Transient Thermal 3D FE Analysis of Headed Stud Anchors Exposed to Fire

In the present paper a transient three-dimensional thermo-mechanical model for concrete is presented. For a given boundary conditions, temperature distribution is calculated by employing a three-dimensional transient thermal finite element analysis. The thermal properties of concrete are assumed to be constant and independent of the stress- strain distribution. In the thermo-mechanical model for concrete the total strain tensor is decomposed into pure mechanical strain, free thermal strain and load induced thermal strain. The mechanical strain is calculated by using temperature dependent microplane model for concrete (1). The dependency between the macroscopic concrete properties (Young's modulus, tensile & compressive strength and fracture energy) and the temperature is based on the available experimental data base. The free thermal strain, which is stress independent, is calculated according to the proposal of Nielsen et al. (2). The load induced thermal strain is obtained by employing the bi-parabolic model recently proposed by Nielsen et al. (3). It is assumed that the total load induced thermal strain is irrecoverable, i.e. creep component is neglected. The model is implemented into a three-dimensional FE code. The performance of headed stud anchors exposed to fire was studied. For a given geometry of a concrete member and for a constant concrete properties three-dimensional transient thermal FE analysis was carried out for three embedment depths and for four thermal load histories. The analysis shows that the resistance of anchors can be significantly reduced if they are exposed to fire. The largest reduction of the load capacity was obtained for anchors with relatively small embedment depth. The numerical results agree well with the available experiments.