Simulation of conventional cold-formed steel sections formed from Advanced High Strength Steel (AHSS)

Simulation of conventional cold-formed steel sections formed from Advanced High Strength Steel (AHSS)

Hamid Foroughi1 and Benjamin W. Schafer2

Abstract The objective of this paper is to explore the potential impact of the use of advanced high strength steel (AHSS) to form traditional cold-formed steel structural members. In this study, shell finite element models are constructed, and geometric and material nonlinear collapse analysis performed, on simulated lipped channel cross-section cold-formed steel members roll-formed from AHSS. AHSS sheet is currently being used in automotive applications with thickness ranging from 0.35 to 0.8 mm (0.0138 to 0.0315 in.) and yield strengths from 350 to 1250 MPa (51 to 181 ksi). However, AHSS has not yet been employed in cold-formed steel construction. To assess the impact of the adoption of AHSS on cold-formed steel member strength a group of forty standard structural lipped channel cross-sections are chosen from the Steel Framing Industry Association product list and simulated with AHSS material properties. The stress-strain models used in this study are based on AHSS in production, including dual-phase and martensitic steels. The simulations consider compression with work on bending about the major axis in progress. Three different bracing conditions are employed so that the impact of local, distortional, and global buckling, including interactions can be explored. Due to the higher yield stresses of AHSS the potential for interaction and mode switching is anticipated to be greater in these members compared with conventional mild steels. The simulations provide a direct means to assess the increase in strength created by the application of AHSS, while also allowing for future exploration of the increase in buckling mode interaction, imperfection sensitivity, and strain demands inherent in the larger capacities. The work is intended to be an initial step in a longer-term effort to foster innovation in the application of new steels in cold-formed steel construction.

1 Graduate Research Assistant, Dept. of Civil Engineering, Johns Hopkins University, hforoug1@jhu.edu

2 Professor, Dept. of Civil Engineering, Johns Hopkins University, schafer@jhu.edu

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1. Introduction Cold-formed steel (CFS) structural members use cold bent sheet steel to provide efficient structural shapes that are noncombustible and highly structurally efficient. Use of cold-formed steel members continues to grow for both architectural and structural applications in building construction. Due to the manufacturing process thicknesses are naturally limited and ultimate strength of practical shapes is thus necessarily limited to a relatively modest value. Advanced High Strength Steels (AHSS) have been developed for the automotive industry over the last 20 years (Keeler and Kimchi 2014). AHSS sheet has yield stresses as high as 1250 MPa and are able to maintain large ultimate tensile elongations (>10%) even for these high yield stresses. As a result AHSS can be readily formed/manufactured and supply material yield values that are significantly in excess of current applications in CFS building construction. With this new strength comes new potential for design, particularly in mid-rise applications for CFS structural members.

Existing design specifications, such as the Direct Strength Method (DSM) in AISI S100-12, provide a potential design framework for CFS members formed from AHSS, but the validity of the provided rules has not been substantiated for higher strength steels. AISI S100-12 strength predictions include local-global (L-G) interaction, but based on experimental results at the time (Schafer 2002, Schafer 2008) excluded local-distortional (L-D), distortional-global (D-G), and local-distortional-global (L-D-G) interaction. More recent experimental research has shown that for higher strength (non AHSS) steels, such as G550 (Fy=550 MPa (80 ksi)), L-D interaction should be included (Yap and Hancock 2008, 2011). The interaction becomes more pronounced because of the additional local post-buckling demands, and because higher strength sections use thinner sheet steel requiring additional intermediate stiffeners, further complicating the response. Lead by Prof. Camotim at TU-Lisbon, Yap and Hancocks’s findings motivated new activity in buckling mode interaction for members. They began by using shell FE simulations and demonstrated conditions where L-D interaction may be significant (Dinis et al. 2011, Camotim and Dinis 2011, Silvestre et al. 2012). They then collaborated on a small test series on G550 columns to physically demonstrate L-D interaction and its related strength erosion (Young et al. 2013). Finally, additional simulations on simple and c

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