Small house using I-beam portal frames

[pir8ped]

I'm planning on building a small house with an I-beam portal frame structure. The place is very sheltered and the whole area gets very little wind, but there's the occasional thunderstorm passing through which can have very strong gusts.

 

The house will have an outer skin of 9mmOSB, glued and screwed to the flanges of the frames. Inner skin the same, but 18mm OSB. Insulation probably rockwool. More details of the general situation are in my profile. Part of the design intention is to allow me to do much of the work here in the UK - making the portal frames, doors and windows. I can take all the parts of the frames for the house with me to France in one shot with my van and trailer. (And yes, I've seen discussions about making DIY I-beams, but I still want to do it - having the 9mm groove in the flanges cut at a local wood engineering shop, and making up the I-beams at home.)

 

I'm hoping to have all the structural stresses dealt with by the frames and the OSB skins. There'll be no interior walls except for a small bathroom in one corner. There will be a mezzanine floor though extending across the width of the building, up to 3 metres from the north wall. I anticipate this will to a degree stiffen the whole building, counteracting the forces that would tend to make the tops of the walls spread.

 

This leaves the 5 metres or so of the southern end of the building with no interior structures, depending solely on the south end wall and the strength of the portal beams to keep its shape.

I've seen photos of Patrick's portal frame house here, and Joe90's. Their frame joints at the tops of the walls and the peak of the roof seem to be just plywood plates glued and screwed through the web. I don't think the frames were the sole providers of the structural strength though - I think there were beams under the peak of the roof. Not sure.

 

I've drawn the house structure in the attached pdf, showing general views and dimensions, and then some options for joining the angles of the portal frames.

I'd welcome any comments or suggestions, but particularly about connecting the I-beams at the frame angles.

Thanks,

John

portalFrameConnectionOptions.PDF

[SteamyTea]

Welcome

 

I am going to follow with interest as I think making I-Beams should be simple.

Take lots of pictures.

[pir8ped]

10 minutes ago, SteamyTea said:

Welcome

 

I am going to follow with interest as I think making I-Beams should be simple.

Take lots of pictures.

Thanks.

 

I think it should be simple too - but I haven't really tried yet. I notice in those pics of I-beam portal frames I mentioned that the webs don't seem to be joined at all - just butted tother. Maybe that's enough, but I'd be inclined to add a 200mm wide OSB plate I think. Am I wrong? It would be nice to be wrong, so I can just butt them together to make the longer ones.

[SteamyTea]

3 minutes ago, pir8ped said:

Am I wrong

I shall have to look at the sketches.

But I don't think you will get away with just butting up I-Beams.

[pir8ped]

6 minutes ago, SteamyTea said:

I shall have to look at the sketches.

But I don't think you will get away with just butting up I-Beams.

I've done no sketches of the joining of the I-beam webs. The max flange length in the frames is 4m, so I can use one length of timber for that. But my OSB sheets for the frames will be 2.4 x 1.2, so I'll have to join the webs together somehow. Butting, or a on OSB plate glued and screwed on one side, or a plate on each side of a join. Those are the options that spring to mind.

[MikeSharp01]

Looks to me like you need a structural engineer to look at this as I have an odd feeling those gusset plates, screws and structural glue and all, won't be up to the job you are expecting of them. 

 

 

[SteamyTea]

While I think some calculations, or at least some testing, is called for, a full report from a structural engineer is probably OTT.

 

I did that DIY I-Beam test a while back, which was more about testing cheap adhesive.

It performed really well.

I am sure a few test pieces joined together in different ways will find out what is strong enough.

All that is trying to be achieved is the joint expanding when the cord (think that is the right term) of beam is in tension.

 

 

[pir8ped]

13 minutes ago, MikeSharp01 said:

Looks to me like you need a structural engineer to look at this as I have an odd feeling those gusset plates, screws and structural glue and all, won't be up to the job you are expecting of them. 

 

 

I think I read of an unwillingness of structural engineers to try to calculate this, as so much depends on the quality of the glue. 

 

I'd have thought the plates for the peak of the roof wood be sufficient. I mean, if I used a whole sheet of OSB to fill in the triangle at the peak, it would be overkill, so it's more a matter of how much of the triangle I need to fill.

