Axial restraint effects on the fire resistance of composite columns encasing I-section steel

This paper presents an experimental study of the axial restraint effect on fire resistance of four unprotected encased I-section composite columns. Axial restraints were applied to simulate thermal restraints from adjoining cool structures onto a heated composite column in a compartment. These real-sized 3.54 m long columns were subjected to concentric axial force at a load ratio of 0.7 at normal ambient temperature. Different degrees of axial restraint are investigated. An electric furnace was used to apply four-face heating condition on the columns for approximating a realistic fire scenario. All columns failed in flexural buckling mode. In the later part of the paper, finite element simulations were conducted to compare with test results. Numerical predictions of both temperature distribution and structural response during heating agree reasonably well with experimental data. Both test results and numerical analyses show that axial restraint significantly reduces the column fire resistance. Moreover, it was also observed that during heating all specimens underwent concrete spalling at mid-height, which noticeably decreased the fire resistance. Column critical times are also predicted according to Eurocode 4 Part 1.2, which are consistently shorter than the numerical predictions.