Keywords: cold-formed steel columns, distortional failure, elevated temperature, direct strength method.

This paper reports an ongoing numerical investigation to assess the performance of the current direct strength method (DSM) distortional design curve to estimate the failure loads of fixed-ended cold-formed steel columns subjected to uniform temperature distributions caused by fire conditions. The columns (i) display five temperature-dependent constitutive laws: two prescribed by part 1.2 of EC3 and three experimentally-based expressions [1,2,3], (ii) exhibit several room temperature yield stresses, (iii) contain (distortional) initial imperfections with small amplitudes, and (iv) are compressed under uniform temperatures up to 600 degrees. The column failure load data obtained, together with experimental results, are used (i) to quantify the quality of the current DSM distortional strength curve and (ii) to appraise how such quality is influenced by the constitutive model adopted. The output of the above assessment is then used to propose alternative DSM distortional strength curves able to capture adequately the temperature effects. The main conclusions of this study are:

(i)

The normalised ultimate strength "clouds" concerning the numerical ultimate loads can be accurately described by "Winter-type" strength curves (the few experimental loads available also follow this trend). The "vertical dispersion" is acceptable in all cases, with the exception of stocky columns analysed with the [1,3] and subjected to very high temperatures.

(ii)

The current DSM estimates are (ii1) mostly safe and accurate for low temperatures (ii2) slightly unsafe for stocky columns under moderate temperatures, (ii3) gradually more unsafe as the slenderness to temperature ratio grows and (ii4) too unsafe for the stocky columns with [1,3], and high temperatures.

(iii)

The incorporation of temperature-dependent parameters into the current DSM leads to fairly good predictions for the vast majority of the numerical/experimental failure loads. Indeed, the proposed, predicted-to-numerical ultimate load ratio averages and standard deviations are in 0.86-0.94 and 0.04-0.11 for all the temperatures the current DSM counterparts are 1.03-1.17 and 0.07-0.11.

1

J. Chen, B. Young, "Experimental investigation of cold-formed steel material at elevated temperatures", Thin-Walled Structures, 45(1), 96-110, 2007. doi:10.1016/j.tws.2006.11.003

2

T. Ranawaka, M. Mahendran, "Experimental study of the mechanical properties of light gauge cold-formed steels at elevated temperatures", Fire Safety Journal, 44(2), 219-29, 2009. doi:10.1016/j.firesaf.2008.06.006

3

C. Wei, Y. Jihong, "Mechanical properties of G550 cold-formed steel under transient and steady state conditions", Journal of Constructional Steel Research, 73(June), 1-11, 2012. doi:10.1016/j.jcsr.2011.12.010

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