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

Thermal conductivity, k i = 0.19 W/m K
Density, ρ i = 775 kg/m 3
Thickness, d i = 0.02 m
5.6.1 Results from a Simulation with the SAFIR Programme:
When analysing the unprotected steel sections with a concrete slab on the top
flange, the wizard pre processor was used to set up the data and structural files
required by SAFIR for the simulation. However, difficulties were experienced
when this was attempted with protected steel. To overcome this problem and
attempt to model the effect of having a concrete slab supported by the steel beam,
the pre processor by John Mason was used. This pre-processor does not have the
option of having a slab on top of the beam, but by protecting the beam with spray
on protection on four sides, and then changing the properties of the protection to
those of concrete for the elements on the top face of the beam, a concrete slab can
be modelled.
The problems with this method is that the thickness of the concrete slab is 20 mm
rather than 150 mm, and the width of the concrete slab only extrudes to the edge of
the insulation, or 20 mm further on either side of the width of the beam. Making
the assumption that this models the effect of a concrete slab is conservative as
using a thicker slab that extrudes further from the faces than shown here will give
lower temperatures than those found using this method.
Figure 5.11 shows the layout of the protected steel beam as inserted into SAFIR,
and the resulting contour lines of temperature after 10 minutes of exposure to the
ISO 834 fire.
In Figure 5.11, the contour lines can be seen to follow the perimeter of the beam
through the insulation, but not through the concrete slab. This is because there is
no fire exposed to the outer face of the concrete slab. The temperature of the slab
increases due to conduction from the steel, but the temperature of the concrete is
much lower than that of the insulation and the steel. The temperature of the
coolest contour line is 84 °C but this is at the steel-insulation interface. It can be
seen from Figure 5.11 that this contour does not extend around to the concrete-
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