Silo Floor Loading 1

[BobCarlson]

I am looking for the load profile as a function of radius (and height) on a grain silo floor. I gather that the shape of this loading profile is something like a "bell curve" as a consequence of Janssen's equation.

Perhaps more to the point, how does one determine how much of the mass at any given height is sitting on the floor, versus "hanging" on the walls? It would seem that with extreme pressures on the grain near the floor, that material will stay with the floor, rather than slide on the floor and create additional pressure on walls. On the other hand, the grain near the top of the silo will almost certainly apply additional pressure on the walls during seismic loading.

This sounds pretty cryptic but I hope it makes enough sense to get some pointers on where to look.

Thanks in advance

Bob Carlson

 

[dbuzz]

Flat-bottomed silo or silo with conical or wedge hopper?

 

[BobCarlson]

dbuzz,

Flat bottom bin/silo. Typically larger Diameter than height.

Thanks

b

 

[Focht3]

I'm going to take a stab at this - it sounds like you are trying to determine:[ol][li]The vertical pressure exerted by the grain at the slab level of a grain silo during a seismic event, and[li]The lateral pressure of the grain on the silo walls during a seismic event.[/ol]The grain will act a lot like a sand - not exactly, because the individual sand grains aren't compressible while the "grain grains" clearly are. This difference means that some common geotechnical engineering principles have to be re-considered before they can be applied to your circumstance.

On the vertical pressure: since the shaking during a seismic event can cause the grain to act like a fluid, I think you have to assume the full weight of the grain will act on the slab, with relatively small amounts acting on the silo itself. This would not apply if the grain was somehow compressed so that it actually "fluffed" during the shaking; but I don't see a mechanism for that compressed condition to occur prior to a seismic event.

I also think that assuming something less than a hydrostatic pressure distribution - with the pressures calculated assuming an equivalent fluid weight based on the density of the grain - could result in an underestimation of the loads. The grain will try to reach a density where neither contraction nor dilation occurs during shearing stresses within the grain mass during a seismic event. I would expect the grain to increase in density, meaning the grain will contract in volume. Arching is unlikely for this circumstance.

But my comments are loaded with suppositions, some (all?) of which may be wrong.



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[aggman]

BobCarlson,
For silos which have a height greater than 1.5*diameter, then the Janssen equations apply, and the floor load would be the least of (unit weight of material * height of meterial) or q = ((gamma * R)/(muprime * k))*[1-e^(-muprime*k*Y/R)] * Cd.

gamma = unit weight of material
R = hydraulic radius
muprime = wall friction factor
k = lateral pressure ratio
Y = depth below surface
Cd = overpressure factor (1.5 for steel floors, 1.35 for concrete)
q = vertical pressure (typically psf)

If the height is less than 1.5 * diameter, then the normal Rankine pressures would apply, where wall friction is neglected, and the floor load would be just the weight of the material times it's height. This is out of ACI 313-97. You can also reference Gaylords Structural Engineering Handbook, 4th edition, for a description also.

 

[Focht3]

Hmmm,

On November 14, 2003 [blue]BobCarlson[/blue] wrote,
Flat bottom bin/silo. Typically larger Diameter than height.

Since Diameter > Height, then clearly Height < 1.5*Diameter. Janssen equations don't apply.

But a pressure reduction does appear to apply; I found the following on the Internet:

http://revobiz.dyndns.org/group/seaint/1999d/msg00129.html

It looks like the change from the 1991 to 1997 code was primarily a move from assuming &quot;active&quot; pressures to &quot;at rest&quot; pressures.

[blue]BobCarlson[/blue]:
It looks like you need to read and understand ACI 313-97...




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[BobCarlson]

Although I have some that have heights approaching 1.5*D it looks like the consensus is ACI 313-97 will get me pointed in the right direction.

Thanks for the help.

b