 

Naturally, I don't want to use the same approach at the sides, as there'd be a weird infill at the join of the walls and roof. Hence the idea of externalising that gusset by filling in the triangle between the rafter and the wall. Plus the plates joining either side of the web. Another option would be to fit plates over the flanges, using blocking pieces to join the plates to the web. Without filling in any of the triangle.

[MikeSharp01]

3 minutes ago, pir8ped said:

23 minutes ago, MikeSharp01 said:

 

I think I read of an unwillingness of structural engineers to try to calculate this, as so much depends on the quality of the glue

You are right. We had the same plan as you - our portals would have been 9m wide and 5m tall. Our SE worked it out and showed it could not be done as a site build and probably only just marginal for a factory build. Our gussets are 15mm thick, we have a ridge beam supporting the top and has four gluelams / kerto beams across the space to stop it spreading. We built it on site with structural glue and process to SE spec but a portal it isn't. I think @Gone West did manage an I beam portal frame built in a factory IIRC.

[pir8ped]

Well, if a structural engineer would turn away the task, might have to get really basic and test a frame.

 

Does anyone here have any experience of load testing a beam or joint of any kind? I'll re-phrase that. A timber joint of any kind.

[MikeSharp01]

29 minutes ago, pir8ped said:

Well, if a structural engineer would turn away the task, might have to get really basic and test a frame.

To get the right answer you will need a number of them so the interactions can be accounted for.

[SteamyTea]

38 minutes ago, pir8ped said:

Does anyone here have any experience of load testing a beam or joint of any kind? I'll re-phrase that. A timber joint of any kind.

Well I did my apprenticeship at a materials testing machine company.

 

How much of a hurry are you?

 

Hopefully @Gus Potter will come along with his SE specs on and impart pearls of wisdom.

[pir8ped]

13 minutes ago, SteamyTea said:

 

How much of a hurry are you?

I'm 67.

[SteamyTea]

8 minutes ago, pir8ped said:

I'm 67.

Plenty of time then, as long as you live the French lifestyle.

Half my family is French descendants, most live well into their 90s.

[saveasteading]

5 hours ago, pir8ped said:

load testing a beam or joint of any kind?

Try a local University. 

 

It is basic stuff, with the application of an increasing load, measuring deflection and failure.

Deflection will always be the main criterion, with 1/180 likely to be appropriate, so you could bodge that test yourself.

A bco won't accept your diy tests.

However the failure of an efficient portal frame with deep webs will often be in the bottom flange kicking away sideways. Again a test can find this, or it can be triangulated out.

 

I'd think the main reason for an SE declining the work will be the small size of the project  (and fee) as compared to the complexity of first principles calculations, and the risk if there are problems.

 

I had the pleasure of renovating a timber portal frame from the 1950s. What's more, it was a tapered frame so very efficient indeed, The webs were made of t@g fixed vertically.

[Nick Thomas]

The description reminded me of a grand designs - https://www.granddesigns.tv/southherefordshire . Particularly the portal frame.

 

Perhaps the suppliers list will be of use? The episode was quite into the portal frames too, watching it might give you some ideas there.

 

Perhaps cut the OSB into V shapes, so there's no joints at the top? I vaguely recall that's what they did.

 

 

Edited August 1 by Nick Thomas
add: V

[Gone West]

14 hours ago, MikeSharp01 said:

@Gone West did manage an I beam portal frame built in a factory IIRC.

We did indeed have a factory built timber I beam portal frame. IIRC there were limitations when designing, such as the roof angle which determined the maximum width for a given height. Ours was only 6m wide. I think PH15 construct their frames in a very similar way to ours.

Edited August 2 by Gone West

[SteamyTea]

@pir8ped

 

Did you say on the other thread that because the place was small, you did not need the French equivalent to building regs?

 

[Dave Jones]

why do you need I beams ? for a shed that size 4x2 for walls/floor and 6x2 for roof is perfectly fine. scarf a ridge board from 8x2 and you set.

 

If your cladding with OSB double check the OSB is not in fact 6 x 4 so you dont set out 400 centres and find they dont land.

 

save a fortune.

[pir8ped]

1 hour ago, SteamyTea said:

@pir8ped

 

Did you say on the other thread that because the place was small, you did not need the French equivalent to building regs?

 

Yes, no requirement for structural engineer, and no building control officer coming round to see it's up to spec.

 

It's an opportunity to open a large can of worms.

[pir8ped]

1 hour ago, Dave Jones said:

why do you need I beams ? for a shed that size 4x2 for walls/floor and 6x2 for roof is perfectly fine. scarf a ridge board from 8x2 and you set.

