10.07: Numerical analysis of the fire resistance of high‐strength steel circular columns

Abstract

High strength steels (HSS) have specific chemical compositions, which primarily depend on rolling tempering techniques, element thickness and the producer. Nonetheless, EN1993-1-12 related to HSS up to S700 grade does not provide any additional note on the design of steel structures subjected to fire and the designer is referred to EN1993-1-2 valid up to S460 grade. This paper presents the results of a numerical analysis on the behaviour at elevated temperature of Circular Hollow Sections (CHS) columns made of HSS. In details, three full-scale HSS CHS columns with average measured yield strength of 822 MPa and 3.15 m long were tested under the standard ISO fire with constant eccentric compression load. Two members were made of a cross section 323.9 mm x 10 mm and one of a cross section 355.6 mm x 12 mm. The constant applied loads during the tests were respectively equal to 10%, 14% and 19% of the axial capacity of the columns at ambient temperature. Then, a nonlinear thermomechanical finite element model made of solid elements was developed by means of ABAQUS. A fully-coupled analysis using Geometrically and Materially Nonlinear Imperfect Analysis (GMNIA) was employed meaning that the temperature field was determined together with the stress-displacement fields. The calibration of the numerical model was carried out against experimental data by exploiting the thermal and the mechanical properties of steel provided in EN1993-1-2. A sensitivity analysis was performed to highlight the main parameters that affect the simulation results, i.e. mesh size, residual stresses, geometrical imperfections etc. The calibration process showed a good agreement between the numerical results and the experimental outcomes both in terms of failure temperature and of failure mode. With the calibrated model, it was finally possible to perform a preliminary parametric analysis by varying the applied load and the column relative slenderness to assess the appropriateness of the buckling curve included in EN1993-1-2 to predict the fire resistance of HSS CHS columns.