# Structural rigidity of bookcase



## city17 (13 Dec 2021)

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.


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## Richard_C (13 Dec 2021)

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.


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## city17 (13 Dec 2021)

Richard_C said:


> 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.


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## Cabinetman (13 Dec 2021)

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


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## dzj (13 Dec 2021)

120cm wide and 15mm shelves...Maybe a vertical piece in the middle?
There might be sagging issues otherwise.


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## city17 (13 Dec 2021)

@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)


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## dzj (13 Dec 2021)

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.


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## TheTiddles (13 Dec 2021)

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?


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## thetyreman (13 Dec 2021)

I would go with 18mm thick stock for the shelves


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## Jones (13 Dec 2021)

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.


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## isaac3d (13 Dec 2021)

TheTiddles said:


> 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?


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## city17 (13 Dec 2021)

Jones said:


> 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:





Regarding wracking / torsion rigidity, would adding three back plates like this be enough?




@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 (13 Dec 2021)

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.


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## Woody2Shoes (13 Dec 2021)

city17 said:


> 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:
> 
> ...


Try the sagulator:


The Sagulator – WoodBin


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.


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## isaac3d (13 Dec 2021)

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.


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## Spectric (13 Dec 2021)

TheTiddles said:


> which I think could benefit from a face frame in my view


That would also add character in my opinion


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## TheTiddles (13 Dec 2021)

isaac3d said:


> 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.


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## Jones (13 Dec 2021)

city17 said:


> 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:
> 
> ...


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.


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## Jester129 (14 Dec 2021)

If you're putting books on those shelves, will dominoes and glue be enough? I would think about rebating the shelves into the uprights.


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## JobandKnock (14 Dec 2021)

Jester129 said:


> 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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## MikeJhn (14 Dec 2021)

Don't know if it's been mentioned, but the up-stand on the front of the shelves will be in the way of getting books in and out and restricts the shelve width, better to have a down-stand.


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## Junah (14 Dec 2021)

You need something like this, still standing after recent tornadoes in America


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## Henniep (14 Dec 2021)

city17 said:


> @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?
> 
> ...


Even 18mm will sag over 1.2M - given time and the weight of the books. I recommend a vertical support in the centre of each shelf.


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## Sgian Dubh (14 Dec 2021)

isaac3d said:


> 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.


If you were to weigh your books you might be surprised at the result. 

The human eye can easily spot a shelf sagging by as little as 1 or 2 mm (1/32”) over a 610 mm (24”) length. These 660 mm (26”) long shelves sag 4 mm (~5/32”) at their centre point. The shelves carry a typical load of books weighing approximately 11 kg per 300 mm length, about the same as 25 lbs per foot length. In this case what you're looking at is a loaded MFC (melamine faced chipboard) shelf supported on shelf pins. Technically the shelves are simple beams carrying a static load, and in this case the manufacture is of the every expense spared type common in cheap and simple furniture mass production. Slainte.


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## Adam W. (14 Dec 2021)

Creep.


It's called creep.


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## RobinBHM (14 Dec 2021)

is this sitting against a wall with skirting?

does the room have carpet fitted with gripper rod?

if so the unit wont sit back flat to the wall

personally id be tempted to put in some random vertical dividers -wide bookshelves arent very practical as its hard to keep the books dead vertical

Im a little concerned your idea of a shadow gap formed by having the lipping short might look like a badly fitted joint -ie an error rather than a design feature -maybe you might want to make a sample and see....please dont see this as a criticism -just thinking aloud!


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## city17 (14 Dec 2021)

RobinBHM said:


> is this sitting against a wall with skirting?
> 
> does the room have carpet fitted with gripper rod?
> 
> ...


It's sitting against a wall with thin skirting at the bottom. Floor is linoleum. I might fix it to the wall if needed, it shouldn't be a problem as the skirting is quite thin.

Good point about the shadow gap, I've tested it by clamping a few pieces together and I think it looks good, but it might turn out differently on the real piece. I'll make a test piece to be sure. If it doesn't look good, I can turn it into a normal lipping.

I've change the dimensions to 1000 mm wide, which should be a little less ambitious...


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## accipiter (14 Dec 2021)

@city17 
To get some idea of the potential sag of the shelves - whether 1200, 1100 or 1000mm in length (for the "width" of the unit you are making) and 15mm or 18mm thick - get a piece and rest the ends on some support to suspend above the floor and then place the intended publications on it. Give a couple of days to see what sag you'll get.

Personally I would have to put vertical supports in if using 15 or 18mm even if facing up with some slightly wider timber. You'll be surprised at the weight of books and magazines.. and 18mm will sag over 600mm long shelves if only minutely to start with... even with say a 25mm wide face timber hiding the ply edges.

I'd also put the bottom shelf supported on a plinth maybe 50mm/75mm in height rather than rest the bottom shelf directly on the floor as your drawings give the impression of doing. I've no experience with dominos for joints but they'd be a weak area on the bottom and top of the unit and a plinth at the bottom would give some rigidity. I've used a biscuit joiner though and I see dominos as a thicker biscuit.

*Or*... consider a double lamination of the ply making *all* parts 30mm... or even 33mm (15+18mm) or 36mm... thick if you don't really want the vertical supports. That may be too "chunky" for you and the design - as well as too heavy to move about - especially if 36mm thick but the shelves shouldn't sag and you could double or treble up the dominos in the shelf thickness to strengthen the joints and rigidity.

Just some thoughts based on the drawings and information given in your opening post.


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## MikeJhn (14 Dec 2021)

Adam W. said:


> Creep.



No need to go name calling. LOL


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## city17 (22 Jan 2022)

The bookcase is almost finished now, thought I'd give an update on how strong the design actually turned out to be.






As to bowing, the shelf that had no connection to the vertical parts (so the lowest shelf) was quite susceptible to bowing if I pushed it down. If you would put a few light things on it, it would be fine, but I don't think it would be a good idea to put heavy books on it.

However, after I added the lipping, I think it is very solid now and won't bow unless you really overload it. All the other shelfs are even more solid.

As to racking, there is a tiny amount possible when I try to twist it from the top. Maybe 3mm of sideways movement on either end. Although it might be less when putting a full backing board panel on it, this seems to be plenty sturdy. 

Now I just need to add another coat of Osmo and make the drawers, and it's finished!


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