Civil Engineering Reference
In-Depth Information
Chapter 5
Composite columns and frames
5.1
Introduction
A definition of
composite frame
is given in Section 4.1, with discussion of
joints
,
connections
, and of the relationship between these and the methods
of global analysis (Table 4.1).
To illustrate the presentation of continuous beams in the context of
buildings, it was necessary to use, in Section 4.6, an atypical example:
a two-span beam with a wall as its internal support, with no transfer of
bending moment between the wall and the beam.
Where a beam is supported by a column through a joint that is not
'nominally pinned', the bending moments depend on the properties of the
joint, the beam and the column. They form part of a frame, which may
have to provide resistance to horizontal loads such as wind (an 'unbraced
frame'); or these loads may be transferred to a bracing structure by the
floor slabs.
Where the lateral stiffness of the bracing structure is sufficient, the
frame can be designed to resist only the vertical loads (a 'braced frame').
The lift and staircase regions of multi-storey buildings often have con-
crete walls, for resistance to fire. These can provide stiff lateral restraint,
as can the end walls of long narrow buildings. The frames can then be
designed as 'braced'.
Unbraced frames require stronger members and joints. The long-
standing empirical 'wind-moment' design method [43], in current use
for frames of moderate height, is not given in EN 1994-1-1.
Some of the design rules of Eurocodes 3 and 4 for semi-rigid and
partial-strength joints are so recent that there is little experience of their
use in practice. The scope of this chapter is limited to braced frames with
beam-to-column joints that are either 'nominally pinned' or 'rigid and full
strength'. The structure shown in Fig. 5.1 is used as an example. Typical
plane frames such as DEF are at 4.0-m spacing, and support composite
floor slabs as designed in Section 3.4. Each frame has ten two-span beams
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