Nominal Fires

The nominal or standard fire curves are the simplest way to represent a fire by pre-defining some arbitrary temperature-time relationships, which are independent on ventilation and boundary conditions. Historically, they were developed for fire resistance furnace tests of building materials and elements for their classification and verification. The main disadvantages and limitations of standard fires include:

1. The standard fires do not represent real natural fire. The differences in the heating rate, fire intensity and duration between the standard and real fires can result in different structural behaviour. For example, a short duration high temperature fire can result in spalling of concrete exposing steel reinforcement due to the thermal shock. Whereas a long duration low temperature fire can result in a higher average temperature in the concrete members resulting in a greater reduction in concrete strength.
   
2. The standard fires do not always represent the most severe fire conditions. Structural members having been designed to standard fires may fail to survive in real fires. For example, the modern offices tend to contain large quantities of hydrocarbon fuels in decoration, furniture, computers and electric devices, in forms of polymers, plastics, artificial leathers and laminates etc. Consequently, the fire becomes more severe than the conventional standard fire.

Although there are disadvantages and limitations of assuming the nominal fire curves and member design, the simplest and most common performance-based approaches have been developed based on the results and observations from standard fire resistance tests.

Considering, in a simplistic form, different fuel types and ventilation conditions, BSEN1991-1-2: 2002 and PD7974-1: 2003 provides the following nominal temperature-time curves:

* Note: This fire curve was developed so that materials such as intumescent coatings which rely upon chemical reactions could be subject to a test that addressed possible concerns regarding their intumescing behaviour. It was not meant to represent a design fire scenario (PD7974-3: 2003).

BSEN1991-1-2 provides three nominal fire curves as follows:
a) For standard fire,

(1)

(2)

(3)

PD7974-1 adopts the same equations for the standard and hydrocarbon fires. However, the code provides alternative fire curve for large pool hydrocarbon fire, as well as a slow growing fire, as follows:
a) For large pool hydrocarbon fire,

(4)

b) For smouldering fire,

(5)

Figure 1 shows the various nominal fire curves for comparison. It can be seen that, over a period of 2 hours, the hydrocarbon fire is the most severe followed by the standard fire, with the external fire being the least severe fire although the slow heating fire represents the lowest temperature up to 30 minutes. It is noteworthy that for standard and smouldering fires, the temperature continuously increases with increasing time. For the external fire, the temperature remains constant at 680°C after approximate 22 minutes. Whereas for the hydrocarbon fires, the temperatures remain constant at 1100°C and 1120°C after approximate 40 minutes.

According to the nominal fire curves, the Eurocodes provide some heat transfer parameters for thermal analysis to structural members such as convection factor, emissivity of fire and surface emissivity of members. The structural response of the members in fire can be calculated. This ‘simple’ performance-based approach will generally allow more economical buildings to be designed and constructed compared to those designed using the prescriptive approach.