Impact of 'NCHRP project 12-49' on performance-based seismic design of bridges in low to high seismicity zones

Design procedure development needs to be based on concepts that can allow an easy implementation of various performance levels to meet the intended objectives. As contained in the current provisions, the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications are, for the most part, based on provisions and commentary from Division I-A (Seismic Design) of the AASHTO Standard Specifications for Highway Bridges. In 1998, the National Cooperative Highway Research Program (NCHRP) initiated a project to develop a preliminary set of comprehensive specification provisions and approaches intended for seismic design of highway bridges, compatible with AASHTO LRFD Bridge Design Specifications. NCHRP Project 12-49 had the primary focus and objectives on the development of seismic design provisions that reflect the latest design philosophies and design criteria of highway bridges. Specific issues considered include: seismic hazard representation; loads and displacements; site effects; advances in analysis and modeling procedures; and requirements for component design and detailing. The current AASHTO specifications are applicable to all seismic zones, and all bridge construction types and materials. However, the new proposed provisions are more complex and more in line with realistic site conditions and bridge behavior.; The specific objective of the proposed research is to compare the difference between the NCHRP Project 12-49 and the Division I-A (Seismic Design) of the AASHTO Standard Specifications for Highway Bridges. Specifically, this comparison will be for the seismic design of highway bridges subjected to seismic activities expected in Southern Illinois. The study focuses on identifying such differences as the design earthquake; earthquake loads, seismic forces on substructures; bearings; substructure designs; and construction cost.; The study revealed that for low seismicities (ground acceleration of about 0.1g), there are almost no differences between the AASHTO and the NCHRP results obtained for design forces at bent columns and piers. However, at seismicities in about 0.2g acceleration, the NCHRP values are about 3 times higher indicating a higher seismic capacity demand than that expected from the AASHTO provisions. Considering forces in foundations, however, the two design methods yield compatible results at all seismicity levels. The differences for the bent columns and pier forces are determined to be due to the inherent risk levels in the two methods, differences in the reduction factor (used to adjust the result from the linear elastic analysis) and the overstrength factors used for plastic hinging behavior.