UKC

I need some input from the UKC engineering section. I'm planning to build a high bed out of steel tube and tube clamps with an office space underneath, to make fullest use of my 3.5m bedroom ceiling height. Originally I was planning to use 48.7mm OD tube, but I found a company selling a bed kit online made from 33.7mm tube (https://www.themetalstore.co.uk/hightown-loft-bed-frame) and now I'm doubting my choices.

The mattress platform will be at 2200mm and will be for a small double (~1200mm width) mattress. Would 33.7mm tube be appropriate (I assume not - in my head I'm picturing far too much deflection on the horizontal lengthways members) - and if not, could 42.4mm tube be appropriate? Obviously the smaller the tube, the lighter it will be, the less of an arse it will be to build, and the cheaper it will be to buy all the bits.  

My plan is vaguely akin to the linked kit, but possibly without a ladder and with diagonal bracing instead of the lower horizontal tubes they've used; something like |/| on the sides and |/ | on the back. All four legs will additionally be screwed to the floor, and the bed platform horizontals could also be attached to the wall if required for stability. Does that sound probably adequate, or would it likely be a death trap without further cross-bracing?

I was originally going to support the desk on more tube (something like the lower horizontals in the linked kit), but it seems easier to brace if I don't make the desk part of the structure.

Edit: the forum appears to auto-edit my wonderful ASCII bracing should be like that, but also mirrored. Two diagonal braces, meeting in the middle at the top.

Possibly helpful 

https://www.omnicalculator.com/construction/beam-deflection

You will need to know the wall thickness of the tubes to calculate a second moment of area ("I"). From that you can get deflection.

We used to have aluminium tower scaffold bed, not as high as you plan as we used it to store my bikes and climbing kit under, but  the platform could have been raised to a similar height that you require. Same width but longer than a standard double. It could be quite noisy in certain situations.

Re your options above I would go for the larger diameter tubing, or kee clamp fittings and tubing.

Possibly a cheaper option is using scaffold tube and fittings, depending on your scavenging skills.

Without knowing the wall thickness of the respective tubing all the rest is meaningless, anyone sensible goes to rectangular tubing anyway.

50mm sounds overkill to me. 30mm too small. My bet would be on 40mm 3mm thick tube.

I have a kit car which has a chassis using 40mm 1.5mm square tube and it's solid. Yes it welded so acts more like a truss, but it's convertible so not a very tall truss. There is some 3mm thick stuff around the transmission tunnel, but its limited to two lengths I think.

If you can get hold of it, skinner wall stuff for the legs will be fine.

The lack of lateral bracing on the commercial kits looks dodgy to me. I have no experience with pipe clamps but if they allow any movement, without bracing it will feel wobbly.

I believe you can get pipe clamps that work at 45 degrees. A pair in each corner would stiffen it all up and reduce the effective length for bending. Remember halving the effective length reduces deflection by a factor of 4.

I used to have a long scaffolding pole. Don't recall if it was 40 or 50mm but it was solid. One would easily be comfortable in terms of deflection at the lengths you're looking at.

Why not make it in timber? 

Sounds like a half day job for a joiner or a full day for DIY.

My DIY joinery skills are non-existent, I don't have any tools and I fancy the idea of making it myself. Whereas I quite often build things at work out of truss, scaff, etc, and trust myself to be able to bolt together some scaff tube and kee clamps without drama.

Fair point - all 3mm in this instance. I didn't realise that tube clamps existed for rectangular tube until you got me Googling, but they do indeed, so that could be an option. 

Kee clamps (or equivalent) are the plan, though I'll probably buy the tube new for ease of finishing (ie none required).

Plan amended to use 40mm steel square tube (2.5mm wall thickness). Pretty confident that will be alright after some very cursory calculation - so thank you for that

The Metal Store are sound, they know what they are doing so I would be happy to go with the 33mm based on that but 42mm would naturally be more robust. There is also the asthetic to think of, 42mm might simply look better or be too chunky.

An engineer is someone who calculates everything to four decimal places, and then doubles the final result.

So err on the aide of caution: https://en.wikipedia.org/wiki/Factor_of_safety

That was going to be my suggestion due to the variety of connections and options available. 

Nah, 'engineers' think that pi=3

On the materials maybe, but the design of that off-the-peg bed kit is very poor.  With nothing but 90 degree KeeKlamp connectors to stiffen it and no diagonal bracing at all it'll be a bit wobbly.  And if it moves it'll creak.  Like the bunk-beds in a not very good YHA.

It's not going to collapse, but it's unlikely to be conducive to a good night's kip either.

The bendyness of the relatively skinny tubes would only really be a concern on the longer spans though - the longitudinal ones supporting the bed platform itself.  That off-the-peg design just has the two tubes with probably quite flimsy wooden slats between them - the slats are most likely spaced just with a couple of lengths of webbing that they're stapled to.  (Meaning that it can be rolled up to flat-pack the thing.)

For a double bed you would probably want three tubes, adding a middle one as well as the one on either side.  But it's also an option to beef up the wooden slats a bit, and stiffen the whole shebang by screwing them to a couple of battens running lengthways instead.  (Or as well.)  If you make them reasonably substantial they'll act as joists to stiffen the platform and take a bit of the bending stress off the tubes.  If it's fairly rigid and a snug fit it'll also have the affect of giving the frame a bit of plan-bracing, you don't need much but a bit might be good as you have no horizontal diagonals.

I'd be tempted to ditch the slats and use 12mm ply stiffened with a longitudinal batten or two personally - but you'd probably want to find a way to make it 1200mm wide (instead of a bit wider than the mattress), and a standard sheet is 1200x2400.  (Perhaps have four longitudinal tubes at the top there - two set in a bit from the sides of the bed to support a platform for the mattress, which stops short of the two outer tubes.  eg:  Make the bed 1300mm wide to accomodate the 1200mm mattress, with a ply 'deck' on tubes 1000mm apart - stiffened with battens that sit snugly against the inside of those tubes so the 'deck' can't slide about.)

