Simplified design procedures for the structural analysis of reinforced concrete columns in fire

Abstract

Simple analytical procedures are described to predict the structural behavior (axial displacements and failure) of reinforced concrete (RC) columns exposed to fire. The analyses assume a concentric load (no initial eccentricity) or an equivalent axial load to take into account first and second order effects. The main objective is to provide reliable and straightforward tools for a preliminary assessment of the structural integrity of RC columns. The procedures compute the total displacements and the failure (time and type) of RC columns with different concrete strengths (normal to ultra-high performance), shapes, fire incidences, load levels and initial eccentricities. The results of 24 experimental tests from different authors are predicted and compared with the analytical solution. The displacements are estimated by the introduction of EN 1992-1-2 (2004) stress–strain curves and a simplified bilinear constitutive law that takes into account the Load Induced Thermal Strain (LITS) as a reciprocal to the modulus of elasticity. In general, a good agreement with the experimental time vs. displacement curves was obtained. Failure is predicted with the adoption of the nominal stiffness method (EN 1992-1-2, 2004) to take into account the second order effects. Sectional analyses (axial-bending equilibrium) and simplified analytical procedures assuming an equivalent axial load replacing the axial-bending force (force equilibrium) are carried out. The results of both analyses were very close, indicating the reliability of the simplified procedure. The prediction of the fire resistance time demonstrated a good approximation, on the safe side, with the experimental data. The results are compared with the EN 1992-1-2 (2004) prescriptive formula.