The continuous strength method for steel cross-section design at elevated temperatures

Abstract

When subjected to elevated temperatures, steel displays a reduction in both strength and stiffness, its yield plateau vanishes and its response becomes increasingly nonlinear with pronounced strain hardening. For steel sections subjected to compressive stresses, the extent to which strain hardening can be exploited (i.e. the strain at which failure occurs) depends on the susceptibility to local buckling. This is reflected in the European guidance for structural fire design EN1993-1-2 [1], which specifies different effective yield strengths for different cross-section classes. Given the continuous rounded nature of the stress-strain curve of structural steel at elevated temperatures, this approach seems overly simplistic and improved accuracy can be obtained if strain-based approaches are employed [2]. Similar observations have been previously made for structural stainless steel design at ambient temperatures and the continuous strength method (CSM) was developed as a rational means to exploit strain hardening at room temperature. This paper extends the CSM to the structural fire design of steel cross-sections. The accuracy of the method is verified by comparing the ultimate capacity predictions with test results extracted from the literature. It is shown that the CSM offers more accurate ultimate capacity predictions than current design methods throughout the full temperature range that steel structures are likely to be exposed to during a fire. Theofanous, M., Prospert, T., Knobloch, M. and Gardner, L. (2016). The continuous strength method for steel cross-section design at elevated temperatures. Thin-Walled Structures. 98, 94-102. 2Moreover due to its strain-based nature, the proposed methodology can readily account for the effect of restrained thermal expansion on the structural response at cross-sectional level.

Elevated temperature material properties of stainless steel reinforcing bar

“…For materials which exhibit these stress-strain characteristics, it is conventional to use the 0.2% proof strength f 0.2p as the design strength. Beyond this point, no further strain hardening is considered in traditional design, though it is considered, and systematically harnessed, in the deformation based continuous strength method [26][27][28]. The non-linear stress-strain characteristics lead to some differences in the structural performance of stainless steel members compared to carbon steel members, which are generally reflected in the design rules set out in EN 1993-1-4 [29].…”

Local Buckling of Steel Members Under Fire Conditions: A Review

“…Local buckling was also reported for column specimens -106 in total, mostly from Rectangular Hollow Sections (RHS) and Square Hollow Sections (SHS) sections-tested under fire conditions as referenced in the paper by Theofanous et al [17]. The majority of these tests were carried out in ETH, Zurich (detailed information about them is given in [18]), with the authors concluding that the reason for the observed local buckling failure was the small magnitude of the global geometric imperfections and the low slenderness values of the columns.…”

“…Fig. 2 shows the local buckling failure of a SHS specimen and a H-section specimen of the programme referenced by Theofanous et al [17]. In other experimental work [19], hot-rolled H-section column stubs were tested under fire exposure with the authors reporting failure due to local buckling at the flanges.…”

“…Failure Temperature ( o C) [11] b/t = 19 (BOX-WELDED) Figure 6: Plot of the load ratio versus the failure temperature for stocky columns with Hsections (welded or hot-rolled) experiencing local buckling under elevated temperatures (experimental data collected from the literature [11], [13], [17], [19])…”

“…As expected, temperature increase leads to failure at lower load ratios despite the noticeable In order to better understand the effect of width-to-thickness ratio on local buckling at elevated temperatures, experimental data (for temperatures lower than 600 o C) from the above figures are tabulated in Table 3 Load Ratio Failure Temperature ( o C) [11] b/t =7 (LFB-WELDED) [11] b/t =10 (LFB-WELDED) [13] b/t =15 (LFB-WELDED) [13] b/t =50 (LWB-WELDED) [13] b/t =40 (LWB-WELDED) [17] b/t = 6.5 (LFB-ROLLED) [19] b/t = 7.5 (LFB-ROLLED) [19] b/t = 10 (LFB-ROLLED)…”