Background to structural (mechanical fire) analysis

1. INTRODUCTION

The increase of the temperature of steel and concrete in composite steel-concrete elements leads to a decrease of mechanical properties such as yield stress, Young's modulus, and ultimate compressive strength of concrete (see Figures 1 - 4). Thus, when a steel or a composite structure is submitted to a fire action, its load bearing resistance decreases. If the duration and the intensity of the fire are large enough, the load bearing resistance can fall to the level of the applied load resulting in the collapse of the structure (see Figure 5). This state is illustrated by:

P_u (θ_cr) = P

where:

P is the applied load in fire conditions

P_u (θ_cr) is the load bearing resistance for a temperature θ_crit (the critical temperature)

P_u is the load bearing resistance at room temperature.

The objective of this lecture is to give a background for structural analysis of this situation.