Building long unsupported spans

[SalvatoreDelorean]

I'm the TD in a college theater (I'm a student), and I have a set designer who wants to build a 4'x16' platform 8' in the air. The 16' sides need to be open so the audience can see through, so it's essentially a 16' bridge. He wants to accomplish this by putting a 4x4 post in each corner, and cross-bracing the 4' sides, but that's pretty much the extent of the reinforcement. He wants to frame it out of 2x6 and space toggles every 12". Is there any safe way to accomplish this? I can't imagine that there would be. Are there accepted industry standards for building long, unsupported spans out of wood?

Thanks!

 

[MNicolai]

Let me guess. After this is built, the director will also want to have a bunch of cast members on it at once?

 

[SalvatoreDelorean]

Yup! They'll be dancing, too. And the orchestra will be sitting underneath it. You can see why this is raising some red flags.

 

[sk8rsdad]

The maximum recommended span for 2x6 is about 10' depending on the material and local building codes, so 16' is in the realm of possibility for a set depending on how the structure is going to be used. 2x10 would be closer to code for typical live loads. It might need to be 2x12 or engineered joist if people are going to be really active up there.

My concern would be midspan sag and risk of collapse due to inadequate diagonal bracing. Sag can be reduced by additional blocking between joists and making any handrail or superstructure structural. Diagonal bracing can be provided by the staircases but they would need to be anchored to prevent them from moving. Are you allowed to fasten into the deck?

 

[SalvatoreDelorean]

The maximum recommended span for 2x6 is about 10' depending on the material and local building codes, so 16' is in the realm of possibility for a set depending on how the structure is going to be used. 2x10 would be closer to code for typical live loads. It might need to be 2x12 or engineered joist if people are going to be really active up there.

Thank you! That's really helpful. The materials would be utility grade 2x6 decked with 3/4" CDX. I'm not familiar with local building codes, but codes don't usually apply to temporary theatrical sets, do they?

We can't fasten into the deck; this whole structure will mostly likely need to be moveable. The designer wants to place it on teflon skids. There will be handrails, how would you suggest building them so that they take some of the load?

 

[LavaASU]

Thank you! That's really helpful. The materials would be utility grade 2x6 decked with 3/4" CDX. I'm not familiar with local building codes, but codes don't usually apply to temporary theatrical sets, do they?

We can't fasten into the deck; this whole structure will mostly likely need to be moveable. The designer wants to place it on teflon skids. There will be handrails, how would you suggest building them so that they take some of the load?

Could you cross brace at least the back of the 16' side with GAC?

 

[Footer]

What? Teflon skids? 16' span of 2x6? Not lagging it into the deck? Do you have an advisor/TD for the department?

First, no way that thing is going to move with teflon on the feet of the posts. Ain't gonna happen. It is going to weigh WAY to much... and it will wreck your deck in the process. There are a few different ways to skin this cat.. first question though... do you have a welder and someone who can weld? What is your deflection criteria for the piece?

 

[len]

I'm no engineer, but it seems without any cross bracing, the legs are going to snap off the first this is moved, or when it gets some weight on it. Personally, I'd just videotape the persons who are supposed to be underneath and hang a video screen on the downstage edge. That way you can build this thing so it's capable of supporting the load, with proper bracing.

 

[techieman33]

Since your on a college campus maybe you could get an engineering student to design something for you as a class project?

 

[lwinters630]

Building codes are not the issue, it is construction standards, load limits and structural safety. This can be done, but only within these constraints. Do not let this director design it, bring in someone who understands construction designs. Above all you are dealing with lives, so don't guess, you must know it will support more than the needed load. You may come into rehearsal one day and find the entire cast 8 ft. up doing the harlem shake ( I did).

Can you split it at 8 ft. and roll each half off SL & SR?

I don't think any of us can tell you how to build it on this forum, but it can be done, you will need money and professional help.

 

[BillConnerFASTC]

For light and thin, stressed skin construction is the practical option for scenery. There was a 16' X 3' stressed skin deck with 1X4's and 3/8 ply in a show I worked on and it was fine for one person to walk on. I can't promise without some calculation time but something like 1 X 6 12" oc and probably 1/2 or 5/8 top skin and 3/8 bottom would work. The columns are a larger challenge but pretty sure 4 x 4 in wood unbraced will not work. Could end supports be like an A-frame/saw horse, maybe a foot or so at the base?

