DEESA Beams tech sheet (1 Viewer)

pride2iceman

Member
Joined
Aug 12, 2010
Messages
22
Reaction score
0
Hello gents!

Where can i find some more technical information for DESSA Beams(450,D78,750X,1.33x,1.5X)?

I have some technical data for simply supported and for 3 point loads, would need for truss out and for bigger bridged spans with 4 and 5 point loads!

Regards!
 
I had that link, Thanks anyway!

As i mentioned in my post i have tech info. regarding simply supported beams, i need details for truss out!
 
They are part of altrad I'm sure trad could give you the information you need
 
The data sheets appear to be based on simple moment & shear calculations - if you take each of the four load cases, either the quoted maximum shear or maximum moment limits the loading. If you take those maximums as accurate, you could check larger spans and cantilevers using straightforward analysis.
Applying the European standards means that every different load case should be checked at component level because secondary effects have a large effect on the welded connections. They don't appear to have done that to arrive at the safe load figures so it could be reasonable for you to do the same as they have done. You will not be complying fully with the requirements of the European standards for analysis by design if you do so.
Since European standards were introduced, it does not seem to be strictly acceptable to take allowable moment and shear values for scaffold beams and apply them to all spans and variations unless they have been derived from a number of analyses that have found the worst conditions. Where manufacturers have done that, the allowable figures are much smaller than we have been using successfully since aluminium beams were introduced. Search within the forum for beam design and you will find this expanded upon. Having said that, I think that most, if not all, designers do use the 'old' published figures and so you will probably be in good company in also doing so.
 
Thanks for the reply!
I have seen a calculation done by a designer where a bridge to support cuplock independent with 4 point loads at various lenghts (2.5m, 1.3m, 1.8m, 2.5m); he just converted total point loads to linear load/m.
Is this practice right?
 
You get almost the same answer if the beam is long and the loads are evenly spaced but it will always give a lower answer than the correct one (that is to say if you treat them as point loads). If you only have one point load and you treat it as a udl, the moment you get is 50% of the actual moment which is quite a long way out.

Because of this, I would say that it is not good practice to convert point loads to udl's

However, the load from a scaffold standard on a beam will change as the beam deflects and the scaffold above distributes the load from the deflected standard to other standards. It is possible that something which fails in theory will actually work in practice because of this but I wouldn't rely on it as a first try in a design.
 
So my understanding is that on a bigger span where there are more than 3 point loads it can be converted to udl, and as per tech sheets provided by Dessa can't span more than 12m.

Just not sure when designing bridge to support cuplock if a partial factor of safety of 1.4 should be applied to self weight of independent?
 
If you haven't done it before, do a comparison for yourself using point loads as one load case and then an equivalent udl as a second load case so that you see the difference. I'm not sure that the data sheets say you can't span more than 12m (and I know that I have designed structures with greater spans), they only go that far because loading capacity drops away rapidly as the span gets greater.
If the data sheets show allowable loads and quote allowable shear force and allowable bending moments, you should check using actual (characteristic) loads. Partial safety factors are only used in ultimate limit state design. The 1.4 you refer to looks like a partial safety factor. Current Eurocodes use similar numbers but they have changed in recent years so you need to look at the latest versions if you are working to limit state.

Is there anyone out there (Alan?) who can explain the difference between limit state & working stress design so that this issue of safety factors is understood? I keep trying to explain when it crops up but I am clearly not succeeding.
 
Firstly, so as not to confuse the issue, ULS is generally taken as being the same thing as Limit State Design. I know that there are two basic limit states, Ultimate & Serviceability but scaffold design is normally carried out to a working stress design. Whether it is only some designs or most designs, I don't know for sure but I have a feeling that it is most. TG20 was produced to give a pseudo version of working stress design based on and complying with limit state design so that the simple minded scaffold designer like me can relate working loads to working capacities.
Secondly, I think that the current gamma factors in the uk for permanent actions in EN1990 are 1.35 & 1.5 rather than 1.4 & 1.6. When I do steelwork design, I consider up to 7 different load combinations with different favourable and unfavourable gamma factors so it tends to get a bit more complex than just putting one factor into the calculations. That mostly affects continuous beams rather than simply supported ones though.
Thirdly, the manufacturer's data sheets show allowable capacities not ultimate capacities. Like I said, secondary effects within fabricated elements like scaffold beams are significant. You need to analyse the beam with the actual configuration of span and loading using the material gamma values to assess the capacity of individual members and joints to deal with the stresses produced by the factored loads. That is not quick or easy to do and is one of my problems with having to use limit state design on scaffolding. I have used limit state design for around 40 years for concrete and steel structures and whilst it has become more complex since CP110 first came out, it is reasonably easy to apply to building structures.
One final issue is that with a system like Cuplock being supported by a beam, the standards may land on chords away from nodes and that has to be considered and defined on the design drawings - this is the case whichever design method you use but it probably means that you shouldn't be simplifying the loads to udl's if you are working on limit state design.
 
I take the permissible limits for the beams and apply in all configurations. These permissible limits are available from all the manufacturer's. Just give them a call, introduce yourself and ask politely.
 
Top Bottom