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Thread: How can I reinforce some shelves to prevent sagging under a load?

  1. #1
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    How can I reinforce some shelves to prevent sagging under a load?

    Hi,

    I am planning some shelves which are 28.5" wide by 14" deep made from 3/4" maple plywood. My initial plan was glue on some 1/4" maple strips as edge banding on the front and back. However, when I put those numbers into the sagulator calculator, the load deflection is a little much.

    If I cut thicker edge banding, like 1", and attach with biscuits, will that provide any additional strength? I could double up the ply, but that would throw off the look of the piece, at least to my eye...maybe.

    Any thoughts? I am open to all suggestions

    Thanks!
    Chris

  2. #2
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    Adding shelf support in the rear helps a lot.

  3. #3
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    like additional shelf pins?

    Thanks!
    Chris

  4. #4
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    The ideas that come to mind for me are to add a solid piece of wood maybe 1 1/2" deep to the front and back of the ply but like you said it could change the look of the piece. Like Kevin said one of the easiest solutions is to just add support to the back. If you are really going for a specific look then other not so desirable solutions would be to add a piece of 1/4" steel plate under the shelf or something similar.

    Before you really get in a tizzy over the sagulator thing I would probably cut a similar piece of plywood and load it up like you are going to and see how much it really 'sags'. You could cut a few different pieces to glue to the front and see how much of an effect that has to reduce the depth of the solid wood piece on the front.

    good luck.
    Greg

  5. #5
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    The taller your edge banding, the better. Thickness of edge banding has a much smaller effect.

    eg, 2" tall and 6" wide will flex more the 6" tall and 2" wide.

    You can test this by bending a ruler one way, and then (attempting to bend it) the other way.

  6. #6
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    If you can tolerate a 1"-tall front edge, you get a much stiffer shelf if the 1" thickness continues the whole depth of the shelf. I can buy 1" maple ply at a local dealer, so that might be a possibility for you. Another way to get thicker plywood is to laminate two thinner pieces -- say 3/4 plus 1/4.

    Another approach is to make the shelf from solid lumber. Solid maple is stiffer than an equal thickness of plywood.

  7. #7
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    You can rout or otherwise recess a piece of angle iron in the shelf. Then cover it with trim piece. I heve done this where I needed longer spans but want a thin profile.
    Shawn

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  8. #8
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    1/4" ply certianly won't take much load before it deflects. I would go with 3/4"x1" or 1 1/2" edgebanding then rabbit the top of the banding 3/8"x1/4", glue and pin nail. It really depends how much weight the shelves will be incurring, how far between shelf supports etc. If you do not go to 1/2" ply but rather stay with 1/4" I would suggest stretchers at least every 2' but again how far between shelf supports and how much weight, those are the real deciding factors. Good luck!

  9. #9
    Quote Originally Posted by Shawn Pixley View Post
    You can rout or otherwise recess a piece of angle iron in the shelf. Then cover it with trim piece. I heve done this where I needed longer spans but want a thin profile.
    I did this with angle aluminum. My shelves were in a closet so I didn't do anything special to hide the angle aluminum, but the shelves are painted so they don't show much. I put a piece of angle aluminum in front and in back of each shelf. I cut a rabbet under the shelf so the angle aluminum would roughly align with the top of the shelf. The shelf span is about 52" and the shelf, itself, is 3/4" Baltic birch. The shelves are about 22" deep. When I show the shelves to friends, I have to point out the angle aluminum - people don't notice it.

    I got the angle aluminum from OnLine Metals.

    Mike

    [Additional info: The shelves are loaded with dishes so the load is fairly heavy - and I don't see any noticeable sag. The shelves are on pins so they're adjustable - but I doubt if I'll ever adjust them.]

    Hall-closets-2.jpg
    Last edited by Mike Henderson; 09-09-2012 at 7:53 PM.
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  10. #10
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    Here is one link that provides a load rating for different materials/thickness/spans http://www.woodworkingtips.com/etips/etip01.html

    You will definitely push the limit with 3/4" ply shelves that deep. Yes a front edge of 3/4" x 1" tall maple (straight in both dimensions) glued on with biscuits will help a lot. The biscuits or dominos are for alignment not for added strength. One on the back too will help even more. A row or two of pins on the back would help for a less deep shelf but I think that for 14" I would add the solid wood edge(s) first then maybe a shelf support at the back. Of course all this depends on the intended loads - towels or books can change the equations.

