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

From Figure 4.11 a-c, the spreadsheet curve is close to the SAFIR 1 curve without
a concrete slab protecting and cooling the top flange of the steel beam. This is
because the spreadsheet method as used in this report makes no allowance for the
effect of a concrete slab absorbing heat. The spreadsheet assumes that all heat
transferred to the steel section is absorbed by the section and all energy absorbed
in turn contributes to the temperature rise of the member.
Differences between the spreadsheet method and SAFIR 1 result from the
spreadsheet assuming a constant thickness over the cross section of the beam,
while SAFIR accounts for the true thickness and allows conduction across the
beam. The effective thickness of the section as used in the spreadsheet method is
increased with three sided exposure due to a decreased section factor as the heated
perimeter decreases, see Section 1.6.3. The conduction feature in SAFIR means
that although the beam is not heated on the top face of the top flange in the SAFIR
1 simulation, there are no outside influences to stop it rising in temperature. The
SAFIR 2 simulation has concrete protection on the top flange so that although it
does heat up, a significant amount of the energy transferred to the top flange is
then conducted to the concrete slab.
Using the spreadsheet results is a conservative method of predicting the
temperature of three sided fire exposure, since the temperatures that this method
predicts are higher than other methods. Since in most ‘real’ cases of three sided
exposure to fire, a slab would be present, the spreadsheet method is possibly too
conservative for the average temperature. A reduction in the formula of the heat
transferred to the beam could be an alternative method of estimating the likely
temperatures that would be reached in a fire. By considering the average
temperature in the slab at each time step, an energy balance could be found to
account for the heat and energy loss to the slab, resulting in a more accurate
estimation of the likely temperatures reached in the steel section.
The SAFIR 2 simulation with the concrete slab resting on the top flange is at a
significantly lower temperature throughout the test run. The difference in
temperature between the two SAFIR results is the effect of the addition of a
concrete slab, which lowers the temperature of the top flange and therefore the
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