In the classical layout, shown Fig. 20 as the left hand bottom corner bearing is restrained in the longitudinal direction, it becomes difficult to estimate the internal forces due to the external lateral loadings.
Proper layout of bearings on a curved span along with estimation of movement is a critical and important activity for the durability of the structure. Due to continuity in superstructure the longitudinal forces due to braking, tractive effect, temperature, creep & shrinkage movement will be transferred to the bearings, not only depending on the shear rating of the bearings but also on the stiffness of the pier and foundation. In case of straight continuous span the distribution of horizontal force can be carried out effectively with relative simple computation. Whereas for curved continuous span the transverse stiffness of deck due to curvature along with the location and direction of movement of bearing becomes an important factor. All stresses in the bearing and as well on the superstructure will be of normal nature and the system can be effectively designed for all loading conditions. The span length and the loading pattern will determine the size of bearing. Similar to that of straight continuous girders, for a stiff system single bearing guided in all directions can be provided on intermediate piers of curved spans.
Fig. 20: Classical & Determinate systems
For straight continuous bridges, normally two bearings are used at abutment and piers location. If the torsional stiffness of the deck is very high like in box girder, intermediate piers can have single bearing (Fig. 21). As shown in the same figure, for curved continuous bridges, the longitudinally guided bearings shall be placed in such a way that the main movement of bearing should be in the axis of the traffic. The movement of other bearings shall have the same angle between the polar line and moving direction.
Fig. 21: Bearing Layout for straight/curved continuous spans
32  Volume 43
Number 4
December 2013
Shock Transmission Units (STU) or Lock Up Devices (LUD) In case of multispan continuous bridges, the horizontal forces due to seismic in longitudinal direction is mainly absorbed by fixed pier as such there is no uniform distribution of horizontal forces among the piers. One way of achieving this is by integral bridge without bearings so that seismic distribution is uniform on all piers. But this gives rise to provision of very large expansion joints which may not be feasible. The shock transmission units designed to connect the superstructure with substructure( Fig. 22) to form a temporary rigid link provides an opportunity to distribute the sudden loads due to seismic and braking etc. uniformly on all piers apart from allowing the movement for slowly induced loads due to temperature, creep and shrinkage. Within the cylinder provided in STU as shown in the Fig: 22, viscous fluid passes from one compartment to another through a small designed passage. Under the sudden load like seismic and braking it gets locked as the
The Bridge and Structural Engineer