Shear connection - Composite Structures of Steel and Concrete - page 27

Fig. 2.1(c), or an encased stanchion, Fig. 5.14, the analogy with re-
inforced concrete suggests that no shear connectors need be provided.
Tests have shown that this can be true for cased stanchions and filled
tubes, where bond stresses are usually low, and also for encased beams
in the elastic range. In design it is necessary to restrict bond stress to a
low value, to provide a margin for the incalculable effects of shrinkage of
concrete, poor adhesion to the underside of steel surfaces, and stresses
due to variations of temperature.
Research on the ultimate strength of encased beams has shown that,
at high loads, calculated bond stresses have little meaning, due to the
development of cracking and local bond failures. If longitudinal shear
failure occurs, it is invariably on a surface such as AA in Fig. 2.1(c), and
not around the perimeter of the steel section. For these reasons, codes of
practice do not allow ultimate-strength design methods to be used for
composite beams without shear connectors.
Most composite beams have cross-sections of types (a) or (b) in Fig. 2.1.
Tests on such beams show that, at low loads, most of the longitudinal shear
is transferred by bond at the interface, that bond breaks down at higher
loads, and that once broken it cannot be restored. So in design calcula-
tions, bond strength is taken as zero and, in research, the bond is deliber-
ately destroyed by greasing the steel flange before the concrete is cast. For
uncased beams, the most practicable form of shear connection is some form
of dowel welded to the top flange of the steel member and subsequently
surrounded by in situ concrete when the floor or deck slab is cast.

2.4.2 Shear connectors

The most widely used type of connector is the headed stud (Fig. 2.6).
These range in diameter from 13 to 25 mm, and in length, h , from 65 to
150 mm, though longer studs are sometimes used. Studs should have an

Figure 2.6 Headed stud shear connector