Determining tipping point

Stan in Oly, WA

Well-known Member
I told my sister and brother in law that I would make a platform for them to put barrels of water on to drip irrigate a vegetable garden they have on a back lot. It will make it possible for them to go on the kind of three to five day vacations they enjoy, without having to make arrangements to have someone else water. I have enough scrap 2" steel pipe to make a 6' high platform 8' long and 2' wide. The 50 gallon water barrels are 3' high and there will be four of them. I'm planning to make the legs in the shape of a truncated isosceles triangle, two feet wide at the top. How wide do I have to make the base to make sure it won't tip over? I know absolutely nothing about the calculations necessary to answer such a question.

Stan
 
Thinking that factor will depend entirely on the surface it sits on...

On a slab or poured curb, they would need very little spread, especially if bolted down.

But sitting in dirt... It's anyone's guess.

My thoughts would be to pour some piered curbs. That would give a permanent footing, keep the steel up out of the wet dirt, which could be quite corrosive if fertilizer is used.

Don't want something that could quietly degrade and turn over some years down the road!
 
for basic stability for overturning figure 20 Psf wind force on the barrels and structure and compute the overturning moment about the base.
the restoring moment is the dead weight times 1/2 width of the base. a factor of safety of 3 is used for overturning
I assume the barrels will eventually be empty so i would use the empty weight.
this is for a free standing structure not tied down and assuming the material it sits on is stable and it won't sink in
 
Just a question; does the property have any natural drop? A 6' platform only generates 2.6 psi head pressure. Including the height of the drum it peaks at 3.6 psi when full. Admittedly I have no experience with drip irrigation and there may be a minimum pressure required but I'm curious if locating the drums on elevated ground would accomplish the same end, avoid building a structure, and make it a bit easier/safer to fill.
 
Online, they advertise a solar powered fountain pump for about $30. I know it's not an answer to the question you asked but it may save you some time and money.
 
Can you build the top of the platform 4 ft x 4 ft and place the barrels in a square rather than in a row? A 4 foot base will be more stable than a 2 foot wide base and the legs may not need to be splayed out at all. You could band or rope the barrels together to make them more stable on top of the platform. The filled barrels will weigh around 400 pounds each.

What is the wall thickness of your pipe? If it's thin I would be concerned about the columns bending or the platform twisting and falling. Some X-bracing will help that.

Will your platform sit on top of the ground or will the posts be buried?
 
I suggest dropping by a farm fuel supply business or a local farm where there are overhead fuel storage tanks & look & measure. I have a 560 gallon tank, about 5 feet at bottom and about 9 above ground at top & estimate that legs angle outward to a width of about 6 feet. Tank is horizontal and is 42 inches in diameter. Sits on level dirt.
examples:

https://www.google.com/search?q=560...XGTSYKHXN5AbgQ7AkIMw&biw=1024&bih=611
 
The simple answer is that the barrel will tip when a line from the center of mass of the barrel (and stand) and drawn straight down goes outside of the feet. When the line goes through the foot it will balance

So in order to tip it, you need to lean it up at an angle to make this occur. This is a fairly easy problem to solve graphically.

If you want to be more sophisticated, you would calculate the wind load on an empty barrel and the moment around the foot to get your answer.

Cliff(VA)
 
My initial thought was that the garden is placed on the highest ground on the property, but in thinking more about it I realized that might not be the case. The land rises slightly toward the road. I'll ask if the barrels could be placed on higher ground without being inconveniently far away. Thanks.

Stan
 
That's a very practical suggestion. It introduces a couple of other problems such as needing to do much more extensive modularizing to make the parts able to be assembled at the site where welding is not an option since I don't own an engine powered welder. The bolting together required for the individual leg assemblies I had in mind seems like it would be quite minor in comparison. But I'll definitely think about it.

Stan
 
Thanks. Good information. Sketching the setup you describe next to the one I am proposing, yours looks more stable. Do I understand it correctly that the top of the platform your tank is on is five feet wide? My calculation says that a 560 gallon tank 42 inches in diameter has to be about eight feet long. Is that about right?

Stan
 
Thanks Cliff;

I appreciate explanations I can understand, and I appreciate graphical illustrations. It's not that I don't trust mathematical proofs, but I'm amazed at how often I'll make a simple key entry mistake and render my calculation worthless. Often it's obvious that the answer can't possibly be right, but sometimes it's not. Those are the times I worry about.

Stan
 
Thanks Steve;

For some reason I didn't pay attention to your last sentence until today. It's a good point, well worth keeping in mind. The concrete curbs idea is what I should probably do, but it makes what started as a volunteer project---largely intended to use up some of my accumulated steel instead of just scrapping it---start to be more than I intended to take on. Nevertheless, I'll give it some serious thought.

Stan
 
I appreciate you taking the time to describe how to compute the overturning moment and the restoring moment, but I have to admit that I don't understand it. Is the overturning moment computed by comparing the square footage of the windward face of the structure times 20 (for a 20 mph wind) against the dead weight of the platform and empty barrels? It seems I must have this wrong because it compares different units of measure---pounds versus foot/pounds, I'd like to understand it, but I don't.

Stan
 
(quoted from post at 12:31:30 06/24/16) Thanks. Good information. Sketching the setup you describe next to the one I am proposing, yours looks more stable. Do I understand it correctly that the top of the platform your tank is on is five feet wide? My calculation says that a 560 gallon tank 42 inches in diameter has to be about eight feet long. Is that about right?

Stan
es, 42" X 8'. Top of "platform is less than diameter of tank. Not really a "platform", but rather like a saddle that curved part of tank fits into.
 
you have most of it Stan---you calculate the force(20 psf x area) then times the distance down from the center of the area to the bottom of the leg on the ground. thats called overturning moment. the restoring moment is dead weight which usually can be considered at the center of the structure which times the distance from center to the outer leg(1/2 the base width)
yes a simple static diagram would explain it much more clearly (a picture is worth a thousand words)
this is the same procedure used to design a retaining wall or bridge abutment, difference being earth pressure acting horizontally instead of wind pressure
 
When a tractor gets to about 45 degrees, I start shaking like a leaf and on the brakes so I don't hit 46 degrees because I figure that's the tipping point and I either won't jump in time, or will but into the wrong spot.

Mark
 

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