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

The values for the thermal properties of the steel and geometry of the shape are
averaged values and not necessarily exactly what the member will be when constructed
in place. This means that high accuracy is unnecessary and often inappropriate when
many other factors, such as the temperature of the fire are also estimated.
The advantages of the spreadsheet programme are that the computer software that is
required for it is a spreadsheet programme such as Microsoft Excel, which is installed
in most office computers. For computer programmes such as SAFIR, files such as
beam sizes, are required as well as the three units of the programme. With the
estimations and assumptions made in fire design and analysis ,the 5 % difference
found between the two methods can not show that one is more accurate than the other.
Since the spreadsheet method usually gives higher temperatures than the SAFIR and
other finite element programmes, the results are acceptable to use in design with four
sided exposure and when analysing the temperature elevation of simply supported
members.
2.1.2 Unprotected Steel:
The temperature of unprotected steel is found from the following method, (Buchanan,
1999):
Time Steel Temp Fire Temp T f – T s h t ∆T s
T s
T f
t 1 = ∆t Initial steel Fire Temp at T f - T so Eqn 2.2 with Eqn 2.1
temp, T so ∆t/2
T s and T f
from this row
t 2 = t 1 +∆t T s +∆T s from Fire Temp at T f - T s Eqn 2.2 with Eqn 2.1
previous row t 1 +∆t/2
T s and T f
from this row
etc etc etc etc etc etc
Figure 2.1: Spreadsheet calculation for heat transfer in unprotected steel members, (Buchanan, 1999)
The difference in temperature of the steel over the time period is calculated from:
∆T
s
H
p
ht
=
A
sc
ρ
s
( T − T ) ∆t
where H p /A is the section factor of the beam (m -1 )
20
f
s
2.1