Over loaded system scaffold standard & Hi!! (1 Viewer)

Georgina Molloy

Active member
Joined
Dec 2, 2010
Messages
28
Reaction score
0
Location
Dublin
Hi All,

I hope everyone is well. Nice to 'meet' you all!

What do ye think of this solution, suggested to me........

If we have a system scaffold standard with is over loaded, the solution is to connect a tube standard onto the side of the system standard with swivels so that the tube standard can take half of the load from the system standard.

I don't believe that this works but I've seen it done a few times and I'm wondering am I correct in querying it?????
To me it seems that the load path is..... load on boards, to system transom, to system star, to system standard, to base jack, to sole board, to ground.
I can't see how half of the load can jump horizontally across to the tube.
Am I right?

Recently I designed a beam in a system scaffold to support two slung standards.
The system standards supporting the beam at each end would have been over loaded so I put in an extra tube at each end of the beam to support the beam load and I let the system standard support its own weight only.
When I saw it on site the beam had been connected to the system standards at each end and there was a tube connected to the side of the system standard with swivels (similar to the situation above and tube standard not connected to beam chords).
To me, this tube standard is not supporting any vertical load.
There was even a gap of 10mm between the tube standard and the base plate so it couldn't be supporting vertical load.

The only benefit I can see to the extra tube is providing a bit of resistance to buckling, but I'm not sure how I would quantify that.

Any thoughts or experience of this?

Much appreciated Folks!
 
Hi Georgina and welcome to the forum.

Like yourself, I can't see how the configuration would be of any use in supporting vertical loading. With no transfer system in place to split the load over the standards, the extra leg is adding no benefits.
 
Swivels are not great at transferring loads because of the play in the fitting but in theory the extra standards could be carrying load if there was no play or less play than the elastic shortening in the system standard. If there are enough fittings in the right places and the play has been taken up, some of the load from the system standard will be transferred into the tube through the upper fittings and back out through the lower fittings. This means that there is a potential for the system standard to be carrying less than the total design loading in the height between the top and the bottom fitting. With a gap at the bottom of the tube, the system will be carrying the full load from the beam to the top fitting and from the bottom fitting down to the ground but if these are short lengths (and the base jack is strong enough), the system standard may not be overloaded.
Having said all that, in this case ask for the tube standard to be put in the way you designed it and, if the scaffold is more than a lift or so above the beam, ask them to put a jack in the bottom of the tube so that they can jack some load into it once it is connected to the beam. Hitting the tube and the system standard with a hammer while the jack is tightened will give a rough indication of whether they are carrying similar loads - when they are, the note should be similar.
 
Georgina, I agree with you and the comments above and I would usually try to support the load in another way when using system scaffold standards, or reduce the bay length/width or the lift height at lower levels to accommodate the greater load. That said, your comment regarding buckling to resistance is something that I have considered and used before, following discussions with one of the system scaffold manufacturers engineer's. It is difficult to quantify but the extra area provided by the additional tube effectively gives you more depth in the member, more stiffness, a higher radius of gyration and hence a higher strength against buckling. As long as the swivels are connected top, bottom and near the centre of the lift which is where most of the theoretical buckling would occur then the added tube will help to prevent the main standard buckling. An analogy would be doubling of a scaffold lifting tube by connecting another tube to it on swivels to create a truss - you're effectively increasing the depth of the tube and increasing its stiffness and hence its bending capacity - I've analysed this and the swivels are within limits when loading the tube to twice a single tube's bending capacity. The fact that the added tube isn't on the ground doesn't really matter as the main standard can comfortably support the compressive load over a short lift height, t's just the buckling that you are trying to inhibit.
 
The second standard on swivel couplers is an age old solution, which I doubt has merit. It do however as mentioned above reduce the bending in standards via increases section depth.

That said it is still common place
 
Many thanks for your replies all.
That's very helpful.

I'm off to explain it to the Contractor......!!!
 
Hi Georgina, welcome to the forum.

The TG20:13 had a piece on load sharing between standards in Appendix G.21 this says load sharing does occur; However 50:50 load sharing does not occur immediately it gives a rough estimation on the rate that the load is shared from lift to lift such that if you need 50:50 load sharing from the 3rd lift then the double standard should extend so far above this level until this is achieved which can be worked out given the ratios mentioned in TG20:13.

When designing a beam that needs supported by a double standard, I will always detail it in such a way that the tube transoms are supported directly by both standards, to ensure that load sharing occurs immediately in this situation.
 
If we have two standards close together, I know that we do take it that they share the load (probably equally) but once the beam starts to deflect, something complicated happens at the support involving vertical movement and a tendency for the second standard from the span to become unloaded or at least less loaded than the first one. How do you justify the load sharing in that situation and how much off 50:50 do you think it goes?
 
Thanks Roarz, I am on site for the day so I don't have access to TG20:13 at the moment but I will check out appendix G.21.

When I am designing a beam that requires 2 standards I will position the beam in the centre of the 2 standards with a tube under each chord of the beam and the tubes connected to the standards with doubles (and supplementary couplers) to ensure that the load is shared equally.

