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

Protected steel exposed to the Eurocode parametric fire gives similar results to the
results found from the comparisons with beams exposed to the standard ISO 834
fire. The results from the spreadsheet give higher temperatures than the results
from the SAFIR programme as with the results with the ISO fire. The curve with
results from the spreadsheet method allows for the heat capacity of the insulation
due to the properties of the insulation being classed ‘heavy’.
This Eurocode parametric fire has a ventilation factor of 0.08, a fuel load of 800
MJ/m 2 , and a thermal inertia of the wall linings of 1160 Ws 1/2 /m 2 K and is the
‘middle’ fire of those used in this report. The beam is the medium weight beam
used throughout this report and sized as BHP 310 UB 40.4.
Figures 6.2 a, the spreadsheet curve and SAFIR curve follow the same path until
the temperature reaches about 650 °C, at a time of about 45 minutes. The
spreadsheet then gives lower temperatures than the SAFIR programme. The lower
temperatures from SAFIR after the steel reaches a temperature of 650 °C is due to
the higher specific heat of steel which is taken into account by SAFIR but not in
the spreadsheet method. The slower decrease in temperature in the decay stage of
the fire is due to the same effect.
The temperature difference between the two methods is more pronounced in
protected steel than unprotected steel due to the time period that the steel is in the
650 – 800 °C temperature range. Since the steel is heating up at a slower rate with
protected steel than with unprotected steel, the time that the steel is in this
temperature range is longer and the results from the spreadsheet method deviate
further from the SAFIR temperatures in these situations.
The results from the spreadsheet turn out to be quite conservative because it
estimates higher temperatures due to this effect and the maximum temperature
reached in the beam from the spreadsheet method is 901 °C while the maximum
temperature found from SAFIR is 800 °C. The difference of slightly more than
100 °C, means the spreadsheet has a maximum temperature of 12.5 % more than
that found from using the SAFIR simulation.
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