System Reliabilities of Planar Gravity Steel Frames Designed by the Inelastic Method in AISC 360-10

Abstract

Current methods for designing steel structures, such as load and resistance factor design (LRFD) are based on safety checks of individual members derived from elastic analysis. The next generation of steel design methods will move from member-based to system-based design. Recent advances in nonlinear structural analysis make it possible to design a steel frame as a system rather than as a set of independent components, and several steel design specifications worldwide have incorporated provisions for designing for overall system behavior. However, requirements for minimum system reliability that have been implemented in such design-by-inelastic analysis methods rely on existing resistance factors originally developed from member reliability considerations. This paper examines the system reliabilities of a number of simple yet representative structures, including a continuous beam, a portal frame that fails from elastic instability, and three related frames with various load redistribution capacities. The paper provides an overview of the strengths and system reliabilities of these structures when designed either by the second-order inelastic method or by LRFD in AISC 360-10, and discusses the reliability implications of these alternative approaches to steel structure design.