System Factors for Highway Bridge Superstructures

Abstract

Although traditional methods of bridge structural design proportion each member separately, the members do not behave independently but interact to form a single structural system. To account for overall system safety during the design process, the current AASHTO load and resistance factor design (LRFD) specifications (AASHTO LRFD Bridge Design Specifications, 2002) provide a load modifier factor, η, which explicitly accounts for the effects of ductility, redundancy, and operational importance. In the absence of precise information, the values used for the load modifiers are assigned subjectively. The results of a research project funded by NCHRP that led to the development of objective measures of redundancy and a method that uses system factors, Φs, during the design and load capacity evaluation process to account for the system's reserve strength are summarized. A set of proposed system factors was calibrated to provide bridges with adequate levels of overall safety and system reliability. Nonredundant...