Columns and Struts - Design considerations - Steel Structures.

The most important property of a strut as far as the determination of its load- carrying capacity is concerned is its slenderness, l, defined as the ratio of its effective length, LE, divided by the appropriate radius of gyration, r. Codes of practice such as BS 5950 used to place upper limits on l, as indicated in Table 15.1, so as to avoid the use of flimsy construction, i.e. to ensure that a member which will ordinarily be subject only to axial load does have some limited resistance to an acci- dental lateral load, does not rattle, etc. Although not now explicitly stated in the 2000 version of the code, it is still good practice to aim for robust construction.

Table 15.1 Maximum slenderness values for struts

Similarly, for BS 5400: Part 3, no actual limits are specified but the user is provided with a general advisory note to the effect that construction should be suitably robust. By contrast, BS 5400 does place upper bounds on local plate slenderness to avoid consideration of local buckling (see Table 15.2).

Strut design will normally require that, once a trial member has been selected and its loading and support conditions determined, attention be given to whichever of the following checks are relevant for the particular application:

(1) Overall flexural buckling – largely controlled by the slenderness ratio, l, which is a function of member length, cross-sectional shape and the support condi- tions provided; also influenced by the type of member
(2) Local buckling – controlled by the width-to-thickness ratios of the component plate elements (see Chapter 13); with some care in the original  choice of member this need not involve any actual calculation
(3) Buckling of component parts – only relevant for built-up sections such as laced and battened columns; the strength of individual parts must be checked, often by simply limiting distances between points of interconnection
(4) Torsional or torsional-flexural buckling – for cold-formed sections and in extreme cases of unusually shaped heavier open sections, the inherent low  torsional stiffness of the member may make this form of buckling more critical than simple flexural buckling.

In principle, local buckling and overall buckling (flexural or torsional) should always be checked. In practice, provided cross sections that at least meet the semi-compact limits for pure compression are used, then no local buckling check is necessary since the cross section will be fully effective.

Table 15.2 Upper limits on plate slenderness of BS 5400: Part 3

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