The seismic evaluation and retrofitting of bridges

Past earthquakes have clearly shown the seismic vulnerability of existing bridges. Despite recent progress by the structural engineering profession in addressing bridge seismic risks, there are still several areas where improvements in bridge evaluation and retrofit practices are needed. The first part of this report reviews the common seismic deficiencies of bridges, procedures and criteria for seismically evaluating bridges, and the engineering techniques which have been used for retrofitting bridge seismic deficiencies. Information on seismic deficiencies, retrofit techniques, and related research has been organized by the author in a concise tabular form. The review indicates several areas where effective retrofit techniques have been established, and other areas where improved procedures or further research are needed. Seismic upgrade measures proposed for Wellington Thorndon bridge, including an innovative retrofit of superstructure linkages, illustrate the benefits of a capacity-design approach to seismic evaluation and retrofitting. The second part of this report describes the laboratory testing and inelastic computer analysis of a 1936-designed bridge which is typical of many of the older, reinforced-concrete, multi-span bridges in New Zealand. The structure has plain-round (undeformed) reinforcing bars and questionable anchorage details, shear strength, and column-transverse reinforcing. Despite the suspected seismic deficiencies, the testing and analysis of the bridge show that its seismic performance will be good. The results indicate that (a) seismic retrofitting is not warranted, (b) code criteria applicable to the design of new structures, with deformed reinforcing, can be overly conservative when used for the assessment of existing structures, and (c) plain-round reinforcing bars under cyclic seismic forces suffer extensive bond deterioration resulting in pinched hysteretic response which, for earthquake inputs with extreme pulses, can lead to greater seismic damage. The third part of the report reviews procedures which have been used to prioritize bridges for seismic upgrading. A new method of managing and prioritizing bridge-upgrade work, developed by the author for New Zealand’s Tasman district, offers several improvements over previously used procedures. The new method is based on cost-benefit and earthquake loss-estimation principles. The method outlines four levels of seismic evaluation and includes a seismic-vulnerability rating system, a flowchart assessment method, and formulas for estimating bridge value and benefit/cost ratios for seismic upgrading. The recommended methods have been implemented on a stock of 445 bridges using a minimum of data, and have also been applied to the in-depth evaluation of retrofit options for the Thorndon bridge