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

The temperature range that equation 4.3 is valid for is non conservative, when
comparing the line with the time temperature curve from SAFIR 1. Neglecting the
results from Figure 4.12 a, due to the section factor for this beam size being out of
range, the upper limit for the temperature based on the results of Figure 4.12 b and
c, should be closer to 750 °C than the present 850 °C in the New Zealand Steel
Code. This is the upper temperature limit in the Australian Code. The ECCS
equation temperature limitations could be modified, by increasing the upper limit,
but by a lesser amount than suggested for four sided exposure in Section 0. The
temperature range for equation 4.2 could be raised to at least 700 °C for three sided
exposure, with a linear interpolation region added to the method here for
temperatures below 400 °C.
Since the formula for equation 4.3 is modified for three sided exposure from
equation 4.1, the line from equation 4.3 fits better with the SAFIR 2 curve with a
slab. This is because the data that the line is formulated to fit to is from results of
three sided exposure with a slab present, from British Steel Institute tests, SNZ,
(1997).
Equation 4.3, therefore, appears to give a more realistic prediction of the average
temperature of steel beams with three sided exposure, but again equation 4.2 is
more conservative, by giving higher temperatures. The designer may wish to
choose a more economical design by using the estimations of the NZS formulas,
but the factor of safety in design is less than when using alternative methods of
temperature estimation.
Although the average temperature of steel beams is best predicted by equation 4.3,
the ECCS formula, equation 4.2, gives a temperature estimation closer to the
maximum likely temperature that will be reached in the steel, which is unaffected
by fire exposure conditions, see Section Figure 4.10. Using the maximum
temperature instead of the average temperature for three sided exposure is more
conservative, as it gives higher temperatures and will give a more likely failure
time for simply supported members. The temperature of the steel section is
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