shelf strength

[forunna]

HI there,
Can I get some advice on shelf strength here please?
Im making a cabinet for the Telly and all its typical attatchments.
Three boxes from 12mm laminated ply screwed to common legs of 18x60mm hardwood of some kind.With extra bits on the bottom of the legs that take the actual weigth of the cabinet.
Then a full sheet across the top of the lot.
The shelves are going to be 752mm wide by 500mm deep. Is that too wide for 12mm ply?
I plan on rebating the shelves in to the sides of the boxes so they are permanent.

[Chumley]

Hi Craig,

I guess there is a scientific answer somewhere (not mine) - but from my experience it will depend not only on what you put on the shelf but where you put it. I've made lotsa bookshelves in my time and 750mm would be ok for 25mm or 30mm think but not 12mm. I would happily use 12mm if the shelf was only holding a dvd player (not that heavy), or if the item was heavy I could make it fit neatly across the whole width of the shelf so that the weight is distributed (even better if the object has feet and the weight is then near the edges). Think of it this way - if you had a 750mm wide, 12mm thick shelf, supported at the edges with dados, putting a small 10kg block (say brick size) in the centre of the shelf would probably cause a sag; however if you position the block at the edges you could probably put one each end and have no sag at all.

Cheers,
Chumley

[silentC]

http://www.woodbin.com/calcs/sagulator.htm

[forunna]

thanks guys.
I really like that Sagulator. :-)
hmm, now to decide, shorten it or step up to thicker ply. maybe even mix it up.