Newbie question glulam beam options

[colinh1301]

Hi there,

This is my first post so please bare with me!!

I'm in the process of making a Shepherds Hut. Its basically a steel chassis with 6*2 floor (hopefully eventually with an oak floor - depending on budget). I'm making the walls from 3*2 CLS. It will have T&G interior. I'm going to sheet it in OSB then a breather membrane and then finish either in corrugated steel or hit and miss york cladding. The roof will be curved corrugated steel.

I would like the roof supported on curved wooden transverse beams. Which I was going to cladd in T&G then fit some longitudinal purlins with the curved tin on top.

My question is how best to make the curved beams.

I tried ripping down some CLS into about 4mm thick strips and then bending them into the curve. My idea was too glue a load together to get the thickness.

It was a bit of a distaster. The kept cracking across knots.

I'm now thinking about cutting a number of curves from ply and glueing together to get the width

or making it out over oversized straight sections and then cutting the curve out.

the hut is 83 inches across and the height of curve needs to be about 350mm at the apex.

Any other general advice would be appreciated - as I'm sure you can guess I'm a bit new to all of this

Cheers
Colin

 

[wobblycogs]

At that sort of width perhaps you could consider steam bending. never tried it myself but watched someone make something like this on youtube a while back

 

[jasonB]

Rather than cutting curves from your plywood buy thinner sheets 1.5mm would be ideal but you could get away with more commonly available 4mm. Cut these into say 50mm wide strips and these will easily bend to your curve and when glued up will form a strong structure.

If you want to stay with solid timber get hold of some Southern Yellow Pine, this is relatively free from knots so less liekly to snap when being bent.

Jason

 

[colinh1301]

wobblycogs - I think steam bending is a bit to advanced for me. Thanks for the reply.

jasonB - Does ply bend the same in both directions ie will one sheet only bend say across the width and another a long the length? If the curve is longer than 8 foot would it still be possible to use this method?

cheers
colin

 

[jasonB]

For that sort of span I would not worry about teh direction of the outer layers but yes a normal sheet of ply with the grain running the 8ft length will make a long small cylinder easier than a short large one.

Provided to keep with the thinner ply bending will not be an issue. The chess table in my avitar is approx 500mm dia and the apron which is smaller is laminated from ply, have a look here If you can wait until tomorrow I have some 50mm wide strips of 4mm ply, I'll bend them and take a picture.

Jason

 

[squib]

I can highly recommend steaming ,once you have tried it it is not only versatile but great fun. If the beams are going to be visible then using clean knot free timber will work the same as ply.

 

[jasonB]

Here you go Colin. Three 50mm wide strips of 4mm ply with light hand pressure on one end, they are about 2000mm long and the deflection is around 600mm. For your span it would be best to buy a 10ftx4ft sheet though you could use a standard 8x4 and stagger the joints, once its all glued together it won't move.

Jason

 

[ChrisD]

I agree with Jason's suggestion. I would use 10x5 foot (don't think you will get 10x4) ply, cut into strips about 10mm wider than your final width to allow clean-up of edges. You will need a moulding shape, and I would use 1" thick MDF, if necessary glued together to give you the required thickness, which ideally will be wider than your strips. You need the two halves (male/female) to obtain even pressure and lots of clamps. Incidentally, if the curve is gradual you can use one saw cut, but if its a tightish ellipse, you will need to allow for the thickness of the laminations. Use waterproof glue, my choice would be a urea formaldehyde type like cascamite, applied with a roller on both faces. It sounds worse than it is. Spread all the glue, build your stack and then clamp. Incidentally, you can buy bendy ply, designed for this purpose but its not cheap and only bends easily in one plane. However, it will cope with tight curves easily and if you are feeling windy about the prospect, it is the easiest solution. Good luck!!

 

[colinh1301]

Blimey Jason

Thanks for doing that - I hope you didnt cut the wood especially?

10 by 5 sheets I've never heard of such a thing. I'm in Trowbridge in Wiltshire near Bath. Does anyone one of where best to buy it. I'm always in the various local builders merchants and I've only ever seen 8 by 4.

I was hoping I wouldn't need to make a mould by securing at the apex and simply pulling the ends down letting the ply take on its natural bend secure the ends and lay up each strip in turn?

 

[Vinny]

Colin

I'm in Corsham and have used Robbins timber in Bristol a couple of times, theyre very helpful, don't know if they do a 10 x 5 sheet, but you could alsways ask.

Vinny