Civil Engineering Reference
In-Depth Information
(2) Simple calculation models
These methods enable the three preceding stages to be applied, in simplified
form, in checks on the resistances of cross-sections. These are normally
done only for the temperature distribution at the specified failure time,
assuming that beams and slabs are simply-supported and columns are pin-
ended at each floor level. The model for a composite slab is explained in
Section 3.3.7.5.
(3) Tabulated data
For cross-sections of beams and columns that are often used in practice,
results of calculations by method (1) or (2) are presented in EN 1994-1-2
as tabulated values of minimum dimensions, areas of reinforcement, etc.,
for each fire resistance class. Methods of this type are used for the beams
and columns of the worked example in this topic.
3.3.7.5
Simple calculation model for unprotected composite slab
It is assumed that the dimensions and properties of materials for the slab
are known, and that its cold design was for distributed loading on simply-
supported spans, for which the bending moments
R
d
and
E
d
are known, so
that
η
fi,t
(Equation 3.32) is known.
It is assumed that the required fire resistance period (
t
fi,d
) is 60 minutes,
and that the profiled sheeting, not protected by insulation, is heated from
below by the standard fire.
Thermal insulation criterion
An equation in EN 1994-1-2 gives the fire resistance time for thermal
insulation,
t
i
, as a function of the dimensions of the cross-section, defined
in Fig. 3.7, and the density of the concrete. For the worked example that
follows, it gives
t
i
157 minutes, so the slab can prevent fire spreading to
the floor above, provided that it does not collapse. This criterion is not
considered further.
=
Load bearing function
For the bending resistance of the slab, the strength of the steel sheeting
after 60 minutes' exposure is very low, and the tensile strength of the
Figure 3.7
Dimensions of composite slab
