Seismic evaluation of existing reinforced concrete bridges in Ottawa region

Many existing bridges, especially those built before 1970, were designed with minimum or no seismic considerations. Past experience has shown that such bridges are very vulnerable when subjected to moderate and strong earthquake motions. This was illustrated during the 1989 Loma Prieta, the 1994 Northridge, and the 1995 Kobe earthquakes, when many existing bridge collapsed or were severely damaged. Such a poor performance of the bridges was attributed primarily to the fact that the seismic effects were underestimated in the pre-1970's design practice. In Ottawa region, a great expansion of highway bridges occurred in the 1950's to 1970's, before modern bridge design codes were developed. Statistics show that the number of existing bridges designed according to substandard seismic codes is significantly larger than the number of new, well-designed bridges. Given this, it is essential to develop methods for evaluation and retrofit of existing bridges in order to reduce the risk from seismic actions. In this study, seismic evaluation was conducted to eight bridges located in the Ottawa region and designed according to the pre-1970's bridge codes. In addition, one new bridge, built 1994, was analyzed and was used as a reference case for comparing the performance of older and new bridges. Inelastic models were developed for each bridge and nonlinear dynamic analyses were conducted by using excitation motions compatible with the design spectrum for Ottawa, prescribed by the latest Canadian national code for bridge design. The performance of the bridges was assessed by analyzing the responses represented by curvature ductilities, shear demands, and lateral drifts. The results indicated that the performance of most of the selected bridges is acceptable for the seismic excitations used in this study