1. 3/4" ply has to be supported every 40cm or 18" and you CANNOT have a 4 x 8 sheet with only one leg at each corner! No matter how much bracing you put under it, it will fail when the first person dares to jump up and down. (And that would be a normal person - have you seen the size of some people lately!!??)
Sorry you are absolutely wrong here. Keeping in mind safety code in your area has to apply...
You DO have to be careful of certain things, for instance the weight of the platform needs to rest on the bracing, not on the plywood top. And you can support every 2', but I wouldn't go over 2' in most cases, but I can assure you, I have several pieces of platforming of various sizes up to 4x8 and legged (Or castered) at the corners, and yes multiple people could jump on (And have as I use them for rock shows) without issue. In this case the bracing is done with 2x10s, which is obviously much more structural than 2x4s, but my point in general stands as you stated 'No matter how much bracing'. You could certainly do it with smaller bracing (And in fact I have in the past).
2. Given correct numbers of legs, braces and supports, your total structure would weigh at least 2.5 tons! Vehicle? And assuming that 2x4 still costs between 50 Cents a $1 a foot (wood yard or retail) your whole construction could set you back $3,500!!! (Those 'pro' systems are not looking so expensive after all!)
3. Each element would be too heavy for two people to carry and would have to be wheeled in - and not every venue is step-less, elevator-less and with wide doors. Four strong men could carry one element (just) but you can't get four men around an upright element going through a door frame or up a flight of stairs!
I believe your math is off here. I do stagehand work as well as my sound engineering/design/consulting and end up loading in a lot of platforming depending on the show, including 4x8 platforms very similar to what is described above, and can assure you that any two stagehands can carry a 4x8/2by4(And often larger bracing) platform with no issue. 2.5 tons seems quite high as well for that size of stage.
Looking at this:
Figuring approx. 100lbs for each platform (Which honestly sounds high, but going off the math and looking up wieghts in the Backstage Handbook) a 4'x8' braced with 2x4's around all sides and every 2 feet.
24x16 stage is 12 platforms
8x8 drum riser is 2 platforms
14 x 100lbs == 1400lbs, or 3/4 ton, not 2.5 tons.
Even with 18" legs of 3qty 2x4s at each corner and another set of 2 in the middle (Standard practice although 2qty 2x4s is more standard across the board) you have added 36 lbs to the overall weight (A little under actually but I rounded down above) of each platform for a total of about 140lbs alltogether accounting for the rounding I did above.
14 x 140lbs == 1960lbs, or just under a ton.
Now those 2x10 platforms I mentioned above weigh about 200lbs each so end up being about the 2.5 tons for that size of deck yes, but are also overkill for many things unless you plan on rolling around genie lifts etc. on the deck, in which case you should be going with at least double thickness 3/4" ply as that is a lot of weight on very small points.
And finally legging, while it is done, as mentioned above isn't really the best way to handle platforming, which really you should be using trussing under the platforming to support on the bracing. But see Dan's comments in general for better ways, structural design for the stage is a great book to read through in general when dealing with anything like this (Which reminds me I need to re-read it sometime). And honestly I tend to agree with Dan in that for most thigns this isn't worth the effort. In some occasions it is, but mostly it is not. Rent in the staging when you need it, honestly.
Again make sure you check all applicable codes for your area.
5. You can still use the cheap 3/4" ply as flooring, but what goes on underneath has to be properly and professionally built and you may need safety certification for some venues - best check in advance. Ebay is full of stage construction companies that can help you.
This I certainly agree with.
EDIT: Forgot which forum I was on for the syntax for quotes.
Edited by seablade (29/05/12 06:50 PM)