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26 The sample size in the survey was limited by the concurrent conditions of non-conventional bridge design and moderate to high seismic regions of the United States. Project examples and project-specific criteria were consistent in approaching design practice using a well-established performance-based design process that has evolved from the time of ATC-32 and expanded in terms of definitions and details to strain-based criteria for assessing member and structure performance in earthquakes. Based on the criteria obtained for non-conventional bridges in high seismic regions compared with other regions, the details of nonlinear analysis and model- ing of inelastic members vary with the relative significance of the seismic input. However, the performance-based design principles are consistent. The survey responses listed both the AASHTO BDS and the Guide Spec as references for project-specific criteria. In some cases, each was listed as the independent criteria document. Performance standards were applied within project-specific criteria that are not terms in either specification but are presented in ATC-32 and referenced in most project-specific criteria. Even though the Guide Spec and the AASHTO BDS are not the independent criteria document for a non-conventional bridge design, the detailing requirements of these AASHTO documents serve as the basis for shear and confinement design criteria applied to non-conventional structures, which is the primary application of these references in the project criteria examples reviewed for this synthesis. Guidance is offered for non-conventional bridges in the AASHTO BDS even though the scope of the document excludes non-conventional bridges. The topical areas of nonlinear dynamic analysis, number and combination of ground motions, and foundation modeling serve as a general reference for project-specific criteria, which often vary from the stipulations in the AASHTO documents due to project-specific studies. Table 6 contains a summary of the current practice for seismic analysis and design of non- conventional bridges as determined through this study. Appendix A includes the full listing of seismic design criteria from selected non-conventional bridges. The findings based on review of the criteria documents in Appendix A as summarized in Chapter 3 and Table 6 represent the current standard of practice for seismic design of non-conventional bridges in the United States. These criteria have their primary origin in the recommendations of ATC-32. As noted in ATC-32, critical bridges (assumed herein to encompass non-conventional bridges) are not designed based on the full plastic moment design principles of conventional bridges and are almost exclusively limited ductility designs based on current practice. The question of how to assess capacity protection with variable ground motion is addressed through the hierarchy of strain limitations assigned to damage limit states for non-conventional bridges. The peer review process employed for critical bridges in the highest seismic regions of California addresses the question of ground motions and acceptance criteria on a project-specific Conclusions C H A P T E R 5
Conclusions 27 basis. There is no guidance for factoring ground motions to address variance in the level of ground shaking assigned to the return periods selected for project criteria. Unlike the case for conventional bridges, where review of ductility demand is based on a simple bilinear moment- curvature response of full ductility columns, evaluations involving increased ground motion input for the more detailed and complex conditions for non-conventional bridges are evaluated by running additional nonlinear dynamic analyses to capture the influence of higher ground motions, which, owing to the nonlinear regime of the structural system, are not determined by scaling results or factoring demand displacements from lower intensity ground motions. Research Needs The general performance basis for seismic design practice for non-conventional bridges is well established. Details for the assignment of performance limit states vary, particularly as they relate to evaluation of the repairable damage performance limit state. In the case of reinforced concrete design for ordinary loads, the strength limit-state design for conventional loads is not strain based for reinforcing steel. If one evaluates rebar strain for a typical under-reinforced section, the computed strain may be on the order of 4 to 5 times nominal yield strain. Assum- ing the target reliability for the seismic case is similar to other geotechnical safety indices, most designers assign a target strain value that is the same or greater than those for the normal strength limit-state values. The research need is to establish a definition of strain values con- sistent with damage limit states that support the criteria for performance requirements and practical requirements for design and construction. There is a need for developing guidelines that include the seismic design of non-conventional bridges. The broad range of criteria and applications in the AASHTO BDS allow for a concise inclusion of performance-based design and damage limit states within the current BDS frame- work, borrowing much of the information presented in ATC-32. Such guidelines would comple- ment AASHTO BDS to address the current conventional bridge limitation in Section 3.10 and integrate the detailing methods specifications for reinforcing within AASHTO BDS that are applied in the current state of practice for non-conventional bridges. The research need in this regard is to write these guidelines so they are compatible with the current AASHTO BDS. Key points for a non-conventional bridge seismic design specification section include the following: 1. Definitions for non-conventional bridges, and the basis for determining when non- conventional criteria apply. 2. A basis for determining the recurrence intervals for multi-hazard analysis in different regions of the United States for non-conventional bridges. 3. Assignment of the variable limit states for limited ductility designs for non-conventional bridges. 4. A basis for establishing strain limits applicable to the range of limit states for concrete and steel elements that are consistent with the general performance limit states of minimal, repairable, and significant damage, and the definition of strain differentials for capacity protection. 5. Acceptable methods for characterizing nonlinear material and element behavior in a non- linear dynamic analysis. 6. Acceptable methods for characterizing foundation structures for analysis within an overall nonlinear dynamic analysis, particularly those sites with soft or liquefiable soils. 7. A basis for establishing the number of ground motion time histories and a rational procedure for combining demands or enveloping demands from multiple events at each ground motion level.
