how to calculate weight applied to structure by flowing material

[minfarm]

I need to hold in place a drive assembly mounted in the center of a grain bin. My plan is to use three "spokes" that will be supported on the concrete foundation on the outside of the bin. They will project to the center of the bin and be supported in the center by the drive assembly. These spokes will be approximately 12" off the floor of the bin. I want to keep the amount of down force applied to the drive assembly mounted in the center as low as possible.

My plan was to use two 1x3x3/16" rectangular tubes for each spoke. The two tubes would be welded together to form a 1x6. This 1x6 would be mounted vertically so that the 1" dimension would interfere with the downward grain flow. According to calculations that were done for me this would give me about 1" deflection on a 15' span.

I recently had the idea to possibly use a fabricated spoke to lessen the amount of pressure exerted on the center drive assembly, but I do not know how to calculate the force applied by the flowing grain.

My thinking for the fabricated spoke was to use two lengths of 3/16"x5" set on edge with a crosspiece of 1/4"x4"? welded between them every 4'. I do not know how far apart I need to have the 3/16"x5" to keep the pressure from the flowing grain to a minimum. I assume that if they are to close the grain would essentially act as though it was bridged and exert that full amount of force. I also do not know the size of flat stock that I need to keep my deflection to 1" or under.

The weight of the grain is 45#'s per cubit foot. The angle of repose of the grain is about 28°. The maximum grain depth will be about 20'. I calculated the total weight (1125#'s) applied to a 1" wide spoke by determining the cubic feet in a vertical column 1" wide by 15' long and 20' high.

Any help or thoughts on this would be greatly appreciated.



Please see attached sketch for an idea of how this looks. (the spokes are labeled stabilizing arms)

 

[desertfox]

NO SKETCH ATTACHED

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[minfarm]

Sorry. After I uploaded the attachment, I didn't see you needed to click on the link to insert the file.

 

[racookpe1978]

Why have a flat area on the top of the horizontal braces?

Streamline them so the falling dust can't stay on top of the brackets. Avoid EVERY "trap" where any dust can get trapped between adjacent braces. 28 degree angle of repose, right?

 

[minfarm]

Thank you for your reply, racookpe1978.

I apologize if I have not made myself clear. The first option that I mention would have a 1" flat area the length of the the horizontal braces (15'). The second option would have two 3/16" wide areas the length of the braces besides the few cross pieces welded between the two sides.

See attached sketch (hopefully) for clarification.

 

[IRstuff]

I think a 20' high pile of grain will behave somewhat like a fluid, i.e., there ought to be something akin to a water pressure. The braces are not completely supporting the entire mass above it; the grain underneath and around it will also share the load. The 28º angle of repose is a measure of how much more it behaves like a fluid compared to behaving like a brick. When the grain fills the space, it should flow around the braces until the braces are covered

Conversely, having hollow rectangular tubes are probably a bigger issue, since it would seem likely that the tubes would be crushed on the 5" sides from the pressure, which will definitely weaken the structure.

What exactly are you "driving?" I would have thought that the torque required to push grain around would be a bigger concern for twisting such skinny braces. While I alluded to the grain behaving like a fluid, it is probably an absurdly viscous, somewhat non-Newtonian, fluid.

TTFN
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[chicopee]

So if I understand this correctly the tripod is to inside the grain bin and the legs of the tripod are to extend thru the wall of the bin and rest on concrete pads outside the bin. Next is to have a motor driving "something" at the apex of the tripod within the bin. Is that description correct and if so what as mentioned what is being driven and for what purpose? Is it for blending grain within the bin?

 

[JStephen]

Assuming all the product above the strut is supported on the strut is probably overly conservative, but probably how I'd do it.
With deflections that high, you may have lateral buckling of a 6"x1" section as well.
You could put an angle turned 45 degrees on top of whatever structural member you were building up.

 

[RolMec]

assuming statical arms / spokes:
proposal for rough workaround for a vertical grain flow: F = m x a --> = m x v/t --> = m/t x v --> = mass flow x (local) velocity. The circle sectors to be suitably split into ring sectors, the whole then back on summed up along the length of the arm to arrive at a single force re momentum. This does not consider anything like impact and internal friction between the grains as well as external friction grains to arm.
But i think the speed of vertical grain flow is rather very small, so this would imo not be the issue.
Then it is down to the beams (spokes) dead weight + weight of the grain above it + hub + gearbox. --> Keep the width of the spokes small is the first thing.
You should experiment with different executions / sections, perhaps for this application, a H - shape would be good, except that it would accumulate grain on the insides.
However, you'd need the calculation model of the tripod, also in order to assess whether you can live with the deflections under different operational circumstances --> I assume you want to connect the gearbox to a protruding shaft and you then need to meet machinerys fits & tolerances?!

 

[minfarm]

Thanks for all the replies.

This post will help explain what I am driving. Link

IRstuff, I would agree that the grain will behave somewhat like a fluid. I would like to have the spokes designed for the maximum load that they will see. When grain is being taken away from directly underneath the spoke, I believe that the spoke may see close to the total weight of the grain on top of it as the grain flows around it to displace what is removed.

JStephen, How much would streamlining the top of the spoke help? I thought about this but reasoned that a small amount of grain would essentially sit on the top and act to divert the flow around the spoke. I suppose though there would be increased friction this way compared to actually attaching something on top.

RolMec, You mention an H shape would be good, except it would accumulate grain on the inside. There would be no bottom in the H shape so the grain would flow through.

 

[IRstuff]

" I believe that the spoke may see close to the total weight of the grain on top of it as the grain flows around it to displace what is removed. "

Possibly, but what I've seen of such things on the web, unlikely, simply because a screw drive sort of system isn't that fast, and doesn't appear to have the motive power to move against the grain itself, i.e., it can move around until the silo is essentially empty.

TTFN
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[Cockroach]

You can also multiply the density of your grain flow with bearing load of the spoke and velocity of the flow squared to get the force exerted by the medium. Basic fluid dynamics, F = p A v^2 = kg/m^3 X m^2 X (m/s)^2 = N.

Regards,
Cockroach

 

[PersonalProfile]

Various guidance is available for assessing design actions on silos [some of them also address "embedments"] by the silo's contents, including:

http://jenike.com/our-services/structural-mechanical-engineering/

Standards Australia, 1996, Standards Australia: Loads on bulk solids containers (AS3774-1996), Standards Australia, Australia

I guess you are based in the USA, and I understand there are "local equivalent" standards to the reference that I provided.

I propose you consider using this guidance.

Regards,
Lyle

 

[tbuelna]

One sketch provided shows a drive system consisting of a 1003:1 gear motor in series with a 50:1 gear reduction. That implies a total ratio of 50,150:1. So even if the lift screw rotates at just 1 rpm, that means the hydraulic drive motor would be rotating at over 50,000 rpm. Is this correct?

 

[minfarm]

The combined ratio is 50,150:1. But instead of 1 RPM I was looking for 1 revolution in three hours or about .005 RPM.

I would like to thank everyone who replied to this post, but the project has been abandoned.

Thanks again.