Fire performance of restrained welded steel box columns

• The paper examines the effect of corner welds, load ratios, and b/t to the fire performance of welded steel box columns.
• The CJP box columns perform better fire behavior than thos of PJP box columns at 500 °C.
• No cracking was observed on CJP box columns at 500 °C.
• Fire resistant of PJP box columns can be enhanced with the decreasing of the applying loads and the b/t ratios.
• Compact section is suggested for PJP box column to avoid the premature cracking at corner weldes.

The objectives of this study is to investigate the effect of weld details (CJP or PJP), the load ratios, and the width-to-thickness ratios on the fire performance of unprotected welded steel box columns at 500 °C. Upon the experimental works, five column specimens were loaded to their ultimate state at room temperature and ten unprotected column specimens were loaded by steady-state method up to 500 °C.In this study it is found that local buckling is the failure mode observed for the column specimen tested either at room temperature or in the fire condition. At 500 °C, unprotected CJP steel box columns show superior fire behavior compared to the columns fabricated by PJP which are more susceptible to premature cracking at the corner welds. CJP box columns with non-compact sections and no discernible cracks are able to sustain design loads of 0.8ϕPn for 2 h under temperature conditions of 500 °C. At 500 °C, the fire resistance of PJP box columns can be enhanced by decreasing the applied loads and/or the width-to-thickness ratios. But, the fire resistance of steel box columns would be reduced significantly whenever the premature cracks were existed in the corner welds. Based on the result of this study, it is found that CJP box columns would be a better choice than PJP box columns for unprotected steel box columns under 500 °C. If CJP is not feasible in practical application and PJP is used instead, it is strongly suggested to design a compact section for those PJP box columns.