An integrated framework for nonlinear analysis of plane frames exposed to fire using the direct stiffness method

A novel coupled framework for analysis of reinforced concrete (RC) and steel planar frames subjected to fire is developed with three-way coupling between heat transfer, mechanical deformations and pore pressure build-up. Structural members are discretized in space using a two-level scheme where the mechanical solver utilizes 1D line elements, and the thermal and the pore pressure solvers work on 2D finite element (FE) meshes for each sub-span used by the mechanical solver. Such a strategy enables consideration of effects of large deformations, temperature-dependent material properties (thermal, moisture transport and mechanical), and spalling. None of the earlier developed frameworks considered a three-way coupling between mechanical, thermal and pore pressure solvers without employing a full-fledged 3D FE scheme. A matrix method type approach, developed herein, enables modeling of the three main physical processes taking place in RC members during fire without the need to consider full-fidelity 3D FEM. Several numerical examples are presented to demonstrate the accuracy and applicability of the developed framework in fire analysis of normal and high strength RC and steel structures.