Tip-over prevention

[jds10011]

Not sure if this question will make a lot of sense. Advance apologies. This question could be about many things, so here is a concrete example. Suppose we want to build a facsimile of one of these items (images borrowed from Uline):

How would one determine a safe distance that the "legs" need to extend horizontally away from the vertical "flat" (aka the length of the "legs")? Obviously, if they are an inch long, it will tip over, and if they are 20 feet long, it won't. But, what is the best practice for how to determine an appropriate safe length? From the physics of the situation, it would seem just that the base needs to be wider than the upright portion, but we all know that isn't true in practice because with very short legs one could very easily push the whole thing over accidentally by knocking into it slightly or attempting to move it (aka getting the center of gravity so it is no longer over the base). Further, we also all know that we would be much more comfortable with a wider base for a taller "flat" -- but is there a good rule of thumb to relate these two quantities?

P.S. One of the reasons for asking about a design like the one above is that you can't easily lower the center of gravity by making the bottom/legs super heavy -- many improbable-looking items like giant moveable orchestra shells can have super-tiny bases, but on inspection, the base is an order of magnitude heavier than the upright portion. Sure, the panel portion shown above likely weighs very little, but the center of gravity is still likely to be above the base.

 

[TimMc]

P.S. One of the reasons for asking about a design like the one above is that you can't easily lower the center of gravity by making the bottom/legs super heavy -- many improbable-looking items like giant moveable orchestra shells can have super-tiny bases, but on inspection, the base is an order of magnitude heavier than the upright portion. Sure, the panel portion shown above likely weighs very little, but the center of gravity is still likely to be above the base.

My PAC has a custom Wenger orchestra shell, the convex side walls are on counterweighted carts. The 90° attachment of the wall to the cart is the highest stress point of the entire assembly. We sweep and dust mop the stage before moving them ever-so-carefully.

The upstage wall and "ceiling" are clamshell hinged at the top, and that assembly rides on a track to take out and bring in. When about 70% in, we pause and connect a pair of hoist lines to the downstage of the ceiling to raise it fully after the back wall is at low trim. A now-departed coworker once told me it was a 30 ton apparatus. That might include the side wall sections. Lots o' weight so not to resonate.

 

[Lasermike]

I found a lot of hits looking for stability triangle which is a term used for powered lift trucks. From there, I picked a few. One has a, for me as a person with numerophobia tendencies, a scary looking equation. Another says to draw a triangle with each half at 18°. The tip is at the CG, the base would be on the deck in your example. This I can do. The third does the math for you but requires thinking and planning. Two things I try to avoid. To add weight low down on the examples, I would fill conduit or black iron pipe with sand and run it across the bottom of the flat as if it were a stretcher.

Michael

Scary, https://demonstrations.wolfram.com/StabilityAndCriticalAngleOfABox/
Not as scary, https://vention.io/blogs/calculating-the-stability-of-your-equipment-89
Automated, https://www.studiored.com/tip-point-calculator/

 

[jds10011]

I found a lot of hits looking for stability triangle which is a term used for powered lift trucks. From there, I picked a few. One has a, for me as a person with numerophobia tendencies, a scary looking equation. Another says to draw a triangle with each half at 18°. The tip is at the CG, the base would be on the deck in your example. This I can do. The third does the math for you but requires thinking and planning. Two things I try to avoid. To add weight low down on the examples, I would fill conduit or black iron pipe with sand and run it across the bottom of the flat as if it were a stretcher.

Michael

Scary, https://demonstrations.wolfram.com/StabilityAndCriticalAngleOfABox/
Not as scary, https://vention.io/blogs/calculating-the-stability-of-your-equipment-89
Automated, https://www.studiored.com/tip-point-calculator/

Interesting. Just out of curiosity, if I'm doing the math right, for a standard 8 foot flat, presuming the CG is at 4 feet, that would suggest two horizontal legs of nearly 16 inches each. Though I suspect 16" on both sides works, I'm wondering if 12" really wouldn't. Not sure where the 18 degrees comes from...

 

[Lasermike]

I'd guess the 18° is a rule of thumb based on a complicated formula. I recently built flats that were 6' high by about 3' wide that represented doors and mirrors for Mailda. Since they were meant to be quickly rolled all over the stage by younger performers, I went with 18" each side or a base of almost 3' square. They are extremely stable, probably overkill. I think I could have reduced the length to 14" each side without issue but my mantra is no crying children due to anything I've built. Our 10' peri are a tip hazard, I use 20 lbs of sand bags in the base to move the CG down as far as I can and they are still a bit tippy at 32" per side. This is partly because they are so tall, actors tend to push from higher up.

 

[egilson1]

http://entertainmentriggingservices.com/uploads/9/3/7/5/93751914/tipping_calculations.pdf

This might be helpful.

 

[Jay Ashworth]

My PAC has a custom Wenger orchestra shell, the convex side walls are on counterweighted carts. The 90° attachment of the wall to the cart is the highest stress point of the entire assembly. We sweep and dust mop the stage before moving them ever-so-carefully.

My theatre has *50 year old* Wenger clamshells, and I'm not sure they've ever been mechaniced in the interim; they're still pretty rugged; Wenger built em nice...