    ... for a more radical approach - I attach the shelf pins (the straight rod style) to 3/4" x 1-1/2" to 2" cleats that fit at the back and sides of the cabinet. These can be moved up or down as needed and the shelves just rest on those cleats. This pretty much solves all the problems of sag though you still must use some common sense depending on the load. I typically attach a riser directly to the front of the shelves if they are only 3/4" thick and will span more than 30" and would do so even with this support cleat system. I have done this with great success on shelves in a cabinet in which the face frame hid the walls of the cabinet with an overlay of 1" or more so that the cleats are mostly hidden.
    Last edited by Sam Murdoch; 09-09-2012 at 5:09 PM. Reason: More info
    Sam

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  11. #11
    Without changing where the supports are, it's all about the stiffness of the material. Simple bending is width X depth X depth X depth. Depth is important.

  12. #12
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    The "standard" answer to your question is this:

    Use a 1-1/2" x 3/4" trim across the front of the shelf. Assuming you have fixed shelves, you could shorten that to 1-1/4" and put some cleats - 3/4" x 3/4" - in the back - screwed into the back and the shelf above.

    Also - just so you know my personal experience - I always check with The Sagulator God, but that has turned out to be the extreme case, and at my spot on the time-space continuum, shelves are frequently loaded only part of the span, and not everything is of max height and depth.

    So, then - when you say "the load deflection is a bit too much", I'd ask a few questions:
    1. How much "too much"?
    2. what loading were you using? Border-to-border with your old Thermo + calculus text books, or a real-life example? You specify 14" depth, and I'd be hard pressed to find any books that are 14" deep, much less enough of them to fill 28.5" span.
    3. Load closer to the ends of the shelf don't make nearly the impact as load in the center. So - a bunch of books that are towards the edges is a completely different critter than a volkswagon on a point load in the very center.

    Having said all of that, I do have some shelves-slash-bookcases that are fully loaded to the gunwales. And, I did not cut any corners there. Span and depth have been adjusted as needed. I have not, generally, used the 1-1/2" front trim - but I have used that trick. Instead, I have put cleats along the back, or - more often - driven screws in through the case back into the shelf, and/or or upgraded from ply to solid wood. No doubt that some of my work could, in fact, support the Vee-Dub as a centered point load.

    Oh - BTW - clamp + glue on the front trim is sufficient. The biscuits add nothing IMO. If you have a pin nailer, that helps hold the trim in line during clamping, but not necessary.
    Last edited by Kent A Bathurst; 09-09-2012 at 4:13 PM.
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  13. #13
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    I have some tool cabinets I built with 36"L x 18"D deep shelves. I have them loaded pretty good, mostly distributed loads with screw boxes, hardware, and craftsman metal toolbox etc. The shelves are 3/4 ply with 3/4"W x 1-1/4"T red oak edging front and rear. The edging is rabbeted to help with alignment. Each shelf rests on four shelf pins in the 3/4" ply sides. I have not had any problems with sagging.

    If you have to keep the profile at 3/4", another thought might be to introduce another 3/4" hardwood strip in the middle of the shelf. In otherwords, each shelf would be a glue-up of a 3/4" hardwood strip on the front, back, and middle, with two 3/4" hardwood panels separating them. The tricky part is getting middle hardwood strip flush.

    Another way, might be to embed a piece(s) of flat steel or aluminum in the shelf. Get stock that is 1/2" wide and stand it on edge. Rip a groove(s) to glue them into and they wouldn't be seen from the top, and they could sit flush on the bottom. You could do one, two, or more as you see fit, but one would likely work if you edgeband the front and back edges.

    Mike

  14. #14
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    You can also add strength by "fixing" the ends and the back.. gluing them into dadoes.. although you've probably already tried that in the Sagulator.
    One can never have too many planes and chisels... or so I'm learning!!

  15. #15
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    It's impossible to answer your question accurately without knowing what the load is, in pounds, and approximately how the load will be distributed across the width of the shelf (as Kent pointed out, putting 40 lbs in the middle 12" of the shelf will cause greater sag than placing 20 lbs at the each of the far ends of the shelf).

    Sagulator says you can load a 28.5 x 13.25 x .75 plywood shelf with an evenly distributed 26 lbs/lin ft, if you attach a 3/4" x 1" hard maple front edging strip.
    Last edited by scott vroom; 09-09-2012 at 8:53 PM.
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