Scaffold pole strength

[Calvin Sambrook]

Hi all,

I'm going to be making a circus aerial rig. It's only for home use so no worries about liability etc but of course it still needs to be strong enough to take the loading and safe in all aspects. I have a good understanding of the mechanics but I've got no experience of the ultimate strength of steel or how to even begin to do the sums to work it out.

Standard steel 48.3mm scaffold pole is often used for such things and I know it's perfectly capable of supporting two rigs simultaneously as that's what I train on right now. For my rig I have the choice of either 33.7mm, 42.4mm or the more normal 48.3mm. The smaller diameter tubes are typically seen as guardrails etc and keep the cost down dramatically while also looking a lot less like scaffolding.

I want to support a static load of at least 500kg, or preferably 1 tonne, at the mid point of a horizontal pole spanning a 5m gap without it permanently bending. Does anyone know how I'd go about finding out if the smaller poles would be suitable? Anyone got a degree in mechanics? Know of a good website?

BTW, just for the record, I don't weigh 500kg!

[Bill Wright]

Does it have to be a single pole? If not a lattice made from two poles perhaps 300mm vertically apart would be far stronger than a single pole. I had cause to make such a thing and I was astonished at the strength. For the record the poles were alloy scaffold tubes and the cross members were lengths of half inch thin-wall alloy tube forming equilateral triangles. I fixed the cross members to the scaff tunes by tapping M6 holes in the tube wall. The whole thing was very light and very strong. It took a bit of working out but I have the drawings somewhere if you want them.

Bill

[A.Lee]

No chance. I used to use an 8ft scaffold pole to lever up tree roots. On one particularly stuck root, I bent the pole by putting my weight on it, and trying to jump on it etc. I weighed less than 14 stone then.

Alan.

[Newshound]

I'd tend to agree. My (unchecked) sum for a simply supported beam gives about 1000 MPa max stress for your figures with standard sized (48.3) steel poles. If you took 300 MPa as a yield stress for mild steel, you'll be starting to bend at 150 kg, which feels about right to me.

[Peter Parry]

that for a 3m length of 2in (48mm) scaffolding tube the safe load at the centre would be 100kg and at that load the deformation would be 25mm.

For a 5m length there would seem to be no hope of supporting 500kg in the centre.

[Newshound]

Yup, that verifies most of the numbers in my calc (a little more pessimistic on strength). But as the other poster says, go to two poles set a few inches apart vertically and tied together by a web or a lattice, and that will take the vertical load, although you would then have to think about buckling under off-vertical loads. Perhaps three poles in a triangle (as used in pop concert lighting rigs)? Can't help thinking a small RSJ might be a better bet.

[Adam Aglionby]

Aluminum lighting truss, 2d ladder, 3d triangle and 4d box, don`t buy it secondhand.

Truss uses very thin wall tube and relies for its strength on the trinagle chords joining it, its easy to overighten a clamp or biff a deny into its , at which point its scrap or on ebay for the unwary.

is a UK distributed `value `brand, though guessing steel I-beam is probably going to be a lot cheaper.

[Bill Wright]

I made a triangular lattice and it was far lighter than an RSJ.

A catnic might be OK though.

Bill

[Calvin Sambrook]

Many thanks for that, the link is really helpful. And many thanks to others who've responded with thoughts too. It's only when you pose the question and start to get answers that you realise how important the details of communication are and I realise that I posed my question carelessly so it's time to add some more detail.

My "500kg" load was actually calculated from 70kg (me) *3 to allow for a 3G drop *2-and-a-bit to allow for a safety margin. Now the 70kg is real (unfortunately!) but the *3 drop is very extreme, that would be a free-fall of 3m followed by deceleration in 1m - if that happens it will be accompanied by a very big scream and the need for fresh underwear! The *2 safety margin is interesting in that if that is ever relevant then I probably don't care about damage to the rig as it will already be damaged. So if I look at the numbers more realistically I'm really interested in two aspects. Firstly I'd like the rig to withstand normal usage without permanent deformation. So that might be 70kg *2 (2G drop) giving a load of

140kg. Secondly I'd very much like it not to collapse with my extreme load-plus-margin of 500kg although in that case I can tolerate damage to the rig itself so long as it or the user doesn't come tumbling down. Remember that the link given already adds an unspecified safety margin in its "safe load".

What this discussion has taught me is that I really need to reduce the span as much as possible and that I really shouldn't be thinking of downsizing to one of the other tube sizes. From that .pdf file I now have the magic numbers for the tube so if anyone has the equations to plug the numbers into I'd be very grateful.

The ideas of using a lattice type beam are very good but I've already seen how expensive those are. The lighting rig type "TriLite" stuff is stupid money while the scaffolding stuff (sold as "beam" is just silly money! If money were no object that would be the way to go but...

Once again, thanks everyone.

[Newshound]

A catnic doesn't normally see a point load; I would worry about buckling. And isn't 5 metres a bit long for a catnic?