Define the Shock Transmission Unit?

Ever since engineers have started designing multi span simply supported bridges, they have been dreaming of connecting all the spans together to distribute seismic loads to more than one pier during an earthquake or other sudden loadings. Thus, the peaks due to sudden force is considerably reduced. One possible solution to this requirement could be continuous construction of bridges so that a large number of piers can participate in load sharing during earthquake. But the requirement of size of expansion joint in such an arrangement would be extraordinarily large and thus not feasible. The other solution has been provided by Shock Transmission Unit (STU) or Lock Up Device (LUD).

It is a simple device which is designed to be connected between superstructure and substructure of a bridge to form a temporary rigid link. The peculiar feature of this device is that it forms a rigid link only under rapidly applied loads such as braking and seismic forces whereas under slowly applied loads such as temperature, creep, shrinkage etc, it allows the free movement. STU, thus, allows load sharing in case of suddenly applied short duration horizontal loads. The unit is connected near the bearing between the superstructure and substructure. It works on principle that rapid passage of viscous fluid through a narrow gap generates considerable resistance while slow passage through the same gap only minor resistance.

In this figure e₁ and e₂ are the movements accommodated in STU due to superstructure and substructure respectively.