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Old 20th March 2014, 11:21 AM   #1
motorphotos
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Join Date: Apr 2011
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Default Effective lengths of beams

Hi Tony

Had a question from a checking engineer regarding the effective lengths of beams and destabilising loads

I normally do domestic stuff with simple padstone bearings so no lateral restraint and use end bearing condition 7 and if there is a wall over I tick the box about destabilising loads.

The query was ' why am I doing that to eurocode as the span is the span' he said he has no problem as the design is more conservative and I am normally one size larger than the beam could be

I can find the bit where L is used to determine deflections Table 7 I think but not about calculating effective length s

Regards

John
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Old 20th March 2014, 11:41 AM   #2
Tony Bryer
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Default Re: Effective lengths of beams

The checking of unrestrained beams in accordance with EC3 is fraught with difficulty since EC3 fails to give useful guidance on some key aspects of this. The subject is discussed at length in the manual, see 'Steel beam design to Eurocode 3'. To quote

" ... The effective length of an unrestrained beam, or lengths of the end segments if there are intermediate restraints is a function of the support conditions at the restraints. In the absence of express guidance in EC3, BS5950 Table 13 provides suitable guidance. The conditions most likely to be encountered are
  • Ends restrained against rotation, flanges free to rotate on plan. 1.0L (e.g. beam built into a party wall)
  • Partial torsional restraint against rotation about longitudinal axis provided by connection of bottom flange to supports. 1.0L + 2D (e.g. beam bottom flange fixed down, top flange free)
  • Both flanges free to rotate on plan; partial restraint against rotation provided only by pressure of bottom flange on support. 1.2L + 2D (e.g. loft conversion beam sits on blocking piece at eaves) ...

The last, condition G/7 is the most onerous but rarely applies, so only select it where it applies.

Re destabilizing loads, again I quote the manual:

"SCI Paper 093 1.4 says “… It is of course fundamental to the discussion in this Section that the loading is applied in such a way that as the beam tends to buckle sideways the loading can move freely with it. If the load cannot move in this way … no destabilizing effect will be present. Thus in practice, destabilizing loads are only considered in cases for which the applied loading offers no resistance to lateral movement, e.g. a free standing brick wall on a beam. Normal loads from floors etc. do not constitute a destabilizing load …”.

Consider the above free standing wall built off a steel beam. If the beam twists fractionally, the wall’s centre of gravity will shift to one side of the beam’s axis, thus tending to twist the beam even further. In contrast, consider floor beams or joists bearing on the top flange of a steel beam. If the beam twists fractionally, the load will transfer to the higher edge of the flange, and this pushes the beam back into line."


So generally you would not expect the loads in the typical domestic situation to be destabilizing, but it's for the designer to make the appropriate judgement.
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Old 20th March 2014, 01:04 PM   #3
motorphotos
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Default Re: Effective lengths of beams

Hi Tony

Many thanks for that

I would consider a beam built into a party wall to be condition 7 as normally I have no site control and have found even if I give the builders a detail they don't follow it so in most cases the flanges are free to rotate with bits of cut brick/block pugged into the web of the beam and bottom flange just sitting on a padstone with no fixing

Had this business with restraint from floors before and it relies of the beam being tight under the floor joists - does the condition still remain if they put the beam under the timber wall plate for example - plus with through lounges etc you could have the beam at door height level so there is a panel of wall between the beam and the floor - again I normally do not get involved with the construction and would not know the position of the beam so again in design I would destabilise the loads to allow for this the of course in newer buildings the floor joists are on hangers and not built in at all

I think I will carry on the way I have been designing

Regards

John
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