Nonlinear Material Behavior of the Bridge Components - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 86

2.6.2 Shear Connectors
In steel-to-concrete composite construction, longitudinal shear forces are
transferred across the steel-concrete interface by the mechanical action of
the shear connectors. The problem associated with this connection is that
it is a region of severe and complex stress. The methods of connection have
been developed empirically and verified by tests. These tests show that at low
loads, most of the longitudinal shear is transferred by bond at the interface.
This bond breaks down at higher loads, and once broken, it cannot be
restored. So, in design calculations, bond strength is taken as zero. Also,
greasing the steel flange before the concrete is cast destroys the bond
between it and the concrete slab. The design of the connectors to ensure
an adequate degree of interaction was first specified in CP 117 [ 2.34 ,
2.35 ]. There are numerous types of shear connectors available in the market
such as channels, bent bars, and T-sections. However, the most widely used
type of connector is the headed stud shown in Figure 2.14 . The British Stan-
dards [ 2.36 ] require the steel from which the studs are manufactured to have
an ultimate tensile strength of at least 450 N/mm 2 and an elongation of at
least 15%. The advantages of stud connectors are that the welding process
is rapid, they provide little obstruction to reinforcement in the concrete slab,
and they are equally strong and stiff in shear in all directions normal to the
axis of the stud. The property of a shear connector most relevant to design is
the relationship between the longitudinal shear force transmitted, P , and the
slip at the interface, d . This load-slip curve should ideally be found from tests
on composite beams. However, most of the data on connectors have been
obtained from various types of “push-off” test. The flanges of a short length
of steel I-section are connected to two small concrete slabs. The details of the
“standard push-off test” given in Eurocode 4 (EC4) [ 2.37 ] are shown in
Figure 2.18 . The slabs are bedded onto the lower platen of a
compression-testing machine or frame, and the load is applied to the upper
end of the steel section. The slip between the steel member and the two slabs
is measured at several points, and the average slip is plotted against the load
per connector. The push-off test must be specified in detail for the load-slip
relationship, which is influenced by many variables. The variables include
the number of connectors in the test specimen; mean longitudinal stress
in the concrete slab surrounding the connectors; size, arrangement, and
strength of slab reinforcement; thickness of concrete surrounding the con-
nectors: freedom of the base of each slab to move laterally; bond at the steel-
concrete interface; strength of the concrete slab; and degree of compaction
of the concrete surrounding the base of each connector. The amount of