Thermal Gradient Estimation due to Surface Heat Exchange in Steel I-Sections

Fire poses a great threat to steel structures. An accurate estimation of the temperatures in structural member cross sections is the first and most crucial step to calculate fire-induced forces and moments and ultimately to evaluate the fire performance of a structural system. This study focuses on the thermal behavior of perimeter I-section columns in steel buildings subjected to the standard fire curve. Because I-sections have a concave enclosure or cavity, the surfaces in the enclosure exchange heat by radiation, which significantly changes the thermal gradient in the section. An efficient algorithm is developed to calculate the radiation heat flux generated by surface heat exchange in I-section enclosures. The results show that the surface heat exchange in the enclosure reduces the thermal gradient as much as 40% for shorter and wider sections. The fire test results confirm the numerical findings. Using a simplified equation of a partially restrained column, it is shown that the thermally induced moment reaches as much 60% of the column plastic moment capacity, which indicates a significant reduction in the load-bearing capacity of the building structure. (C) 2017 American Society of Civil Engineers.