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

1
T
s, cr
= 39.19ln
−1
+ 482
3. 833
3.7a
0.9674µ
0
or, for values of µ 0
from 0.22 to 0.80, the limiting temperature has been
calculated and entered in a table. The degree of utilisation is a factor giving the
ratio of design load to load bearing capacity of the member at elevated
temperatures, and the same effecitve factor as r f in NZS 3404.
When this equation is rearranged to produce a formula in terms of µ 0
, this
becomes:
− 1
3.833
T − 482
µ
0
= 0.96741
+ exp
3.7b
39.19
Fitting the results from this equation against those in Figure 3.2 gives a curve that
fits the data points included in the table in EC3. This is expected, as the
calculation of the limiting temperature in NZS 3404 is a direct rearrangement of
the yield stress formulas also.
3.1.4 Temperature rise of unprotected steel:
According to NZS 3404:1997
The time (t)at which the limiting temperature is attained shall be calculated
for:
a. Three sided exposure as follows:
0.433T
l
t = −5 .2 + 0.0221T
l
+
3.8
SF
b. Four sided exposure as follows:
0.203T
l
t = −4 .7 + 0.0263T
l
+
3.9
SF
H
where: SF is the section factor of the steel member p
, m
A
-1 .
These formulas have limitations for the steel temperature and the section factor of
the beam. These are:
500 < T l < 850 °C in NZS 3404, and 500 < T l < 750 °C in AS 4100
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