Legging stress skin platforms

mightymel

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I feel like Ive been on here a lot lately. You guys have been so helpful and I really love this community.

I’m at a new school this year and for the last 4 years they have been using these 4x10 platforms to extend the stage. The platforms go over the pit. (See attached picture).

Last year one of the platforms cracked and it really could have been a disaster. I found said platform and after inspecting it I know why it failed, aside from being overloaded and not enough bracing. Im looking to make them stronger and have rebuild the broken one.

My thought is to make them into stress skinned platforms but I’m having trouble with ideas on how to leg them. Especially since they are in the pit and its carpeted.

The director is very attached to this runway design so I’m trying to make it work this year. Ive talked to our fire marshall and the only restriction is that the ramp/pit access cannot be blocked.
 

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RonHebbard

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I feel like Ive been on here a lot lately. You guys have been so helpful and I really love this community.

I’m at a new school this year and for the last 4 years they have been using these 4x10 platforms to extend the stage. The platforms go over the pit. (See attached picture).

Last year one of the platforms cracked and it really could have been a disaster. I found said platform and after inspecting it I know why it failed, aside from being overloaded and not enough bracing. Im looking to make them stronger and have rebuild the broken one.

My thought is to make them into stress skinned platforms but I’m having trouble with ideas on how to leg them. Especially since they are in the pit and its carpeted.

The director is very attached to this runway design so I’m trying to make it work this year. Ive talked to our fire marshall and the only restriction is that the ramp/pit access cannot be blocked.
Hello @mightymel While you're waiting for the pro's to post, a few thoughts from a long retired sound and lighting geezer.
To me, it appears you don't require much additional lateral support in either direction, merely legs to limit vertical deflection and hold gravity at bay.
Consider two vertical legs, one on each side; think dressed 2" x 8".
Use glue and screws throughout to brace across their lower ends possibly 2" above their lower ends / the floor of your pit.

Across their upper ends, hold your horizontal cross braces back 4" to 6" from their ends and position your upper horizontal braces such that they tuck up tight against the under side of your existing platforms.

As for not damaging the pit's carpeting, consider a double layer of 3/4" ply across the lower ends attached flat across the bottom and carpeted on its underside such that the weight is distributed over a wider area and your supports mate carpet to carpet.

Looking at your cradle, or cradles; consider gluing and screwing diagonal cross braces on both the up stage and down stage sides of your vertical legs.

Clear as mud, right?

Glue, screws, good quality lumber and keep all vertical joints in compression. With all vertical loads in compression, very little should be required to secure the upper ends of your legs to your platforms, clamps or one decent size screw or bolt per leg per side ought to do it.

Remember, you're reading the ramblings of some old geezer on the internet whose suggestions are worth every penny he's NOT charging for them.
With apologies for my lack of clarity. Perhaps @Van or @bobgaggle will drop by and add their comments.
Addendum: IF you actually rebuild employing stressed skin construction; consider installing multi-pronged T-nuts within the side members or underside panels of your platforms to receive 5/16th or 3/8th inch bolts to secure your legs to your platforms.

Referring back to my previous ramblings, when you attach your horizontal cross members to your vertical legs, consider notching your legs and insetting the horizontal cross members to keep all vertical loads in compression.
Toodleoo!
Ron Hebbard
 
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BillConnerFASTC

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Do you want to just replicate the runway or a full pit cover?

make them into stress skinned platforms
And do you mean replace them or actually modify existing? I would advise strongly against modifying existing for something so particular as a stressed skin platform.

And why leg? Why not use stage and pit rail if it's strong enough? Stressed skin can easily span it.
 
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RonHebbard

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Do you want to just replicate the runway or a full pit cover?

And do you mean replace them or actually modify existing? I would advise strongly against modifying existing for something so particular as a stressed skin platform.

And why leg? Why not use stage and pit rail if it's strong enough? Stressed skin can easily span it.
@mightymel ALSO said in she/he's original post: "I've talked to our fire marshal and the only restriction is that the ramp/pit access CANNOT BE BLOCKED. "
Toodleoo!
Ron Hebbard
 

RonHebbard

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Do you want to just replicate the runway or a full pit cover?



And do you mean replace them or actually modify existing? I would advise strongly against modifying existing for something so particular as a stressed skin platform.

