Deflection (1 Viewer)

Getting figures for both bending stiffness and shear stiffness for scaffold beams is pretty much impossible and assessing the effect of wear and tolerances on connections is difficult. Working out how to add in the stiffening effect of any scaffold above or below the beam is as complicated an art as I could think of in design.

If you think that you are doing an accurate calculation for deflection you are probably deluding yourself but I wait to hear what everyone thinks is reasonable.

I know that some manufacturers have tried to provide standard solutions for some fairly well defined load cases but don't think that the people who drafted the Eurocode were ever involved in day to day design with the pressures of deadlines and economics bearing down on them and what is written therein is not practicable.
 
Getting figures for both bending stiffness and shear stiffness for scaffold beams is pretty much impossible and assessing the effect of wear and tolerances on connections is difficult. Working out how to add in the stiffening effect of any scaffold above or below the beam is as complicated an art as I could think of in design.

If you think that you are doing an accurate calculation for deflection you are probably deluding yourself but I wait to hear what everyone thinks is reasonable.

I know that some manufacturers have tried to provide standard solutions for some fairly well defined load cases but don't think that the people who drafted the Eurocode were ever involved in day to day design with the pressures of deadlines and economics bearing down on them and what is written therein is not practicable.

Amen TG,
Too many variables in joint play to get anywhere near an accurate figure, not to mention the already mentioned stiffness of the scaffold above.
The only time I now look at span deflections in beams is when I use steelwork!
 
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I agree with the comments above but we are now using 2d or 3d frame analysis a lot more which gives far more accurate results in terms of bending moment, shear and deflection in the beams and also takes account of the stiffness of the adjacent scaffold. We can model fairly quickly so it doesn't cost the client any more and can lead to more efficient designs. When modelled correctly it also proves the increased beam bending strength achieved by connecting through 2 standards at each end of the beam and proves that the reactions on the inside standard or uplift on the outside standard are far less than a crude simply supported or continuous beam analysis done in isolation would suggest.
 
Some manufacturers have down rated their beams after carrying out the sort of modelling you are doing but I don't think that is quite what you mean when you say that you prove the increased beam bending strength.
Just so I understand what you are doing:
Do you model the scaffold beam as a fully welded sub element using the section properties for each member, allowing for out of plane alignment and tolerances and also allow movement releases at all of the connectors in the chords?
How do you allow for site changes in beam types or lengths?
Do you model the scaffold above with weak points at ledger & standard connections and partial rotational stiffness at ledger to standard junctions?
Do you allow slip at concentrated load transfer points to model the rotation and dig in of the couplers?
Do you really do all of that for every job without it taking time and costing more?
 
The increased bending strength comes from the fact that you have a continuous member through 2 connections at each end (its not really an increased beam strength, more that the bending moment is less). There are 2 ways to model it, either as you describe above and including all beam members separately, or make up a section using the stiffness of the beam, by manual calculation or manufacturers figures (we have both for all the beams that we use). The movement at the joints could be modelled but as deflection isn't critical i think this just adds unnecessary complexity. We've modelled complex scaffolds with and without the out of plane and away from node bracing using rigid end offsets and it makes little difference to the overall result. Site changes in beam type or length would of course require approval from us. We usually release moments form the ledger to standard connections although you can also include the coupler stiffness depending on the job complexity and level of detail required. We wouldn't (and couldn't) model slip as there is no way of knowing how much each individual coupler slips (if at all). No we don't do it for every job but do use it where we can gain an advantage or where the client requests a 3D model.
 
Thanks for that. Having done a fair bit of research in the past on fabricated beams and chatted with manufacturer's engineers, I think that the concept and effect of shear stiffness is something that is not fully understood (if at all) and so the quoting of overall stiffness of a scaffold beam is something that needs some sort of standardisation based on research and practical testing. It is possible to get a figure for shear stiffness by calculation but small errors in fabrication can have significant effects and so I prefer the empirical approach.
The one other thing to remember and emphasise to everyone is the effect of planting the standards away from the nodes. Obviously it can have a ruinous effect on the bending moment capacity but it also affects shear stiffness in a disproportionate way.
 
And they all want to come off the spanner to be an Engineer because it's the easy option.
Cut and paste :)
 
The grass is always greener isn't it?
 
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