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

For all three sizes of beam, the spreadsheet method of calculating the temperature
of an unprotected beam over time correlates well to the results given from the
SAFIR programme. The temperatures from the spreadsheet method are lower than
the average temperatures over the beam as exported from the SAFIR programme
for approximately the first 10 minutes. After this time the spreadsheet results are
slightly higher than the SAFIR results until they merge to be within 2 °C of each
other. At temperatures of over 800 °C, this is an error of within 0.25 %.
Differences between the spreadsheet and SAFIR programme can be accounted for
by considering the different methods of evaluation of temperature. The
spreadsheet uses constant values for thermal conductivity, density and specific heat
for steel and protection when used. The SAFIR programme uses more accurate
graphs of these properties varying with temperature. The spreadsheet method does
not consider variances in temperature over the cross section but assumes a constant
temperature instead.
4.2.3 Comparison with NZS 3404 and ECCS formulas for four sided
exposure:
The New Zealand and Australian codes contain formulas to give an estimate of the
temperature of four and three sided fire exposed steel beams. These formulas are
based on results from experimental standard fire tests performed by the British
Steel Institute. The ECCS recommendations are also formulated empirically and
have been compared with experimental data in ECCS, (1985).
The formulas existing in the codes are:
t = −4 .7 + 0.0263T
A
l
+ 1. 67Tl
4.1
H
from NZS 3404 and AS 4100
p
This formula is valid in the temperature range of 500 °C to 850 °C by the New
Zealand code, but the Australian code has an upper temperature limit of the steel as
750 °C. Below 500 °C, both codes allow linear interpolation between the time
that the steel becomes 500 °C and a temperature of the steel of 20 °C at the start of
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