ACI 341.3R-07 Seismic Evaluation and Retrofit Techniques for Concrete Bridges.
2.2.5 Hinges and supports—Failures due to unseating of superstructures have, historically, been some of the most common and disastrous (Fig. 2.10). In an effort to minimize nonseismic forces, many bridges were designed with seat widths that were just large enough to accommodate movements due to creep, shrinkage, and temperature changes. For example, to minimize the moment in a dapped end of a hinge, the length of the dapped end would be designed to be as short as possible. Structures supported at hinges and abutments by steel rocker bearings or by elastomeric bearing pads with insufficient displacement capacity are susceptible to unseat completely, which may lead to collapse. These bearings are also prone to instability when subjected to large displacements, which can lead to the superstructure dropping a maximum distance equal to the height of the bearings, leaving the bridge out of service. Even moderate damage, such as damage to the bearings or concrete at the expansion joint, may result in temporary bridge closure (Fig. 2.11). Some bridges built or retrofitted in the 1970s after the San Fernando Earthquake have cable restrainers or high-strength bolts at hinges. Some of these retrofits have insufficient strength or elastic deformation capacity, which can result in failure of the restrainers or diaphragms in a seismic event (Klosek et al. 1995; Ranf and Eberhard 2002; Selna et al. 1989a,b).
3.2—Seismic capacity evaluation To determine the necessity and the extent of a seismic retrofit, both the component and system capacities of the existing system should be estimated. The current design philosophy adopted in the United States for bridges in seismic regions assumes that the earthquake will mobilize the inherent strength and, therefore, a key retrofit design consideration is to ensure the formation of ductile mechanisms (suppress brittle shear and anchorage failure mechanisms) to permit inelastic structural deformations without significant loss of lateral load resistance. This design philosophy requires realistic capacity estimates and comparisons of capacities of local mechanisms within an element with respect to the adjacent elements to ensure a global ductile mechanism. The evaluation of actual member capacities requires that realistic bounds on strength be determined for individual structural components. To guard against brittle modes of failure, conservative estimates of demands should be compared with lower-bound estimates of capacities (ATC 1996). For example, shear demands computed from upper- bound flexural strengths are compared with lower-bound shear capacities. This approach can be applied to assess the seismic vulnerability of existing bridge structures and to determine, if necessary, appropriate retrofit schemes. The key component in a comprehensive capacity-based seismic design approach is adequate evaluation of the component and system behavior under combined gravity and seismic loads. A number of research efforts describe procedures and provide equations for estimating the critical capacities (Priestley 1991; Priestley and Seible 1991; McLean and Marsh 1999; ATC 1996; Priestley et al. 1996; California Department of Transportation 1999a,b; FHWA 2006). A realistic estimate of the component capacities and critical mechanisms of an existing bridge structure may be based on the following steps.ACI 341.3R pdf download.