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

This Annex makes a distinction between whether or not members are engulfed in
flame, depending on their locations relative to the openings from the fire
compartment. It states that a member that is not engulfed in flame is assumed to
receive radiative heat transfer from the openings that the member can ‘see’ and
from the flames projecting from the openings. A member that is engulfed in flame
is assumed to receive convective heat transfer as well as radiation from the
engulfing flames and from the fire compartment openings from which the
engulfing flame projects. Other openings and extruding flames may be neglected.
Member not engulfed in flame:
The average temperature of a steel member, T m, not engulfed by flame can be
determined by solving the following heat balance:
4 4
m
+
m
z f
σT αT
= I + I + 293α
7.13
where σ is the Stefan-Boltaman constant (5.67 x 10 -12 W/m 2 K 4 )
α is the convective heat transfer coefficient (kW/m 2 K)
I z is the radiative heat flux from a flame (kW/m 2 )
I f is the radiative heat flux from an opening (kW/m 2 )
Member engulfed in flame:
The average temperature of a steel member, T m , that is engulfed in flame should be
determined from the following equation:
4
m
4
m
σ T + αT
= I + I + αT
z
f
z
7.14
where T z is the flame temperature (K)
The radiative heat flux from an opening is to be determined from
I
f
f
4
f
( 1−
az
) σT
f
= φ ε
7.15
where φ f is the overall configuration factor of the member for radiative heat
transfer from that opening
ε f is the emissivity of the flames
a z is the absorptivity of the flames
T f is the temperature of the fire (K)
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