Civil Engineering Reference
In-Depth Information
Figure 3.12
Composite slab - design for fire resistance
be 780°C, at which the design tensile stress for the steel has fallen to
about 56 N/mm
2
. The temperature of the profiled sheeting is higher, and
its mean design strength is about 26 N/mm
2
. Hence the tensile force avail-
able to resist bending is about
1.178
×
26
+
0.168
×
56
=
40 kN/m
(3.51)
Assuming that the concrete at the top of the slab has not weakened, its
stress block is about 3 mm deep, so the mean lever arm for sagging
bending is about 95 mm, and
3.8
kN
m/m
M
Rd,fi,sag
=
40
×
0.095
=
(3.52)
This is less than one-quarter of the required value (Equation 3.50), so
the contribution from crack-control reinforcement at the supports is now
considered, using data from EN 1994-1-2 to find the hogging moment of
resistance.
It is assumed that 8-mm bars at 150 mm spacing (336 mm
2
/m) are
provided with 20 mm of top cover, as shown in Fig. 3.12(a). The yield
force per unit width is
336
×
0.500
=
168 kN/m
As A193 mesh has been provided at the bottom of the slab, to resist
local bending, the total transverse reinforcement above the beams is 336
+
529 mm
2
/m, which satisfies the need for 380 mm
2
/m for control of
cracking (Section 3.4.5).
The compressive resistance of the concrete is reduced by its exposure
to the fire. The concrete within the ribs is (conservatively) neglected,
leaving a uniform slab of effective thickness 95 mm. The effective depth
is 95
193
=
−
20
−
4
=
71 mm.
