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

4.1.2 Analysis Between Methods of Temperature Evaluation:
When looking at the comparisons between different methods of analysing
temperatures of the steel, it must be established what is conservative and what is
non conservative.
1200
Predicted
temperatures
1000
Temperature ( o C)
800
600
T crit
400
200
C
SAFE
B
A
D
UNSAFE
Actual
temperature
0
0 10 20 30 40 50 60 70
Time (min)
Figure 4.1: Schematic graph showing the comparisons made between approximate formulas and
time temperature curves
If the formula is used to calculate the temperature, at a certain time, the formula
predicts a temperature that is too low, ie. from Figure 4.1, the formula will predict
a temperature at the point A, whereas the actual time temperature curve gives a
higher temperature relative to point B. This is unsafe because a member which can
carry a certain load at temperature A could fail at temperature B.
If the formula is used to calculate the time that a limiting temperature is reached,
the formula is again unsafe. The actual fire curve gives a time as seen in Figure
Figure 4.1 at C, while the formula gives a time located at point A. Since point C
occurs at a earlier time than point A, the formula gives a fire resistance rating time
for the element which is longer than it actually is safe for.
Graphically, if the line is below the curve, as from the origin to point D, then the
formula is giving unsafe or non conservative results. If the line is above the graph
as from point D to the end of the line, the formula is giving conservative and safe
results.
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