeric bearing types. For large shear strain, CDPs may split and crack or result in girder slip on the CDP. The limited shear deflection capacity is frequently overcome by the addition of a polytetrafluorethylene (PTFE) sliding surface to accommodate large movement. When PTFE surfaces are used, they are often combined with stainless steel sliding surfaces, similar to that shown in Figure 10.2. The overall capacities depend on the stiffness and deformation capacity of the CDP and vary from manufacturer to manufacturer. To assure adequate performance from CDP, quality control (QC) testing measures and design recommendations have been developed and incorporated into the LRFD specifications  

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