Conical steel hopper supported by columns

[odradek7]

Hi all!

I am studying the (in my opinion remote) posibility of bulding a large steel hopper in order to install it inside a cylindrical steel silo bin storing cereals (wheat, maize...). The silo would be quite big, aproximately 60 feet diameter and 90 feet height, so the hopper would have to be designed to bear quite high pressures.

The structural model i have worked with until now is a conical hopper hanged from a circular quite rigid beam connecting with the upper part of the silo. This way the stresses to calculate the hopper section and joints follow a meridional-tangential distribution as it works as a membrane. They have suggested me to somehow reinforce the hopper coupling a "belt " supported by columns at a height between upper ring and the outlet. This way the construction of the hopper wold be done with two independient levels of steel sheets (from the ring to the belt and from the belt to the outlet).

I don't know why but i don't like this idea very much. In my opinion supporting the hopper sheets from its upper and lower part wold change the membrane stress pattern to a bending moment patter where higher sheet thicknesses wold be needed.

What is your opinion? Do you think a finite element model unavoidable to study this kind of structure?

Thousand of thanks!!!

Odradek.

 

[mrMikee]

odradek7,

There's some good info in thread507-123307, in particular I suggest the text “Design of Steel Bins for the Storage of Bulk Solids” by Gaylord and Gaylord. This book is out of print but I've seen it available used. Also worth a look is the ACI 313-97 specification.

Generally a conical hopper will have only tension stresses as long as it has a central discharge point, but an intermediate ring and columns could change this. Your silo is much bigger than anything I have worked on so I can't give too much detailed advice. Hopefully someone else in this forum can help more.

Regards,
-Mike

 

[aggman]

I haven't designed a silo that big in diameter but I would be concerned with the buckling of the hopper wall above the ring. Assuming a 4' diameter opening at the bottom you would be designing the 60' diameter bin to sit on a 32' diameter grid of columns. That is going to require stiffer support structure to control deflections. I would also be concerned with the localized stresses induced. You would get some pretty high bending moments that would have to be accounted for, plus the thrust loads on the ring girder. I would start by designing it by hand and see what you end up with. I would then do FEA on the transition to see if the stresses seemed similar. I would think it will come down to your comfort level and experience with the design after you do your hand calcs but I would be concerned with the transition stresses. I have never seen any large silos designed this way.

 

[mrMikee]

odradek7,

I think we need more information. Will this be a skirt supported silo where the shell goes down to the foundation, or a bin supported by columns around the perimeter.

Is the intermediate ring and columns extra support for the hopper, or is it meant to support the whole silo.

-Mike

 

[odradek7]

Thank you for your answers.

The silo is skirt supported. To hold the hopper we are thinking in four column rings: one in the external upper ring (joined to the silo skirt), one around the outlet and two intermediate. This columns are intended to support only the hopper.

Another problem i have found is the load determination. I usually work with the DIN 1055-6 (1987) standard that results in almost 250 kPa pressures in the upper part of the hopper. Attending to the Eurocode 1 - Part 4 pressures of 175 kPa wold be applied. I have the ANSI/ASAE standar formulae from an article and it seems that no overpressure coefficient is applied resulting in pressures of 100 kPa. I knew that DIN was quite more conservative standard but i can't belive such big differences. The silo height/diameter ratio is 0.9 (cylindrical part).

¿Do you work with any of this standards?

Best regards.
Odradek.

 

[mrMikee]

You mention that the H/D ratio is 0.9 which would suggest this is a shallow bin. According to the Gaylord and Gaylord text mentioned before, "shallow bins may be designed for Jansen, Reimbert, DIN filling pressures, or Rankine active pressures without regard to differences between filling and emptying pressures." So it seems they are saying overpressure coefficients are not required for calculations on shallow bins.

But your bin is much larger than anything I have done so I don't have a 'feel' for this one. I would be very careful, which is what you seem to be doing.

As far a deep bin theory is concerned I generally follow the ACI 313 specification because it is the only published spec in the US that I know of. Emptying pressures are calulated based on the filling pressure and an overpressure coefficient but use different equations to accomplish this in the 77 and 97 specs.

Regards,
-Mike

 

[odradek7]

Hello.

Than you very much for your replies, they have been very helpful.

I have convinced the people here that such a big steel hopper was not the correct way. Finally we have decided to do a mixed solution: the outer part wil be constructed in concrete (as a hollow cilinder) and the inner part (inside a 7 m diameter) with steel sheets as they were a funnel.

My first approach has been to design a conventional hopper with trapezoidal steel sheets, supported only in its upper-outer perimeter, attached to the concrete ring. My workmates insist in installing beams under the radial joints of the sheets and columns at midspan of this beams. I think that this is a hazard because sheets between beams are induced to bend and i wold have to weld them some kind of siffeners.

I'm trying to defend my option but i don't have any bibliographical reference. And i think to myself: If nobody "puts legs" to hoppers at its midway it has to be becaouse something.

¿Any help?

Odradek

 

[aggman]

Off the top of my head I would suppose you could do this. The upper and lower rings will have to support the beam reactions and the beam would have to be designed to span. I would think from a theoretical standpoint it would work, as long as there were no eccentric loads. What concerns me is the effects that the stiffeners will have on the hopper. I think it would hinder the membrane action of the shell itself. I would also think that the traditional hopper would be a more efficient and cheaper design overall. Your co-workers should help you determine why there is a desire to do the design as they are requesting. Not knowing whom you are dealing with be sure that the request is coming from someone who understands structural behavior somewhat. Too many times you get a lot of input from people who don't really understand how it works. While the dreamers of the world are needed in their own right, sometimes you have to put your foot down when they get off into left field.

 

[mrMikee]

Theoretically the steel cone will carry its load with only tension in the plate, or as aggman says the "membrane action of the shell itself." A result of this are large forces pulling in on the ring beam around the perimeter of the cone which would need to be carried by the concrete part of the bin bottom. The addition of radial beams would or could affect stresses but at this time I don't really know what to tell you. Note that a basic assumption to this analysis is that loads must not be eccentric, otherwise there could be some bending.

I can understand the desire to put in beams however, coming from the thinking that you need to hold something up. But that's not the mechanism that carries loads in a bin. As I mentioned in a previous post though, I don't have experience with bins as large as yours.

Regards,
-Mike

 

[flamby]

Before you worry too much about membrane stresses, give a thought how you will handle the vacuum- partial of full, created when unloading the vessel.

Actually, this size of structures are far more likely to buckle due to external pressures (sometimes just erection forces)than due to combination of internal loading conditions. You will end up using large number of stiffeners to your silo and there will be some redundant safety.

Ciao.

 

[JoeTank]

There are a lot of 60+ ft diameter cone-bottom steel bins and silos out there. They are skirt or column supported. Don't hestiate to proceed in this manner. The true design expertise for such structures normally resides with the contractor.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com