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

the slab except to reduce the section factor for a reduced heated perimeter.
Reducing the section factor in effect increases the effective width, making the
temperature rise of the steel decrease to a slower rate. The difference is very small
however so a substantial difference in the temperature results from the spreadsheet
method is hard to distinguish when comparisons are made between the spreadsheet
method and SAFIR.
The maximum temperatures found in the beam are not affected by changing the
fire exposure conditions. For conservative results for the temperatures reached in
the member, analysis of the member as if exposed on four sides to the fire by the
spreadsheet method gives results close to the maximum temperatures as found by
the SAFIR programme. Even though the average temperature is much lower when
a concrete slab is present as found in SAFIR, this only because a small percentage
of elements are at a much cooler temperature which subsequently lowers the
average temperature of the steel. Throughout the section of the steel however,
only the cooling effects of the concrete affect the top flange that is in contact with
the slab. Since it is the lower flange that yields and forms a plastic hinge, this
cooling effect does not affect the failure temperature of the beam.
5.6.3 Comparison with the ECCS Formula:
The ECCS equations for three sided exposure are the same as for four sided
exposure but with a reduced section factor. The equation for protected steel beams
with heavy insulation is equation 5.1 in Section 5.2.3, and is below:
40( 140)
d
= −
i
A c
+
i
ρid
t T
i
l
2
ki
H
p
cs
ρ
s
0.77
The properties of the insulation are listed in Section 5.6, and are of Fendolite by
Firepro.
Figure 5.14 shows the comparison of the ECCS formulas with three sided exposure
compared with the results from the spreadsheet method and SAFIR.
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