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Shown here are plans for my office/bedroom build (bed is a Japanese futon and pillows that fold and fit into the compartment of the 3 benches). As you can imagine, if I need to sleep, I can set the futon on the floor or, put these benches inline, and set them on cushions (latches on the bottom to prevent them from coming apart). These benches will be set on 3" casters (not shown).

The problem I'm trying to solve is how to design in a sturdy deployable backrest. What's shown is a crude T-bolt and knobs implementation but I don't think it'll hold up when a person is leaning against it. It might work if I use two knobs per slot but the short height of the backrest won't leave room for a second knob.

The benchseats carcass and backrest will be made from 3/4" hardwood plywood (Home Depot/Lowes). Where stiffness is needed, I prefer to double-up on the plywood.

Willing to use metal (1/4"-thick will do I think) in the design if it solves the problem. I'm thinking a metal L-bracket that latches onto the underside and sides of the bench (that's how my computer chair solved the problem).

Sketchup Model of Room

Seat Height (Raised 3" off ground)

Update 1 Found an OEM Testrite that makes telescopic tubing/square tubing with spring button locks (and YT: Telescopic tubing lock (latch), Product demo).

I'm thinking to widen the slot to fit one of these tubes. The base of the tube, fixed to the base of the seat, would be 12" tall. The inner telescopic arm would be attached to the lifted backrest. Assuming the base tube and the telescopic arm overlap 4", the backrest should come up 8". If I attach the base tube inside of the seat, I can hide the hardware and have it extend lower to the ground by 2". The cushion piece can fit over the 3/4" plywood (bridle joint) and increase the height by another 4".

Update 2 Telescopic tube could work but is more complex to integrate into the design. I think a ratcheting bracket might work better. Something like Amz: 180 Degree 5 Gear Folding Chair Lift Bracket Sofa Hinge. Mount that upside-down so that when the bracket is fully opened (the bracket arms are 180 deg to each other), the backrest is "deployed". When it's closed (bracket arms are pointing in the same direction, downwards towards the floor), the backrest is "stowed". Amz: This looks pretty promising

Update 3 Another idea (I'm on a roll today): Slotted backrest! I think I saw this on youtube somewhere ... Modern Builds/Plywood Sofa? enter image description here

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    I think you could fit in two knobs per slot since the slot doesn't have to be 'stopped', i.e. it can run out the bottom (as is commonly done on the supports for headboards). I think this would be sturdy enough to withstand use; not sure if this would solve the problem of flex in the system, but I think it is worth a try.
    – Graphus
    Jan 18 at 5:16
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    You show horizontal dimensions, but not vertical dimensions, so it's hard to know how much room there is. Maybe make the backrests taller so that when stowed, the bottoms just clear the ground, leaving enough room to wheel the bench around. Maybe even make it come up (when stowed) to just below the cushion level. This way, when fully extended, there is extra material to overlap the base allowing for a more sturdy connection.
    – FreeMan
    Jan 18 at 16:27
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    Congrats on the clearly stated Q, especially for the images which give us an unequivocal idea of the original plan. But despite this and the updates you've neglected to mention what you expect to/will build this from! This is a key detail in terms of resultant stiffness and strength of the components. The two points I see as critical are 1) how much flex there might be in the backrests themselves (assuming firm and strong anchoring) and 2) the inherent strength of the material of the box, separate from the T-nut. Since I think the knobs are a potential solution IF mounted v strongly [contd]
    – Graphus
    Jan 18 at 19:46
  • ....and if the board material is inherently strong (so not standard MDF), one further point becomes a possible issue 3) how much internal space needs to be maintained? I think even if you build this from top-quality 3/4" / 18mm birch ply (do not assume all birch ply is made equal, despite its inevitable high cost) I think internal reinforcement where the knobs will go would be desirable (critical for only one knob per slot), and additional reinforcement of the box structure itself might be desirable because of the levering forces that will arise in use, trying to peel apart the box at the top
    – Graphus
    Jan 18 at 19:56
  • @Graphus Great catch regarding material info, thanks. I'm going to use 3/4" hardwood plywood for the carcass. The plywood will mostly serve as backing for the foam cushion, which is traditionally thinner 1/4" plywood (so this will be fairly rigid). Update 3 proposes a tab-and-slot joint which is simple and requires no hardware but depending on the load (e.g., will an adult forcefully lean against the backrest? No), the tab might need to be reinforced (metal glued to the tab surface).
    – Minh Tran
    Jan 18 at 20:48

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