Elastomeric Bearing in Bridges
knowledge of experienced bridge engineers.
Basic Design Consideration
For elastomeric bearing, the shear stiffness is an important parameter for design because it influences the force transfer between the bridge and its piers. In essence, elastomers are flexible under shear deformation but it is relatively stiff in compression. However, elastomeric bearings should not be used in tension.
Elastomeric bearing should be designed in serviceability limit state only. The cross sectional area is normally determined by the compressive stress limit under serviceability limit state. The shape factor, i.e. plan area of the laminar layer divided by area of perimeter free to bulge, affects the relation between shear stress and the compressive load. In essence, higher capacity of bearings could be obtained with higher shape factor.
The long side of the bearing is usually oriented parallel to the principle axis of rotation because it facilitates rotational movement. The thickness of bearings is limited and controlled by shear strain requirements. In essence, the shear strain should be less than a certain limit to avoid the occurrence of rolling over at the edges and delamination due to fatigue. Hence, it follows that higher rotations and translations require thicker bearing. On the other hand, the vertical stiffness of bearings is obtained by inserting sufficient number of steel plates. In addition, checks should be made on combined compression and rotation to guard against the possible occurrence of uplifting of corners of bearings under certain load combinations.
Steel Plates Inside the Bearings
In the design of elastomeric bearing, the bearing should be allowed for bulging laterally and the compression stiffness could be increased by limiting the amount of lateral bulging. To increase the compression stiffness of elastomeric bearings, metal plates are inserted. After the
addition of steel plates, the freedom to bulge is restricted and the deflection is reduced when compared with bearings without any steel plates under the same load. Tensile stresses are induced in these steel plates during their action in limiting the bulging of the elastomer. This in turn would limit the thickness of the steel plates.
However, the presence of metal plates does not affect the shear stiffness of the elastomeric bearings.
Elastomeric bearing is normally classified into two types: fixed and free. For fixed types, the bridge deck is permitted only to rotate and the horizontal movements of the deck are restrained. On the other hand, for free types the deck can move horizontally and rotate. To achieve
fixity, dowels are adopted to pass from bridge deck to abutment. Alternatively, in case there is limitation in space, holes are formed in the elastomeric bearings where anchor dowels are inserted through these holes. It is intended to prevent the “walking” of the bearing
during its operation.
Limitation of Elastomeric Bearing
Elastomeric bearing has the potential advantage of low cost when compared with other bearing types. In particular, it requires little long-term maintenance to enhance its performance during servicing. Moreover, it demonstrates good performance in seismic condition because of its relatively large plan areas with low height and the natural dampening effect of elastomer.
In the event of high vertical loads combined with large angle of rotations, rubber bearings are undesirable when compared with pot bearings. For instance, elastomeric bearings require large bearing surfaces so that compression can be maintained between the contact
surfaces between the bearings and piers. Moreover, it also leads to uneven distribution of stress on the piers and some of these highly induced stresses may damage the structure. Consequently, pot bearings are better alternatives than elastomeric bearings in such an scenario.