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1 Design of conventional bridges for seismic demands in the United States is based on one of two AASHTO documents: the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications (AASHTO BDS) (1) or the AASHTO Guide Specifications for LRFD Seismic Bridge Design (Guide Spec) (2). The stated scope of these documents for seismic design is limited to conventional bridges. The objective of this synthesis of practice is to document the current state of practice for seismic design of non-conventional bridges that are outside the scope of the two AASHTO documents governing seismic design of conventional bridges. Non-conventional bridges outside the scope of these two AASHTO documents, such as cable-supported bridges and long-span arch bridges, are typically high value investments designed with special project criteria. There is no current AASHTO standard seismic design criteria document specific to these non-conventional bridges. Seismic design criteria for these non-conventional bridges are typically part of a broader project-specific criteria docu- ment that addresses the special character of the bridge type. AASHTO BDS is a comprehensive design specification that includes a range of approaches to seismic design, with design options ranging from an unreduced force-based elastic design to a ductility-based force reduction for inelastic design (1). The Guide Spec is limited to inelastic seismic design for conventional bridges and calls for a displacement-based design that is predicated on design for fully ductile columns as a capacity protection mechanism (2). In regions of higher seismicity, a ductility-based capacity protection design method is typi- cally applied using either AASHTO document: the force-based method of the AASHTO BDS or the displacement-based method of the Guide Spec. This capacity protection design pro- tocol is a design procedure for conventional bridges that addresses the extreme limit-state behavior and allows the designer to optimize the structural design for the single no-collapse limit state that AASHTO applies to conventional bridge designs. Application of the capacity protection principle takes a different form for non-conventional bridges. In moderate seismic zones, wind or vessel impact forces can control the lateral forces design for large, non-conventional bridges. In high seismic areas, the high value and criti- cal nature of the long-span non-conventional bridge often leads owners to adopt multi- level seismic inputs (a lower return period event as an operating condition and a longer period return event as a safety event) and set criteria for non-conventional bridges that limit ductility levels throughout the primary structure as a means of protecting investment and providing for continued service even after a major seismic event. All these characteris- tics of non-conventional bridges make them special cases that fall outside the prescriptive approaches in the AASHTO BDS and the Guide Spec. S U M M A R Y Seismic Design of Non-Conventional Bridges
2 Seismic Design of Non-Conventional Bridges The approach to this synthesis was to conduct a literature review about seismic design for non-conventional bridges, survey the 50 state DOTs for their current practice, and collect project-specific criteria for seismic design used in recently constructed non-conventional bridges in the United States. Forty-three responses to the state DOT survey were received. Ten responses were from states with low, moderate, or high seismic regions having recent experience with seismic design of non-conventional bridges as defined for this synthesis. The criteria documents received through the survey and from practitioners document the current standard of prac- tice for seismic design of non-conventional bridges. These documents are reviewed in detail in the body of the synthesis. They represent non-conventional bridges designed and con- structed in the last 20 years in moderate and high seismic zones, with some bridges in the moderate zones having lateral design controlled by non-seismic criteria. The common feature of the criteria reviewed for the non-conventional bridges in mod- erate and high seismic zones is that all include performance-based design standards that require a limited ductility design basis that is not typically used for conventional bridges. The standard practice of a performance-based design approach for non-conventional bridges provides a basis for developing limited ductility designs through either a grada- tion of descriptive damage conditions, or a specification of specific strain-based perfor- mance standards at assigned damage levels in the lateral resistance system. The framework for these performance standards was established in the 1996 Applied Technology Council (ATC) report titled âImproved Seismic Design Criteria for California Bridges: Provisional Recommendationsâ (ATC-32) (3). While there are a few exceptions revealed in the survey, non-conventional bridge seismic criteria typically include a multilevel seismic hazard, with a more frequent earthquake level being associated with functional requirements for contin- ued operation, and a longer return period event being associated with a life safety require- ment. The project-specific criteria for major non-conventional highway bridge structures require limited ductility designs based on a repairable damage level for the higher safety evaluation seismic event instead of the significant damage associated with the no-collapse limit state in the AASHTO BDS and Guide Spec. The descriptive performance limit states are based on the original ATC-32 document (3). Advancements since the development of ATC-32 in computational capabilities, nonlinear analysis, and material behavior in seismic events have allowed for more rigorous analysis and demand forecasting that have led to quantitative criteria that are used in some cases to validate compliance with the same performance limit states first described in ATC-32. Advancements in materials and model testing correlated with nonlinear behavioral models has greatly improved the capability for predicting member resistance to seismic demands and establishing a quantitative design basis for new non-conventional bridges. This advancing technology has been assimilated into the standard of practice as the technology has evolved. Current practices for modeling structures for nonlinear dynamic analysis and for model ing soil-structure interaction within a nonlinear regime vary among practitioners and projects. Research needs for the methods of practice for seismic design of non-conventional bridges fall within the general evolution of tools for engineering design. The primary research needs relate to codifying the standards of practice for non-conventional bridges into a concise guideline document for reference in designing non-conventional bridges. The elements of practice that are unique to non-conventional bridges are combined with elements of prac- tice that are common to both conventional and non-conventional bridges only through project-specific criteria and applications. New guidelines are needed to clarify the process and criteria for seismic design of non-conventional bridges.
