Implementation of the fire beam element method into OpenSees for the analysis of structures in fire

摘要

The fire beam element method is a tool for structural fire analyses that simplifies a structure into a skeletal frame consisting of only beam and column elements. It considers a shifting neutral axis of each beam element, which is updated throughout an analysis. This method was implemented in the OpenSees software environment by adding two subclasses: one for the fire beam element added to the element class, and one for the member section, in which the neutral axis is iteratively adjusted for non-uniform temperature profiles. To validate the implemented model, three benchmark case studies were sourced from literature: (1) a heated cantilever beam with an analytical solution, (2) a steel beam in a furnace with high axial and bending forces and (3) a two-dimensional steel frame in a fire with complex behaviour such as non-linear heating, restraint and buckling. For (1) the fire beam element predicts deformations identical to an analytical solution. For (2) the fire beam element method simulates deformations with good accuracy across the entire time domain relative to experimental data, and simulations in the literature using Vulcan, although with experimental deflections typically being underestimated. For (3) fire beam element predictions are compared to experimental data and models developed in CEFICOSS, ABAQUS, SAFIR and LS-DYNA. Trends are typically accurately captured, with percentage differences varying. Runaway failure is predicted with 2 min of experimental data. A sensitivity analysis of the fire beam element model on mesh size of elements and fibres showed the runtime to be more sensitive to the number of elements than the number of fibres per element.