Simple Structural Models to Compute the Fire Resistance of Restrained Steel Columns using Energy Principles

Abstract

AbstractA simple structural model to represent the behaviour of restrained steel columns in fire is presented. The heat transfer and the mechanical problems are split and the mechanical one is analysed in the temperature's domain, in order to allow the application of any appropriate fire curve. The structural model is composed by two rigid rods connected by plastic hinges. These respect the bending moments – axial forces interaction and degrade their resistance with heating. At the model's top cross section an axial spring is inserted, to represent the restraining of the surrounding frame to the column's axial elongation, and an axial load is applied in order to model the serviceability loading applied prior to the fire event. In the worst‐case scenario, fire occurs only in the neighbourhood of the analysed column while the surrounding frame remains unaffected. Therefore, both the serviceability load and the restraining spring stiffness remain constant throughout the fire event. Longitudinal temperatures gradients are accounted for. Both pinned‐pinned and clamped‐clamped boundary conditions are considered, to assess the influence of the column's statical indeterminacy on its fire resistance. Equilibrium is obtained by means of energy balance: when the temperature rises, the variation of the work produced by the serviceability load equals the variation of the internal strain energy stored at the restraining spring plus the variation of the energy dissipated at the plastic hinges. A set of illustrative examples closes the work.