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Steel Designers' Manual - 6th Edition (2003)
418 Columns and struts
how the effective length of the critical segment will be reduced if more stable segments
(shorter unbraced lengths in this case) are present. A practical equivalent for
each case in terms of simple braced frames with pinned beam-to-column connections
is also shown. It is also necessary to recognize that practical equivalents of pin
joints may also be capable of transferring limited moments. This point is considered
explicitly in BS 5950: Part 1 for both building frames and trusses; the effect on the
design of compression members is considered in detail in Chapter 18 for the former
and in Chapter 19 for the latter.
For compression members in rigid-jointed frames the effective length is directly
related to the restraint provided by all the surrounding members. Strictly speaking
an interaction of all the members in the frame occurs because the real behaviour is
one of frame buckling rather than column buckling, but for design purposes it is
often sufficient to consider the behaviour of a limited region of the frame. Variants
of the ‘limited frame’ concept are to be found in several codes of practice and
design guides. That used in BS 5950: Part 1 is illustrated in Fig. 15.11. The limited
frame comprises the column under consideration and each immediately adjacent
member treated as if its far end were fixed.The effective length of the critical column
is then obtained from a chart which is entered with two coefficients k1 and k2, the
values of which depend on the stiffnesses of the surrounding members KU, KTL, etc.,
relative to the stiffness of the column KC, a concept similar to the well-known
moment distribution method. Two distinct cases are considered: columns in nonsway
frames and columns in frames that are free to sway. Figures 15.12 (a) and (b)
and 15.12 (c) and (d) illustrate both cases as well as giving the associated effective
2' '
(a) (b)
Fig. 15.11 Limited frames. (a) Internal column. (b) External column