Introduction - Composite Structures of Steel and Concrete - page 3

was then found that encasement reduced the effective slenderness of the
column, and so increased its buckling load. Empirical methods for cal-
culating the reduced slenderness still survive in some design codes for
steelwork.
This simple approach is not rational, for the encasement also carries its
share of both the axial load and the bending moments. More rational
methods, validated by tests, are given in EN 1994 (Section 5.6).
A composite column can also be constructed without the use of form-
work, by filling a steel tube with concrete. A notable early use of filled
tubes (1966) was in a four-level motorway interchange [4]. Their design
is covered in Section 5.6.7.
In framed structures, there may be steel members, composite beams,
composite columns, or all of these, and there are many types of beam-to-
column connection. Their behaviour can range from 'nominally pinned'
to 'rigid', and influences bending moments throughout the frame. Two
buildings with rigid-jointed composite frames were built in England in
the early 1960s, one in Cambridge [5] and one in London [6]. Current
practice is mainly to use nominally pinned joints. In buildings it is expen-
sive to make joints so stiff that they act as 'rigid'. Joints are usually
treated as pins, even though many have sufficient stiffness to reduce
deflections of beams to a useful extent. Intensive research in recent
years [7, 8, 9] has enabled comprehensive design rules for joints in steel
and composite frames to be given in Eurocodes 3 [10] and 4. Some of
them lead to extensive calculation, but they provide the basis for design
aids that, when available, may bring semi-rigid connections into general
use.

1.3 Design philosophy and the Eurocodes

1.3.1 Background

In design, account must be taken of the random nature of loading, the
variability of materials, and the defects that occur in construction, to
reduce the probability of unserviceability or failure of the structure during
its design life to an acceptably low level. Extensive study of this subject
since about 1950 has led to the incorporation of the older 'safety factor'
and 'load factor' design methods into a comprehensive 'limit state' design
philosophy. Its first important application in British standards was in 1972,
in CP 110, The structural use of concrete . It is used in all current British
codes for the design of structures.
Work on international codes began after the Second World War, first on
concrete structures and then on steel structures. A committee for composite