Investigation on Postfire Residual Capacity of High-Strength Steel Columns with Axial Restraint

This paper experimentally and analytically investigates the fire and postfire behavior of axially restrained high-strength steel columns. Tests on the buckling behavior during heating as well as the residual load-bearing capacity after heating are first conducted on the column specimens made of Grade Q550 steel with a yield strength of 730 MPa. A numerical model is established and validated against the test results, and parametric studies are conducted with influencing factors including the maximum temperature, load ratio, slenderness ratio, and steel grade. A simplified calculation method is proposed to determine the residual capacity of restrained high-strength steel columns. The experimental results show that restrained high-strength steel columns buckle at a low temperature of about 300°C. The postfire residual capacity and axial stiffness of the columns are significantly reduced to a level of 55% and 32% of its initial values, respectively, due to the presence of a residual bending deformation. The numerical results show that the residual strength decreases greatly if the maximum temperature of a column is higher than its buckling temperature. The axial stiffness ratio and slenderness ratio have a significant influence on the residual capacity of restrained high-strength steel columns at buckling temperatures, while the axial load ratio has a significant influence at critical temperatures. However, the steel grade has little influence on the residual capacity of restrained high-strength steel columns.