Structural rigidity of bookcase

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MikeJhn

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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.
 

Junah

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Screenshot 2021-12-14 at 08.54.48.png


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

Henniep

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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)

View attachment 124063
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.
 

Sgian Dubh

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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.
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.

TW-26-12_9.jpg
 

RobinBHM

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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!
 

city17

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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!
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...
 

accipiter

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@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.
 

city17

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

DSCF1657 Large.jpeg


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!

DSCF1659 Large.jpeg
 

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