Strut Slenderness
The minimum slenderness limits for compression members previously given in BS 5950: 1990 have been removed from BS 5950-1: 2000.
I personally disagree with this change to the design code and would ask all structural / civil engineers to review the following warning: -
The shortening or lengthening of any member subject to axial load is generally taken to be equal to (P.L / (A.E)). This formula, used by most linear elastic computer programs, ignores shortening due to strut lateral buckling deformation. When a strut is not too slender the error in ignoring shortening caused by strut buckling is trivial, but as a strut becomes more slender this secondary shortening effect can become dominant.
The following table provides an example of the magnitude of error (Dv / x) for various strut slenderness values. For L / ry = 100 the classic assumption of strut stiffness would be in error by 10% whereas at L / ry = 200 the error increases to 280% !
|
Horizontal and Vertical Deflections for a Strut at full stress |
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|
Length |
L / ry |
Dh |
Dv |
Ratio of Dv / x |
|
1585 |
50 |
3.8 |
2.3 |
1 |
|
3170 |
100 |
19.9 |
2.7 |
1.1 |
|
4755 |
150 |
53.6 |
3.3 |
1.8 |
|
6340 |
200 |
101.5 |
5.4 |
3.8 |
|
7925 |
250 |
162.9 |
9.4 |
8.1 |
|
9510 |
300 |
237.6 |
15.6 |
16 |
|
11095 |
350 |
325.5 |
24.4 |
28.9 |
|
12680 |
400 |
426.6 |
36.2 |
48.6 |
|
The infamous Strut:- 26.9dia x 3.2thk in Grade S355 x 10m long! |
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|
Length |
L / ry |
Dh |
Dv |
Ratio of Dv / x |
|
10000 |
1182 |
1322 |
436 |
6239 |
|
x = P.L |
i.e. The classic assumption for strut shortening when displacements are small. |
|
A.E |
|
| Dh = Mid length lateral buckling displacement (mm) | |
| Dv = Total of (end shortening due to Dh) + x (mm) | |
The above tables show the worst possible condition with a strut at full stress. For a strut at 90% of full stress and L / ry = 200 the error in assumed strut stiffness is reduced to 80%!
I would be interested to hear the views as to the solution from other engineers (reinstate slenderness limits or another?).
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| Problem with Strut Deformation |
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CADOSS (c) 2004 |