Torsion Box Recommendations Request

[Brian Kehlmeier]

I wonder how often golf comes up in this forum, but I am going to rip that band-aid off...

Some backstory:

I have setup a golf simulator in my garage. The sensor that makes it all work is an Optishot:

os2_test_render_rev3.jpg

It works in unison with a computer, and has a set of lights and sensors behind and after the ball. It tracks the club, not the ball. I use a third-party replacement turf to add depth to the hitting surface. I can take divots with my irons because the turf is so thick:

323434_6c41fc343774428c8033304bde225dce~mv2_d_4032_3024_s_4_2.jpg

To raise my feet to the same level as the turf, I built a hitting platform. The one I have used for two years was just proof of concept; 1/4" OSB sandwiching some insulation. The simulator has worked out wonderfully, and I want to make a new platform, hence this post. I have reviewed many threads but this one was by far very informative and I thank you guys for this info: http://www.sawmillcreek.org/showthread.php?174228-How-Thin-Can-I-Make-a-Torsion-Box

Project:

I want to build a torsion box that I can raise and lower in my garage on a hand-operated pulley system. So, weight is a consideration. The total height from outside skin to outside skin will be 1-1/2". I would have to add structured blocking to receive mounting hardware in the corners, and that I can figure out. The platform with be 5' x 5'. Here are my questions:

1. To lighten the weight, can I use 1/2" or 3/4" plywood instead 1/2" MDF for the webbing?
2. Will glue and brads work to adhere the skins to plywood webbing?
3. With 1/2" skins, is 1" webbing going to be enough structural support?
4. Being larger than 4' x 4', will skins made up of two pieces be alright?

Thank you for any feedback.

[cody michael]

I made shed doors out of 1/2 plywood with 2x2 material inside. each skin is 2 pieces of wood. They are very stiff, feel like a bank vault door when opening and closing. if it is only going to be 1 and 1/2 inch thick I don't think that leaves enough room to whave 1/2 inch plywood.

I don't know much about torsion boxes but I would put 1/2 inch ply on top 1/4 on bottom. saves weight and should be plenty strong. I would think plywood would be better then MDF for webbing. I remember a picture on here of one made from card board. so you have lots of options.

[Brian Kehlmeier]

We will obviously be walking on it but no jumping around. The only part that takes a jolt will be the sensor itself.

If I use 1/2" plywood for webbing, will glue and brads be strong enough, or would I need screws? Screwing into that thin of plywood seems odd to me.

The more I think about this, the construction is more floor joists than torsion box. The torsion box construction with overlapping lap joints just makes the assembly easier. I'm not so worried about twisting as I am support. I also don't want it to squeak over time while we walk on it.

[Rick Alexander]

We get water bottles at work packed in really nice boxes that have a top and bottom cushion that is actually a thick piece of cardboard - like 1 1/8 inches thick. I took some home and made a torsion box as a planer sled using two layers of those cardboard pieces and 1/4 inch skins. Amazingly strong - I can easily stand on it and it doesn't flex at all and very light. Those torsion boxes will surprise you. It's the structure rather than the material that is the key. Cardboard is nothing more than torsion boxes really but these thick pieces we get have an amazing amount of "torsion" structure that made this planer sled really strong and flat. I bet you could do the same with several layers of cardboard glued in there and skinned with ply that would work.

[Mike Henderson]

A torsion box works on the same principle as an I-beam. The webs can be fairly thin, maybe 1/4", but you should glue them to the skins. The skins need to be able to support the weight between the webs without too much deflection.

Mike

[Brian Kehlmeier]

We get water bottles at work packed in really nice boxes that have a top and bottom cushion that is actually a thick piece of cardboard - like 1 1/8 inches thick. I took some home and made a torsion box as a planer sled using two layers of those cardboard pieces and 1/4 inch skins. Amazingly strong - I can easily stand on it and it doesn't flex at all and very light. Those torsion boxes will surprise you. It's the structure rather than the material that is the key. Cardboard is nothing more than torsion boxes really but these thick pieces we get have an amazing amount of "torsion" structure that made this planer sled really strong and flat. I bet you could do the same with several layers of cardboard glued in there and skinned with ply that would work.

Man 25 sqft of cardboard to make up a layer of 1" might be tough to come by.

[Adam Herman]

check out your local door prehanger for cardboard. we use to get our exterior doors stacked on pallets with a door size of nice, rigid and thick cardboard on the bottom and top. used that stuff for everything! We even built a room in the un heated warehouse to restore a car in with it, makes decent insulation.

[Chris Parks]

First principle of torsion boxes is they are supposed to be light and strong and most people over build them and they finish up heavy and strong but no stronger than if they were built using lighter materials. As an example I built a torsion box 11 feet long, it is constantly loaded with heavy sewing machines and it does not bend. The skins were 6mm MDF and the internal webs were 3mm MDF fully glued top and bottom. I think the grid was about 4" square from memory but more would not hurt. It will be light and you won't break it with normal use.

[Brian Kehlmeier]

We will be making swings while standing on it, so I am concerned about the skin to web connection. Would I be better off placing glue on the webbing, and holding the skin with weights? Or would brads be a better clamping mechanism? If I need brads, I don't think I can get away with 1/4" webbing.

[Chris Parks]

I used common PVA to tell the truth. The perimeter edges were glued to a T section of timber all round for good support as that is most probably the most easily damaged part and the web built inside that. I used weights and a few sand bags or such should do it over a small area. If weight is not too much of an issue then by all means use thicker webs but the brads will add nothing strength wise. Here are a few pics. it is entirely unsupported apart from being anchored to the wall for safety









Sewing Room Bench 2.jpgSewing Room Bench 3.JPGSewing Room Bench 4.jpgSewing Room Bench 5.JPGSewing Room Bench 1.JPG

[Brian Kehlmeier]

So maybe 1/4" plywood as the webbing would be better, adhered with wood glue and clamped with weights until dry...

[Chris Parks]

I would not use plywood as the glue does not adhere to end grain. The only logical choice for webbing is MDF, it is cheap, glues well and is very strong in this application. It may not appeal to the average woodworker but put simply for torsion boxes it can't be beat.

[Brian Kehlmeier]

Estimated weight for a 4' x 8' sheet of 1/2" plywood is 50#. 1/2" MDF would be 65#. For weight purposes, 1/4" MDF webs, 1/4" plywood bottom skin, 1/2" plywood top skin. That sounds like a plan. Would brads hold in MDF while the glue holds?