Civil Engineering Reference
In-Depth Information
The mid-span effective breadth of 2250 mm was found for a 8.6-m
span. The increase for a 9.3-m span has little effect on the properties and
has been ignored.
Other properties, not in Table 4.5, are as follows:
for the steel section:
10
−6
W
a,pl
=
1.194 mm
3
10
−6
I
az
=
12.0 mm
4
V
pl,Rd
=
697 kN
4.6.2
Flexure and vertical shear
A rough check on the adequacy of the assumed beam section is provided
by rigid-plastic global analysis. The value of
wL
2
/8 that can be resisted by
each span is a little less than
M
pl,Rd
+
0.5
M
pl,a,Rd
=
829
+
212
=
1041 kN m
neglecting the reinforcement in the slab at the internal support, B in
Fig. 4.11. This is well above
wL
2
/8 for the loading (659 kN m, from
Table 4.4).
Minimum reinforcement at the internal support
It is assumed that the exposure class is X0 or XC1 (Section 4.2.5) and that
the limiting crack width is 0.4 mm under quasi-permanent loading. It is
assumed initially that the top longitudinal reinforcement at support B is
six T12 bars (
A
s
679 mm
2
) at 200 mm spacing. The cross-section is then
as shown in Fig. 4.1. It was found in Section 4.2.1.2 that its hogging
resistance is
M
pl,Rd
=
1.225 m.
The area
A
s
may be governed by the rules for minimum reinforcement
(Section 4.2.5.2). These require calculation of the distance
z
0
in Fig. 4.14(a)
for the uncracked unreinforced composite section with
n
=
510 kN m, with effective flange width
b
eff
=
10.1. Initial
cracking is likely to occur above the small top ribs of the sheeting, where
the slab thickness is 80 mm, so the assumption
h
c
=
=
95 mm, made for
serviceability checks at mid-span, is not appropriate.
The transformed area of the uncracked section is
A
=
7600
+
1225
×
80/10.1
=
17 300 mm
2
Taking moments of area about the top of the slab for the neutral-axis
depth
x
c
,
17 300
x
c
=
7600
×
353
+
9700
×
40
whence
x
c
=
177 mm
