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SUMMARY PERFORMANCE-BASED SEISMIC BRIDGE DESIGN Currently, bridge seismic design specifications in the United States are based on prescrip- tive design methodologies that only marginally relate important design parameters to the performance of a bridge during an earthquake. With the current specifications, the designer does not directly control the seismic performance of bridges. This methodology has served the bridge community reasonably well, but techniques are being refined that would permit the designer, with appropriate owner input, to select and instill desired seismic perfor- mance into new bridges, and to some extent into retrofitted bridges. This new methodology is called performance-based seismic design (PBSD), and although it includes some features of the current design approaches it extends those features to a level at which designers and owners can make informed decisions about seismic performance. Such features include the ability to consider different earthquake inputs, or seismic hazard levels, and different operational classifications, such as bridges that have designated functions required after an earthquake. These functions could include postearthquake access for emergency respond- ers or immediate availability to all traffic in order not to disrupt the regional economy. For these reasons, PBSD shows substantial promise in helping designers and owners build bridges whose performance in earthquakes is better understood and better quantified. PBSD has been developed to such a level that it has been deployed on a limited number of large projects, and some departments of transportation have even developed approaches to apply PBSD to ordinary bridges. As this technology is promulgated, a clear and consis- tent approach will be crucial. This means that easy-to-use tools should be developed for relating typical engineering demand parameters (EDPs), such as displacements, force, and strains, to potential damage and to the risks arising from such damage. Damage might be in the form of concrete spalling, steel fractures, or permanent displacements. Damage can then be related to the direct risks of loss of use, loss of life, substantial repair costs, and downtime, in addition to the indirect risk of economic loss to the region. As a profession, bridge engineers can relate earthquake loading to structural param- eters, such as EDPs, through well-defined seismic hazard and structural analyses. The correlation of structural behavior to damage and then loss is less well understood, although various industriesâbridges, buildings, and waterfront/marineâare working to develop loss calculation tools. A significant amount of relevant research and performance-based specifications from other practice areas was reviewed in this synthesis, and, although much work is incomplete, the profession is definitely moving toward fully probabilistic PBSD. Ultimately, the PBSD method may be able to address uncertainties in loading and resis- tance and to relate damage likelihoods, which can then be related to losses, including the randomness of the processes and uncertainties in our knowledge. This goal may take con- siderable time and effort to achieve. In the near term, however, PBSD can be implemented on a deterministic basis with a design that includes multiple seismic hazard levels and targets specific performance levels. A guide specification or other nonmandatory guideline document could provide a consis-
2 tent basis for engineers to use for projects where PBSD makes sense. Typically, these would be large and important projects. A survey of all 50 states was undertaken as part of this synthesis project, with 41 states responding (82%). Of the states that have regions in the higher seismic zones (34 states), 31 states responded (91%). It is clear that some states are using elements of PBSD, and, in reviewing various project-specific documents, there are some variations in EDP limits and expected damage between agencies. It is also clear that tools are needed to help frame the questions and produce the answers that policymakers will need when deciding seismic per- formance for future projects. Thus, developing a document that clearly defines terms and helps users to consistently apply performance-based concepts would be a beneficial first step in implementing PBSD. In the longer term, research and feedback from initial implementations of PBSD will likely fill in much of the data needed to implement fully probabilistic PBSD. In the building industry, significant effort is underway to accomplish this point, but years will be needed to achieve the current goals. The bridge industry has done less work in this area, but the work will nonetheless be required. However, PBSD can help owners decide what performance they want and what modern seismic design can achieve.