And why leg? Why not use stage and pit rail if it's strong enough? Stressed skin can easily span it.
How can you determine this without knowing the potential live loading to be applied?
What if the live load was 4 persons abreast, shoulder to shoulder in end to end rows of over weight students, or adults on a rental, all bouncing up and down on synchronized pogo sticks or even dancing rhythmically??
What if, what if, what if??? @egilson1 and @What Rigger? Would you care to comment?? (PLEASE!)
Toodleoo!
Ron Hebbard
 
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BillConnerFASTC

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Usually 50 psf is used as a live load for design of stage platforms. That's a classroom, office, or living room live load and pretty conservative. If I thought they'd be packed and all jumping up and down in unison, I'd do a little research and maybe go to 60 or even 75 psf. Some judgement required. I think you'll find most commercial systems - StageRight or Wenger for example - use 50.
 
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TimMc

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Well then... put me in the category of "I don't modify commercial products." Why? Because then I *own* any subsequent liability for failure or unintended outcome from the use of the modified product. If the manufacturer is still in business I think consulting with them would be the very first order of business.

That it has cracked and begun to fail is a clear indication that it needs to be replaced or repaired IN ACCORDANCE WITH THE MANUFACTURER'S RECOMMENDATIONS. Absent any finding of defect of materials or workmanship on the part of the manufacturer, the emerging failure is the result of either under-specification or mis-use (overloading; excessive dynamic loads*), or both.

And Mel, the word for this use of staging is called "passerelle". The classic design is a kind of semi-circle out over pit and maybe into the house as well, but I think the director's use of this qualifies it as a passerelle. The word is also a synonym for "gang plank" or similar means of boarding a boat or ship. I think the literal translation is "little bridge."

* footie note: In the time I've been here on CB there have been at least 2 pit cover collapses in school auditoriums due to excessive loading; kids were injured, some seriously. Directors do not know and mostly do not consider things like load bearing - in their minds, that it is a structure means that it will take whatever they throw at or on it, no matter how much jumping is involved. IMNSHO, a static load rating of 150 lbs/s.f. is the minimum acceptable specification because "people of vision" cannot foresee negative outcomes from their "art." /curmudgeonliness, cranky old guy rant.

Edit ps: people getting seriously injured or killed in the production or pursuit of entertainment is unacceptable and why I joined Jim Digby's Event Safety Alliance after he formed it following the Indiana State Fair roof/stage blow down a few years ago. NOBODY should die for a good time, especially someone else's.
 
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What Rigger?

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Man, it's been so long since I've had to touch anything like this. I have only the tiniest thing to "contribute" here- do whatever it takes to find out beforehand what kind of load these sort of pit covers can take. Because when it fails- like at Servite High School in Anaheim, Ca not too long ago- it will be uglier than you imagine. No, the whole "everyone jumping up and down on the pit cover at once" idea was not discussed ahead of time.

 
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BillConnerFASTC

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If the manufacturer is still in business I think consulting with them would be the very first order of business.
I think the existing are home built.

There were three collapses of HS pit fillers in the same year - all homebuilt and obviously of very poor design - like 1/2" flake board with toggles 32" on center or a ledger barely nailed to the fascia trim.
 
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TimMc

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I think the existing are home built.

There were three collapses of HS pit fillers in the same year - all homebuilt and obviously of very poor design - like 1/2" flake board with toggles 32" on center or a ledger barely nailed to the fascia trim.
I hadn't heard that some of the pit covers had been "home brewed" but I wouldn't be surprised. A lot of shortcuts and workarounds are done to "make theatre" and I respect the efforts expended to fulfill a director's artistic vision, but building bridges for unknown loads, and supporting school kids? If I were a parent in these districts, I'd be asking why quasi-structural things of unknown design pedigree are supporting my kids over a 8 ft deep abyss, when the ladders used by the maintenance staff have to meet certain minimum criteria...

/ moral outrage... because 'good enough', isn't
 

MarshallPope

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Without making any detailed recommendations without seeing the space in person, I would personally consider building stressed skin platforms and stud walls. Depending on what architecture is available to anchor to, perhaps stud wall pillars would be a decent solution. (Picture one stud wall across the end of a platform with another stud wall 18 inches or so away, with significant crosss bracing between the two. Repeat on the other end.)

Also, I’m not sure if I’m looking at the photo correctly, but are the platforms currently leaning up at an angle onto the stage deck proper? If so, I would be worried about tripping over the end and taking a nosedive into the pit. I’d personally want to see a platform with a curve cut into it to match the apron.
 
