I built this montser about 5 years ago for
Anna in the Tropics. It was wider in the front than the back. I want to say it was 20' across the front and about 16' across the back. I believe those are 8"x8" posts with 2x12 joists and 2x4 decking on top. I had additional 4x6 bracing underneath which you can sort of see in this picture, I believe the 4x6's bolted the 8x8's together and the 2x12 joists sat on top of them. Lots of cross bracing to itself and big metal brackets bracing it to the floor. The stairway across the back was a giant cross brace. Lots of large heavy
lag screws and metal plates bolting it all together. It's not that hard to do this, but
you MUST do the math. I knew exactly what my weight load would be. I never had more than 2 people on top, neither was dancing. There's no way this sucker was going to move, it was way to heavy plus we had it bolted to the floor for stability. Note the lack of
handrail across the front, this was a serious concern. The director was dead set against at
handrail. Fortunately the guy who was the actor in the scene was a professional occupational
safety inspector. We developed a plan to keep the actors in the central part of the back half of the
deck, always more than 4' from the front
edge. You can see that only a small part of the
deck is lit to help actors stay in the safe zone and there was a do not cross
line marked on the
platform. It was plenty sturdy, but it was only sturdy for 2 actors standing still as that's what it was designed for. There's no way I would allow 6 people up there to dance because it wasn't designed to handle those forces.
I can't stress enough the importance of doing the math on this project. Right now, your set designer is flat out guessing and is guessing wrong. Guessing gets people killed. Charts that tell you how much load you can put on a given joist size are easy enough to find. Use them.
I would seriously consider a
truss solution instead of wood.
Truss companies do this kind of stuff very safely all the time, but it's going to be big, expensive, and may not meet your prefered design aesthetic.
One other thing, don't forget to design in a
safety factor. This is just as dangerous as rigging, if not more so, you need to follow the same
safety rules.