Fig. 2.9. Graph of allowable stress as given in BS 449: 1964 (in tabulated form) against slenderness ratio.

If, however, design is based on the safety factor method instead of the load factor method, then N is omitted and ay/n replaces ay in the formula, where n is the safety factor.

2.8. Struts with initial curvature

In ยง2.6 the Perry-Robertson equation was derived on the assumption that strut imperfections could be allowed for by giving the strut an initial curvature. This proof applies equally well, of course, for struts which have genuine initial curvatures and, provided the curvature is small, the precise shape of the curve has little effect on the end result.

Thus for an initial curvature with a central deflection Co, maximum deflection =

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