PD79743 (2003) adopts the simple calculation
method proposed by Wickström (1986) for calculating the temperatures
in concrete members exposed to the standard fire in accordance
with BS476 or real fire conditions. It should be noted that this
method does not take into account of possible spalling of concrete.
The fireexposed surface temperature T_{s} of
a concrete member at a time t is first given by:

(1) 
with 

where
n_{s} 
is the ratio between gas and surface temperatures of concrete
member [°C]; 
T_{g} 
is the gas atmosphere temperatures [°C]; 
t_{s} 
is the scaled time as given by Eq.(2) [hr]. 
The scaled time t_{s }, accounting
for the variation in thermal properties between the concrete being
considered and a nominal standard mix for normal weight concrete,
is given by:

(2) 
where
ρ_{c} 
is the density of concrete at elevated temperatures [kg/m^{3}]; 
A_{t} 
is the total internal area, including openings, of the enclosure
[m^{2}]; 
A_{w} 
is the area of the openings [m^{2}]; 
h_{w} 
is the opening height [m]; 
c_{c} 
is the specific heat capacity of concrete at elevated temperatures
[J/kg L]; 
k_{c} 
is the thermal conductivity of concrete at elevated temperatures
which is assumed to reduce linearly from approximately 1.25
W/m K to 0.5 W/m K between 100 °C and 1200 °C [W/m
K]; 
t 
is the time [hr]. 
It is worth noting that when predicting the response
of normal weight concrete exposed to the standard fire, the scaling
of time is unnecessary and t_{s} may be set to equal t.
For uniaxial heat flow condition, such as in a
slab, the temperature rise T_{x} at any depth
x [m] beneath the fireexposed surface of the member is a factor n_{x} of
the surface temperature T_{s} with n_{x} given
by:

(3) 
with 

where K_{c} is the thermal diffusivity of concrete
[m^{2}/s].
Hence, the temperature rise T_{x} is given by:

(4) 
The method can be applied to concrete members
heated on parallel faces simultaneously, in which n_{x} is
simply the superimposed total of the n_{x} values
calculated with respect to each face.
The method can also be used for corners of beams
where there is accommodated heat flow from two directions, through
superimposition of the contributions from the orthogonal faces n_{x} and n_{y} as
follows:

(5) 
where n_{y} is calculated in the same way as n_{x} .
In case of normal weight concrete exposed to the
standard fire in accordance with BS476, the scaling of time is
unnecessary and t_{s} in Eq.(2) may be set to
equal t. Hence, Eq.(4) can be greatly simplified as follows:

(6) 