Regarding the ladder/access etc.. don't forget that you also have to make the bed as well as sleep in it, and possibly turn the mattress now and then.  And if it's your only bed, you'll also need to be able to crawl in there when you're not feeling well, have a twisted ankle or whatever.

Also you're going to want a bit of storage and a lamp - you won't find a bedside table that's 2m tall so at the very least you'll want a shelf or something so you have somewhere to put eg: an alarm clock, pair of specs, book, phone charger etc..

I remember once over-hearing some astrophysicists arguing about whether pi=1 or pi=10...

Put one of these together recently for a small studio space-like giant meccano!

https://www.tecrostar.com/

https://xkcd.com/2205/

All fantastic points, thank you.

I did think about using ply as a base, but had settled on some chunky flat slats to allow better airflow. The width is halfway between a single and full double . If this isn't adequate it wouldn't be difficult to add another longitudinal beam after building. Fixing the slats to the frame is a good shout - I hadn't considered that this would give a bit of additional bracing.   

I was very tempted to build it with no access whatsoever until I remembered that beds do indeed need changing. I haven't settled on a solution yet, but it'll be either a ladder climbed internally with a hole at the top and a storage platform to the side of it (and the bed longer than the mattress), or possibly some sort of campus rung-type ladder separate to the bed. I think it'll be a later addition once I see how the desk fits underneath - ladder-and-hole or not, I'm planning for space at the foot end for a storage trunk.

At the head end I'll probably include a little shelf on top of the headboard with a wireless charger, place for book, clock. Lighting will probably, in the fullness of time, become wall-mounted. There is already a downlight circuit in the room that could be repurposed, but it would require some rejigging to end somewhere useful for a reading light. The height of the bed has been partially set to clear one of the current downlights, which is in the right place to become lighting for the desk space. 

I know you don't want to use timber, but as an example of what is possible, I built this for my son a couple of years ago. It's only a single bed, but it is a full-sized one (200cm x 90cm). It's all just softwood from the local B&Q equivalent, to my own design. The posts are 70mm x 70mm timber, and it's wooden slats under the mattress, with space between for ventilation. The slats are actually just stapled to two sturdy bands of fabric tape, which means it can just be rolled up should I need to take it apart to move house.  It's all pocket-hole screwed together which pulls the timber parts together nicely. Granted, it's not as tall or as wide as your plans, but last weekend there were 3-4 adults sitting on it and 3 kids climbing the bar at the top and it didn't move at all - didn't even creak. I've slept in it plenty and it is very sturdy and quiet. I think the key is the timber being pulled firmly together, but also the vertical width of the longitudinal aprons, which gives it a bit of cross-wise rigidity. I would be confident to use the same size timber for a raised 1 1/2 or double, potentially with a little bit of x bracing on the sides if I decided I needed extra rigidity. My gut feeling is that you will struggle to get that out of the kind of bed you linked to - not necessarily because of the rigidity of the poles, but because of the rigidity or otherwise of the connections. 

I guess at those scales a order of magnitude estimate is enough lol

Looks great but are those posts really only 70x70? They appear quite a bit chunkier in the photo, maybe more like 90x90.

All assuming sensible wall thicknesses (2mm+).

I'd not worry at all about 34mm tube for the horizontal or vertical members in a bed, especially so if using the design you linked where the horizontals are pretty much doubled up by the guard-rails (assumes the ladder is retained). We use 34x3mm with tube clamps at work for various bits of racking, it's plenty strong enough for negligible deflection under human sized loads at the sort of scale of a bed. Worst comes to the worst it'll bend gracefully.

Unless you can tie it to a wall you're going to need some diagonals and lower level horizontals or shear webs otherwise it'll wobble like a jelly even if you're just sleeping in it.

49mm is complete overkill unless you want that scaffold aesthetic. Plenty of cheap metal beds are made in roughly 25x1 square tube and that's full of holes and creases.

I've been thinking of doing similar for my daughter's room but I'll probably do it in timber supported from walls and ceiling to clear more floorspace.

jk

I tried to ASCII-art some bracing in my original post, but the forum software apparently removes backslashes.  The bracing plan is for diagonals from the top of each rear leg to the centre of the bottom longitudinal beam, and from the bottom of each rear leg to the top of each front leg. Or from the top of the rear legs to the bottom of the front legs. I can't picture whether one is better than the other, so I was going to put it together and see how it felt

I assume that not having the side and rear bracing meeting at the top of the rear legs would be better, but I can't explain why that seems correct to me. I guess because if they don't meet, it has the effect of bracing the rear legs at both the top and bottom, increasing the overall rigidity?

If verticals are fixed to floor and rear of bed to wall, that does the diagonal bracing for the rear legs that form a rectangle with floor and rear of elevated bed.
Fixing to the wall also does the diagonal bracing that stops the end rectangles turning into parallelograms.

Yup, 70mm x 70mm - I just measured as you made me doubt myself! The long side apron is 140mm in the vertical direction as well, for reference. 

I think they just look better rising from rather than to a common point, not sure why. The effect is much the same.

I mentioned in my previous post putting in horizontal members low down but thinking about it, that's overkill unless you're using diagonal wires for the X bracing (which is where my brain went because it looks cool).

I think you'll probably end up wanting triangulation (or a web) in the bed deck, especially if one of the 4 upright faces remains open, lacking diagonal bracing. Personally I'd just tie it all into the wall at the head end, use a ply/OSB bed base and be done with the bracing for a clean look and solid build.

jk

Sorry about that, to me they really do look beefier in the photo!