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

The spreadsheet method gives temperatures greater than that of SAFIR even
though SAFIR accounts for more heat transfer modes, because the spreadsheet
method assumes that the insulation protection is in contact with the steel section
for the whole perimeter, and transfers energy by conduction to the beam over the
full perimeter.
The H p /A value is smaller with box protection so the spreadsheet distinguishes that
as a thicker beam. For a BHP-530 UB 82.0 steel beam, the H p /A value for spray
on protection is 178 m -1 , which gives a effective width steel thickness of the
inverse of this which is 5.6 mm. With box protection added to the same beam, the
H p /A value changes to 140, giving an effective steel thickness of 7.1 mm. The
thicker section should give lower temperatures for the beam, and when the H p /A
value is decreased to those more equal to that of three-sided exposure, the results
become closer to those found from SAFIR. With the spreadsheet ‘seeing’ the
beam as a thicker section, it is assumed the temperatures would be lower than the
results from SAFIR as thicker members take longer to heat up than thin members
do. Since this does not occur this suggests that the energy received by the beam
through conduction through the insulation as calculated in the spreadsheet, is much
more than the radiation and convection components as modelled in the SAFIR
programme.
5.5 RESULTS FOR FOUR SIDED EXPOSURE WITH THICK
PROTECTION:
In this section, the thickness of the insulation has been increased to 40 mm, rather
than 20 mm as used in Sections 5.2-5.4. The one dimensional model which the
spreadsheet equations are based on requires that the ratio of the outside perimeter
to the inside perimeter is not too large, so that the one dimensional assumptions are
valid. If the ratio becomes too large, the calculations will give temperatures that
are too low, and a two dimensional heat flow analysis would be required.
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