Effects of material nonlinearity on the global analysis and stability of stainless steel frames

Abstract

In structural frames, second order effects refer to the internal forces and moments that arise as a result of deformations under load (i.e. geometrical nonlinearity). EN 1993-1-1 states that global second order effects may be neglected if the critical load factor of the frame cr is greater than 10 for an elastic analysis, or greater than 15 when a plastic global analysis is used. No specific guidance is provided in EN 1993-1-4 for the design of stainless steel frames, for which the nonlinear stress-strain behaviour of the material will result in greater deformations as the material loses its stiffness. A study of the effects of material nonlinearity on the stability of stainless steel frames is presented herein. A series of different frame geometries and loading conditions are considered. Based on the findings, proposals for the treatment of the influence of material nonlinearity on the global analysis and design of stainless steel frames are presented. KeywordsContinuous Strength Method, Frame stability, Global analysis, Numerical Modelling, Second order effects, Stainless steel IntroductionStainless steel is widely used across a range of industries with its key advantage over ordinary carbon steel being its corrosion resistance and durability. Although a number of design standards for stainless steel currently exist, their provisions have generally been based on a presumed equivalence with the carbon steel design rules. However, stainless steel and carbon steel have distinctly different material characteristics; while carbon steel is accurately characterised by a bilinear (elastic, perfectly plastic) stress-strain response, stainless steel exhibits nonlinear rounded stress-strain behaviour with no sharply defined yield point. It is important that the codes reflect these differences so as to ensure safe, efficient, and structurally sound design. The current European rules for global analysis and design are examined in this paper, assessing whether the methods provided are suitable for stainless steel. Particular attention is given to the extent to which the differences between the behaviour of stainless steel and carbon steel frames are adequately recognised.EN 1993-1-4 [1] gives supplementary guidance for the design of stainless steel structures that complement the design rules given for structural carbon steel in EN 1993-1-1 [2] . Section 5 of EN 1993-1-1 outlines the rules for structural analysis with detailed subsections on structural modelling, global analysis and imperfections, as well as methods of analysis considering material nonlinearities. The guidance on material nonlinearities largely relates to the occurrence of traditional idealised plastic hinges, as seen in carbon steel structures. However, such hinges do not form in stainless steel structures; instead zones of plasticity with gradually reducing stiffness, but with peak capacities well in excess of the traditional plastic moment are exhibited [3] . EN 1993-1-1 also states that "elastic global analysis may be used in all cases" and that...

Behavior and Design of Normal- and High-Strength Steel SHS and RHS Columns

“…A local imperfection amplitude of ωl = c / 200 was assumed, as recommended in EN 1993-1-5:2006(CEN 2006c) and successfully employed in several previous numerical studies e.g. and Walport et al (2019). Note that the measured local imperfection amplitudes (Ma et al 2016;Meng and Gardner 2019b) for the considered SHS and RHS profiles were, on average, slightly smaller than the value of c / 200 recommended in EN 1993-1-5:2006(CEN, 2006c, and thus, slightly conservative resistance predictions from the FE models were anticipated.…”

Stability and design of stainless steel structures – Review and outlook

“…Material nonlinearity also affects the behaviour of the stainless steel structures at the frame level, but no explicit guidance on the treatment of material nonlinearity in global analyses is currently given in EN 1993-1-4. In the absence of guidance, an elastic analysis may be assumed to be acceptable for stainless steel frames, though recent research [78] has shown that this is not necessarily the case. The gradual degradation of material stiffness was shown in some instances to significantly affect the characteristics of the structural system and the subsequent distribution of internal forces and moments.…”

“…If material nonlinearity is considered in the global analysis of a frame, greater deformations result due to the loss of material stiffness; if plasticity is ignored, peak moments are typically underpredicted. It was therefore recommended [78] that plastic analysis (employing the rounded material stress-strain response described in Section 2)…”

A method for the treatment of second order effects in plastically-designed steel frames

“…As discussed in Walport et al [17], for elastic analysis, the amplification factor kamp is equivalent to the ratio of internal forces from a second (GNA) to a first order (LA) analysis (MGNA/MLA) at any point in the frame. However, for plastic analysis, due to the redistribution of the internal forces and moments following material nonlinearity, kamp must be calculated by determining the magnitude of the amplification of the horizontal loading in a first order analysis (MNA+ kamp) required to align the sway deflections to those in a second order analysis (GMNA) at a given applied load factor, as shown in Fig.…”

“…As presented in Walport et al [17] for stainless steel, the influence of plasticity on the sway stiffness of frames may be considered by defining a modified elastic buckling load factor αcr,mod, as given by Eq. (4).…”