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

This time-temperature curve shows the results from the spreadsheet method and
the SAFIR programme. The curve of the fire is also shown, which remains at 20
°C once the fire decays to this temperature. This Eurocode parametric fire has a
ventilation factor of 0.04,0.08 and 0.12 from top to bottom respectively; a fuel load
of 800; and a thermal capacity of wall linings (√kρc) of 1160. See Section 2.3.2
for more details regarding the Eurocode fires.
Figure 6.1a-c shows that the temperatures of the steel from both the SAFIR
simulation and the spreadsheet analysis reach a temperature close to the
temperature of the fire and that the temperature of the steel begins to decrease
immediately after the temperature of the fire begins to decay. At the peak fire
temperature, the steel temperature is less than 2 °C lower than the fire. The rate of
temperature decrease is slower in the steel than the rate of decrease of the fire
because the temperature decrease is based on the difference of temperatures.
When the temperature difference between the fire and the steel is small, the
decreasing rate of temperature of the steel is small because the steel can not lose as
much heat energy to its surroundings if the surroundings are at a similar
temperature.
As in Section 4 and 5 with the ISO fire, the curve from the SAFIR analysis reaches
a plateau when the temperature is around 650 –750 °C. The curve from the
spreadsheet calculations continues with a steady increase of temperature, until the
steel temperature nears the temperature of the fire. This occurs during the decay
region also when the steel is cooling down. This is due to the specific heat of steel
changing in the SAFIR programme, and remaining constant in the spreadsheet
analysis. As with the heating of the steel, when the beam can absorb more energy
before the temperature increases, when the steel temperature is between 650 – 800
°C, the steel must lose more heat energy before it can cool down in this
temperature range.
Figure 6.1 a-c confirms the results seen with the ISO fire in Section 4, that the
spreadsheet method gives very close results to those found in SAFIR for
unprotected steel with four sided exposure.
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