The Elizabeth Bridge—which carries Route 5 over the Little Kanawha River in Wirt County, W.Va.—features neoprene pads in its construction.
Neoprene bearing pads are commonly used in bridges due to their durability, simple details, and maintenance-free qualities. Neoprene pads are a subset of elastomeric bearings that includes both neoprene and natural rubber pads. First fabricated in the 1930s, their use was not widespread until the 1950s when laminated neoprene pads were introduced. In 1958, the American Association of State Highway and Transportation Officials published the first formal specification for elastomeric bearings, and their popularity grew.
Neoprene pads are idea for short- and medium-span bridges with low to moderate vertical reactions and normal movement and rotation demands.
Two types of neoprene pads are commonly used in bridge construction. Plain pads are molded, cut, or extruded pieces of neoprene. Laminated bearings are made of alternating layers of neoprene and steel vulcanized together during the fabrication process. The shape of pads used varies according to bridge movement. Rectangular pads suit applications in which movements and/or rotations occur in only one direction, while structures with movements and/or rotations in two directions warrant circular pads.
Neoprene bearings must facilitate three functions: support dead and live loads, permit longitudinal movements, and accommodate girder rotations.
Load support is controlled by limiting compressive stress. Neoprene is an incompressible material, and vertical strain of the pad results in lateral displacement of the elastomer. Therefore, compressive deflection of the pad is controlled by limiting the thickness of the elastomer layers. As layers become thinner, lateral displacement from compression of the elastomer is reduced.
The shape factor—defined as the elastomer layer plan area divided by the perimeter area—is important in the design of neoprene bearings because compression is carried by shear stress in the elastomer and adds to that stress due to horizontal loads. Thinner elastomer layers and a larger shape factor allow laminated pads to support larger loads and movements than plain pads. Also, compressive deflection of rectangular pads exceeds that of circular pads with the same area and shape factors because the corners of rectangular pads are poorly confined and support little load.
Neoprene pads permit horizontal movement through shear deformation of the elastomer. As horizontal movements increase, the elastomer stiffens. To allow larger movements, the height of the pad or thickness of the elastomer is increased.
Anchorage is another important factor. Depending on the applied loads present, the shear force in the elastomer may exceed the friction force between the bearing and beam, and the beam will slip relative to the pad.
Rotation increases the compressive stress at one edge of the pad and decreases the compressive stress at the other. When anticipating large rotations, the designer must ensure that uplift due to rotation is not greater than the compressive force of the pad. One way to minimize this effect is to ensure the pad is as short as practical in the direction of rotation.
Neoprene pads have numerous advantages over mechanical bearings. They provide functional simplicity because the pads allow movements without any internal moving parts, and they allow multi-directional movement as compared to the uni-directional movement permitted by other bearings. Neoprene pads are easier to detail and fabricate than bearing types that require fabrication from individual parts. Furthermore, neoprene pads provide consistent performance without significant maintenance activities.
Neoprene pads are not the answer for every project. Their compressive stress limits eliminate them from consideration for bridges with large reactions. Also, neoprene pads are capable of limited longitudinal and rotational movements unless other measures are provided to alleviate the stresses in the pads.
— Jim Bintrim, P.E., is a project engineer with HDR Inc. in Weirton, W.Va.