Effective length query

[billy dale]

Hi all,

Please see attached pdf, can you please explain why i can have a beam with such a high slenderness ratio.

Billy Dale

[Tony Bryer]

Firstly, for anyone else reading this who hasn't looked at the attachment, you've got a 9m span beam with an o.w. of 0.25kN/m and a PL of 6.34m at 0.3m from R1. The resulting BM and SF are much the same as this point load being applied at the centre of a 2.1m span, thus the small section, 125x65x15 PFC. BS5950 does not set hard limits for dead load deflection, but the calculated figure of 31.3mm (L/287) should raise questions.

When it comes to the calculation, the actual slenderness, 623.8, is multiplied by the buckling parameter (u), 0.942, and slenderness factor (v), 0.282. The latter is calculated using the formula in BS5950 B.2.3, which doesn't (unless I've missed something) have any limits, whilst Table 19 runs out at L/x of 20 (in your case the value is 56.2) and a slenderness factor of 0.47. Were this to be used, the slenderness would be 276, and the member would fail.

This is definitely one of those cases where you look at the computer output and ask yourself whether the suggested section is appropriate, regardless of what the numbers say. In particular this section provides next to no margin for accidental loads - if it's a beam running across a loft and someone was to stand on the middle of it, it would probably collapse, and it also has minimal resistance to sideways loads (which might or might be capable of being imposed depending on where the beam is located. Also, as stated in the manual, I would generally counsel against the use of single channels as unrestrained beams as torsional effects (which are not checked by the program) may be material.

I will take a longer look at this and if I have anything to add will post it here.

[billy dale]

Tony,

Thank you very much for your prompt reply, while I have adopted this section for the design I have opted to provide a fully lateral restraint to the beam in the form of bracing thus reducing the effective length of the member.

Billy

[Tony Bryer]

I have addressed this in ProSteel 5.40 by adding a warning note to the printout where the slenderness exceeds 250: "Slenderness, xxx exceeds 250: stability during construction and resistance to accidental impacts has not been checked but may be a relevant design consideration".

Registered users with current update can download ProSteel 5.40 using the Internet > Check Web Site for Updates option within a currently installed copy of ProSteel. The key change in this release is the addition of beam splice checking - flange plate and end plate splicing are supported.

For a full list of changes see http://www.superbeam.co.uk/userpsw.htm