Platform Cross Bracing

[hbarker]

Need advice on platform cross bracing.

At what height is cross bracing necessary? I want to do a row of 4, 4'x8' platforms (32' total) at 6' tall. The platforms will be made from 2x4s, with (6) 2x4 compression legs per platform. We're doing a drop behind the platforms, and aesthetically, want as little cross bracing as possible. There will be a set of stairs on either end of the row of platforms.

I seem to see a lot of sets with tall, unbraced legs, and don't know what the "rule" is for bracing. I normally would brace and mask it in some way. Thanks for the help.

 

[DRU]

At 6' tall, cross bracing is definitely needed. An X pattern from the top of one leg to the bottom of the adjacent leg and vise versa is ideal. A situation where there would be less cross bracing is either A) wrong, or B) engineered with stronger materials and stronger connections than you can achieve with 2x4 and compression legs.

If you build your platforms with larger 2x material, you can reduce the number of legs required to support the spans, which can reduce the amount cross bracing, but you have a thicker platform. The rule of thumb with that is the second number in the 2x framing is the max span between legs (2x4-max 4' span, 2x6-max 6' span, 2x12-max 12' span, etc). Again, it wouldn't negate the need for cross bracing, just the number of legs needed to cross brace.

 

[BillConnerFASTC]

Bracing in both directions is required. For the 32', it's possible to brace the ends off stage or to the walks, so the platform all together cannot move left or right. Takes some expertise. You can't just toenail a couple of 2x4s to the floor. There is much less redundancy compared to Xs on each platform, so the design and especially the connections have to be very well designed and done. It also needs a stiff frame, so it doesn't buckle in plan.

Up and down stage sway is tougher - 6' tall and 4' wide. Perhaps Xs there are acceptable visually. And good anchoring to the floor so it can't lift and tip even if cross braced.

 

[StradivariusBone]

For the 32', it's possible to brace the ends off stage or to the walks, so the platform all together cannot move left or right.

I've done one where we braced the left to right sway with stair units bolted on the ends and that reduced the need for additional cross-bracing. That won't help as much with your upstage-downstage sway though.

 

[Butch!]

At 6' tall you definitely need some kind of diagonal stabilizer. I do some kind of bracing for anything over 2' tall.

I have never been a fan of single legs under raised platforms for a few reasons:

1) The bottoms of the legs are free to move around if they're not attached to the floor (and many places I work will not let you attach to their precious floor).
2) While the legs may in compression because they're against the plywood deck, they're really not as you're depending on the screws that are holding the plywood to the framing which are in tension and they could either pull out of the framing or the heads could pull through the plywood (think about how easy it is to sink a screw through what passes as plywood these days).
3) Where two platforms meet you wind up with two legs that are 3" apart. Not a very clean look and to me a waste of wood as one leg can support the load.
4) They're a pain to assemble as you're either screwing legs to an upside down platform and then flipping the beast over or you're holding the platform in the air while trying to screw the legs on.

What I like to do instead is build a studwall with a stud every 4 feet. By doing this the legs are all anchored at the floor, the platform assembly is truly in compression as the plywood deck rests on the framing which rests on the top of the studwall, and I've eliminated the double legs at the corners for a cleaner look. It also makes assembly easier as you stand the studwalls and then place the platforms on top of them. Diagonal bracing is also easier as you really only need one diagonal brace per studwall as the entire wall is one unit (though I always do one in each direction). The two walls do have to be cross braced to each other at the ends so the whole thing doesn't fall upstage or downstage and I usually run something between the studwalls at floor level to keep them evenly spaced.

If I were in your situation I would build two 32' studwalls with studs every 4' and set the platforms on top and screw up through the top of the wall into the bottom of the platform framing to secure them. Put X bracing between the studwalls at either end. Put a a 2x4 at floor level at each end and in the middle to keep the bottoms of the studwalls 4' apart. I would build the stairs on the ends as one piece with oversized stringers and then run 2x4's from the bottom of the stairs to the stud wall. That would turn the stairs into the diagonal braces and in my eye eliminate the need for any other diagonals. Of course the stairs need handrails and don't get me started on what OSHA thinks you need to do for railing on the platform and securing the stairs so that only safety trained performers can use them during performances.

That's my advice . . .

