Civil Engineering Reference
In-Depth Information
Figure 3.15
Resistance to sagging bending of composite section in
Class 1 or 2
In deriving the formulae below, it is assumed that the steel member is a
rolled I-section, of cross-sectional area
A
a
, and the slab is composite, with
profiled sheeting that spans between adjacent steel members. The com-
posite section is in Class 1 or 2, so that the whole of the design load can
be assumed to be resisted by the composite member, whether the con-
struction is propped or unpropped. This is because the inelastic behaviour
that precedes flexural failure allows internal redistribution of stresses to
occur.
The effective section is shown in Fig. 3.15(a). As for composite slabs,
there are three common situations, as follows. The first two occur only
where full shear connection is provided.
(1) Neutral axis within the concrete or composite slab
The stress blocks are shown in Fig. 3.15(b). The depth
x
c
, assumed to give
the position of the plastic neutral axis, is found by resolving longitudinally:
N
c,f
=
A
a
f
yd
=
b
eff
x
c
(0.85
f
cd
)
(3.56)
This method is valid when:
x
c
≤
h
c
