Weight/load Calculation

[Jerry Bittner]

I need help designing a shelf that will carry the weight of a plasma tv approximately 135 lbs. Basically, how thick should it be to prevent sagging? I'm not able to furnish a sktech so I'll simply try to expalin the dimensions.

The shelf will be mounted between two exisiting vertical boards which are each about and 1 1/4 " thick and 8" wide. The horizontal distance between these two is 80 inches. There is no other loading to speak of on these two being a frame around a mirror that forms the backdrop for a liquor bar. I believe they are made of Oak.

Also the weight of the TV will be carried on a TV stand that is approximately 3 feet in length and 12 to 14 inches wide. So the shelf has to be wide enough to meet that requirement but be able to taper back to the upright supports.

"Cleats" could be used to support each end of the shelf and depending upong the thickness of the shelf, I thought of running a "stringer" the length of the shelf for added support.

Hope I'm not confusing too many with these terms but that's the best I can do with a limited woodworkers vocabulary.

[Jim Becker]

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

Jerry, that's a hefty load and you'll need to build a beefy shelf structure. I'd probably consider a torsion box to keep the weight down or something similar that you embed some angle iron inside, even if there is intermediate support.

Of course, there is always the option of mounting the screen on the wall and building the rest to fit under and around it...which is what I'm doing in our new multi-purpose/media room. The 52" LCD is going on the wall. :)

[alex grams]

I would suggest to do what Jim said. Mount it on the wall and just make a lighter duty shelf below it for whatever else you want to put up there. Depending on what you plan on mounting the TV on the shelf with, it's center of gravity could be as far as 1' away from the wall, which would make a substantial moment you have to counter on the wall, and would make it difficult to hide enough reinforcement to not make any shelving look bulky and cumbersome.

file:///C:/DOCUME%7E1/agrams/LOCALS%7E1/Temp/moz-screenshot.jpgHere is what i did:

http://www.sawmillcreek.org/attachment.php?attachmentid=73720&d=1192647551

[Kent Parker]

[Also the weight of the TV will be carried on a TV stand that is approximately 3 feet in length and 12 to 14 inches wide. So the shelf has to be wide enough to meet that requirement but be able to taper back to the upright supports]

Jerry,

You say the weight of the TV will be carried by a stand under the shelf so is the load capacity of the shelf an issue? By saying that the shelf (outboard edges?) has to "taper back" to the verticals, how far are the verticals from the center of load?

A "cleat" under a shelf could easily stiffen the shelf to allow for the weight of the TV. As example, I recently created a 3/4" plywood table (about 20" wide) for my RA saw with a 3" x 3/4" plywood fore edge to stiffen and straighten. I could stand on it without flexing it (I'm about 150 lbs wet). The panel should have cleats at its fore and after edges.

Given the width necessary, the "cleat" could be incorporated into the design so that the shelf has visual thickness (to visually balance its length).

Cheers

Kent

[Lee Schierer]

I would use the cleats to support the ends of the shelf as you have suggested. I would also run full length pieces of 1 X 2 (3/4 x 1-1/2) full length under the shelf along the back edge and across the front. Attach these pieces to the shelf with screws and glue. The trapazoidal portion sticking out should be okay since it is only 4-6" wider than the shelf, but I would put 1 X 2 pieces under that so it looks like the cross braces. Make sure you attach the frame securely to the shelf. Tie the ends that touch the front cross piece into tthe cross piece with screws from the back side. Most of the weight is going to be on the overhanging part of the shelf so you are going to need to attach your shelf to the cleats with screws. I would not recommend pine or any type of particle board material for this. Use hardwood like Oak, Maple or Poplar.

You can run calculations using the sagulator. With 2.25 thickness using the supports I described you should see about 1/8" deflection. sagulator (http://www.woodbin.com/calcs/sagulator.htm)

[Chase Gregory]

Get the point of contact of the base as far from the shelf center as possible. Hopefully the base just bears on the ends.

With a 36" base you'll be 22" min from each of the uprights and your 8" bearing cleats. 70lbs 22" away from the ends can introduce a good bit of deflection - it would be easy to mock this up w/ a couple of concrete blocks and an 80" board. Just get some cap blocks and start loading it up
at the base points of contact and watch what it does.

The center of gravity of the tv needs to be as close to the center of the 8" bearing cleats as possible also or the torsion will twist your shelf like a giant wing nut over time. And torsion failure can be instantaneous once it reaches the tipping point.

A boxed section with some dimension to it (4" to 6" - which is still under 1:12) will probably look better in an 80" clear span that trying to do something really thin that'll just look flimsy - especially once it deflects.

A boxed section also has the opportunity to do something attractive w/ the face whether it be reveals/niches/penetrations/etc...

Or break out the MIG and some 1" 11ga sq tube and build a structural skeleton to veneer.

Just connect it REALLY well to your uprights whatever the case.

Chaser

[Jerry Bittner]

Some clarification.

I can't mount it on the wall because of aesthetics - theres a large mirror 80" x 72" or so in the space.

The stand for the TV will sit on the shelf.

Given the data as before, and using Jim's idea of a torsion box, or running stringes on the front and back,can I get by with 3/4" thick shelving -- White Oak.

Also, since I don't believe I'm giong to come up with a board that wide, can I get by with edge joining with glue and biscuts and running stringers across the additional width to span the edge joining?

I really appreciate the ideas gus and gals.

People who don't contribute to the forum must not realize the quality of the advice you get for a nominal fee.