FIRE DESIGN OF STEEL MEMBERS - Civil and Natural Resources ...

Figure 4.2 shows the variation of temperature between the different points on the
cross section of the beam. The maximum and minimum temperatures shown in the
graph are the maximum and minimum temperatures of an element at each time
step over the time period examined here. The element in which the maximum or
minimum temperature is found is not necessarily the same throughout the entire
time period of the graph. During the later stages of the simulation, however, the
same element in the cross section has the maximum and minimum temperatures.
The average temperature on the curve is simply an average of all elements at each
time step. No consideration due to the size or position of the element on the cross
section has be made. In the later stages of the test this is especially acceptable
because these temperatures of all the elements merge to within a small temperature
range, and the later temperatures are more vital when considering the structural
behaviour of the beam.
In each of the SAFIR curves there is a small plateau region in the temperature of
the steel, when the beam has been subjected to the fire for around 20 minutes.
This reduction in the rate of temperature rise is due to the increase in specific heat
of steel, which occurs when the temperature of steel is between 650 °C and 800
°C. The same behaviour and similarities are present in all three sizes of beams
tested.
The specific heat changing has an impact on the rate of temperature increase
because it governs the amount of energy that is required to be absorbed by the steel
for its temperature to be raised. The value of the specific heat of steel changes
from being between 600 – 800 J/kg K at temperatures below 650 °C, to a
maximum of 5000 J/kg K when the steel reaches a temperature of 735 °C as shown
in Figure 1.1. This variation in magnitude of about 8 causes the significant
flattening of the curves.
The maximum, minimum and average temperatures of elements in the SAFIR tests
are shown in Figure 4.2. The maximum difference between the maximum and
minimum temperatures is 49 °C, which occurs 5 minutes after the start of the
simulation when the temperature range is 353 to 402 °C, and the average
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