System-based design of planar steel frames, I: Reliability framework

• Direct Design Method (DDM) by advanced analysis can design a steel frame as a system.
• A reliability framework of developing system resistance factors for DDM is presented.
• A planar rigid frame is used to demonstrate the reliability framework.
• The simplified reliability analysis method based on First-Order Reliability Method is verified against the direct Monte Carlo simulations.

The design of steel frames by advanced analysis (second-order inelastic analysis with imperfections) of overall system behaviour is permitted in the American steel specification AISC360-10 and the Australian Standard AS4100. In both specifications, the strength of a structural frame can be determined by a rigorous system nonlinear analysis in lieu of checking member resistances to the specific provisions of the Specification, provided that the limit states covered by the Specification equations are detected by the inelastic analysis, and a comparable or higher level of structural reliability is achieved by the inelastic analysis than by member-based design. This system-based, design-by-advanced analysis approach is termed “Direct Design Method” (DDM). In DDM, a system resistance factor is applied to the frame strength. The system factor in AISC360-10 was adopted without considering its impact on frame reliability. This paper describes the framework for developing reliability-based system resistance factors suitable for use with DDM. A simple frame is used to demonstrate the procedures. Appropriate system resistance factors for various load cases and design recommendations are presented in a companion paper [1] based on the system reliability analyses of a series of steel frames.