Do I need more support on this?

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02-22-2017, 11:21 AM
Joe Kasier

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Do I need more support on this?

I am making this credenza (https://imgur.com/a/MjEC3) for a friend. It is 8' long and they plan on putting a 50" TV and some decorations on it. (Ignore the doors on the model. They are going to be a sliding barn door style I just dropped them in for size reference)

The idea I am working through is a long open space in the middle. (i.e. no vertical dividers) The top rail in the front and back is poplar at .75" thick and 3.5" wide (or tall as pictured here). I have a 1.25" deep tenon going into the supporting legs on each side.

There is also going to be a back on it. Probably 1/2" plywood or something.

Is that going to be enough support for the kind of weight mentioned above? I could add vertical supports connecting the top/bottom rails but I would rather not.

to be clear, I am just worried about sagging on the top. The top piece of wood is 1" thick solid oak. My guess is it will distribute the weight across the whole top (legs, side assemblies, and the long rails) but I have never built anything at this scale.

Attachment 354608
02-22-2017, 12:49 PM
William Fretwell

No it will sag. With no vertical dividers it's too long
02-22-2017, 1:09 PM
Adam Herman

install a length of 2.5 or 3 in steel angle iron behind the front "apron" screw into both the top and the apron ever few inches.

with only the wood, it will sag and prevent the doors from operating.
02-22-2017, 1:12 PM
daniel lane

Check out the Sagulator (http://www.woodbin.com/calcs/sagulator/) to calculate this. With your numbers and assuming 24" deep, you're looking at about a third of an inch sag. Too much, especially if you have doors (they will jam)!

d
02-22-2017, 1:59 PM
Joe Kasier

Quote:

Originally Posted by daniel lane

Check out the Sagulator (http://www.woodbin.com/calcs/sagulator/) to calculate this. With your numbers and assuming 24" deep, you're looking at about a third of an inch sag. Too much, especially if you have doors (they will jam)!

d

I must be putting something in wrong...

The top will be oak (stained top, painted base. Hence poplar)

96" wide. 20" deep 1" thick. 100pounds centered weight. It says the oak will sag .15 in total. "Acceptable"
02-22-2017, 2:11 PM
Pat Barry

Regardless of the sagulator calculation, I'd want to have a vertical center divider of some kind to support the top over this span. It doesn't have to be fulll depth of the cabinet, just the front side in the middle. I'm assuming the back side of the top will be supported just fine by the back itself.
02-22-2017, 2:40 PM
Jim Koepke

Quote:

Originally Posted by Joe Kasier

I must be putting something in wrong...

The top will be oak (stained top, painted base. Hence poplar)

96" wide. 20" deep 1" thick. 100pounds centered weight. It says the oak will sag .15 in total. "Acceptable"

My results were the same as yours. What is different is my understanding this is initial sag. Reading the information below the calculator also reminds users of this:

Quote:

1. The deflection calculations use average wood stiffness properties of clear, straight-grained samples measured in controlled laboratory conditions (mainly from the U.S. Forest Products Lab). Expect some real-life variation from the calculated results.

Also to consider is your finished top going to be a full 1"?

Will there be absolutely nothing else placed on the top of this item?

My guess would be as a preventative measure it would be wise to include a vertical support in the center of the piece. It wouldn't have to be a big piece.

It may only sag 1/64" when first set up, over time or with changes in humidity it could sag enough to jam a door.

It looks like your plans have a piece across the front. This also might be enough as a sag prevention.

There are good reasons why most furniture isn't built this long without some internal support.

jtk
02-22-2017, 2:41 PM
Joe Kasier

Ok. One divider at 4' should be enough then?
02-22-2017, 2:47 PM
Joe Kasier

Quote:

Originally Posted by Jim Koepke

It looks like your plans have a piece across the front. This also might be enough as a sag prevention.

jtk

You mean the top rail? Yes that is the 3.5" piece I have tenoned into the "legs" as I am calling them.

I think I'll add a small vertical support to the front in the center. The back will have a solid piece of plywood rabbited in and I would think that will be sufficient there
02-22-2017, 4:18 PM
bridger berdel

Quote:

Originally Posted by Joe Kasier

You mean the top rail? Yes that is the 3.5" piece I have tenoned into the "legs" as I am calling them.

I think I'll add a small vertical support to the front in the center. The back will have a solid piece of plywood rabbited in and I would think that will be sufficient there

Bbuild 1/4" of crown (upside down sag) into that top rail.
02-22-2017, 6:47 PM
Joe Bailey

Quote:

Originally Posted by Adam Herman

install a length of 2.5 or 3 in steel angle iron behind the front "apron" screw into both the top and the apron ever few inches.

with only the wood, it will sag and prevent the doors from operating.

Given that the OP set the "no divider wanted" parameter, this is an excellent solution.
02-22-2017, 7:24 PM
Gary Focht

What about shelf sag also? I would be concerned about an 8 ft shelf.
02-22-2017, 8:11 PM
Stanley Covington

Quote:

Originally Posted by bridger berdel

Build 1/4" of crown (upside down sag) into that top rail.

Amen. The best and most craftsman-like solution.

Especially if the crown is natural and not contrived with a plane, since a long span with a live load can cause progressive deflection and set over time. And don't forget that there is more than just live load to cause deflection. In this case, you need to worry about warpage due to long-term heat-induced moisture-content expansion/contraction from electronics resting on the cabinet top. I experienced a similar problem (although not caused by electronics heat) in a 7' long mahogany shoe cabinet I built and installed in a Japanese-style genkan I built for a customer in Las Vegas. Being located directly below a South facing window, it received a lot of direct sunlight, which caused the top to warp down in the summer months binding the sliding doors. It would rebound back to level in the winter months. I ended up adding a vertical support mid-span and a cambered apron on the bottom (floor) panel (the cabinet was mounted to the wall and was unsupported underneath) to stiffen the cabinet front to solve the problem. I should have planned better.

In the job I am finishing up now, the AV cabinets house routers and other electronic equipment below the wall-mounted monitors. Adequate venting was not provided in the design because the AV vendor insisted his equipment would not exhaust significant heat. Wrong. Very hot. The cabinet lids have warped (bowed up) in over ten instances. These are beautiful, high-end, urethane finished, lumber-core cabinets that cost nearly $100k each (not including electronics). We are still working on a fix that is not ugly. The point is that you need to plan how to deal with heat in your design.

Stan
02-22-2017, 8:35 PM
Pat Barry

Build 1/4" of crown (upside down sag) into that top rail.

Quote:

Originally Posted by Stanley Covington

Amen. The best and most craftsman-like solution.

Do you really want a humpbacked table top? I doubt this is the goal.
02-22-2017, 8:59 PM
Stanley Covington

Quote:

Originally Posted by Pat Barry

Build 1/4" of crown (upside down sag) into that top rail.

Do you really want a humpbacked table top? I doubt this is the goal.

Silly boy, you forget the intended loading conditions. 5/8 of the camber will deflect the instant the point load is applied. I strongly suspect there will be more than .15" long-term deflection over a 8' span. The remainder will settle out over time. Nature abhors a straight line anyway.

Stan