Structural rigidity of bookcase

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city17

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I'm working on a design of a small bookcase/cabinet made from 18mm birch plywood. It's about 150cm high and 120cm wide, will have four shelves (15mm ply) and use dominos for joinery.

However, I'm not entirely sure how rigid it will be. Specifically I'm wondering:

A: If it needs a back plate, or at least some small spacers between shelves at the back to prevent torsion? There will be a back plate behind the drawers. For the design I'd prefer it to not have a back plate.
B: What the strongest configuration is with the side and top and bottom panels, using dominos? I've currently got the top board between the two side ones, and the bottom one underneath.

Below is a drawing from the front perspective. The shelves will be 15mm, they seem thicker on the drawing but that's just a decorative edge.

Any advice would be much appreciated.

Schermafbeelding 2021-12-13 om 14.08.14.png
 

Richard_C

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Can't help with domino's, not used them, but general thoughts.

I made a smaller bookshelf 30 years ago, still fine. No back, no drawers, your drawers might give you all the rigidity you need. The bottom shelf was between the uprights with a small slightly set back upstand underneath it and glued to it which gives extra rigidity. Plus the bookshelf then stands on 2 ends/1 edge rather than a flat bottom which might give you issues on carpet as it can end up on top of carpet gripper bars and forever lean towards you. No back, but I did add a c. 15mm deep cross piece about 12cm above each shelf so the books don't end up pushed over the back edge.

For appearance your top might be better sitting on top rather than between unless you plan to edge the ply. As it is you will see the ply edges on the front and a bit on the top. If it's on top you will see it full width and ends as a continuum.
 

city17

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Can't help with domino's, not used them, but general thoughts.

I made a smaller bookshelf 30 years ago, still fine. No back, no drawers, your drawers might give you all the rigidity you need. The bottom shelf was between the uprights with a small slightly set back upstand underneath it and glued to it which gives extra rigidity. Plus the bookshelf then stands on 2 ends/1 edge rather than a flat bottom which might give you issues on carpet as it can end up on top of carpet gripper bars and forever lean towards you. No back, but I did add a c. 15mm deep cross piece about 12cm above each shelf so the books don't end up pushed over the back edge.

For appearance your top might be better sitting on top rather than between unless you plan to edge the ply. As it is you will see the ply edges on the front and a bit on the top. If it's on top you will see it full width and ends as a continuum.
Thanks Richard. I indeed forgot to mention that I'm planning to use edgebanding the plywood edges, that's why I planned to do the top this way, to keep the continuous grain on the side panels. But was wondering if had any impact on the rigidity.

Regarding the bottom, my idea was to add small adjustable feet (max 2 cm height), so it wouldn't lean on the floor with the full width of the panel. 2 on each side, and two in the middle.
 

Cabinetman

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As it is I think it will wrack, @Richard_C 's idea of a small upstand to stop books going too far back would stiffen it up significantly.
Another way to stiffen it would be to incorporate a plinth, behind which you could put some fairly substantial blocks under the bottom shelf and fastened to the sides. Ian
 

city17

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@dzj Do you think the 15mm shelves will sag? I could upgrade them to 18mm, the cost difference is not that much, if that would solve it.

I would have a cross piece behind the drawers that would function like the upstands Richard mentions (although just being the single one). Do you think that would be enough to prevent wracking?

One option would be to add another cross section/back piece from the bottom to halfway to the first shelf.

Edit: like so: (view from the back)

Schermafbeelding 2021-12-13 om 15.14.36.png
 
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dzj

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Do an experiment. Clamp all the cross pieces and lipping to a shelf and load it with books and such. See how it fares in a day or 2.
 

TheTiddles

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Yes, needs a back.
15mm is very thin for the shelves unless you put a chunky drop front and maybe upstand at the back, but you could go thinner and make them as torsion boxes but then they’d look odd compared to your frame, which I think could benefit from a face frame in my view.

Out of curiosity, have you bought a domino jointer but haven’t made anything like this before?
 

Jones

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15mm way too thin for a shelf spanning 1200mm. Normally 18 mm is ok up to 1000mm but 1200mm might be a stretch. If the lipping on the sketch is more than decorative say 28 mm deep by 40 mm wide that would stiffen the shelves or use 25 mm plus solid timber or ply. A plinth will help with racking as will a piece under the top which will also give you a place to fix back to the wall to prevent toppling.
 

isaac3d

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Yes, needs a back.
15mm is very thin for the shelves unless you put a chunky drop front and maybe upstand at the back, but you could go thinner and make them as torsion boxes but then they’d look odd compared to your frame, which I think could benefit from a face frame in my view.

Out of curiosity, have you bought a domino jointer but haven’t made anything like this before?
I'm also building set of bookshelves and was planning to use 15mm thick timber for the shelves. Your comment gives me pause for thought. The carcass of my unit (1640mm high by 770mm wide and 180mm deep) will be made from an old ikea table (so, machined solid pine). I have milled the carcass pieces to 25mm thick. The shelves will also be recycled ikea furniture (machined solid pine). The unit will have a back but it is only hardboard (yep, more recycled ikea stuff). It will be rebated in to the carcass and nailed so it might add a bit of torsional rigidity (it will also be nailed in to the back of the 7 shelves) but I'm not sure it will add very much strength to the shelves. I could add an upright in the middle of the back (also machined solid pine). Is this something that you would recommend for a shelf span of 770mm (720mm internal) with 15mm thick timber?
 

city17

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15mm way too thin for a shelf spanning 1200mm. Normally 18 mm is ok up to 1000mm but 1200mm might be a stretch. If the lipping on the sketch is more than decorative say 28 mm deep by 40 mm wide that would stiffen the shelves or use 25 mm plus solid timber or ply. A plinth will help with racking as will a piece under the top which will also give you a place to fix back to the wall to prevent toppling.
Thanks, I'll be going for 18mm then. The lipping will be solid birch, 18mm deep and 36mm high. Front and back. Would that be sufficient you think? I'm also thinking about reducing the width to 1100 as 1200 seems to be pushing it a little.

