Time Equivalence

The concept of time-equivalence is used to relate the severity of real fires to the time-temperature relationship in a standard fire test. Figure 1 illustrates the concept of time-equivalence, relating the actual maximum temperature of a structural member from an anticipated fire severity, to the time taken for the same member to attain the same temperature when subjected to the standard fire.

Figure 1: Concept of time-equivalence

There are a number of time-equivalence methods which take into account the amount of fuel load, compartment size, thermal characteristics of the compartment boundaries and ventilation conditions, including:

• Law (1971)
• Pettersson (1976)
• CIB W14 (1986)
• Harmathy (1987)
• BSEN1991-1-2 (2002)

Generally, time-equivalence can either be determined by using a simple equation or taken from experimental data from natural and standard fire tests. Although simple to use, the time-equivalence is a crude approximate method of modelling real fire behaviour and bears little relationship with real fire behaviour. In addition, the limitations of the method should be clearly understood. The main limitation is that the method is only applicable to the types of members used in the derivation of the adopted formulae.

In the following, the time-equivalence method given in EN1991-1-2 and the background research will be discussed.