This may be either monolithic or bearing supported, as shown in Figure 15 (Priestley et al., 1996). Monolithic construction is normally used for slender columns and small bridges. The energy absorption capacity of this configuration is in general larger than bearing supported systems (Elnashai and McClure, 1995) due to the double curvature of the former leading to potentially two plastic hinging zones, instead of one in the case of bearings. For multi-column configurations, use of monolithic systems enables the stiffness in the longitudinal and transverse directions (when using circular and square columns) to be equal, thus eliminating the potential for a preferential response direction. Two more advantages of monolithic construction are that it allows consideration of using a fixed or pinned column-foundation condition (see section on foundation-pier connections) and that it leads to higher redundancy of the lateral response system.
One of the major disadvantages of monolithic deck-pier systems is that large moments are transmitted to the deck that add to the moments from gravity forces thus creating critical conditions there. This is especially true for single-column systems with wide decks, since the effective deck section resisting these high moments is relatively small. Another problem with this type of connection is the difficulties of ensuring that capacity design is respected, in terms of connection overstrength. Large-diameter bars from the pier should be adequately anchored in the relatively shallow cap beam, thus reinforcement congestion may become a problem.
Another problem is the imbalance between the longitudinal and transverse directions for single-pier systems. In the transverse direction, the torsional stiffness of the deck only is acting against the pier behaving as a cantilever, thus the behaviour is close to such a condition. Longitudinally, the deck is monolithic with all piers, hence the restraint it applies on pier heads is much larger than in the transverse direction. Therefore, the pier is normally stiffer longitudinally than transversely. If a balanced design is sought, then a rectangular section will be required.
Thermal expansion imposes large displacement demand on monolithic systems longitudinally and therefore requires short spans between expansion joints. This is an unfavourable feature of monolithic pier-deck construction.
The second option is the provision of bearing support between pier and deck, allowing one or more translational and/or rotational degrees of freedom. The most significant advantage of bearing supports is that the deck is not subjected to seismic forces, hence configurations not amenable to high moment resistance may be utilised. Also, the period of the bridge is elongated as compared to monolithic
bridges. This may be advantageous when the bridge is founded on rock or stiff soil, but is not suitable for soft sites. Another important advantage of bearing supports is that by suitable adjustment of bearing characteristics, stiff bearings may be placed on top of flexible piers and vice versa. Hence, a more uniform distribution of stiffness and strength than the case of monolithic structures would be easy to achieve.
Bearing supports have serious disadvantages, such as the effect of period elongation of the structure in areas of soft site conditions subjected to large distant earthquakes. Also, the bridge is subjected in general to larger displacements than its monolithic counterpart. For multi-column structures, the piers are placed in double curvature transversely while the longitudinal response is that of a cantilever. Since the option of pinned pier-foundation condition is no longer available, footings are subjected to high seismic forces and are susceptible to uplift. Also, due to the existence of one potential plastic hinge in a single-column structure, the ductility demand on the single pier is much higher than that of the overall structure (a global displacement ductility of 5 may correspond to a pier ductility demand of 14; Priestley et al., 1996). Finally, bearing systems may undergo large inelastic displacements that are not restored at the end of the earthquake, hence impairing the use of the bridge. This drawback can be offset by using self-restoring bearing systems.
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