Beams failing in LTB (Buckling)

[shove2001]

Hi,

I have had a checking engineer claim that beams designed using SuperBeam are failing in buckling. I am investigating which beams (of four) and re-printing "with Details" shown to show the Buckling checks shown. Has anybody else had similar claims?

Mark

[Tony Bryer]

I would like to know more on this. Is he suggesting that the calculation is wrong, or does he think that you have used the wrong effective length? Recent discussions in the Structural Engineer suggest that checking engineers do not always agree on what effective length factor should be used.

[shove2001]

I will check this effective length issue and get back to you.

Mark

[shove2001]

The Building Inspector has changed his tune and does not have a problem with Superbeam. He now says that my loads should be increased...

" For steel beams, BS 5950 requires the loadings to be factored (ie increased) by a factor of; -Dead loads x 1.4 -Live loads x 1.6. This gives a factor of safety overall of +50% ."

Only when he adds this 50% do the beams fail in buckling. Is this factoring taken into account in Superbeams section selection?

Mark

[Tony Bryer]

It looks like the BCO has failed to realise that SuperBeam calculations are done using BS449 which is a permissible stress code (BS5950 is a limit state code). You do not factor the loads when using a permissible stress code - the factor of safety comes from the fact that the permissible stress shown (e.g. 180N/mm2 for a restrained S275 steel beam) is well below that which will cause failure. If you were doing the calculations to BS5950 then you would need to factor the loads by 1.4/1.6 but the resulting stresses would then be compared against the actual strength of the material (275N/mm2 for S275 steel).

For a discussion on the use of BS449 see our web page http://www.sda.co.uk/info/sbw/bs449.htm