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

Figure 5.3 a-c show the results from four sided exposure to the standard ISO 834
fire, for 66 minutes. The graphs show a very close correlation between the
temperatures predicted by the spreadsheet and the average temperatures found
from the SAFIR simulation. The spreadsheet gives slightly higher temperatures
than SAFIR at lower temperatures of the test, but the difference between the
average SAFIR temperature and the spreadsheet temperature increases as the beam
gets hotter, with the spreadsheet temperatures exceeding those determined from
SAFIR.
This is again due to the thermal properties of steel remaining constant with the
spreadsheet method, but varying with SAFIR. The curves tend to separate more as
the temperature increases, and the spread increases greatly at around 650 °C to 700
°C as this is where the thermal properties of SAFIR deviate most from the constant
value of 600 J/kg K that is used in the spreadsheet.
The deviation from the spreadsheet is more pronounced with protected steel than
from the unprotected members considered in Section 4 due to the rate of heating
that the beam experiences. Since the rate of heating is much slower due to the
protection applied to the beam, the time period that the temperature of the steel
remains at around 650 °C to 700 °C is longer than when no protection is added.
Therefore overall the difference in temperature between the results from the
SAFIR programme and from the spreadsheet method from keeping the properties
constant in the spreadsheet is more marked for protected steel than for unprotected
steel.
The properties of this insulation are such that the equation used in the spreadsheet
formula accounts for the heat absorbed by the insulation. If a lighter insulation
was used, the equation could be simplified by assuming the heat absorbed by the
insulation is negligible as described later in Section 5.3.
The heavier beams appear to have better agreement between the results from
SAFIR and the spreadsheet method than the light 180 UB 16.1 beam. This is
because in the time scale used in SAFIR for the comparisons, the heavier and
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