Keywords: finite element, steel, columns, elliptical, fire, hydrocarbon, restraint.

This paper presents the first ever validated finite element study on steel columns of elliptical hollow sections (EHS) subjected to severe hydrocarbon fire temperatures reaching 900°C in the first 4 minutes of heating. EHS present a recent addition to steel sections available to structural engineers [1]. However, despite the extensive interest in their use, there is currently a lack of dedicated studies on their structural performance under high temperatures inhibiting more widespread use in construction.

The paper presents a numerical study where a three-dimensional finite element model of the elliptical columns was built and validated using twelve specially conducted experiments on axially restrained elliptical columns subject to a hydrocarbon fire curve. The numerical analysis was performed using a staggered transient heat-flow stress path. The model considered thermal conduction and radiation heat transfer. During the thermal analysis eight-node isoparametric brick element was merged with four nodes of the three-dimensional linear boundary flow element. The boundary element was used to describe the thermal convection and radiation to the brick element surface. The temperature values resulting from the heat flow analysis were then tabulated into a time-temperature form, and used for the structural analysis. During the structural analysis the boundary element was deactivated, and the potential flow element was converted to the three-dimensional twenty node isoparametric solid brick structural element. The EC3 geometrical imperfection and accidental eccentricity were incorporated in the model when calculating the Euler buckling load which was supported by an eigenvalue analysis of the columns. The axial restraint imposed on the columns was modelled using a one way spring with constant stiffness to simulate the surrounding structure. The finite element model demonstrated an excellent agreement with experimental results including failure modes of overall and localised buckling. The verified finite element model was used to conduct a parametric study involving parameters of loading level and slenderness.

The study has shown that the greater the slenderness of the column the lower the fire resistance of the columns. The parametric study has also shown a non-linear relationship between the loading ratio and the slenderness of the elliptical columns. It was also concluded that increasing the loading level form 0.2 to 0.8 has reduced the maximum displacement of the columns by 47% and decreased the failure temperature by 28%.

1

EN 10210-2, "Hot finished structural hollow sections of non-alloy and fine grain steels - Part 2: tolerances, dimensions and sectional properties", CEN, 2006.

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