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

1.6 BACKGROUND INFORMATION:
1.6.1 Forms of Heat Transfer:
Finite Element computer software, such as SAFIR use three forms of heat transfer to
evaluate the temperature of the steel member. These heat transfer forms are also
accounted for by the equations used by the spreadsheet method.
Radiation:
Radiation is the strongest form of heat transfer as the energy transferred between two
bodies is related to the fourth power of the temperature. Radiation transfers energy via
electromagnetic waves and these will be intercepted by any object which can ‘see’ the
emitter. Unlike convection and conduction, heat can travel by radiation through a
vacuum (a space with no particles), although on earth there is always a medium, air,
which radiation must travel through (Tucker, 1999).
The general formula for radiation between an emitting surface and a receiving surface
is:
4 4
q = ϕεσ ( T e
− T r
)
1.1
Where T e is the temperature of the emitting surface (fire temperature), and T r is the
temperature of the receiving surface (steel element temperature).
When using this formula in the spreadsheet analysis, the emissivity, ε, is assumed to be
0.5, which is the default setting in the SAFIR programme. The temperatures are
expressed from the absolute temperature scale (Kelvin), and the Stefan-Boltzmann
constant, σ, has a value of 5.67 x 10 -8 kW/m 2 K 4 .
The configuration factor, ϕ, is an estimate of the fraction of the area that the emitter
occupies of all that the receiver can ‘see’. This has been assumed to have a value of
unity during this project.
Convection:
Convection arises from the mixture of fluids, either liquid or gaseous that are at
significantly different temperatures to result in different densities. Heat transfer takes
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