Branch-Switching Procedure for Buckling Problems of Steel Elements in Fire

In numerical shell-based simulations employed to capture both local and global buckling of steel members, the definition of a consistent initial geometrical imperfection plays a primary role. However, the choice of an appropriate amplitude and the introduction of such an imperfection is not straightforward, above all if the behaviour at elevated temperatures is being investigated. In fact, when the temperature of a steel member increases due to a fire, the material properties degrade and redistribution of the load may also occur. In order to tackle the outlined problem, an investigation was performed to analyse the effect of the imperfections on compression steel plates by means of an advanced approach based on a triangular thermomechanical co-rotational shell element that incorporates a branch-switching procedure. This procedure allows for a complete description of the physical behaviour of a structure by checking the existence of possible critical points along the fundamental path at each time step and then switching to the secondary path. Analyses were performed on perfect and imperfect plates representative of web panels. The study confirmed that the introduction of imperfections is not trivial and that the branch-switching procedure may represent an appealing preliminary tool to understand the structural behaviour.