During is the technique performed to estimate the

During engineering design of a structure or building, it is
important to design and perform the fire hazard analysis. Fire hazard analysis
is the technique performed to estimate the potential impact of fire. This fire
hazard analysis technique can be divided into two categories which are the risk
based and hazard based. Risk based method analyses the likelihood of the
occurrence of an event, however, hazard based method does not. The goal of a
fire hazard analysis (FHA) is to determine the expected outcome of a specific
set of conditions called fire scenarios. These expected outcome can be made
based on expert judgement, by probabilistic methods using the past incidents’
data and by deterministic means such as fire models.

Fire hazard analysis is usually used as part of the performance
based design process. It is a straightforward engineering method to perform a
fire hazard analysis. Firstly, it is essential to select a target outcome. One
of the most specified target outcome is to avoid any fatalities in a building.
Next, is to develop a design fire scenarios which helps to determine the fire
source. Determining the fire source is one of the most important part in the
fire hazard analysis. Fire scenario is a set of conditions that defines the
development and the spread of combustion products. However, for a design fire
scenario, it is a set of conditions that defines the critical factors of
determining the outcomes for trial fire protection designs of new building or
modifications to existing buildings. The development of design fire scenarios
are essential as the data determined are very useful for future quantification.
Design fire scenarios should be based on the reasonable expected fires and
worst case fires. Past fire records for the specific building or similar
building are very useful in identifying the conditions that should be avoided.

After determination of the design fire scenarios, a design
fire curve is developed for the design fire scenario or the portion of the
design fire scenario of interest. Once the fire curve is estimated, fire
effects can be predicted. The design fire curve (as shown in Figure 2) is a
graphical representation of the heat release of fire over a period of time. In
another words, intensity of fire is said to be a function of time. Design fire
curve is divided into four phases; ignition, growth, steady-burning and decay.
The design fire curve usually starts at the ignition. Ignition phase is divided
to into piloted and non-piloted. In piloted case, a spark initiates the flaming
whereas in non-piloted case, flaming occurs spontaneously as a result of heat
even in the absence of flame or spark. Different calculation methods can be
used to determine the occurrence of an ignition. The selection of calculation
methods usually depends on the state of fuel whether they are solid, liquid or
gases type. For instance, for a thermally thin solid materials, method of
Mikkola and Wichman can be used, i.e.  where Tig and T0 = the
ignition temperature (oC) and initial temperature (oC)
respectively; tig = time to ignition (sec); ? = the density of material (kg/m3);
L0 = thickness of the material (m); c = specific heat of the
material (kT/kg.c);   and  = the external heat flux and critical heat
flux ignition (kW/m2) respectively.

Following ignition, fire might grow. The growth of fire
depends on the first item that ignites the fire or the spread of fire to the
neighboring items. The rate of fire growth depends on the arrangement and the
type of materials near the fire scene. For the selection of an appropriated
design in fire’s growth, several data need to be obtained; such as the
realistic prediction of the activation of the detector and sprinkler, the time
to start evacuation and the initial exposure of occupants. In 1972, Heskestad
proposed a formula to predict the early fire growth assumption. This is a power
equation that is expressed as:

 

Where Q = rate of heat release (kW),  = fire intensity coefficient (kW/secn),
t = time (sec) and n = 1,2,3.

            A steady burning will continue to
occur when a fire scenario involves a fire in an enclosed area. The rate of
burning usually depends on the available ventilation (i.e. the amount of
oxygen) and the available fuel. Simple algebraic calculations can be used to
calculate the fire temperatures in the room. A flashover normally occurs when
the fire will continue to grow until all the combustible items in the enclosed
room are involved. The time at which a flashover occurs can be estimated by
predicting when the fire will reach the minimum heat release rate. One of the
method to predict the minimum heat release rate necessary for a flashover is
known as the method of Babrauskas. In the method of Babrauskas, the minimum
heat release rate is expressed as:  where  is the area of opening into the compartment
and  which represents the height of opening into
the compartment. Lastly, when the fire eventually decrease in size due to the
consumption of the available fuel, depletion of oxygen and suppression, this
process is known as decay. Usually decay is omitted from the analysis.