Issues of Thermal Collapse Analysis of Reinforced Concrete Structures

Under extreme temperature exposure thermal softening of stiffness and strength properties as well as thermal expansion and drying shrinkage of cement-based materials have come to the forefront in the safety assessment of tunnel and jet fuel fires in high rise buildings. It is indeed unfortunate that several accidents in the recent past have reinforced the need for a more comprehensive understanding of concrete and steel materials exposed to fire environments. Thereby the transient nature of rapid heating and cooling and the thermal mismatch between concrete and steel may introduce extensive spalling in reinforced concrete because of pore pressure build up in regions of severe temperature gradients. In this paper we explore the temperature sensitivity of concrete and steel and their effects on the thermal collapse of a reinforced concrete benchmark structure (RCS) representative of a fire scenario. Specifically we review the mismatch of thermal expansion and the thermal softening of the mechanical response behavior of concrete under heating and cooling. In the sequel, we illustrate some of these issues with the thermal collapse of a RC box structure subjected to an increasing temperature gradient. Whereas structural collapse is well understood using limit analysis concepts of plastic yielding in skeletal structures, failure under combined hygro-themio-mechanical load histories deserves more attention in view of the absence of upper and lower bound theorems for softening and transient conditions. Computational failure studies using finite element simulations introduce additional issues which give raise to fundamental questions what does constitute collapse, and how does numerical failure relate to physical collapse in the presence of viscous regularization and global stabilization strategies. In the last part of the paper we explore the classical collapse problem of a portal frame for which plastic limit analysis techniques provide well-established reference solutions to validate numerical failure simulations using the incremental damage-plasticity model for concrete in ABAQUS.