Similarly, if a beam needs to be 'double beamed' and I want to save vertical space, I will position the beams side by side with tubes over both chords of the beams and the slung standard connected to the tubes so that the load is shared by both beams.

If the beam support standards were connected directly to the beam and positioned beside each other (as you look at the beam in elevation), I imagine that the standard closest to the span takes the majority of the load.
It is good practice for the beam to catch 2 standards at each end but that for stability and so you can have a braced bay at the ends.
 
There is an urban myth that a beam has to go through 2 standards at each end, this can present problems with uplift on the outer standard, the important thing is to model the beam or beams as you intend to connect them , you don't get any nasty surprises that way like frantic calls from site saying the outer standards have lifted off the floor. Remember you may need to consider temporary conditions before the scaffold is complete.
 
There is an urban myth that a beam has to go through 2 standards at each end, this can present problems with uplift on the outer standard, the important thing is to model the beam or beams as you intend to connect them , you don't get any nasty surprises that way like frantic calls from site saying the outer standards have lifted off the floor. Remember you may need to consider temporary conditions before the scaffold is complete.

Hi Steve,
as always correct, I seem to remember showing this some time back on the forum think I even posted a picture, Will pop it on if I come across it
regards
Alan
 
Hi Alan

Well found, thats a great example of what happens, hows life treating you back in the UK at least you brought the weather with you.

regards

Steve
 
All good thanks Steve, getting used to the cold slowly, p1ssing down here today so missing the sun :)
 
Alan / Steve, to expand on the above regarding lifting of the outside standards; If you model the scaffold as it is actually built 90% of the time when connecting through 2 standards at each end i.e. a braced tower at each end of the beam span, the face bracing on the towers ends up transferring this"uplift" back to the inside standards and there is little or no uplift on the outside standards (less than 1mm usually). We always connect beams through 2 standards at each end, unless there is a reason not to do so, and have never seen the outside standards lift on site. If you have 2 standards right next to each other in the plane of the beam as shown in TG20:13 Op. guide fig 6.59 then that is a different issue and some uplift may occur theoretically but the outside standard will invariably be carrying a high enough load above the beam to prevent this uplift occurring.
 
Morning Biffo,
Many thanks for your input. I have seen this numerous time to be honest and the uplift on the back standard of your will rise and fall dependant on how you choose to erect the scaffold.
None the less the fact that the standard has raised 1mm would tend to prove the case that the standard is in uplift and as shown on my previous attachment the inner leg on the span attracts a greater load whilst trying to resist the uplift.
Figure 6.59 relates mote to the twin standard picking up the Beam as opposed to the second standard on say a 1.2m tower which would tend to show the lift more
 
I was not sure about the effectiveness of the 6.59 detail, given that the load onto the outside standard goes through the horns of the beam as cantilevers off the end of the beam proper. Because of this and the effect of shear stiffness of the beam, I would have said that the outer standard carries little or no load either up or down. If the beam continues past the outer standard, something else will happen. I have played around with analysing the supports as springs to see what happens with tiny vertical movements - differential shortening of the two standards or slight movement of the fittings and I am fairly sure, although I can't prove it, that load sharing occurs pretty effectively through these secondary effects rather than through the primary effects. It does seem that if you follow the spirit of limit state design, you should do that secondary effects check on every job and make 100% sure that the connections are exactly as modelled - something that is never going to happen in practice.
On long span beams with end towers in the region of 1m, I have seen serious loading differences occurring on site when checking the ring tone of the standards but even that can vary depending upon how the beams were lifted into place and how accurately the two transoms are levelled at each end.

Best not to overthink the problem I think.
 
Sorry to jump in on this. But what happens if you place an upright beam as a standard on either side?
 
Hi Gannit

depends how you connect to it. If you connect all chords to chords i.e. 4 double you get the same issue with the tendency for the outer of the chords to lift off, if you connect to the middle of the rungs (assuming a steel ladder beam) with structural transoms then the problem virtually goes away you will never eliminate it because the joints in scaffolding are always eccentric. Frankly never liked using beams as standards, always struck me as en expensive and inefficient way of getting a double standard, not to mention lifting the things up! long live aluminium i say
 
With two couplers per chord and a vertical ladder beam, the safe moment capacity is getting up to 4 kNm so you have a portal frame which carries moment around the corner, thus reducing the mid span moment in the spanning beam by 4 kNm. The bottom of the vertical beam has to resist that moment with a horizontal force equal to 4 divided by the height but the moment in the beam has dropped to zero at that level so the tension in the outer chord decreases from the 12.5 at the top to zero at the foot tie. With a few standards connected with a foot tie, the lateral load probably works through friction due to the scaffold weight so the portal frame works and reduces mid span moment. Unfortunately, and I may be wrong here, I think that the load from the beam all goes down the inner chord and as it is also carrying a compression force from the moment it may be more overloaded near the top. Overall, I don't think that it is a good idea.
It may be worth someone doing a computer analysis to see what actually happens if the 4 couplers stay rigid.
 
Top Bottom