Civil Engineering Reference
In-Depth Information
Figure 3.6
Effective width of composite slab, for concentrated load
a
m
=
a
p
+
2(
h
f
+
h
c
)
(3.25b)
The width of slab assumed to be effective for global analysis and for
resistance is given by
b
em
=
b
m
+
kL
p
[1
−
(
L
p
/
L
)]
≤
width of slab
(3.26)
where
k
is taken as 2 for bending and longitudinal shear (except for
interior spans of continuous slabs, where
k
1.33) and as 1 for vertical
shear. These rules become unreliable where the depth of the ribs is a high
proportion of the total thickness. Their use is limited in EN 1994-1-1 to
slabs with
h
p
/
h
=
0.6.
For a simply-supported slab of span
L
and a point load
Q
Ed
, the sagging
moment per unit width of slab on line AD in Fig. 3.6(a) is thus
≤
m
Ed
=
Q
Ed
L
p
[1
−
(
L
p
/
L
)]/
b
em
(3.27)
which is a maximum when
L
p
L
/2.
The variation of
b
em
with
L
p
is shown in Fig. 3.6(a). The load is as-
sumed to be uniformly-distributed along line BC, whereas the resistance
is distributed along line AD, so there is sagging transverse bending under
the load. The maximum sagging bending moment is at E, and is given by
=
M
Ed
=
Q
Ed
(
b
em
−
b
m
)/8
(3.28)
The sheeting has no tensile strength in this direction because the corruga-
tions can open out, so bottom reinforcement (Fig. 3.6(b)) must be provided.
