plywood sag

I'm building some wall mounted book shelves out of 3/4" birch plywood. Do I need to add vertical supports for spans that are 36" wide and 11" deep?
Thanks
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Yes, or reinforce the shelves, especially if you plan on putting books on that book shelf.

I
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I
If I was doing it, I would attach a 1" hardwood strip to the front to a) hide the side of the plywood and b) add stiffness.
todd
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Yes. As Nathan said, use some hardwood face frame, but also place stiffeners along the back of the shelf. You also want to make sure that the stiffeners you apply to not cause the next shelf down to bend also.
JAW
Nathan wrote:

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Please define 'wall-mounted' for us. In fact, maybe a clearer description of what you'd like to do would be helpful.
I had a response all written, and then re-read your post, and seen that I'd made a bunch of unjustified assumptions about what you wanted to accomplish.
Patriarch
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Hey Patriarch,
It's 1 shelf book case that has 4 sides and a back. The only difference
======================================| | | | | | | | | | | | |====================================== | | | | | | | | ======================================"patriarch snipped-for-privacy@nospam.comcastDOTnet>" <<patriarch> wrote in message

I'd
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Hey Patriarch,
It's single shelf book case that has 4 sides and a back. By "wall mounted", I mean that it will be attached to framing studs on a wall and won't be free standing. Sides are attached to top/bottom with 1/2" deep rabbets. All material is 3/4" birch plywood. Here's some crude ASCII art of what a front view of the bookshelf will look like if I don't make any modifications for sag:
36" ======================================| | | | | | | | | | | | |======================================= 24" | | | | | | | | | | | | ====================================== I'm considering adding a vertical support on the bottom shelf:
36" ======================================| | | | | | | | | | | | |======================================= 24" | | | | | | | | | | | | | | | | | | ====================================== Your thoughts?
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OK. Good thing I didn't send what I wrote earlier, because I had envisioned this thing somewhat differently. But, hey, it's not MY shelf.
Were I to build YOUR shelf, I think I would add a face frame, like one of the other posters mentioned, and I would add horizontal cleats (3", 3/4 ply, mounted flat against the back somehow, on the outside. These would have two functions:
    * stiffening the shelves against sag     * functioning as a french cleat, for hanging on/against the wall
Of the two, the french cleat is probably most important. The 3/4 Birch ply is a pretty substantial backer. If you need explanation of a french cleat, googling this group archive will give you all sorts of excellent references. IMO, there are few better ways to hang heavy things on walls. And by the time you're done, this piece will have most of a full sheet of birch ply in it. Maybe 60 lbs, empty?
If the vertical piece you have drawn adds apparent strength, then go ahead and add it. You could mock this up pretty easily, if you do the joinery with pocket screws, or screws with the intent of covering the heads with plugs. Then you could test the apparent strength of the construction in your intended application, and make any adjustments you felt were required.
We live in earthquake country, here in Northern California. I have a wife who is 'concerned' about bookcases. Hence, the ones I build aren't particularly dainty. I built a bookshelf of similar scale and construction last month, but mine has 4 shelves, and sits on the floor, on a 5/4 hardwood base, to be loaded with engineering texts this week. Your needs may vary greatly.
Enjoy the discovery. Plan and work safely. Take pictures. Share.
Patriarch, whose father, again yesterday, said 'It may not be pretty, but it's hell for strong.' Some lessons stay around a while.
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Nathan wrote:

if you can support the rear edge of the shelves, no need for vertical supports. does the cabinet have a rear panel??
Google on "sagulator". it's a cool webpage to give you sag statistics on shelves supported only on the ends.
dave
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3/4 ply should do fine on a 36in span
John

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John wrote:

do you know how much weight he plans to carry on those shelves?
dave
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Hi Nathan,
I just built a bookshelf last week using cherry ply for my office. I wanted it to fill a specific space so I made it 37 1/2" wide. The shelves themselves are exactly 11" ax 36 inches. You can see the shelf at http://www.redway.com/shelf /
I have a lot of heavy EMS and Firefighting textbooks and they caused a slight bow in the shelf. The picture shows a more pronounce bend, but that is exaggerated by the camera lens. I have lighter books and software boxes and there is no bow at all. So I guess it all depends what you are going to be putting on the shelf. A 32 inch shelf would probably have been better, but I wanted the larger size. I think I will spread out the books a bit so that the bend is reduced, and there will be no need to insert vertical braces.
James Makers of The Small Table Making program http://www.knightlite.com/creative

I
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36" span is a lot for books. Put a 1.5" solid wood lip on front and on the back of each shelf. This will make the shelf appear thicker.
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