Reliability of Steel Frames Designed with Advanced Analysis

The design of steel frames by advanced analysis can result in more efficient structures while maintaining satisfactory levels of reliability by incorporating nonlinear structural analysis. The structural reliabilities of a series of two-story, two-bay steel frames designed by both load and resistance factor design (LRFD) and advanced analysis methods are compared. Yield strength and gravity loads are modeled as random variables. Strength distributions and probabilities of failure are calculated by Monte Carlo simulation and first-order approximation for two failure criteria—plastic collapse and first plastic hinge. The results indicate that LRFD methods successfully enforce the target reliability on first plastic hinging. In contrast, design by advanced analysis results in acceptable reliabilities against plastic collapse, but relatively large probabilities of plastic hinging at service load levels. Specifications based on advanced analysis may require greater attention to serviceability criteria. Resistance factors for advanced analysis are calculated based on the strength distributions for two levels of target reliability. System-based failure criteria, such as plastic frame collapse, present a fundamental difficulty in determining resistance factors applicable to a wide-range of structures.