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I decided to make a bunk bed for my room as it is small and I need a desk. I made a first design in this thread Make a bunk bed

first design first design

But I realised that all the strength would be on the angle brackets and the screws. So I made a new design where the two main wooden beams are laid on the four vertical beams. It reduced the number of 3m wooden beams by two because instead of having 4x2m beams I had 4x1.4m beams.

second design second design

I made some tests and saw that the main wooden beams could support about 1000kg but the slates could only support about 50kg. With the help of the bed, the weigh should spread on the other slates but I still would like to have your opinions.

sagulator sagulator

If I had a center horizontal beam, the slates would have ~60cm on each side of the central beam and the support capacity would be much higher but I'm not sure wether it's well attached to the rest of the structure or not.

sagulator final

Did I use the sagulator tool right ?

Do you think I should put the center beam ?

Should it be attached better to the rest of the structure ?


Material :

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    "But I realised that all the strength would be on the angle brackets and the screws." That's perfectly acceptable — this is how floor joists are commonly held up for example. There is no wood-to-wood connection, just a metal bracket or cup of some kind which is fixed in place by some screws. – Graphus Oct 15 '15 at 16:45
  • But I guess it's still less durable than my design (which uses less wood) if you regularly apply dynamic forces on it. – Servietsky Oct 15 '15 at 16:56
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    if you regularly apply dynamic forces on it. It's easy to over-engineer a piece of furniture (I'm guilty of this myself from time to time). I don't think the forces you'll be applying to it (even if you engage in, ahem, adult activities with a consensual partner on the bed) will be enough to break your connections. Most furniture is way overdesigned for the loads that it takes since the sizes and proportions are based on what's visually-pleasing. – grfrazee Oct 15 '15 at 17:51
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    It should be fine. If you're really concerned, tenon the center beam into the ends for added support. – grfrazee Oct 15 '15 at 19:49
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    A joist hanger would be a better choice for some of those, particularly the top center beam (e.g. ep.yimg.com/ay/yhst-4829792992329/… ) and other various simpson (etc) ties that can handle sheer forces better than plain old angles (e.g. manasquanfasteners.com/images/ACE_applic.gif). These are all pretty standard for floor joists and construction. They're all engineered to transmit forces to screws in the safest way possible. They're also easy to mount square. – Jason C Nov 10 '15 at 1:14
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Did I use the sagulator tool right ?

The folks who made Sagulator made the default condition for shelf attachment to be fixed. The connections that you have in your design would all be classified as floating. To be fixed they would need to be clamped tightly with no room to flex at the ends. Nailing would not be fixed, screws would not remain clamped tight for long. Screwed and glued - might work. The brackets definitely should be considered as floating connections. So, I would say that you did not use it correctly from the beginning.

Sagulator can not be relied upon if the loads are unreasonable high or low. In the case of the long beams, a 1000 kg concentrated load is a bit of an over estimate and would have an excessive deflection. If you used a more reasonable total load of 250 kg it would be fine.

For the slats, I think you switched the the depth and thickness (your diagram shows the depth being greater than the thickness.) Sagulator would show excessive deflection in both cases and even with much smaller loads.

Do you think I should put the center beam ?

Even though, Sagulator only shows deflection, and not strength, I would guess that the slats would actually break if you used them for the entire 1200 mm span. So, yes, you need the center beam.

Should it be attached better to the rest of the structure ?

That is a very good question and shows insight on your part. If you are attaching your brackets with carriage bolts - no problem. If you use screws, use the longest ones that you can.

| improve this answer | |
  • Screws or bolts, they're going to hit eachother inside the corners if all those angle brackets have identical and limited mounting holes. If angle brackets are used be prepared to drill some holes in them and try to avoid turning the wood to swiss cheese. Note also that angle brackets will put pulling force instead of sheer forces on some of the screws/bolts if there are lateral forces on the structure. – Jason C Nov 13 '15 at 1:18

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