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| tipping point of stage scenery | |||
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| Posted by: norm ® 04/24/2007, 16:11:35 Author Profile eMail author Edit |
I'd like to know roughly how much force would have to be applied to tip over a freestanding wooden box that is 48" wide, 4" deep, 98" tall, and weighs about 45 pounds. The box frame is made of 1x4 lumber (¾ x 3½) and a 4'x8' sheet of ¼ inch plywood attached on each side of the frame to the narrow (¾) edges of the 1x4. BACKSTORY - With no experience in stagecraft I answered the call to aid the Set Designer in building scenery flats for a production in which my granddaughters were acting. The flats were to be made of 1x4 lumber with 4'x8' sheets of ¼ inch plywood attached on each side of the frame. The assembled flats had to be remain upright, be rotatable, and move on and off the stage. Stagecraft books contain rules of thumb about keeping stationary flats upright but have little to offer about moveable flats and caster placement. With no rationale whatsoever I decided to place casters 12" from the face of the flat at each end of the flat. I added 12" vertical and horizontal braces to keep the flat upright and the casters from shifting relative to the flat. The braces were connected to caster supports (two pieces of 1x4 each 28" long and screwed into the bottom of the flat.) You can imagine my relief and delight when the flats with casters attached were lifted into an upright position and remained so and rolled about the stage floor with little effort. Only once during the run did an actor slam into a flat. It rolled back a few inches but remained upright. The caster frames create a problem because they extend into the performers' space. After the run I did two tests to determine how stable the flats really were. I first struck one flat forcefully with a 3' wide push broom at the 4', 6', and 8' heights. The flat moved a few inches back but didn't topple and the casters never left the floor. I then replaced all four 12" vertical braces with 3" braces and repeated the test and had similar results. To my regret I didn't do a push broom stability test with the casters attached closer to the flat, say 14-16" apart. When I posted my experiences in a stagecraft forum I was gratified to learn that others had achieved similar stability with the widely spaced (approx 26" apart) casters. But I've never ever just stumbled into a perfect solution to anything so I'm figuring there's a fair size safety factor in play here somewhere. I'd like to bring the casters in closer to the flat so that the caster supports only extend 6" into the performing space without making the upright flat unstable. Initially, I'd like to know roughly how much force would have to be applied to tip over the freestanding flat, just standing upright without casters. I do know about the center of gravity and that if it moves outside the flat that the flat will tip over. I need help to determine how to calculate the force that would cause the flat to tip over. Secondly, the casters, even if only 14" apart rather than 26" would seem to provide more stability to the flat. Is there a way to translate caster spacing to the force needed to tip over the flat? Thanks for reading this far and any help you may offer. I know things can get complicated real quick, so make any simplifications you think necessary or helpful to come up with some results. Regrettably I wasn't able to get a photo of the casters and frame reduced to the attachment size. |
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| Re: tipping point of stage scenery -- norm | Post Reply | Top of thread | Forum |
| Posted by: jboggs ® 04/25/2007, 10:49:33 Author Profile eMail author Edit |
A sum of moments calculation will give you the answer. Use the caster nearest to the CG as the center of rotation. Locate the CG's of all the other components, calculate their moments about that center, and add them up. Also, the more dead weight you can add on the back side, the less likely it will be to tip forward. I've seen stagehands carrying plastic jugs full of sand or water or something to place on the supports behind uprights to stabilize them. |
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| Posted by: swearingen ® 04/24/2007, 17:04:14 Author Profile eMail author Edit |
Sure, there are calculations that can be made to predict how much force it would take to tip them, but there's a simpler way. Attach a scale to the end of your broom stick and slowly apply load until it tips. Check what the maximum load was on the scale and there you have it. A more fool-proof, but more complex, way to measure the force is to attach some twine to the flat, run it over a pulley, and hang some weights on the end. Keep adding weight until the flat falls - there's your load. You can do the same experiment with the castered flat, but you'll need to chock the casters. It's a much more involved calculation (and experiment) to see how much load will tip a free rolling flat. The good news is that wheels make it less likely to tip, so test your chocked casters at different spacings until you are satisfied and then you know you've got some built-in safety factor when the wheels are un-chocked. |
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| Posted by: norm ® 04/26/2007, 08:31:59 Author Profile eMail author Edit |
Thank you both for the advice. Unfortunately I don't have access to the flat materials anymore and not even a place with enough clearance to set one up and do the experiments. Future tests will have to wait until the next March. Calculating moments is something I might have done on a good day 50+ years ago but now I barely recognize the term. I was naively hoping that there would be a not-too-difficult calculation that would solve the tipping point problem. I knew that casters would complicate things. Again, thanks for taking the time to consider my problem. Norm |
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| Posted by: Kelly Bramble ® 04/26/2007, 09:16:43 Author Profile eMail author Edit |
Even if there is a simple way to calculate the tipping angle, how would you relay that to the real world? If this equipment is moved around often a test and solution could be that you attach a simple plumb-bob at the approximate mid-point - high on the equipment. Then with the help of another, tip the equipment untill it clearly wants to fall over. While you are tipping the equipment the plumb-bob is pointing down (gravity). Simply put an indicator(mark or equivalent) on the equipment where the plumb-bob is when the equipment begins to want to fall. If required, this could provide a visual aid to determine when a un-safe tilt has been achieved. Of course you will want to adjust the indicator for saftey reasons. |
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