Civil Engineering Reference
In-Depth Information
method, the presence of the concrete is allowed for in two ways. It is
assumed to resist a small axial load; and to reduce the effective slender-
ness of the steel member, which increases its resistance to axial load.
Resistance to bending moment is assumed to be provided entirely by the
steel. No account is taken of the resistance of the longitudinal reinforce-
ment in the concrete.
Tests on cased struts under axial and eccentric load show that this
cased strut method gives a very uneven and usually excessive margin of
safety. For example, Jones & Rizk [45] quote load factors ranging from
4.7 to 6.7, and work by Faber [44] supports this conclusion. The method
was improved in BS 5950, but is still generally very conservative. Its
main advantage is that it is simpler than the more rational and economical
methods now available.
One of the earliest methods to take proper account of the interaction
between steel and concrete in a concrete-encased H-section column is due
to Basu & Sommerville [46]. It has been extended to include bi-axial
bending, and agrees quite well with the results of tests and numerical
simulations [47, 48]. It was thought to be too complex for routine use for
columns in buildings, but is included in the British code for composite
bridges. Its scope includes concrete-filled steel tubes [49], which have been
used as bridge piers, for example in multi-level motorway interchanges.
The Basu and Sommerville method is based on the use of algebraic
approximations to curves obtained by numerical analyses. For Eurocode
4:Part 1.1, preference was given to a method developed by Roik, Bergmann
and others at the University of Bochum. It has wider scope, is based on
a clearer conceptual model, and is slightly simpler. It is described in
Section 5.6, with a worked example.
5.3
Beam-to-column joints
5.3.1
Properties of joints
Three types of joint between a steel beam and the flange of an H-section
steel column are shown in Fig. 5.2, and a short end-plate joint is shown in
Fig. 5.22. They are all bolted, because they are made on site, where weld-
ing is expensive and difficult to inspect. The column shown in Fig. 5.2(a)
is in an external wall. At an internal column, another beam would be
connected to the other flange. There may also be minor-axis beams, con-
nected to the column web as shown in Fig. 5.2(c).
Where the beams are composite and the column is internal, longitudinal
reinforcement in the slab will be continuous past the column, as shown in
Fig. 5.2(c). It may be provided only for the control of cracking; but if it
