Understanding how to prevent wood structural sag sag in future projects - Woodworking Talk - Woodworkers Forum
 
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post #1 of 5 Old 08-19-2013, 02:16 PM Thread Starter
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Understanding how to prevent wood structural sag sag in future projects

Hello,

I recently completed a project for a client that involved making a 9'L x 15"D x 12"H TV base. It's a 5 sides box (made with 45" mitre cuts, and a slide in back panel), and it has 2 internal walls evenly spaced creating 3 inside spaces of roughly 3'L x 13.5"D x 10.5"H. There are three doors hinged at the bottom that fold down via touch latches. That is to say the doors offer pretty much no structural support.

It was all made with 3/4" birch ply.

Now, all is well except in the middle of the doors the ply sags about 1/8". It's not a big deal since this is mostly hidden by the doors, but I was wondering what options would exist to prevent this sag. The obvious come to mind which are:

- More internal supports
- Thicker/stronger top material
- Smaller internal spaces

but I was wondering if there are any tricks that I'm missing?

thanks!
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post #2 of 5 Old 08-19-2013, 02:27 PM
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3' is a bit much to span with plywood. A 1 1/2" or 2" strip of hardwood on the edges would help but I try not to span more than 30" if I can help it. Sometimes you don't have a choice on the span though. Usually if I have to span a length longer than I think I should I will incorporate a piece of steel on the back side of the shelf edging to prevent the sagging.
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post #3 of 5 Old 08-21-2013, 05:27 PM Thread Starter
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I'll keep 30" in mind as a rule of thumb, thanks!
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post #4 of 5 Old 08-21-2013, 06:22 PM
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If you tack the back in the cabinet, you secure the back edge of the shelf through the back. Before tacking the back in, square the cabinet.

On the front of the shelf, you can use a solid wood edge to the face of the shelf, " x 1"...the 1" vertical (90 deg) to the shelf. For shelf length, I get 3 crosscuts (max) from plywood, so they are 32" or less.









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post #5 of 5 Old 08-22-2013, 07:15 AM
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when designing a project for specific use, i also consider the load, and place support in the critical points when possible. if not, treat those spans with extra support.
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