System-based reliability analysis of stainless steel frames under gravity loads

Abstract

Current structural codes for steel and stainless steel structures such as AISC 360-16, AISC 370-21, AS/NZS 4100 and Eurocode 3 are based on the traditional two-step member-based design approach, in which internal actions are first obtained from a structural analysis, usually elastic, and the strength of each member and connection is subsequently checked using a structural design standard. However, the most recent versions of these standards already incorporate preliminary versions of the direct, or one-step, system-based design alternative, which is based on the design-by-analysis concept and allows evaluating the strength of structures directly from numerical simulations, although the standards in their current form do not provide reliability requirements for structural systems. Therefore, it is necessary to build a rigorous structural reliability framework to investigate acceptable target reliability indices for structural systems and to provide adequate system safety factors and system resistance factors. While this framework has been developed based on advanced Finite Element analysis for hot-rolled and cold-formed carbon steel structures in recent years in the form of the Direct Design Method (DDM), the framework does not exist for stainless steel structures. This paper presents an extension of the DDM to the analysis of stainless steel structures, in which system reliability calibrations are presented for six stainless steel portal frames under gravity loads covering the three most common stainless steel families and different failure modes using advanced numerical simulations. From the derived reliability calibrations, suitable system safety factors γ M,s and system resistance factors ϕ s are proposed for the direct design of stainless steel frames in the European, US and Australian design frameworks under gravity loads. KEYWORDSadvanced analysis; probability-based design; reliability calibrations; stainless steel structures; structural reliability HIGHLIGHTS • Extension of the Direct Design Method to stainless steel structures is presented.• Rigorous reliability framework is built and presented for the analysis of stainless steel structures under gravity loads.• Six cold-formed stainless steel portal frames are analysed, including different stainless steel families and failure modes.• System reliability calibrations are presented for the Eurocode, US and Australian design frameworks, covering different live-to-dead load ratios.• System safety factors γ M,s and system resistance factors ϕ s are proposed for the target reliability indices typically adopted in national design frameworks.

Direct Design of Stainless Steel Frames: Recommendations and Case Studies

“…The Direct Design Method (DDM) is a system-based design approach that allows the design of steel structures using geometric and material nonlinear (advanced) numerical analyses. Current international design standards already include preliminary versions of the DDM but the calibration of suitable system resistance or partial safety factors that guarantee that the target reliability levels are met are still under development for the different types of structures covered in these specifications, including the recently proposed system factors for stainless steel structures [9,10]. This paper presents the fundamental aspects of the use of the DDM for the design of stainless steel structures, summarizing the key features of the advanced finite element models and the DDM design checks for strength and serviceability.…”

“…The system factors recommended in [9,10] for the direct design of coldformed stainless steel portal frames in the Eurocode framework are γ M,s =1.15 and γ M,s =1.20 for the gravity load and gravity plus wind load cases, respectively, and ϕ s =0.95 and ϕ s =0.90 when considering gravity load and gravity plus wind load cases in the ASCE design framework.…”

“…The type and behaviour of connections can also have a remarkable effect on the response of steel structures, since internal forces and deflections will depend on the stiffness and strength of connections. While the effect of the connection behaviour on the structural performance of the frame can be accounted for by modelling the moment-rotation response of the joint (for example, by using a bilinear moment-rotation diagram, as per in [7,9,10]), additional considerations are required to actually verify the strength of the connections. All research efforts made to date in the development of the DDM have assumed that joints possess sufficient strength and ductility for the frame ultimate limit states to be reached before their failure occurs [10].…”

“…To address this issue, recent research efforts have focused on the calibration of such system factors for different structure types and materials by building rigorous reliability frameworks. In this regard, advances have been made in the proposal of system factors for hot-rolled steel planar frames [5], hot-rolled and cold-formed 3D steel frames [6], cold-formed steel portal frames [7], scaffolding systems [8] and, more recently, cold-formed stainless steel portal frames [9,10].…”

Towards a practicable GMNIA procedure in the footsteps of the Direct Design Method

“…in which R, G and Q are random values for the resistance, permanent load, and variable load. The system safety factor is determined based on the assumption that structures are designed economically at their limits, thus characteristic values for actions are obtained by [11]:…”