Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design (2020)

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1 This research report details the development of the proposed AASHTO Guidelines for Performance-Based Seismic Design of Highway Bridges. Starting from the work presented in NCHRP Synthesis 440: Performance-Based Seismic Bridge Design (Marsh and Stringer 2013), a methodology was developed to implement performance-based seismic design utilizing primarily strain-based engineering design parameters to control the performance and expected damage levels of bridges under a two-level seismic hazard approach. The methodol- ogy included analysis and capacity determination requirements based on the desired perfor- mance, the seismic hazard, and the parameters of the bridge structure. Due to limitations in available performance data, the proposed guidelines are not fully probabilistic. NCHRP Synthesis 440 provided an in-depth background to the issues surrounding implementation of performance-based seismic design (PBSD) for bridges and examples of how different organizations have approached implementation. Recent implementations of PBSD were reviewed in this report, and similarities summarized. Based on the survey of the state of the practice of PBSD, the development of the methodology for a set of guide- lines for implementation under the auspices of AASHTO was initiated. A number of key points were determined as follows: • There does not currently exist enough research data to enable a fully probabilistic implementation of PBSD, in particular when earthquake-resisting systems beyond flexure in reinforced concrete columns are considered. • A two-level approach to ensuring desired performance, consistent with that used in the FHWA’s Seismic Retrofitting Manual for Highway Structures: Part 1—Bridges (Buckle et al. 2006), would provide a robust control of performance levels while maintaining continuity with existing methods and practice. • Using strain limits, rather than ductility, would provide better control of performance while simultaneously allowing for systems beyond reinforced concrete columns to be easily incorporated. With the previous criteria established, a methodology was developed that implements PBSD for design of highway bridges in the form of guidelines proposed for adoption by AASHTO. These guidelines consist of establishing three major categories of bridges based on their importance in the transportation system; establishing the performance goals for each of these categories for both a lower level and a higher level seismic event; tying the performance goals to engineering design parameters (EDPs), which the designer uses in proportioning the structure; and laying out analysis and capacity determination require- ments consistent with the level of behavior indicated by the performance levels. In the course of developing these guidelines, it became apparent that the damping values often used in seismic analyses, in particular nonlinear response history analyses, may be S U M M A R Y Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design

2 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design overestimated and result in nonconservative estimates of bridge performance. A discussion of this issue is contained in an appendix to the guidelines. Among the available analysis types, the substitute structure approach, also known as direct displacement-based design (DDBD), is a powerful tool that can simplify the design approach for PBSD for structures that can be reasonably approximated as a single degree of freedom system. These guidelines represent the first time this method has been proposed for inclusion in an official AASHTO publication. Appendix B details the method. A series of case studies were carried out as part of this project, which resulted in single column studies for a variety of different seismic hazards, as well as detailed column designs for specific hazard locations. As a result of the column studies, deficiencies in the “equal displacement” rule that underpins the elastic analysis approach for seismic design were identified and relate to the damping issues discussed in Appendix A of the proposed AASHTO guidelines. Further research is recommended to explore this issue.