Distortion control in structures

[chunkolini]

Distortion control on this job. I am making a dome based on a whole lot of influences. There will be a rigid framework with a network of thin bars as cladding, like a mesh, maybe.....
Size 4m diameter by three tall, made in sections. it will be built in one piece and dismantled for shipping. Plan A is to use pairs of curves rolled in strip for the uprights and maybe 25mm tubing for the rest of the frame.
Edit, this job is in mild steel.




Anybody got any ideas.
My concern is that when I come to put the web of bars in the whole thing might warp.
I had a gruesome time september before last making some stainless steel cobwebs. Is there more distortion in stainless than mild steel?

Will I be best off having the frame bolted to the floor before I start welding the lattice, or having it sat loose on the floor. My gut feeling is fixed, also fitting the lttice, I think if there are multiple joints along a long bar, start off at one end and work away form the centre of the structure.

Order of work? start at one side and work around the thing or fix one piece on panel one, the do the same to panel two etc etc to avoiding heating one large section to much.

I have started a seperate thread to the projects one as this is an issue that must affect all of us at one time or another.

Still wake up in a sweat about warped cobwebs.

Chunko'.

 

[shenion]

Stainless has a higher expansion rate with temp so, distortion is worse. Supposedly about 2x what mild steel is.

 

[cookie munsta]

yeah mate stainless doesn't distribute heat as efficiently as some other materials and therfore warps freely so use heaps of tacks and weld it in seqeunce.
if you can bolt it down than do, I can't imagine it would harm it but should make it easier. with a circular structure try to weld it like you would bolt up your car tyre but be careful not to weld the opposites straight after each other for instance if you weld a square and do the opposites first they may contract when cooling and pull out of sqaure before you get the other welds in. I would say if you had 8 points number them clockwise then weld 1, 3,5,7 2,4,6,8.
on a long beam with lots of welding starting at the centre and working out may warp as well, I usually try to divide it up evenly so as to give 50mm max welds number them and say 10 then weld 5,let it cool a little and so on 1 ,10, 2, 7,4,9,3,8,6 or some other random ish order, if it warps then on the back of it run a weld along where it bent and let it cool and grind off, if it goes back straight before it completely cools then grind it off then with out putting excess heat into it cause the more weld metal the more contraction when cooling and may cause
it to bend the other way

 

[chunkolini]

Sounds like I am barking up the right tree, Cooka Munsta, that is a useful tip.
My cobwebs were 2m in diameter, they came out shaped like pringles. The only time my job has had me in tears of rage, in the end I had to cut them along one axis, beat hell out of them and reweld them.
Looked good though.


I just never want to be in that situation again. Oh this job is in Mild steel which might avoid a lot of nonsense.

Cheers, Chunko'.

 

[mike os]

hate telling my granny how to suck eggs & all that....me being an hamature & all

can you fit "heavy" temporary braces between the key points?

Bolt down

cut the web bars into smaller lengths?

Do it in sections & check allignment as you go, start in the middle of each component.

tack tack tack.......

giving me stress just thinkin about it

 

[Hitch]

I cant quite envisage what you plan on doing chunko.... but holding the feet at fixed points, bolted down is a good idea to stop it from moving around.

Id Make one panel up solid, then make each consecutive panel off of that first one, or at least a jig to maintain hole centres whilst welding and cooling.

The mild steel will not warp as much as the stainless, but i fear if you are welding lots of bars ontop of another bar, you will have a lot.

 

[Rudi McAnichal]

Tee section might be the best material for the frame, if you have access to some suitable section rolls. Much more rigidity than you could get from flat bars, and avoids the internal corrosion problems associated with tubes.

Looking at the model and scaling up to 4m diameter, I wonder whether you could use the Tee section ribs from old Nissen huts. There used to be lots of Nissen hut skeletons on farms, after the corrugated sheets and timber purlins had rotted away. With three different sizes of hut (16ft, 24ft and 30ft wide) you might get all your frame components, ready-rolled to the desired radius, at scrap metal price.

Have you decided on a design for the cladding network, and will the cladding have to contribute to rigidity of the structure? If the main frame provides all the structural rigidity, the cladding network bars could be curved (to form concave or convex “surfaces”), in which case welding them to the main frame ought not cause much distortion.

 

[malcolm]

From a car restoration point of view, the way to reduce distortion is to allow the metal somewhere to go as it expands and contracts. A fairly good example of both good and bad ways of doing things here: http://www.renault4.co.uk/gordini-apex-repair.htm

I can't see how distortion would be a problem with the structure in your photo. It's 3D so there is a load of scope for movement. For 2D (like a spiders web) or stainless (like a spiders web) I would leave one arc un-welded and do that last.

Did you ever make the spider?

 

[cookie munsta]

Sounds like I am barking up the right tree, Cooka Munsta, that is a useful tip.
My cobwebs were 2m in diameter, they came out shaped like pringles. The only time my job has had me in tears of rage, in the end I had to cut them along one axis, beat hell out of them and reweld them.
Looked good though.
View attachment 15839

I just never want to be in that situation again. Oh this job is in Mild steel which might avoid a lot of nonsense.

Cheers, Chunko'.

it looks good though from that photo. some times you can pin down the sides and pull the centre in flat with a chain block or rachet tie down. nice work as usual chunko.

 

[Drains]

How many ways to skin a cat?
Chunko, I'd start by tacking the whole thing together, well enough that the tacks won't break.
Then look at the welds you're going to do which will cause the most distortion - in this case, welds going around the tube, but not all the way round, will distort the most. Welds on the strip won't matter, as you can easily bend the strip back into place.

Also, if you do as suggested above and weld like you were bolting up a cylinder head, don't do opposite corners, and don't lay on loads of weld in the same place without allowing to cool, yuou should be fine.

Also, it's a totally different job to your web, which was a 2D shape. As long as you make any distortion even all the way around, will it make any difference to the final outcome?

Hold on... what do you mean, "it'll be built in one piece and dismantled for shipping"?

 

[chunkolini]

Hi Si
Taking it apart?
the vertical lines on the structure are going to be made of mild strip 30x5mm bent into the curves and bolted together in pairs, ie the centre line in the photo'
then the other components will be welded onto them, to give six segments. When welded up I will undo the bolts and dismantle it. The bolt it back together on site.

I figure that as you say tack it together then work around it finishing off the welds. by the sound of it bolting it to the floor sounds a good place to start from.

Tube? I reckon now that I wont be using tube, just flat strip for the uprights and and maybe 13mm bar for the rest of it.

Does that make sense? Must go,The Brat is hassling me for the computer.

Chunko,