This means that when a metal structure is exposed to the action of fire, its internal temperature tends to increase quite rapidly, the effect of which is an equally rapid reduction in its strength and rigidity. As soon as the internal temperature reaches the critical temperature value, which usually varies between 500°C and 650°C, depending on the static framework, the conditions under which it is exposed to fire and the load it is bearing, structural collapse happens. In more technical terms, the collapse condition happens when the resistance of the material, as a result of the temperature increase caused by exposure to fire, falls below the tension induced by the external loads. The internal tension does not vary over time because both the load applied and the geometry of the section are kept constant.
The speed at which a steel element heats up depends on the ratio between the surface exposed to fire (Am) and its volume (V). This ratio is called the Section factor or Mass factor. The larger the surface of the element exposed to fire, the more capable it is of absorbing heat; the greater its volume, the less capable it is of absorbing heat.
In the presence of fire therefore, given surfaces of an equal size exposed to fire, more slim-line structures can reach collapse temperature in under 10 minutes, while more solid structures may take over 30 minutes.
For steel framework in all categories, whether it is open or closed, it is easy to calculate the approximate section factor using simple formulae and/or tables. Once the section factor and critical temperature are known for the element to be protected, in addition to the length of time required, it is possible to identify the most suitable type of protective coating and the necessary thickness to be applied.