Hardboard Torsion Box?

[FriscoSoxFan]

My Sister-in-law would like me to make a platform for her to use to stand on while conducting a high school band.

I've been given the dimensions and they are to be 36" X 36" X 8" Tall. Since it can move around from room to room to concert hall, it needs to be as lightweight as possible, yet strong enough for her to stand on and move around. For safety's sake, I'm intending on building it so two people of large size can stand on it in case a kid joins her univited on the platform.

I'd like to build this using the materials I have on hand, which I think should be doable. I've got plenty of 3/4" Plywood and 1/8" hardboard. In addition I have various lengths of 2x4, 1x3, 2x2, etc, but not enough of each to do much with it.

I was thinking a torsion box would be perfect for this setup. However, I'm not sure of what materials to use to minimize weight while maximizing strength.

Here are some options:

1. Hardboard top and bottom

3/4" Plywood frame with bisecting crossmembers forming 4 18x18 squares in the middle of the sandwich. Pros: Strong perimeter and crossmembers. Negatives: Probably too large spand for the harboard

2.

3/4" Plywood Top and bottom 3/4" External skirt hardboard honeycomb center on 9" centers forming 16 9X9 squares in the middle Pros: Probably heavy, probably strong Cons: would hardboard be adequate as the honeycomb material from a strength perspective?

1. Some sort of hybrid I haven't thought of.

ANy THoughts?

[Pat]

Why do you need a bottom? One cross piece 4" deep under a 3/4" Plywood Top is enough.

[Chris Friesen]

I wouldn't worry about it too much.

I'd build a plywood box (no bottom) with the top sitting on the sides for weight transfer. One 4" deep plywood "joist" to help stiffen the top. All joints glued and screwed. Cut handholds to simplify moving it around, paint the whole thing matte black.

Chris

[Lee Michaels]

Two ideas based on what I have seen for similar platforms.

One is to put four recessed casters on one side. This side could face the band. Put a recessed handle or handhold on the other side. This way you can flip it up and roll it to its new location or storage. That way the weight will not matter as much.

I saw a round platform used before. Just flip it up and roll it to its new location.

[George]

Then go buy some 2x8 and frame for your plywood top. By the time you screw around with bracing and make-do stuff you'll have the same weight involved, and unless Sis-in-Law is a blimp, you don't need anything to underbrace 3/4 ply at that span.

Ours were on casters for moving about, cam release on the caster for use.

[Puckdropper]

"George" wrote in news:E9Xsi.37340$ [e-mail]:

If I were doing this, I'd probably brace the 3/4 ply anyway. Not necessarily with 2x8s, but maybe 1x4s. That way, you don't get any unwanted movement with the 3/4" ply. My 2' x 4' table with no bracing cupped towards the middle because it wasn't supported. A 1x4 on each side would have kept the table top straight.

Puckdropper

[Larry Blanchard]

How about 4" structural foam insulation stacked two high with a 1/4" plywood skin?

[Pat Barber]

I would even use 1/2" ply for this job. If you put several cross members inside the box about 8" apart, it will hold several people and will be much lighter than 3/4".

No need for massive bracing... put some handles(cutouts) to move it around.

FriscoSoxFan wrote:

[J. Clarke]

I needed to reach something up high the other day and didn't have a stool, ladder, or chair handy. Friend of mine thought I was a loon when I stuck a piece of plywood on top of a corrugated box and climbed up on it (I weigh 250 pounds in my underwear). He was very surprised when the box didn't collapse and then I jumped up and down on it a couple of times to _really_ upset him.

You don't need massive construction to support several people if most of the perimeter is going to be on a floor or other level surface.

For the sides and cross pieces 3/8 ply should be plenty and 1/4 would likely be sufficient. For the top, 1/2 would as you say be fine if the cross members are closely enough spaced.

[Gordon Airporte]

Of course you have to take into account the acoustical properties of the riser: the resonant frequency and various modes of the cavity. You'll want to place the braces so as to make the platform as dead as possible. You should also consider dampening material like Dynamat, which will add a great deal of weight but is pretty much essential. After all, you wouldn't want to do anything to distract from the subtle and moving experience of a high school orchestra performing in a gym ;-)

[Larry Kraus]

I can see high heels going right through this one...

[Mike Marlow]

Maybe with an active imagination.