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

4 CALCULATION OF STEEL TEMPERATURES FOR
UNPROTECTED STEEL – ISO FIRE:
4.1 INTRODUCTION:
The temperature rise of unprotected steel can be modelled using a variety of
computer programmes and calculation methods. In the New Zealand code,
formulas provide an estimate of the time until a limiting temperature is reached,
which is sufficient for situations where a designer is confirming the structural
stability of a steel member. These formulas however are valid for a limited time
period, temperature range and for a limited range of steel member sizes. They also
only provide an estimate of the time at which a member will reach a particular
temperature, based on test results from experiments performed with unprotected
beams exposed to the standard fire.
The spreadsheet method is a simple calculation method which estimates the
temperature in a one dimensional heat flow or ‘lumped mass’ approach. It was
developed using basic heat transfer principles and the amount of temperature rise
of the steel is based on the energy being transmitted to the member. The major
variable to influence the rate of temperature rise of unprotected steel beams when
using the spreadsheet method is the H p /A value, which is a ratio of exposed
perimeter to cross sectional area, and the inverse of the average or effective
thickness. This is discussed further in Section 1.6.3.
In this section, the computer programmes SAFIR, Firecalc, and FIRES-T2 are
compared, to examine the repetitivity of these finite element software programmes,
and to determine whether the most user friendly, SAFIR, could be used in place of
the others with the same accuracy of results. The results from SAFIR are also
compared with the results from the spreadsheet method to examine the accuracy of
the spreadsheet method. It is also investigated whether the spreadsheet method can
be used with the same confidence as with the finite element computer programmes
for simple fire related steel member problems such as the average temperature of
the steel after a given time.
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