An advanced beam-column element for analysis of frames in fires

Abstract

A novel advanced fiber beam-column element, which utilizes the stability functions and the fiber distributed plasticity model to capture the geometric and material nonlinearities, is proposed for the first time to predict the nonlinear inelastic thermo-mechanical behaviors of framed structures in fires by using the Fortran programming language. The element stiffness matrix is integrated via the Gauss-Lobatto numerical integration scheme, and geometric nonlinearity is considered by using the stability functions and a geometric matrix for P-δ and P-Δ effects, respectively. A nonlinear thermal incremental-iterative solution scheme based on the Newton-Raphson algorithm is also developed and operated to solve the nonlinear problems due to thermal expansion and material degradation in fires. The proposed element has the novelty of simultaneously taking advantage of the benefits of the stability functions with low computational cost and the capacity of tracing the gradual space spread of plasticity of the fiber model. At the moment it is not possible for the normal beam-column element in commercial software that allows for considering the effects of temperature to provide both low computational cost and accurate representation of the spread of plasticity. To demonstrate the proposed element's capacity in dealing with various scenarios, it will be investigated with both uniform fire, compartment fire, and localized fire with members made of steel and concrete. The accuracy of the proposed element is verified by comparing the obtained results with those from test data and the Abaqus program. Results show that the proposed element can provide exact solutions with excellent computational performance, which is more significantly effective than the traditional finite element in commercial software like Abaqus.