Continuous beams and slabs, and beams in frames - Composite Structures of Steel and Concrete - page 134

There are several reasons for the apparently conservative requirement
of EN 1994-1-1 that full shear connection be provided in hogging regions:

(1) To compensate for some simplifications that may be unconservative:

•

neglect of the tensile strength of concrete,

• neglect of strain-hardening of reinforcement,

• neglect of shear due to reinforcement (e.g., welded mesh) provided

for crack-width control that is neglected at ultimate limit states.

(2) Because the design resistance of connectors, P Rd , is assumed not to

depend on whether the surrounding concrete is in compression or
tension. There is evidence that this is slightly unconservative for
hogging regions [25], but slip capacity is probably greater, which is
beneficial.

(3) For simplicity in design, including design for lateral buckling

(Section 4.2.4) and for vertical shear with tension-field action [39].

The worked example in Section 4.6 illustrates the situation where the
design resistance to hogging bending is that of the steel section alone, so
that N t 0 in Equation 4.14, even though light reinforcement is present.

It would be prudent then to provide shear connection for that reinforce-
ment, as otherwise the uniform spacing of connectors could lead to under-
provision in the sagging region.

=

Transverse reinforcement
As explained for regions of sagging bending, this reinforcement should be
related to the shear resistance of the connectors provided, even where, for
detailing reasons, their resistance exceeds the design longitudinal shear.

4.2.4 Lateral buckling

Conventional 'non-distortional' lateral torsional buckling occurs where
the top flange of a simply-supported steel beam of I section has insuffi-
cient lateral restraint in the mid-span region. Both flanges are assumed to
be restrained laterally at the supports, where the member may be free to
rotate about a vertical axis. The top flange, in compression, is prevented
by the web from buckling vertically, but if the ratio of its breadth b f to
the span L is low, it may buckle laterally as shown in Fig. 4.3(a). The
cross-section rotates about a longitudinal axis, but maintains its shape.
It has to be checked that lateral-torsional buckling does not occur dur-
ing casting of the concrete for an unpropped composite beam; but once
the concrete has hardened, the shear connection prevents buckling of this
type. The relevant design methods, being for non-composite beams, are
outside the scope of this topic.