Substructures such as shoulders and stacks of a bridge are connected to the deck by means of devices called restraint devices, so called because of their function of fixing the substructures to the superstructures. These elements are subject to the following actions: those transmitted by the deck (exercise excursions) and those transmitted by the substructures (seismic actions).
In the first case the devices have the function of absorbing the service actions that the deck transmits to the restraining devices and transfer them to the substructures: for this reason they are called support devices. In the second case they have the function of absorbing the seismic actions that the substructures could transmit on them and therefore to the deck: for this reason the name of "anti-seismic devices" is given.
The bridge and viaduct bearings are therefore special constraint devices that allow the displacements caused by various factors such as temperature and seismic activities and facilitate the absorption of translations and rotations due to traffic loads, wind loads, cornering actions etc .
The constraint scheme, that is the arrangement of the bearings for bridges, must be designed in such a way as to allow the transmission of forces unambiguously from the over to the substructure.
There are two large sub-groups of bearings for viaduct: the steel bearing devices and the rubber bearings. The steel bridges bearings group include the POT bearings (also referred to as confined elastomeric disc bearings or encapsulated elastomeric disc bearings).