 

If your cladding with OSB double check the OSB is not in fact 6 x 4 so you dont set out 400 centres and find they dont land.

 

save a fortune.

It's not a shed. It's a house, so it needs insulation. Temperatures vary between 40 degrees and -7. I want to minimise my heating/cooling costs, so we're looking at 300mm thick insulation and minimal cold bridging. 

 

I've built a shed there already, which I live in while I'm building. It's 11mm OSB framed with 2 x 1. 25mm celotex in the walls. 75mm celtotex in the roof, and 100mm celotex in the floor. It's 4.8m x 2.4m floor area.  It's fine it all weather except the top temperatures, when it gets far too hot.

[pir8ped]

2 hours ago, Gone West said:

We did indeed have a factory built timber I beam portal frame. IIRC there were limitations when designing, such as the roof angle which determined the maximum width for a given height. Ours was only 6m wide. I think PH15 construct their frames in a very similar way to ours.

Mine is 6m wide too.

 

So how were the joints made at the wall/roof joins, and at the roof peak? Did you support the roof peak with a beam, or was all the load taken by the portal frames? On the front of the PH15 booklet, there a picture of IBeam portal frames used to make a house with a big beam supporting the top of the roof.

 

I might have to use a beam after all, but that would spoil the head room in the mezzanine bedroom area, unless I raise the roof a big higher, which I'm reluctant to do.

 

I haven't ruled out having my I-beams made for me. It'll depend on the quotes.

[pir8ped]

I went to sleep daunted at the prospect of really testing the portal frames myself, or paying someone else to do it, but also at the prospect of using a beam under the rafters for support. The beam would be pretty big and heavy (AI did a quick calc and said 360mm deep), and I'm most likely putting up the building alone, and then I'd have to think about suitable supporting columns for the beam

 

So I've continued along the line of incalculable designs, and made the connections at the peak of the roof as strong as I reasonably can. There's a loss of headroom under the peak (which only matters over the mezzanine area) but as drawn it's less than the beam. And I could make the gussets and blocking pieces in a batch, glued and screwed together and I think it would aid and simplify the construction.

 

 

peakSupportStructure2.PDF

[Gone West]

2 hours ago, pir8ped said:

So how were the joints made at the wall/roof joins, and at the roof peak?

There were no beams used. A picture paints a thousand words, so

 

 

IIRC the joining pieces were 18mm ply both sides, glued and screwed. I think the roof angle has to be around 45 degrees to prevent spread. I am not a SE though, so could be wrong about that, it was many years ago.

 

[Alan Ambrose]

Ah, I think you can do some back-of-the-envelope calcs to satisfy yourself. If you're a CAD guy then you could do the stress analysis in Fusion or similar and get some approx results. Approx because wood, strictly speaking, is anisotropic (i.e. it has different characteristics depending on the orientation of the load to the grain) and Fusion doesn't do that. You're going to use a safety factor of 2 or more likely 3, so rough approximations all wash out.

 

(1) Do a simple loadout calc using the quantities and densities of the various materials. That'll also give you the quantties to order. Don't sweat the detail i.e. the fixings and the light/small stuff. Add some for live load, wind load etc (standard BC values). This is a super lightweight building anyway so the numbers will all be small.

 

(2) Maybe read the beginning chapters of an undergraduate structures book. Ignore most of the maths, but get a feel for where the shear and bending forces are and which directions. For example, the roof loads you have will 'push the eaves away from each other'. Maybe consider a metal rod or smallish beam in tension between the eaves. You're building a larger version of a shed, so you don't have to stress yourself too much, but some basic understanding of structures would help. Otherwise find someone who can help, maybe informally, so you're not on their PII. If you like, make a balsa wood or cardboard model and see how it breaks when you load it.

 

(2) To convince yourself and maybe anyone else. Take one truss, maybe full dimensions rather than scaled. Load it approximately with the loads you calculated, maybe measure the defelection. Then stress with 3x the load and/or until it breaks. That gives you your safety factor and some confidence.

 

p.s. (a) You can pretty much see from inspection that it'll be strong enough, maybe with a bit of attention to the joints and the eaves support I mentioned.

(b) Is there a worry that any moisture ingress will destroy the MDF? As others have said - would you consider just simple treated CLS rather than I-beams?