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Shelf sag?

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Shelf sag?

Postby MattS » 12 Jan 2021, 22:06

I'm designing a corner shelving unit, a lot of the measurements are determined by some mid-century units it is fitting within. So very unusually I've been working in inches! I'll give both below! Anyway I plan to have corner shelves and am concerned about sag but don't know quite how to calculate or what's acceptable?

I've used the website http://www.woodbin.com/calcs/sagulator/ and the following information
- 18mm MDF
- L shaped shelf has overall dimensions of 17.5" (445) x 18.5" (470)
- depth consistently 9.25" (235)

I've added both lengths together, as a bit of a worst case. If the shelf is fully fixed to the back panel along it's length I think this added length overestimates the sag and it comes out as 2.11mm total and 2.3mm per m.

Does this sound acceptable? I don't know quite what we'll have the shelves, a few books but it won't be packed hard. I used a weight of 30kg in the calculation.
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Re: Shelf sag?

Postby Phil » 13 Jan 2021, 07:20

What about an 'L' shaped lip edge glued?
This should help some way to preventing sag. Also a support along the back.

HTH (my 2.5 bits)
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Re: Shelf sag?

Postby Argus » 13 Jan 2021, 10:01

Phil wrote:What about an 'L' shaped lip edge glued?
This should help some way to preventing sag. Also a support along the back.

HTH (my 2.5 bits)


I agree..... This has worked for me with book-shelves, though I try to restrict the shelf-length to no more than about 30 inches where books are involved. Weigh a foot-length of solid hard-backs to get an idea of the weight involved!
If you are inclined, the added strength of a solidly glued front lip can be enhanced, sag-wise, by a similar piece along the back that has the shelf-edge enclosed in a groove; the back edge then protrudes above and below. It's inherently strong and also acts as a rear edge stop for books and other objects.
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Re: Shelf sag?

Postby Cabinetman » 13 Jan 2021, 10:08

I built a bookcase once using shelves made in a sandwich construction, two sheets of oak veneered MDF with a lip at the front and structural strips between the sheets, about an inch thick altogether, incredibly strong but very time-consuming to make I never did it again as I reasoned that solid oak didn’t cost anymore and was a lot quicker. Ian
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Re: Shelf sag?

Postby RogerS » 13 Jan 2021, 13:55

Cabinetman wrote:I built a bookcase once using shelves made in a sandwich construction, two sheets of oak veneered MDF with a lip at the front and structural strips between the sheets, about an inch thick altogether, incredibly strong but very time-consuming to make I never did it again as I reasoned that solid oak didn’t cost anymore and was a lot quicker. Ian


Sounds like you built yourself a torsion box. The proper thing to stop sag.
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Re: Shelf sag?

Postby Cabinetman » 13 Jan 2021, 14:00

Yes I did Roger, lots of them, you’d think I’d be able to remember the technical term wouldn’t you!
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