Ok, you have the supervision and training that is needed. No offense, but having apprenticed I'm sure you can understand my hesitation in saying go for it.
Lights follow the structure. They are amomost inconsiquential given the span. For starters, let's pluck your brain on what you mean about late 70's early 80's metal clashing odd lights and costumes set piece.
I assume the structure is going to be upstage and spanning the width of the
stage. Given that, what's the intent of the below the
stage? Do we assume it's more of a open space/acting area and to be supported by posts? Proper bracing at this
point I would assume covered. What's the floor
thickness that's desired?
I assume the intent is not to
build a
house with proper joists and floors, much less a across the
stage wall below the
deck, or major framing members spanning sections and beams. Were this the case, I might
build three stud walls upstage to
down stage spanning the
stage, brace it than
lay platforms or 2x6 joists and plywood atop it. Simple done. Lots of lumber.
Over the past couple of years I have worked for a rock and
roll lighting company and had some access to old rock sets from the last 20 years. On such things even than it was all steel and aluminum box tubing for the set's frame, and either steel grating or steel framed platforms for the
deck. This would cost a lot of money and because the lower section was not meant to be seen, more legs or
truss legs were allowed to be used with shorter spans of framing members which also made them lighter. Largest span - (Wolf) would be about 6 to 8' depending upon materials used than. Last time I touched such a set it was about a year and a half ago and I supervised it's construction in the shop for a band rehearsal. I assume that steel for more than a few things such as the below center span is not an option. This will be a lumber install?
Scaffold companies often rent stages also. They are similar to hotel
portable dance floors in that they have a steel or aluminum 4'x4' "L" or "C" frame with the rental
stage version having say a double layer of plywood in the thick frame instead of plywood and
veneer in a lighter frame. In either case, it's a 4x4 section that hooks together
in one of many ways. sTrong welded hooks or coffin locks. In the case of the
stage, it's supported by two opposing legs from the other
stage section and two of it's own normally. 4x4 double layered plywood - probably cross grained should span the gap sufficiently given "L" steel frames and laminating. That puts your legs every 48" on center about, but
deck only slightly larger than 1.1/2". Touring decks are more normally 4x8 or in some cases 6x8. This requires say double the framing size and support but allows for larger spans. Such sections are frequently placed on a
truss wall. It can be pipe legs but it gets elaborate with the bracing unless a simplified and engineered say 4 or 6'
leg brace - but that would be later 80's.
The
truss wall is either similar to a stud wall or
truss roof depending upon engineering. In your height, I would think stud wall that's well sway braced, braced between walls than bolted to the building.
As a rock and
roll solution, I might look to doing well supported and braced stud walls for legs that are 8' on center if steel and rated for the span. Otherwise, if lumber or rented
stage decking it's about a 4' span you will have to cope with. Dad and you can probably do some walk ways larger with beams but for the most part, even if wood and lumber platforms, think upstage/
downstage stud walls. Atop that just platforming that's bolted and screwed to the studwall. 24" on center will probably be fine as long as properly sway braced - again the GC thing to verify with the bracing to supplement 16" oc necessity. Normal
stage platforms need a 50# live load, in your case you could plan on a 30# live load for the most part given a lack of furnature and refrigerators, as long as you weigh on the side of
safety and do the math for bracing and support where needed. 10 people on a 40' span is not a lot. Note your platforms in spanning such things need to be glued well, in good condition and stronger than a hack
platform with lots of bullet holes.
Now given the double
platform deep say placed
stage right to left instead of upstage to
down stage, you might be able to just span the 8' between optimum stud walls given the stud walls are diagonally sway braced back to the
truss well enough. With beams you might even be able to increase that span. Figure for the front and rear of the spans you
bolt the
platform to a 2x8 so it's a 2x4
platform doubled up to a 2x8. Check the load ratings but this - and I'm guessing should give you about a 30# or even a 45# live load. Platforms conform to post and timber baloon framing for your calculations. Otherwise you having checked the span charts might bring it up to a 2x10 on the front and rear - I'm guessing that moves it up to a #45 live load but that's again without engineering the set or checking the charts on double timber of differing sizes beams. I'm not specifically trained for that so the math would take a
bit of time for me.
For the center, I would not trust a bolted double 2x4
platform edge to span a 8' gap. I would like to see it bolted
thru some 1.1/2x3.1/2 or so box tubing but otherwise what about the double 2x4 bolted to a 2x6? Should be able to carry the weight yet still span the gap with minimum of materials. You will have to notch it around the stud wall which will probably require some kind of support at the stud wall for it's notched section. Easy enough to do with a 2x6 spanning say three studs.
You now have a
platform made up of ten normal 4x8 platforms and 6 stud walls at 8' on center. Good
base to start from. On the
deck it's going to be say 7.1/4" thick at the center span giving you a 15'-4.3/4" high by 7'-5" playing space between stud walls. Should you wish to just make it more of a post and beam support instad of stud walls, it's a
base to start from. At least you have head height for beams but swaybracing the walkway will be a bitch unless you
bolt the platforms to the back wall of the theater. In that case, perhaps you won't need much. You will need some as with sway braces off the stud wall to the
platform to some extent.
The most difficult part of the problem and I'm sure most people's hesitation including your's is the stud walls. 16' Tall is very high for a 2x4 post or stud. Much less the span above when added to the basically unsupported height is difficult to figure upon.
For now, let's consider a 2x6 stud wall 16" on center that's still well supported and has a tie bracing at 8' and various firewall/bridging between studs say 48" on center. We will also assume that bracing is on both sides of the stud wall and if not plywood sheed bracing is 1x4 countersunk into the studs on at least one side. Both sides might require a 2x8 stud. Normal GC type stuff. Given the length however, I would for bridging/sway braces between stud walls use L-Stiff's/Wailers which are "L" shaped laminated 1x4 giving much more strenght to it. I'm still thinking three per section. Those in 8 to 12' sections could run bracing between stud walls well enough. I would go 3/4" plywood in at least 30" triangles and flanges where bracing was not able to be used such as acting areas. Think
stage jack in how to
build it.
You might still be able to go 24" on center but I think I would at least double up the ends and center of the stud wall so they act more like posts. Also in bolting the
platform to the backwall, it's going to releave most of the movement problems. For doorways or spans, normal
header design should do say at 5' span as long as not more than 2/3 the
deck area is being done like this. Than at that
point I would go plywood sway bracing. This again is off the top of my head.
Any help? Didn't mean to just write you off but this is something that while designing and specifying it's gotta have it's math and engineering checked and supervised upon. The depth and span are not much of a problem, more the height and expected acting area below.
Similar to what you were thinking or how does your intent differ? I'm interested in what you and Dad think for my own corrections in this and future work.