INFLUENCE OF SUBSTRUCTURE YIELDING ON SEISMICALLY ISOLATED BRIDGES DESIGNED ACCORDING TO THE AASHTO GUIDE SPECIFICA TIONFOR SEISMIC ISOLATION DESIGN

摘要

Although the basic intent of seismic isolation of buildings and bridges is similar, one notable difference for the design of isolated bridge systems is that the flexible substructure column bents below the isolation interface may be allowed to be designed at their elastic limit at the systems design displacement. This feature is facilitated by use of force reduction factors which eliminate overstrength in the demand-capacity equation for the case of earthquake loading (AASHTO, 1999). The AASHTO Guide Specification for Seismic Isolation Design of Bridges states, "...the lower R-Factors ensure, on the average, essentially elastic substructure behavior in the design-basis earthquake". In this respect, it is an inherent assumption in the formulation of the AASHTO code that substructure yielding will be tolerated in the event of system response above the mean. Since no further guidance is given within the code to assess or provide for this substructural yielding, the code formulation further implies that this yielding will not adversely effect the performance of the isolated system, nor inflict ductility demands on substructure components beyond their inherent limits. Studies were performed to systematically evaluate the efficacy of this new code formulation in its ability to reliably meeting these specified performance objectives, over a broad range of seismically isolated bridge systems, for demands imposed by earthquake motions representative of design basis seismic events. The effect of substructure yielding on simple isolated bridge systems designed optimally to these AASHTO provisions is found to be only slight on peak total and isolator displacement and base shear demands on average when subjected to a suite of spectrum compatible motions. However, peak structural displacements are found to be considerably larger on average for these systems when the effect of substructure yielding is considered. More notably, ductility demands for these systems designed according to code provisions with yielding substructures are found to be much larger than unity on average over a wide range of isolated bridge system characteristic parameters when subjected to a suite of spectrum compatible motions. Further research is needed to establish whether the AASHTO code procedures provide sufficient nominal ductility capacity in their component design specifications to meet the larger substructure ductility demands implied by the results of this study.