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66 Conclusions and Suggested Research Conclusions The preceding chapters detailed the development of the proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design. A comprehensive literature review was conducted that focused on developments subsequent to the work contained in NCHRP Synthesis 440. Based on these findings, a methodology was developed that implements performance-based seismic design using three main performance levels and two seismic hazard levels. The heart of the methodology is a set of EDPs that relate design parameters to the desired performance levels and are primarily based on limiting strains in structural members. A robust matrix of required analysis methods and capacity determination methods is specified consistent with the desired performance levels. Appendices are included to provide more detailed guidance on the level of assumed damping and the DDBD method for seismic analysis and design consistent with PBSD. Case studies are included in an appendix to demonstrate how the guidelines can be applied to the design of a reinforced concrete bridge column. Suggested Research During the creation of the proposed AASHTO guidelines, it became apparent that there were significant differences between the results of elastic analyses and results from the use of the substitute structure method. The main source of the differences is thought to be the estimates of damping that were used in the original development of the equal displacement assumption. Resolution of this issue requires additional research, which is strongly recommended. The approach implemented in these guidelines is not fully probabilistic; the tie between desired performance level and the resulting loss estimates has yet to be made. Sufficient research exists to make this connection for earthquake resistance systems that utilize reinforced concrete columns, but only for a relatively narrow class of column designs, and not for other elements that may be used to resist seismic motions in the inelastic range. Further research is needed in this area to develop fully probabilistic PBSD. C H A P T E R 5