I've finished my stage. Here's the final shot:
The main part of the stage is simply two frames made of 2x10's, each about 8' long by 2' across. I made them separately and then screwed them together and toe-nailed them into the floor.
The front curved part is more complicated. I used Google Sketchup to model it in 3-d, then measured the length and front angle of each of the front to back 2x9's.
Their length will determine the curvature of the upper part of the front face, so they need to be pretty accurate. I used my miter saw to cut those pieces to length/angle.
Next to each 2x10 is a 2x6 (cut/planed down to 4.25") that will act as a step support and will determine the curvature of the lower part of the front face. I measured the angle for all of those, cut them, and nailed them to the 2x9's.
I cut a 12' 2x10 with a complex left and right ends to act as the back edge of the curved part of the stage. I then nailed the 2x10/2x6 front to back pieces to that, and put in some cross supports to space them out:
The main stage is covered by 3/4" OSB (basically plywood), 2' wide. I scribed out the curved part of the stage with a pencil and a 7'5" wire nailed
to the floor (the top of the stage is an arc of a 7'5" radius circle). I had to do it in two parts (left/right) as the curved part of the stage is
10' wide and wouldn't fit on a single 4x8 sheet of OSB. I cut them out with a jigsaw.
The front of the step is a 8'1" circle. I cut the front curve of the step pieces, set them on top of the stage above where they'd end up and marked
underneath where the front of the supports where. I pulled the pieces off and cut the back edge to match my markings:
[to be continued...]