A Shelf With No Sag...

One of my hobbies is woodworking. I needed a few shelves in the shop that would be carrying some heavy loads. So, I made these T-boxes. Each is 4' long, 15" deep, and 1-3/8" thick. Take a look a the "sag":
http://i156.photobucket.com/albums/t...oxShelves4.jpg
That's me standing in the center, about 200 lb. The shelf is supported only at the very ends by a couple of benches I made. I used the chair to help get up there, so it's a foot behind the shelf. The steel tube is resting as an indicator. This shelf is mostly hollow and weighs less than 2 lb. Kind of defies gravity, doesn't it? Wonder what Sir Isaac Newton would say about it? :rolleyes:

Nice dovetailing.

I think its time for some new shoes

Your pants is getting shorter.

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Originally Posted by MartinLiss

Nice dovetailing.

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Thanks, Marty. I did that for practice. Those were cut by using my band saw and a jig that I made for it. Most of the time I hand cut dovetails. About any joint could have been used on the corners because that means little to the strength of the T-box.

Just to be sure that the Pic is not a fake, here is another shot of the same event that LOML took:
http://i156.photobucket.com/albums/t...oxShelves5.jpg

And, here are the four shelves that are now securely in place:
http://i156.photobucket.com/albums/t...oxShelves1.jpg

BTW, LOML tried to fit a sheet of typing paper between the steel tube and the shelf while I was standing in it. It would not slide underneath the tube.

Are you going to put a veneer on the edges that are in view?

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Originally Posted by MartinLiss

Are you going to put a veneer on the edges that are in view?

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Well, I could have done that, but I elected not to because I made them for the workshop. Besides, I wanted to remind myself how I made and assembled them.

Considering that these could have been made for formal furniture, it's an interesting proposition. If that were the case, I would have used oak plywood to have a really hard, formal shelf surface. The plywood I used here was the cheapest I could find--1/4" "mahogany" from a big box store, that is actually only 0.2" thick. The internal framing was salvaged cedar that I saved from my old outdoor deck flooring.

Regardless, can you imagine the bending strength of these T-boxes? Few engineers can formally explain how or why it works. It just plain does.

I have also seen them lag screwed to wall studs prior to the skins being attached in a cantilever fashion (floating shelves). After doing so, you can sit on them and they will not budge. :eek:

Please post in the woodwork forum :mad:

kkthanksbai.

I did some wordworking years ago. I was no expert. I don't know what a T-box is. Maybe you could explain exactly what they are and what the internal make up is. Perhaps provide some diagrams.

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Originally Posted by wossname

Please post in the woodwork forum :mad:

kkthanksbai.

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What "woodwork forum"? Please advise. Can you supply a URL? :confused:

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Originally Posted by EntityX

I did some wordworking years ago. I was no expert. I don't know what a T-box is. Maybe you could explain exactly what they are and what the internal make up is. Perhaps provide some diagrams.

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Thank you, EntityX, for your polite question.

A T-box is short for Torsion Box. The idea is to build a simple internal frame rather than rely on a solid board or any other solid material that is automatically very heavy compared to the T-box. Solid materials add very little to bending strength and ultimately the solids bend somewhat under their own weight.

Here is a shot of the interor scrap cedar frame of the T-box that I show above:
http://i156.photobucket.com/albums/t...oxPattern2.gif
Each piece is about an inch thick but could have been even less. After asembling the frame, I applied the skin surfaces using lots of glue on the frame pieces and stapled from the outside to hold them together until the glue between the skins and the frame dried. Small brads could have been used rather than staples. I used dovetailed joints on the outer corners, but that is complete overkill. A common butt joint would have worked, and metal fasteners mean nothing here.

Presto, you have a T-box. The strength-to-weight ratio of this assembly is enormous. :eek:

It's a common setup for floating shelves too. That's usually where I've seen it used. Pretty good job there CodeDoc.

-tg

What would be interesting to know is the breaking point differences between a T-box and a non T-box shelf of the same outer dimension using the same wood for both. For instance find out what amount of force it would take to break a T-box like you made applying the force at the mid point and suspending the very ends and what amount of force would it take to break a solid board of the same dimension with the force applied in the same way using the same materials for T-box and solid board. Probably studies have been done on this.

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Originally Posted by EntityX

What would be interesting to know is the breaking point differences between a T-box and a non T-box shelf of the same outer dimension using the same wood for both. For instance find out what amount of force it would take to break a T-box like you made applying the force at the mid point and suspending the very ends and what amount of force would it take to break a solid board of the same dimension with the force applied in the same way using the same materials for T-box and solid board. Probably studies have been done on this.

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Agreed. The soilid wood beam would probably bend ten times as much as the T-box prior to fracture under the same point load at the center. However, it is possible that before fracture, it would support about the same weight, even though it would weigh ten times as much.

But, nobody enjoys looking at a sagging shelf because it always appears ready to collapse. Note that I-beams are a variant of the T-box idea and they are used about everywhere to build structures. Consider log splitters, and garage beams, for example. Your house main floor beam is likely a steel I-beam.

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Originally Posted by Code Doc

http://www.vbforums.com/images/ieimages/2009/11/1.gif

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I don't think I understand why a T-box would sag less than a solid board of the same material. Consider your diagram. If you took a solid board and removed material from it so that you were duplicating the diagram then you'd have a jointless T-box. Would that sag less and if so why?

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Originally Posted by EntityX

I don't think I understand why a T-box would sag less than a solid board of the same material. Consider your diagram. If you took a solid board and removed material from it so that you were duplicating the diagram then you'd have a jointless T-box. Would that sag less and if so why?

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Well, it's a bit difficult to understand, but I believe there are at least two reasons. First, the solid board has to support its own weight, which could easily be 10 times greater than the T-box. Second, the skins of the T-box tend to work as a team. The top skin is under compression and the bottom is under tension.

The solid board doesn't have this teamwork going for it, so the deflection is much greater under the same load. The strength-to-weight ratio for the T-box is thus enormous and cannot be matched by anything of comparable materials. My 4' long T-box shelves are now in place and I can hang on the center of them with minimal deflection. LOML is still in disbelief! ;)

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Originally Posted by Code Doc

What "woodwork forum"? Please advise. Can you supply a URL? :confused:

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Only members with 5621 post can view it at the moment... unfortunately... :afrog:

Im going to build a T-box with popsicle sticks....AND elmers all purpose. Prison style shelf ;-)