Civil Engineering Reference
In-Depth Information
There are 13 troughs at 0.3 m spacing in the 3.84-m length AD in Fig. 4.11.
The choice of
n
r
is discussed in Section 3.11.2.
Assuming that
n
r
≥
2, the
number of studs needed is 1250/42
30, so four studs are provided in
each of the two troughs nearest to support A, and two in each of the other
11 troughs; total, 30.
From Fig. 4.11, the length DB is 5.46 m. Its sheeting has 5.46/0.3
=
18
troughs. The force to be resisted is 1250 kN plus 295 kN (Section 4.2.1.2)
for the reinforcement at cross-section B, at yield; total, 1545 kN, requir-
ing 37 studs at 42 kN each. The provision of four studs in the trough nearest
to support B, and two in each of the other 17 gives 38 studs, which is
sufficient.
The transverse reinforcement should be as determined for the sagging
region in Chapter 3.
=
4.6.5
Check on deflections
The limits to deflections discussed in Section 3.7.2 correspond to the
characteristic combination of loading (Expression 1.8). Where there is
only one type of variable load, as here, this is simply
g
k
q
k
, but three sets
of calculations may be required, because part of the permanent load
g
acts
on the steel member and part on the composite member, and two modular
ratios are needed for the composite member.
In practice, it is usually accurate enough to combine the two calcula-
tions for the composite member, using a mean value of the modular ratio
(e.g.,
n
+
20.2 here).
For design purposes, maximum deflection occurs when the variable
load is present on the whole of one span, but not on the other span. The
three loadings are shown in Fig. 4.13, with the bending-moment distribu-
tions given by 'uncracked' elastic analyses, in which the beam is assumed
=
Figure 4.13
Loading for deflection of span AB
