Prescriptive vs Performance-Based Approach For Structural Fire Design

Friendly fire can in seconds turn into ferocious flames if failed to be tamed in reasonable period. Structural bare steel’s performance in natural fire has always been a difficult task to predict despite countless efforts to mimic this actual scenario by full scale fire testing and standard fire test in furnaces. Fire engineers and researchers worldwide has taken more rigorous approach in design of structural steel elements to reciprocate severe and aggressive fire loading, whilst not compromising the aesthetic appearance, ease of maintenance and overall performance of the steel.

Nevertheless, prescriptive method or commonly known as deemed-to-satisfy method, used by many engineers around the world as a general rule of thumb served its intended purpose where individual building elements were assessed for its fire resistance periods. However, complex and refined building design added the need for an enhanced method. This led to the birth of performance-based approach for the assessment of structural fire design for improved safety and functionality. Bukowski et al [1] pointed out that the drive towards performance-based design took place in Japan in 1982. This method gave an extra edge over the prescriptive design approach in many aspects. Furthermore, this added additional duty for engineers to re-engineer themselves to adapt to the all-new performance-based approach to understand how the whole structure behaves under extreme loading and fire conditions. This advanced approach takes into consideration the fire behaviour, thermal response, and structural analysis.

Fire Behaviour
Each fire bears its own distinctive behaviour in terms of fire intensity, fire density, distribution, smoke amount released and damage to internal and external structural elements in an event of fire. Fire severity can be summarized as shown in Figure 1. Nominal fires comprise of standard fire, hydrocarbon fire, external fires, and smouldering fires. The fire models, which relate the temperature-time relationship, are considered simple in terms of application, this includes time equivalent, and compartment fires because it assumes uniform gas heating and uniform fire spread whilst not considering the smoke movement.

Additionally, a parametric fire provides simple design method to approximate post-flashover compartment fire and takes into account the fuel load, ventilation conditions and the thermal properties of compartment walls and ceilings. The use of advance models such as zone models and CFD models will give detail input on the fire behaviour that will simulate the heat and mass transfer.

1) Standard fire curve
For many years, the standard fire curve, ISO 834, was extensively used to determine the relative performance of construction materials. The temperature-time relationship is given below and set out in EN 1363 [2].
qg = 345log10 (8t+1) + 20

where:
qg is the gas temperature in the fire compartment (ºC); and
t is the time(min).

The limitation of this curve is that there is no descending branch, i.e. no cooling phase. Cooling phase can be very important with regard to structural performance, particularly when large thermal restraint is present.
2 External fire curve
The external fire curve is used for structural members in a façade external to the main structure. The external fire curve is given by:
qg = 660(1-0.687e-0.32t – 0.313e-3.8t ) + 20
where :
qg is the gas temperature in the fire compartment (ºC); and
t is the time(min).. 010020030040050060070080001020304Time(min)Temperature(°C)
Figure 3: External fire curve
3 Hydrocarbon curve
Most of the actual fires that happen domestically can be related to hydrocarbon fire. In the presence of carbon related products, i.e. plastics and petrochemicals, the temperature rise is very rapid due to the much higher calorific values of the materials. Temperature-time-curve for this situation is given below,
qg = 1080 ( 1- 0.325e-0.167t – 0.675e-2.5t) + 20
where :
qg is the gas temperature in the fire compartment (ºC); and
t is the time(min).
02004006008001000120001020304Time(min)Temperature(°C)
Figure 4: Hydrocarbon curve
4 Equivalent time of fire exposure
The code EN 1991-1-2 [3] incorporates a method for determining the appropriate fire resistance period for design based on a consideration of the physical characteristics of the fire compartment. The method given in Annex F of [3] is material dependent. It is not applicable to composite steel, concrete and or timber constructions. The method also relates the severity of a real fire in a real compartment to an equivalent period of exposure in a standard test furnace.
te,d = ( qf,dkbwt)kc
where:
te,d is the equivalent time of fire exposure for design(min)
qf,d is the design fire load density (MJ/m2)
kb is a conversion factor dependent on thermal properties of linings
wt is the ventilation factor;
{ wt = ( 6 / H )0.3[0.62 + 90(0.4-av)4] }
in the absence of horizontal opening(roof lights) in the compartment,
where:
H is the height of the fire compartment (m) and
av = Ventilation area (Av) / Floor area (Af)
kc is a correction factor dependent on material.
(the value of kc is taken as 1.0 for protected steel and reinforced
concrete and 0.09 if no detailed assessment of the thermal properties is
made)
Therefore, the fire resistance period is then that the fire resistance of the member is greater than the time equivalent value.
Thermal analysis
Thermal analysis is used to determine the temperature distribution, heat accumulation or dissipation, and other related thermal quantities in an object. The primary heat transfer mechanisms are conduction, convection and radiation. Often an object will fail because of stresses induced by uneven heating, rapid temperature change or differences in thermal properties. The strength of all engineering materials reduces, as their temperature increases. Steel is no exception. However, a major advantage of steel is that it is incombustible. During a fire, steel absorbs a significant amount of thermal energy. After this exposure to fire, steel returns to a stable condition during cooling to ambient temperature. During cycles of heating and cooling, individual steel members may become slightly bent or damaged, generally without affecting the stability of the whole structure.
Structural analysis
By determining the thermal properties of steel at elevated temperature, it is then possible to calculate the mechanical behaviour of steel in similar conditions. The difference in computing the behaviour at ambient and elevated temperature would be the use of the right stress-strain curve. Since the stress-strain relation at elevated temperature is non-linear from initial as shown in Figure 2, the linear elastic theory cannot be used for fire design, whereas at ambient, it is possible to employ the linear elastic theory. 0501001502002503000.000.020.040.060.080.100.120.140.16Total strain [%]Stress [N/mm2]
Figure 5: Stress strain curve for average tensile stress 295N/mm2 to EC3-1-2:2005
References
[1] Bukowski, R.W., and Tanaka, T., “Toward the Goal of a Performance Fire Code,”Fire and M,aterials Vol. 15, No. 4, pp. 175-180, 1991.
[2] British Standards Institution (1999) Fire Resistance Tests-Part 1: General
Requirements. BSI, London, BS 476-20.
[3] Eurocode 1: Actions on Structures. EN1991-1-2:2002 General actions.
Actions on structures, European Committee for Standardization, Brussels
Renga Rao Krishnamoorthy
Postgraduate researcher
Extreme Loading & Design Group
The University of Manchester, UK