There's one complicating factor for the lipping, which is that I had designed it to leave a shadow gap, to give the impression of floating shelves. So the shelf would go all the way, but the lipping would leave a ~1-2mm gap, like so:

Schermafbeelding 2021-12-13 om 18.40.54.png

Regarding wracking / torsion rigidity, would adding three back plates like this be enough?
Schermafbeelding 2021-12-13 om 18.53.57.png

@TheTiddles I usually work with solid wood, made some desks / small tables with my Domino. First larger plywood project I'm doing.
 
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Ozi

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I built something similar about 25 years ago but used 18mm block board instead of ply for the shelves, two bays wide 4' span each. That started to bow after a few weeks with very heavy books on it until I added a back board of hardboard which hid two 5" x 1" vertical braces. Not the materials of choice but my budget was very limited and the hardboard and braces were bits I had. It looked Ok ish all it lacked was skill and budget. When our old house came up for sale about 5 years ago my MIL sent us the estate agents details and it was still there.
 

Woody2Shoes

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I'm working on a design of a small bookcase/cabinet made from 18mm birch plywood. It's about 150cm high and 120cm wide, will have four shelves (15mm ply) and use dominos for joinery.

However, I'm not entirely sure how rigid it will be. Specifically I'm wondering:

A: If it needs a back plate, or at least some small spacers between shelves at the back to prevent torsion? There will be a back plate behind the drawers. For the design I'd prefer it to not have a back plate.
B: What the strongest configuration is with the side and top and bottom panels, using dominos? I've currently got the top board between the two side ones, and the bottom one underneath.

Below is a drawing from the front perspective. The shelves will be 15mm, they seem thicker on the drawing but that's just a decorative edge.

Any advice would be much appreciated.

View attachment 124061
Try the sagulator:
Personally, I think that you need a back or some other bracing element(s) to prevent racking and to prevent sagging - even with 18mm ply. This is a situation where solid timber makes a better shelf than ply.
 

isaac3d

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The Sagulator is a cool little tool. Thanks.
It says that as long as I don't exceed 20kg on a shelf the sag should not be noticible. I won't be loading anywhere near that. 72 cm of paperbacks is probably less than 5 kg.
 

TheTiddles

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I'm also building set of bookshelves and was planning to use 15mm thick timber for the shelves. Your comment gives me pause for thought. The carcass of my unit (1640mm high by 770mm wide and 180mm deep) will be made from an old ikea table (so, machined solid pine). I have milled the carcass pieces to 25mm thick. The shelves will also be recycled ikea furniture (machined solid pine). The unit will have a back but it is only hardboard (yep, more recycled ikea stuff). It will be rebated in to the carcass and nailed so it might add a bit of torsional rigidity (it will also be nailed in to the back of the 7 shelves) but I'm not sure it will add very much strength to the shelves. I could add an upright in the middle of the back (also machined solid pine). Is this something that you would recommend for a shelf span of 770mm (720mm internal) with 15mm thick timber?

Hardboard for a back is fine, remember whilst it’s all wibbly-wobbly (that’s a technical term) if you compress it, nailed into the back you will be placing it in tension to move your unit out of square and it’s rather stronger that way.

Your shelves are less wide, so that may be ok, (unless you’re putting lots of books on them) nailing to the hardboard won’t do much for the shelves but it’ll stiffen the unit further. But a small drop edge on the front of each shelf and you have plenty of strength for sure.

There’s straight elastic deformation of the shelf, which may not be apparent, but they can also slowly sag over time if there’s enough weight on them. Particle board is worst for that.

Some of it also depends on the look or need of the piece, thick shelves are certainly stronger and look that way too, but they use valuable storage space if you are maximising capacity.
 

Jones

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Thanks, I'll be going for 18mm then. The lipping will be solid birch, 18mm deep and 36mm high. Front and back. Would that be sufficient you think? I'm also thinking about reducing the width to 1100 as 1200 seems to be pushing it a little.

There's one complicating factor for the lipping, which is that I had designed it to leave a shadow gap, to give the impression of floating shelves. So the shelf would go all the way, but the lipping would leave a ~1-2mm gap, like so:

View attachment 124069
Regarding wracking / torsion rigidity, would adding three back plates like this be enough?
View attachment 124070
@TheTiddles I usually work with solid wood, made some desks / small tables with my Domino. First larger plywood project I'm doing.
I think the three bits in the back will stiffen it up enough to stop any racking. 36 mm edging on 18 mm shelves should be ok for a 1200 mm span as well. I don't think the lipping has to extend to the end so a shadow gap will work as the maximum bending stress in the shelf is in the middle so that's where you need the most depth of material . To fix the shelf and edging together well it might be worth putting a small rebate 18x4 mm on the edging piece.
 

JobandKnock

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If you're putting books on those shelves, will dominoes and glue be enough? I would think about rebating the shelves into the uprights.
Dominos are beech loose tenons and are perfectly good enough for the task at hand. Rebates and housings will potentially weaken the plywood ends and make for problems in lipping the plywood. We were always taught that housed joints like that were OK for "back of house" shelving but were not good quality work, a bias I've had all my working life
 

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