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BillConnerFASTC

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Also, I’m not sure if I’m looking at the photo correctly, but are the platforms currently leaning up at an angle onto the stage deck proper? If so, I would be worried about tripping over the end and taking a nosedive into the pit. I’d personally want to see a platform with a curve cut into it to match the apron.
+1. I was almost going to comment that the blocking of egress from the pit is less a hazard than the many fall into the pit hazard possibilities. I know of many stage injuries from pit falls, none from fire (US) - the reason for unimpeded egress - in over 100 years.
 
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BillConnerFASTC

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I hadn't heard that some of the pit covers had been "home brewed" but I wouldn't be surprised.
The one in Indiana was especially sad. There was a complete manufactured pit filler. I know the consultant and the pit filler manufacture well. The HS teacher in charge (HSTIC?) wanted more storage (surprise) so removed the system with it's scaffold support system and built a wood one that spanned between stage edge fascia and pit rail.

You can bet there was some alarm before the consultant and manufacturer found out it wasn't there's.

Unrelated, the building code says platforms are to be designed for 100 psf (and 20% of that laterally). (Stages are to be designed for 150 psf.) 100 psf is roughly 10-11 people jumping in unison on a 4 x 8. Figure a design factor of 2 or 2.5. Set a 4 x 8 on the floor and try to squeeze that many on it and have them all jump in unison repeatedly.
 
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Van

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A. Build 'real'Stressed skin platforms: If the platforms are 10' long then buy 4x10 plywood Cap the top and the Bottom and filled the voids completely with EPE foam.

B. Build at least 2 four foot wide stud walls with studs 16"0.C. cover one side of them with 3/8" OSB, at a minimum screwed 6" on perimeter and 12" in the field. and install them at the center of each platforms running across the width. . Everything will
have to be painted with flame treated Paint. I prefer FR-1.

You have to cover one side to provide shear strength and to prevent the studs from bouncing and bowing under pressure. The Orchestra director will complain about sound bounce and eye contact with areas of the ensemble. Your job is to stick to the "It's engineered this way or it doesn't go in' policy. Use the video above to show why.
 

macsound

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Being someone who grew up in theatre, everything was made in house. Sometimes ramps like this, sometimes enormous sets for West Side or Lady, but always by carpenters, home builders or landscape contractors and since they were the parents, friends or also the actors, they put alot of stress on how strong the pieces were.

My question seeing this platform is - is it attached to anything or just sitting on top? It looks like you have a channel at the front of your stage for footlights that isn't stage flooring. Would it be possible to add a threaded machanism in that channel to keep the ramp affixed? Then adding legs into the pit that could clamp onto the railing as well? Anything to prevent lateral movement.
I could see someone running for the ramp, it sliding out from beneath them, hitting the person in the front row and falling halfway into the pit.
 
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Back in grad school we used to have a saying that you designed and built for Curtain Call; as that's likely when you'd have the entire cast crammed up onto a platform or some such that otherwise would never see that kind of loading during the actual production....

If the pit rail can take that kind of load (I'm fairly certain the stage itself can...), then a stress skin platform should more than handle a 10ft span. 2x4 framing w/ toggles every 16in and skinned w/ 3/4" ply on top and something like 1/2" on the bottom (it'll be a cold COLD day before I'd build any kind of platforming - top or bottom - out of OSB or similar ...).

If you really feel you need legs of any kind for additional support, can you do stud walls down along the pit rail, perhaps with angled crossbraces going U/S (so long as it doesn't block any part of the pit). I don't see this being necessary, honestly, unless you need to pin them in place - which could also be solved with some low(er) profile brackets or toe-screws.

Good luck!
 
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BillConnerFASTC

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then a stress skin platform should more than handle a 10ft span. 2x4 framing w/ toggles every 16in
If a stress skinned platform is spanning 10', then all framing should be in the long direction, not across as toggles implies. Of course both skins glued to framing and assuming you are splicing 8' ply, the correct overlap splice plates joint the skin - basically a 1 ft overlap to each side of joint glued and screwed.

Stressed skin construction has a lot of rules and you have to follow them all. Design guides available at APA (American Plywood Association.)
 
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Bill's correct, of course; I'd be careful about taking *any* construction advice straight from any forum, much less straight from ME....! I should have said something along the lines of, "A (properly constructed) stress skin platform should more than handle a 10ft span."

ALWAYS DO YOUR OWN HOMEWORK.
 
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TimMc

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Back in grad school we used to have a saying that you designed and built for Curtain Call; as that's likely when you'd have the entire cast crammed up onto a platform or some such that otherwise would never see that kind of loading during the actual production....

Good luck!
On the quoted text we absolutely agree.
 
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