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

5 CALCULATION OF STEEL TEMPERATURES FOR
PROTECTED STEEL – ISO FIRE:
5.1 INTRODUCTION:
Although steel is non-combustible, it is still affected by fires because its strength
becomes severely impaired by the increase in temperature. At elevated
temperatures steel loses a significant amount of strength, so precautions are often
required to prevent the steel from heating up too much. The methods prescribed in
the New Zealand code require that results from standard fire tests be used, ie. data
showing how the member will behave when subjected to elevated temperatures
with the protection in place. The tests are required to give an equal or worse result
than the members being designed, which means that the geometry of the beam,
thickness of the protection or loading patterns must be worse or equal to the beam
being considered.
The most common methods of protection include spray-on vermiculite or perlite
plaster, sprayed mineral fibre or gypsum plasterboard. Varying the thickness of
the insulation changes the protection offered to the steel beam, and as in Section 4,
the H p /A value of the steel member has a major influence of the energy transfer to
and temperature rise in the steel. The properties of the protection also have a large
impact on the results found from simulations. Slightly different methods are
recommended for heavy and light protection, which is also discussed in Section
2.1.3.
The simplified method for light protection, which neglects the heat capacity of the
insulation, is compared with retaining the heat capacity term in the formula.
Comparisons are also made here between the spreadsheet method, SAFIR and the
equations recommended by ECCS for protected members.
5.1.1 Assumptions:
The properties of steel used in the spreadsheet method are kept constant, namely
the density, ρ = 7850 kg/m 3 and specific heat, c s = 600. SAFIR uses varying
values according to information stored in the programme from EC3, as with
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