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3 Problem Statement There are two standards for seismic design of conventional bridges in the United States, the Load and Resistance Factor Design (LRFD) Bridge Design Specifications (AASHTO BDS) (1) and the AASHTO Guide Specifications for LRFD Seismic Bridge Design (Guide Spec) (2). The forewords or scopes of specifications of the AASHTO BDS and the Guide Spec bound the scope of application to conventional bridges. The AASHTO BDS and the Guide Spec are formulated with a focus on the practical design of ordinary beam and column bridge structures; these make up the vast majority of bridges constructed in the United States. The definition of ânon-conventionalâ for this synthesis is intended to be consistent with AASHTO BDS definitions (Art C3.10.1). Non-conventional bridges include long-span cable- stayed, arch, and suspension bridges, and these are the primary focus of this synthesis. One dis- tinguishing feature from a seismic design perspective is that non-conventional bridges may not retain a desirable kinematic system in a high seismic event if a conventional strong-beam weak- column behavior with full inelastic hinging in the columns (in this case, single towers or arch ribs) is assumed as a basis for design. For certain long-span bridges in moderate seismic zones, the design solution may also be affected by the relationship of lateral wind or vessel impact load to the level of seismic load, where seismic loading does not govern in the elastic sense, but where simple plastic-hinge capacity protection principles applied against towers or columns controlled by wind or vessel impact load can make foundation design overly expensive. Virtually all bridges in the United States are designed for a range of service and strength con- ditions based on the AASHTO BDS. Non-conventional bridges are generally designed accord- ing to project-specific criteria to extend the AASHTO BDS design framework to the particular requirements associated with the non-conventional bridge type. Seismic design criteria for non- conventional bridges are typically part of this broader project-specific criteria document. Project-specific criteria for non-conventional bridges address the special conditions required for the specific design, as well as provide context and clarity to the range of standard criteria such as those in the AASHTO BDS and the Guide Spec. Non-conventional bridges in regions of low seismicity are generally designed for seismic loads using the provisions of the AASHTO BDS (see Chapter 3). Seismic design of non-conventional bridges in moderate to high seismic regions require interpretation and extrapolation of either the AASHTO BDS or the Guide Spec to extend criteria beyond the scope of those documents. The project criteria also establish special perfor- mance standards for the design of non-conventional bridges, which are particularly significant for seismic design of non-conventional bridges where either the structure type is not representa- tive of the structural systems addressed in the AASHTO BDS or Guide Spec, or where owners have a need for a more exhaustive set of performance criteria than the single return period no-collapse design limit state in the Guide Spec or the force-based design of the AASHTO BDS. C H A P T E R 1 Introduction
4 Seismic Design of Non-Conventional Bridges Special owner performance requirements can be the result of either financial or operational incentives. Non-conventional bridges are often high value investments and critical lifeline struc- tures where the single no-collapse criterion in the AASHTO BDS or the Guide Spec may not satisfy the ownerâs performance needs for the highway network served by the structure. At the present time, there is no codified standard for non-conventional bridges that provides either separate comprehensive criteria or a guideline for selective application of certain provisions within the AASHTO BDS or the Guide Spec. Objective of the Study The objective of this synthesis is to summarize the approaches and criteria currently used by engineers engaged in non-conventional bridge seismic design practice, and how these practices relate to the Guide Spec, the AASHTO BDS, or other guidelines and references. The AASHTO BDS allows for seismic design methods ranging from elastic to inelastic, including displacement- based design. The Guide Spec criteria is based on displacement-based design exclusively for conventional frames. Both AASHTO standards are formulated to address conventional bridges. This synthesis documents the current seismic design practice for non-conventional bridges, including how project-specific criteria address the operational and performance requirements for non-conventional bridges. Generic questions of ground motion levels, target reliabilities, and special design strategies, such as base isolation or mechanical damping, are not unique to non- conventional bridges and are therefore not reviewed in this synthesis. The Wind Conundrum One example of the special conditions applicable to non-conventional bridges is the effect of wind on design for lateral forces. Wind loading on long-span non-conventional bridges can be a controlling load case for lateral load design. Figure 1 illustrates the condition that can occur in the case where wind controls lateral force design, and one attempts to apply the single limit- state capacity protection design principle of the Guide Spec to the foundation design. Since design for wind is based on pseudo-elastic principles (with reference to concrete foundation elements, which are not truly elastic, even though typically analyzed as being so), simple applica- tion of the inelastic pushover approach of the Guide Spec would result in over-strength factors Î´y Î´p Î´over Fr Fe Fw 1.2Fw Fe Elastic earthquake force level Fr Reduced ductility-based force level Fw Wind force level Î´ Î´ Î´ y Yield displacement for seismic design p over Over-strength displacement consistent with wind design section Seismic demand displacement (inelastic displacement level â See Guide Spec Fig C.3.3-1 ) Figure 1. Over-strength effect of wind-controlled design.
Introduction 5 being applied against a design section required for wind design forces. The procedure would increase foundation design requirements without a commensurate benefit for either wind or seismic resistance. This capacity-protection requirement could result in at least a 20% premium in foundation design, which for a long-span bridge can be a significant portion of the total bridge cost. Wind design for conventional bridges is rarely significant in the design for lateral forces, and so does not enter into the practical design for lateral forces. This is one example of where design procedures that are effective for conventional bridges may not be appropriate for non- conventional bridges. Controlling design conditions similar to this wind case can occur in the case of lateral designs for vessel impact, or even from dimensional requirements for loads during construction. Dimen- sional requirements for strength and service limit-state design for non-conventional bridges can complicate the straightforward application of the capacity protection limit-state provisions as they are applied to conventional bridges.