Elevated Structure Framing

[BSchend]

For an upcoming show I have to create an elevated area 8.5' off the ground that is 32' w x 14' d. This area will be use for a 9 piece orchestra (roughly 32'x8' blue area of picture below) and space utilized by actors (32'x6'). There will be dancing in the show (don't know how much will take place on that second level, but I'm planning for it anyway).

The structure underneath needs a 8.5' span in the middle, two 8' spans to either side, with the remaining 3-4 feet outside them simply completing the remainder of the 32'. The three middle spans will be used for set pieces that will move on and off from under the structure, so those spans can not have any legs in the middle. The middle 8.5' will have a single unit that will never change, but I'm hoping to utilize the ability to change a few pieces in and out of the two 8' spaces.

I plan on minimally having legs/posts for the structure where the black tick marks are. Every 4' for the orchestra area and every 3' for the actor space. I could possibly use stud walls for the sides of the 8.5' section. If I do "legs" they'll be multiple layer 2x6 compression legs, and bolted in to the headers/beam. Don't want to use 4x4 as they never seem to not warp after being stored for later use. I figure 3 2x6s gives you a

Upstage framing I will laterally brace as needed, but the downstage portion will essentially be "wedged" between a concrete proscenium (our proscenium is 16' from the front of the stage) so it won't move laterally at that edge even if it wants to.

I have no issue switching to 12' on center joists for the actor portion to account for live load, but I'm still not sure how beefy I need to go. Sure you always want to air on the side of caution, but I don't want to build something that will be up for 3 weeks that could out-survive the building it's in. The joists will be decked with 3/4 rated ply.

Should I be using 2x8's for the longer span joists? (I think 2x6 is at the end of it's span rating at 8.5', even at 12" on center) Should I then be using 2x10's for beams? Or can I use 2x8's if I'm using stud walls instead of legs and joist hangers?

Recommendations greatly appreciated. As I said as always going to air on the side of caution, but I don't want to build (or more precisely spend the money on) something that could be done another way.

Thanks

 

[BillConnerFASTC]

This is a little bit like asking here how to rig something. Generally we don't answer that.

I'd design for 100 psf and then 20% of that for lateral force, all directions.

I'm not sure which way you intend to run joists. I assumed beams left to right and joists us ds but I think you may be opposite.

But bracing for lateral loads is trickier.

 

[jhochb]

Good Morning

Have you thought about engineered laminated I-beams instead of 2Xs

 

[BSchend]

I understand the reluctance (and fear of liability) about answering questions like this, but if people were willing to entertain a discussion on a 16' span that could move (even though most rightly said it was stupid to even try and should be redesigned), I would think asking how to frame a 8.5' span that doesn't move wouldn't be that crazy. Hell people give cantilever advice which can almost be as bad. That's fine if no one wants to respond, or if the mods want to lock the thread, but I'm still going to talk it out anyway if that's ok (or until they lock the thread if that's the route they want to go).

I had intended on running the joists sl/sr because having a beam run 32' would require splicing/doubling up (which I may have to double up anyway, but that would make it necessary). Also if I run the beams that direction and have the joists run across the top of them (a la deck construction) that would drop my 8.5' height down to almost 7' at the span. If I run the beams us/ds then the largest span the beams cross is 4ft.

Using 100psf it looks like I'd have to use 2x8 to span 8.5'. http://www.awc.org/codes-standards/calculators-software/spancalc

I don't know if I'm reading the span tables correct that I could use 2x10 (doubled, one on each side of the support structure/post/leg) for the 4' span between supports. Or should I use 2x12? Seems like 2x12 would be overkill for 4'.

