Abstract

Cold-formed steel has been increasingly used in storage rack upright systems because of its high specific strength and economy.This paper presents a numerical investigation into the axial strength of steel storage rack uprights exposed to a localised fire(the fire source is at the bottom of member)shown as Figure 1.Finite element analyses(FEA),considering perforations and complex cross-sections,were carried out to investigate the heat transfer and buckling behavior.Firstly,the heat translation analysis was carried out to calculate temperature distributions along the length of members.Based on the temperature field,the linear perturbation analysis was performed to investigate eigenvalue buckling analyses in which elastic critical buckling loads and buckling modes were obtained.Finally,the Riks method was employed in nonlinear analysis to predict the ultimate loads and the failure modes of steel storage rack uprights exposed to localised fires.In addition,modified direct strength method(DSM)curves were reported in this paper for predicting the axial strength of coldformed storage rack uprights under localised fires.

Figure 1　Temperature gradient of uprights under localised fires

Current structural fire design methods are based on international organization for standardization(ISO)834 fire,where the temperature of gas and steel members exposed in fires are uniform.However,it was highly probable in large compartment that compression members were exposed to non-uniform heat flux in real fires[1].The temperature gradient of structural components has important negative influences on buckling behaviors,which had been reported by Zhang et al.[2]Therefore,the temperature gradient along the length of uprights was considered in analytical models to consider a localised fire.According to EN 1991-1-2[3],the flame length,the diameter of the fire and the rate of heat release were considered in the finite element models.The material properties and thermal performance of cold-formed steel at elevated temperatures were based on EN 1993-1-2[4].The results of verification between the numerical results and existing experimental data show that the FEA have good performance for heat transfer[5],elastic critical buckling loads[6]and ultimate loads[7]of coldformed compression members.

This study employed verified FEA to investigate the buckling behavior of 280 models with types of cross-sections,multiple lengths,different thicknesses and various temperatures(20℃,200℃,400℃,600℃and 800℃)aimed at assessing the performance of the current DSM for such uprights under localised fires.Detailed comparisons of ultimate loads and the failure modes between room temperature and elevated temperature were demonstrated in this paper.The results showed that failure modes of uprights were significantly influenced by temperature gradient.Comparison of axial strengths predicted from the current DSM and FEA,this paper reported the predictive equations modified from the current DSM for cold-formed steel uprights exposed to localised fires shown as Figure 2 for 800℃.

Figure 2　Comparison of the current DSM global curve and modified curve