You have an obligation to the performers to guard them from falls with rails or other fall protection while on the deck and handrails and guards on the access to it.

And I hate to disagree with Larry but building codes do apply - you simply are not - in most jurisdictions - required to have a building permit.

 

[BillConnerFASTC]

Looking at all the posts - not sure 2 X 10 would really work. Some species and grades might - on 12" spacing - but if there dancing - that's suggesting something in the 100 psf range - and I'm not even sure 2 X 12 in most commonly available species and grades will work. Google joist span tables and look carefully at the loads as well as grades and species. You won't find most of the better ones at big box stores - probably mostly SPf #2.

 

[derekleffew]

Structural Design for the Stage, by Alys Holden.

 

[rsmentele]

laminated beams.... expensive but strong... might be worth a look... but again, it will not be cheap...

here is a table on span lengths and beam sizes for a few different engineered beam types:

http://www.google.com/url?sa=t&rct=...usVpN5yFiTiFCah8zyvtryg&bvm=bv.57155469,d.aWc

 

[Robert]

I did a 4 foot x 24 foot span using two runs of 12" box truss. I contacted the manufacturer to verify the capacity of the truss in this use. They had actually done several installs of the same nature using truss to support catwalks. My buttresses were very sturdy and built to lock the truss in place. I U-bolted the platforms to the truss and bolted the platforms together. It was a musical comedy, so there was some dancing and singing on the platform. The truss was rental from a theatre friendly AV company. As far as rolling the unit around, with a well constructed base and step unit and everything tied together you might get away with that. Call a local AV company and their truss manufacturer and get their advise.

 

[BillConnerFASTC]

Please do not assume that the live load of 40 psf - as often noted in the span tables for residential - is sufficient for a chorus line. I'd design for 100.

 

[MPowers]

Structurally this is easy. Esthecticly or directory pleasing may be a different matter.
First, the platform(s) doesn't have to span 16', only 4'. Think about the way many bridges are built with very little structure below the roadway and the structure above. Think two trusses (handrails) mahout 48" deep. That's a major truss. That would give you a 42" high rail and 6"'for decking. Themtrusses are 4' apart and the decking only has to span between them. Triscits would work. The support towers are a bit more complicated and depends on how you intend to provide access, self contained or separate. If the unit moves the towers MUST be connected at ground level. If the unit is always pulled to move, the connection can be cable for instance. If pushed, the connection must be ridgid. Also, the units MUST roll. Sliding is not practical. Pneumatic casters or pneumatic brakes are in order.
The towers will have to be constructed to contain all the structural bracing needed. How much depends on whether the unit is totally free standing or if it is supported by another structure, for instance if one end can be attached to the flyrail
For actor access.
Notice I haven't told you how to do it, only how to approach
it. It can be done, but there is a lot more to it than a 16' deck and 4'corner posts.

 

[cdub260]

At the Pageant of the Masters, we built a similar structure for this last summer's show. We had a bridge going across the orchestra pit, a span of roughly 20 feet. We used 2x12 engineered joists with 3/4 ACX ply for the decking. The major difference between what we did and what your director wants is that our bridge was solidly anchored on both ends, steel joist hangers on the edge of the stage and a stair structure anchored to the concrete on the down stage side of the orchestra pit.

Frankly, the apparent lack of solid anchor points on the ends concerns me far more than 16 ft. span. If this were my project, rather than using 4x4 posts at the corners, I would anchor the ends of the bridge to 4 ft. x 4 ft. platforms at the same height as the bridge and brace the snot out of those platforms. This would give you a nice, solid piece of scenery and you could even put it on casters for mobility. Build in a shelf down low on each of the platforms for sandbags or arbor weights for stability. Two people should be able to move this structure easily. If space is an issue, compress the design and shorten the span of the bridge.

 

[gafftaper]

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.

 

[David Ashton]

what you want to do is quite straightforward, I have done similar things quite often, but the structure needs to be of steel truss which is readily available with ratings, then clad with plywood to look like wood, however a 8' high platform on a 4' base is not acceptable, it has to have diagonal bracing or wire bracing to the grid.With people moving on a 6' platform their bodies c-o-g at about 11' above the floor the tipping moment would easily tip over a free standing unit. I know they're only actors but if you kill or maim them the paperwork is horrendous. Probably best to get some basic structural advice.