System Reliability of Steel Frames Designed by Inelastic Analysis

The paper is concerned with the design of steel frames under gravity loads by geometric and material nonlinear analysis, also referred to as “inelastic analysis” in Appendix 1 of the ANSI/AISC360-10 Specification. In this approach, the strength of a structural frame is determined by system analysis in lieu of checking member resistances to the specific provisions of the Specification for tension, compression, flexural members etc., provided a comparable or higher level of structural reliability by the analysis. In this paper the reliability of steel frames is evaluated by performing Monte Carlo simulations for a series of 2D low-to-mid-rise moment resisting frames, including regular and irregular configurations. The analyses treat the material properties, initial geometric imperfections, residual stresses and loads as random variables and suggest suitable system resistance factors for different system reliability levels. Member cross-sections are selected in a way to provide different system failure modes such as sway instability and/or member failure. In designing by inelastic analysis, the system resistance factor (𝜑𝑠) is applied to the frame strength determined by analysis, and provided the reduced system strength exceeds the loads, the design is deemed adequate, requiring no further check of individual member resistance. The procedure is more efficient than current procedures based on elastic analysis and provides the designer with a greater understanding of the behavior of the frame. It promotes a more holistic approach and greater innovation in structural design and is likely to become increasingly used by structural engineers as commercial software packages increasingly make geometric and material nonlinear analyses available.

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