Background to structural (mechanical fire) analysis


Demonstration of calculation of load bearing resistance of structural elements submitted to an increase of temperature. Guideline for the calculation of the fire rating of steel and composite elements.


The theorical failure resistance of a structural element subjected to fire is calculated from the applied load during the fire and plastic theory. Differentiation is made between bending elements and axially loaded elements and between uniformly heated sections and sections with thermal gradients. The main factors influencing stability in fire are presented. Structural analysis of composite columns and composite slabs is also discussed.


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:

Pucr) = P


P is the applied load in fire conditions

Pucr) is the load bearing resistance for a temperature θcrit (the critical temperature)

Pu is the load bearing resistance at room temperature.

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

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