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

7850*600*10500 > 2*775*1100*(1869*20)
4.93 x 10 10 > 6.37 x 10 10
A i is the cross sectional area of the insulation. For the purposes of this inequality,
this is taken as the thickness of the insulation multiplied by the perimeter of the
steel. Since this equality in not satisfied, the protection can not be termed ‘light’
and the formula must be modified to allow for this.
Two methods of modifying this equation to allow for heavier insulation have been
found, the ECCS recommendations, (1985), suggest substituting
power bracket with
d
i
A
H
p
in the
d
i
A
H
p
mod
=
p
s
2
i
di
A ci
ρid
+
H 2c
ρ
s
5.2
Purkiss, (1996), recommends a slightly different variation of this, as below:
d
i
A
H
p
mod
=
d
i
A ρid
+
H ρ
p
2
i
s
5.3
The formula has been simplified from that suggested by ECCS by assuming that
for heavy insulation the specific heat of the insulation will be approximately twice
that of steel, or around 1200 J/kg K. This value is close to that assumed for
vermiculite protection, which is 1100 J/kg K in this report. This modification
effectively increases the thermal capacity of the steel by adding half the thermal
capacity of the insulation.
For the purposes of this report however, the original ECCS modified equation has
been used to compare the formulas for protected steel members from the results
calculated in the spreadsheet formula.
89