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How much weight can this 2x8" shelf support?
Old 05-24-2014, 12:09 PM   #1
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How much weight can this 2x8" shelf support?

How much static weight can a 2x8" board support when installed at an angle between two walls without sagging? I am building a shelf in a corner to hang a projection screen from. Each end of the stud will be supported by a bracket that is screwed through 1/2" drywall into a stud. See diagram. Along the span of the shelf there is no support. I want to hang ~40 lbs on the shelf not including the weight of the shelf itself which I estimate at another 20 lbs. Will this sag over time? If so, can I improve the strength by using two 2x8s screwed together?

Any structural engineers out there?
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Old 05-24-2014, 12:48 PM   #2
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You can find data for shelf span capacities pretty easily with a search.

But I think your bigger problem will be finding a 100" long 2 x 8 that isn't warped, and if you find one, will it stay straight?

I'd think about a 100" 1x8 (or plywood pieces) supported by 2 x 2's or 2 x 4's. As you screw/glue the 2x2 or 2x4's, you can align them to the shelf edge, and everything will be straight (you might still get a little twist, but this usually is controlled pretty well with this method.

Kinda like this, looking from the end:

Code:
 

=========   < ply wood or 1 x 8
[]     []   < 2 x 2 or 2 x 4 on edge

Not a structural engineer, but one of them told be that doubling the thickness increases the resistance to sag by 8x (cubed effect). Some slight variance depending on the support details, but close enough).


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Old 05-24-2014, 12:48 PM   #3
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I'm too lazy to do the calculation, but you want to add some vertical section to make it stronger. In cross section it should be L shaped.
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Old 05-24-2014, 01:00 PM   #4
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Wow, check this link:


The Sagulator - Shelf Sag Calculator


The 'sagulator'! I entered 'pine, western white (I have no idea what big-box lumber variety is), floating support (conservative, I'm not sure your brackets would really reduce sag much?), 100" span (I guess you could use the mid point?), center load (not sure how the screen will be mounted), and 7.5" x 3/4".
But based on that, no way, > 2" sag! Even 5# is 'borderline'.

Try it with the edging options as I suggested.

edit/add: I'm pretty sure 'fixed support' means that you are talking about the sag in the center of a chain of supports, like:

Code:
================================================================
 X          X < sag >  X          X          X         X
Where the board is fixed at each point, so the other supports also reduce sag in the center. But I am not a mechanical/structural engineer.

further edit/add - I entered 1 x 8's, above, now I see you are talking 2 x 8's. Looks like a single 2 x 8 is 'borderline', and two of them are 'acceptable'.

But 1/2" plywood and 2x4 edging also works.

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Old 05-24-2014, 01:09 PM   #5
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I am an engineer but find it easier to test.

place two saw horses 100" apart place your board and measure to the floor than add weight and measure again.

I do not think time will be much of a factor as it is a static load.
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Old 05-24-2014, 01:21 PM   #6
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Do you actually need the depth for a shelf, or is it just to hang the screen? The calculator also shows a single 2x6 on edge is acceptable. I'd probably screw/glue two 1 x 6's together to reduce warp if I went that way.

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Old 05-24-2014, 01:28 PM   #7
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7.5 " by 1.5" not 3/4" as below

dug fir works very little sag .03 inches

Quote:
Originally Posted by ERD50 View Post
Wow, check this link:


The Sagulator - Shelf Sag Calculator


The 'sagulator'! I entered 'pine, western white (I have no idea what big-box lumber variety is), floating support (conservative, I'm not sure your brackets would really reduce sag much?), 100" span (I guess you could use the mid point?), center load (not sure how the screen will be mounted), and 7.5" x 3/4".
But based on that, no way, > 2" sag! Even 5# is 'borderline'.

Try it with the edging options as I suggested.

edit/add: I'm pretty sure 'fixed support' means that you are talking about the sag in the center of a chain of supports, like:

Code:
================================================================
 X          X < sag >  X          X          X         X
Where the board is fixed at each point, so the other supports also reduce sag in the center. But I am not a mechanical/structural engineer.

further edit/add - I entered 1 x 8's, above, now I see you are talking 2 x 8's. Looks like a single 2 x 8 is 'borderline', and two of them are 'acceptable'.

But 1/2" plywood and 2x4 edging also works.

-ERD50
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Old 05-24-2014, 01:54 PM   #8
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Would you be better off to use a piece of metal bar or iron pipe instead?
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Old 05-24-2014, 03:36 PM   #9
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The weight can be supported, but I would reinforce as suggested.

I'd be concerned about the 2x4s you are anchoring to. Over time the walls will sag out due to the weight. That is the guess of a non-engineer.
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Old 05-24-2014, 03:43 PM   #10
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Great help everyone! I googled but had not seen the sagulator before. It does say I am borderline with a single 2x8.

Can't mount the 2x8 vertical because of where the wall studs are.

Can't use metal pipe because I need to be able to attach the screen brackets, and a need some shelf space to hold a speaker.

I think I will get a 1x2 and run it along the length of the 2x8 to add a vertical member to provide added support.

Thanks!
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Old 05-25-2014, 05:16 AM   #11
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As others said, a 2x6 vertical will work, would that work to attach to the studs? Good luck!
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Old 05-25-2014, 06:05 AM   #12
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Another strengthening option is screwing a 1/4" thick piece of steel to the sides of the 2x8.

Or rip the 2x8 into 4 pieces and glue 3 pieces of 1/4" plywood 1-1/2" tall between the pieces of 2x8. I would use baltic birch plywood because it has no voids.

But the best solution may be a torsion box shelf.
AW Extra 3/6/14 - Stronger Shelves - Woodworking Techniques - American Woodworker Look at figure F.

I would use this option maybe 4" tall (thick). There may be some info out there that states how tall it needs to be to support your weight for your length.
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Old 05-26-2014, 04:50 PM   #13
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Ok, here is what I put up, I don't think I'll put the 20 lb center channel speaker on the shelf and instead put it on the media cabinet. Not totally done yet, need to clean up the wires and mount the speakers, but coming along nicely. Now I have a 100" TV!

Thanks all.
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Old 05-26-2014, 06:29 PM   #14
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Quote:
Originally Posted by soupcxan View Post
Ok, here is what I put up, I don't think I'll put the 20 lb center channel speaker on the shelf and instead put it on the media cabinet. Not totally done yet, need to clean up the wires and mount the speakers, but coming along nicely. Now I have a 100" TV!

Thanks all.
It looks like it is supported on brackets mounted close to the wall. This is hugely stronger than a using a single mount in the center, as most of us imagined.
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