 

[BSchend]

So below are pictures of what I came up with. Using essentially doubled 2x6 compression legs (the beam sits where the frame of a platform usually would) and 3 joist/rim joist units sit on top. Using the doubled compression legs is like notching in a 6x6, but in this case the supporting "leg" is a full 6" in one dimension. (plus I don't have to lug around 10' 6x6s to cut down and notch). Not only does each joist have 1.5" of bearing length on each end (calculator says between .65 and .76" needed at each end), but all the rim joists are supported by the flush 2x6 part of the legs as well. I could easily up the beams to 2x12 instead of 2x10, but I'm wondering if anyone has a load table for a beam with a 4' span.

The 2x8s are spaced 16" on center where the orchestra will be and 12" on center down in the acting space.

Thoughts?

 

[BillConnerFASTC]

You shouldn't need anywhere near as many columes as you show. 14*32 is 448 sq ft (?) and without looking ought to be easily doable on 6 columes. The 2*10 vs 2*12 is a calculation based on span.

For the beams, two 2*10s in a table should be nailed together - there are nailing schedules - and not spaced apart as it seems you indicate. I can't be sure.

Double ply on top would do a lot more than changing to 12" spacing for deflection.

The biggest concern remains lateral bracing. Just keep thinking 20% of the vertical load. With 30 columes, that's the equivalent of 6 columes laterally - in each of 4 directions - overdesign being equal. And at a diagonal it needs to be stronger than axially loaded.

I'd probably look at using 2*4 walls us ds and no beams, with 2*6 or maybe 2*8 joists running across, studs and walls 16 OC, aligned, single top and bottom plates on walls. Sheath walls or add diagonals. Figure out how to brace left to right. Follow nailing schedules. Look at joist hanger hardware for anchoring joists to top plates. A lot less material. Easy to build the six walls ( and could eliminate 2 easily with 2*8 - probably keep ends and pair flanking center bay) and tip up. 12' joists - gives 2' overlap splices over the center pair of walls. Just saying. I would go back and check every span. And still like a double layer of ply - does a lot for feel and hollow sound.

 

[BSchend]

The reason I split the beams (and you can do so, many deck posts get notched on each side for just such a method) is so that the joists had something to bear on (which is what span tables and calculators have indicated). If I double the beam the rim joists only would be resting on the beam and none of the joist.

For the plywood, I wouldn't do overlap splices. I would keep all sheets perpendicular to the joists, and alternate between a full sheet downstage and a 6' long sheet upstage, then flip 6' down/full sheet up. Possible to double the ply, though that is way more costly than a couple extra joists.

How could I span 11.5' with 2x8? (if I eliminate an extra wall on each side) Calculations put that at 2x12 territory for that span at 100psf, or am I wrong?

 

[BillConnerFASTC]

How could I span 11.5' with 2x8? (if I eliminate an extra wall on each side) Calculations put that at 2x12 territory for that span at 100psf, or am I wrong?

You may not be able too. I don't know how important the middle section not grow a little and don't know if end "walls" couldn't be inset a little and let the platform cantilever. Even if you had to go to 12" spacing it seems simpler and less expensive - 4 walls and 2X8s set on top. Go to 2X10s maybe. Its the speed and ease of construction that appeals to me.

Again species and grade plays into this. I double checked the 100 psf live load and feel that is right per the building code. Probably 1/180 deflection is adequate, and unless apply a "ceiling" under, you could probably reduce dead load to 5 psf.

Are you using http://www.awc.org/codes-standards/calculators-software/spancalc or similar? I get 2x10s to work on 12" centers in SPF no 2s. 2x12s on 16 SPF no.2 is pretty low.

If you don't want to double ply and go to 12" centers, consider blocking seams.

Did you look at TJIs? Biggest advantage would be you can lift the 32' piece yourself if you can maneuver it onto stage of course. This is all why ist so hard to do remotely and know all of the issues.

Did you look at the stage floor and the concentrated loads of the columns?

 

[BSchend]

So here are two pictures of what I ended up doing. 2x6 stud walls with 2x8 headers and joists. Longest span was 102". The structure was so solid a few members of the orchestra said they have felt more movement on high school permanent stages.

 

[BillConnerFASTC]

Very nice!