Q. Explain Shear stiffness in elastomeric bearing?
For elastomeric bearing shear stiffness is a significant parameter for design since it influences the force transfer between bridge and its piers. Basically elastomers are flexible under shear deformation though it is relatively stiff in compression. Conversely elastomeric bearings must not be used in tension.
Elastomeric bearing must be designed in serviceability limit state only. Cross sectional area is generally determined by the compressive stress limit under serviceability limit state. The shape factor it implies that the plan area of the laminar layer divided by area of perimeter free to bulge, affects the relation between shear stress and compressive load. Basically higher capacity of bearings can be obtained with higher shape factor.
The long side of bearing is generally oriented parallel to the principle axis of rotation since it facilitates rotational movement. The thickness of bearings is limited and controlled by shear strain requirements. Basically the shear strain must be less than a certain limit to avoid the occurrence of rolling over at edges and delamination because of fatigue. Henceforth it follows that higher rotations and translations need thicker bearing. Instead the vertical stiffness of bearings is attained by inserting sufficient number of steelplates. Additionally checks must be made on combined compression and rotation to guard against possible occurrence of uplifting of corners of bearings under certain load combinations.