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ADVISORY DESK

AD 270

Strut Slendernessand BS 5950-1: 2000Since the revision to BS 5950-1 in 2000, our Advisory Service has

received many questions concerning the removal of the maximum

slenderness values for struts (180, 250 and 350) that were found in Cl.

4.7.3.2 of BS 5950-1: 1990.

The last paragraph in Cl. 4.7.3.2 of BS 5950-1: 1990 has also been

removed. This covered struts of a slenderness greater than 180, and

required that if self weight deflection exceeded length/1000, thebending effects needed to be taken into account in design.

The removal of the slenderness values and the comment about

bending effects was not a mistake. They have not been lost from the

revised Code by some unfortunate error during editing or printing at

the time of the revision, as has been suggested several times. The

removal of the maximum slenderness values was, in fact, a

deliberate decision of the Code Committee.

The compressive strength (pc) of a strut is obtained from Table 24 in

BS 5950-1: 2000. These values come from the equations of the

various strut curves found in Annex C, in particular C1 and C2. Atypical strut-buckling curve from BS 5950-1 is as shown in Figure 1,

together with the Euler buckling curve.

In the high slenderness region of the strut-buckling curve, Fig. 1

shows that there are no cut-offs, breaks or steps at certain values of

slenderness as was implied by Cl. 4.7.3.2 of BS 5950-1: 1990. In this

region the curve is smooth, continuous and, in theory, continues to a

slenderness of infinity. However, as the slenderness of a strut

increases, the compressive strength reduces and in the high

slenderness region it tends to zero.

Consequently, the maximum values of slenderness in Cl. 4.7.3.2 ofBS 5950-1: 1990 were simply custom and practice that had been

incorporated into the main UK steel design code and first introduced

into BS 449 many years ago. As can be seen from Fig. 1, these

maximum slenderness values were arbitrary and had no particulartechnical justification and thus the Code Committee decided to

remove them. In addition, the paragraph concerning self-weight

deflection has also been removed because it too was arbitrary,

because some bending effects due to self weight are present in all

horizontal or inclined struts.

Although the values or limits in Cl. 4.7.3.2 of BS 5950-1: 1990 were

arbitrary, when designing struts in the high slenderness region of a

strut-buckling curve to BS 5950-1: 2000 it would be prudent for the

designer to bear in mind the following points:

If the strut is horizontal or inclined then the designer shoulddetermine whether bending and/or lateral torsional buckling due

to self-weight needs to be taken into account.

If struts to BS 5950-1: 1990 were governed by the slenderness

limits and not the calculated capacity, some reserve of strength

existed. This reserve of capacity may not exist in struts designed

to BS 5950-1: 2000, and hence designers should carefully

consider all loading cases.

Bearing these points in mind, struts in the high slenderness region of

a strut-buckling curve may be designed safely to BS 5950-1: 2000.

Contact Thomas Cosgrove, SCI

Telephone 01344 623345 Emailt.cosgrove@steel-sci.com

Fig. 1. Typical strut (flexural) buckling curve (BS 5950-1, 1990 & 2000)