 

[TheaterEd]

Here is a picture of what we did last year for Les Mis. Platforms were 2x4 with plywood tops and we added plywood strips to the bottom to help support the lateral supports which were spaced 16" apart. Used 4 x 4 for the legs and connected them with HeadLok fasteners. We had staircases on either end and a ton of diagonal bracing on those to help with US/DS sway. The thing was solid, but before I went with this plan I had made sure that there would be no dancing or jumping on the platform.

 

[BillConnerFASTC]

2) While the legs may in compression because they're against the plywood deck, they're really not as you're depending on the screws that are holding the plywood to the framing which are in tension and they could either pull out of the framing or the heads could pull through the plywood (think about how easy it is to rsink a screw through what passes as plywood these days).

I thought he meant the legs were under the frame, not beside it, but I agree that much better if the load is on the columns rather than shear connections.

 

[hbarker]

Thanks for the advice!

What do you think about using steel legs instead? I've seen a row of platforms like I want that sit on 3" steel square legs.--two 2x4 framed platforms sit on one leg (the leg sits in the middle of the two legs). The tubing has steel plates welded at the top on either side, and these plates sandwich the two 2x4s of the two joining platforms, and the tubing also has a plate with a large footprint welded to the bottom that can attach to the stage floor (I can screw into my deck). If I legged my platforms in this way, is cross bracing needed?

Thanks!

 

[Butch!]

Even with steel legs I would still cross brace in some way as screws can pull out or shear off. The only way I would consider not cross bracing is if the plates on the bottom of the legs were at least 3/16" thick and at least 6" by 6", the stage floor was at least 2" thick and you could run a 3/8" bolt through each of the corners of the plate on the bottom of the leg through the stage floor and through a matching plate under the floor. That would, in my opinion, probably be strong enough not to cross brace, but I would build it, then get up on it and see how it moved and only then make a final determination. Of course this also assumes that the platform framing is strong enough to handle an 8' span and that the person who welded everything knew what they were doing.

I don't want to seem harsh, but people will get hurt if you don't do this right. And don't trust the person who tells you 'There will only be two people on it'. Did a show a long time ago where the props people borrowed an old horse cart from a farmer. The farmer told them it was old and weak and not to put more than two people on it. The props people told the TD and the SM the limit was two people. It worked great all through rehearsal and then on opening night the Director decides that for curtain call they should bring the 6 leads out on the cart. That didn't go well and we spent the next day making a new axle for the cart while the Director frantically trained someone from the ensemble to cover for one of the leads who was now on crutches.

 

[MarshallPope]

There is also the possibility of tensioned cable stabilizers. I've only ever tied offstage using this method, but in theory, I wouldn't be opposed to making an X with cables, as long as the line of platforms was securely affixed together and heavily legged at the ends. There would likely need to be some steel involved.

 

[lwinters630]

2x6 or 2x10 will span 8' or 16 ' depending your o/c. Plywood can be cut to more creative designs for cross support.

When you least expect it, the entire cast and crew will be up there doing the Harlem Shake or something similar. Keep in mind you can only control what you do ( or build ) not what others do.

So build for the maximum load. If in dout have an architect sign off on it.

Also consider how to block access to stair when not in use with safety chains, caution tape, or stanstions.

Let us know how it ends and post pictures.

 

[BillConnerFASTC]

Thanks for the advice!

What do you think about using steel legs instead? I've seen a row of platforms like I want that sit on 3" steel square legs.--two 2x4 framed platforms sit on one leg (the leg sits in the middle of the two legs). The tubing has steel plates welded at the top on either side, and these plates sandwich the two 2x4s of the two joining platforms, and the tubing also has a plate with a large footprint welded to the bottom that can attach to the stage floor (I can screw into my deck). If I legged my platforms in this way, is cross bracing needed?

Thanks!

As others have said, no, this won't work. I doubt you could get a strong enough moment connection like you describe on a wood stage floor.

It may be a surprise but what you are building on stage has to meet the building (and fire) codes. You can probably get a structural engineer for not too much for an hour to give you good help.

 

[danTt]

I think the easiest (though certainly not cheapest) way to avoid crossbracing would be to look at building the support structure out of 1' box truss, and affixing platforms to the top of that. You might be able to find a local production company that could rent it.