Failure of lightly reinforced concrete members under fire. I: Analytical modeling

This paper is concerned with the failure of lightly reinforced concrete members under fire conditions, with particular emphasis given to the catenary action arising from axial restraint at the supports and the ensuing rupture of the reinforcement. The relevance of this work stems from the need to make a fundamental step toward understanding the conditions that influence the failure of a steel-decked composite floor slab, which is shown to become effectively lightly reinforced at elevated temperature. A new analytical model is proposed for lightly reinforced members subject to axial restraint, which accounts for the compressive arch and tensile catenary stages, bond-slip, yielding, and rupture of the steel reinforcement as well as the effect of elevated temperature. The versatility of the proposed model and the conditions which govern its validity are illustrated in this paper through comparisons with detailed computations based on nonlinear finite element analysis. The companion paper utilizes the proposed analytical model to perform a parametric investigation into the factors influencing the failure of lightly reinforced members, and to highlight key implications for structural fire resistance design.