Shelf Length - Possibility of Sagging?

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humanfish

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Hi there
My parents would like me to make this sideboard for them out of oak, it's a big bu**er!

http://i4.photobucket.com/albums/y120/b ... eboard.jpg

It will have doors by the way :)

I have a couple of questions

The shelf lengths are 720mm using 20mm stock. As you can see in the image my brother included some battens to prevent sagging of the shelf.
I was wondering whether the sagging would be likely to happen without these battens inplace and if so what thickness material should i use to prevent the sagging without the battens. Would 20mm or 25mm be enough over the length or would 30mm be advisable. It will be holding general sideboard stuff, trust me in my parents case stuff is the word. :)

My other question is i will be using some veneered mdf for certain panels in the construction. The chosen finish for the overall sideboard is oiling, is this compatible with veneered board or will it have adverse effects like raising the veneer.

Thanks for any help
 
the most effective way to make wide shelves without sagging is to make them as "torsion boxes" with veneered tops and bottoms, and solid fronts.

basically lay out a spine like the wings of a model airplane, and then cover with veneered sheets/mdf. this is a very popular system in the states, and can i am sure be found on the FWW web site.

paul :wink:
 
Thank's for the link Adam

Using white oak with a load of 35kg uniformley across the shelf

A shelf 720mm wide x 315mm deep x 20mm thick has deflection of - 0.65

A shelf 720mm wide x 315mm deep x 25mm thick has a deflection of - 0.33

A shelf 720mm wide x 315mm deep x 30mm thick has a deflection of - 0.19

Is a deflection of 0.65 on a 20mm thick shelf alot? Is that just over half a millimeter in which case it's not an issue i guess. It doesn't seem there will be an obvious curve in the shelf.
 
latest issue of fww has article about bookcases, and to them such a deflection is quite noticeable, although behind doors who knows??

thing is to try it on a pair of bricks, and see what it looks like to see whether you can live with the bend :roll:

paul
:wink:
 
The calculated deflection is less than one mm over 720 - sounds minimal to me. Also the load of 35KG sounds very high for a sideboard. Unless you are storing lead weights in it, I don't see such a load arising from the usual contents of a sideboard on a shelf of the size you mention.
 
bad_hypertension":hdiazrs6 said:
A shelf 720mm wide x 315mm deep x 20mm thick has deflection of - 0.65

A

Realistically, you could have more than that movement naturally from the oak that you use. Of course, you have to maintain accuracy in your build but IMO it would not be a worry and more importantly who would be able to see that amount of sag.
 
Thanks for the link engineer

By the sounds of the reply's i guess it shouldn't be much of a problem, it's good to confirm this. I will do a check when i've machined the shelves up before they go in like you suggested engineer

cheers guys
 
housings or sliding dovetails at the self/cabinet join help prevent sag too (from the same article in fww)
 

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