Silo/ Tank Shell Question 2

[dmitriy555]

I have a 106' tall by 32.5' diameter tank. It has a hopper that starts at 50' elevation and goes down 23' and has a 7' diameter opening. The tank has 22 W-shape columns that extend only up to the elevation of 80'. the thickness of the tank plate is 3/16" from top to about 80' and than it becomes 1/4" and goes all the way to the ground elevation. My question is:
Does the tank need the shell plate around the columns below the top of the hopper and if so what purpose does it serve? What loads does it experience? As it seems to me it is a non-structural skin. I need to know this in order to design an opening in the bottom portion of this shell.
I can provide a sketch.

 

[ishvaaag]

So you have a cylindrical silo with an inner discharge cone and as well a good number of columns. In my view quite likely the cylindrical plate below the upper part of the discharge cone must be there just to allow for lesser columns or something similar but then an analysis can say what you can do and what not. In any case even if strictly necessary you can find some mode of restoring the basic attempted general behaviour by properly framing around your opening. So make your 3D model and decide.

 

[chrislaope]

The shell plate below the hopper portion serves as cross bracing members in weak axis direction of the W-shape I beam column. If there are strong enough cross bracing members and the W-shape is properly selected to support the silo/hopper, then there is no need to have shell plate below the hopper portion.

 

[dhengr]

Dmitriy:
You need a sizable compression ring at the 50' level; at the top of the hopper, at the transition btwn. vertical shell to sloped shell, to accommodate that direction change. Below that level the shell material serves no good purpose other than adding to the wind loading surface area. It is not a particularly good lateral stiffening element, because it is not truly a plane surface which might act as a tension field, it will buckle out-of-plane. The columns may well have to be braced below the 50' level, but there are better ways of doing that btwn. the columns than with light plate. You say “shell plate around the columns,” you don’t mean the columns are inside the tank do you? You may design the compression ring to reduce the number of columns so as to accommodate a drive-through opening. Aren’t the 22 columns from the 80' elev. to the 50' elev. actually part of the tank shell design, in part to control tank plate buckling? And then, from the compression ring to grade the columns are truly legs for the structure to take both vert. and lateral loads?

 

[dmitriy555]

to dhengr
Yes, the columns are inside the tank. so they go from ground all the way to 80'. the silo has a drive-trough opening, with two moment frames on each end. from 80' to 50' I assume the serve as tank stiffeners. It seems to me that that the plate around the tank columns below 50' would NOT be a good bracing.

 

[chrislaope]

On the contrary, if you do detailed analysis, you will find the 1/4" shell plate on the hopper portion have a very strong bracing capacity, which will be in the range of minimum 40kips to 80kips bracing capacity. With the 2% bracing load rule, it means that the 1/4" shell plate can brace a minimum of 2000 kips axial load W-shape column.

 

[dhengr]

Dmitriy:
Is this tank already designed and built, and you are trying to do something with it; or are you in the process of designing it from the start? Aren’t the columns inside the tank a bit unorthodox, and don’t the add to the problem of stored materials hanging up in the tank during loading and unloading? You want the inside of the tank to be a clean surface, don’t you? And, you don’t want anything, including structural components, adding obstructions to this. You don’t want the stored materials bridging in the tank or hopper and you don’t want to retain materials on shelves caused by structural elements.

From foundation, at grade, to the compression ring at the 50' elev. you do need a rigid frame or a braced framework for the gravity and lateral loads of the whole storage structure. But, the hopper hangs off the compression ring which takes the lateral load components from the hopper sheets. The compression ring also supports the tank walls above it and allows you to transition from 22 columns/tank shell stiffeners to fewer tank legs in the rigid frame below. How did you arrive at 22 shell stiffeners? How many stiffeners would you eliminate if you went to a thicker shell? You can buy a lot of 1/16th or 1/8" thicker plate for the fab. and welding costs of a few stiffeners. Where are you designing this storage tank, what codes are you using and what reference materials are you using as a design guide?

 

[dmitriy555]

dhengr

The tank/silo is already built. We just need to design an observation opening around 10' elevation in the steel plate. That is why I wanted to know what purpose the plate serves on the lower portion of the tank.

 

[chrislaope]

That should be a quite simple job.

1. get the whole weight of the silo/hopper including its content weight (mineral ore? or anything else).

2. applying ASCE7 or any relevent codes to get the worst load combinations and find what exactly the W-shape is.

3. check if the braced W-shapes alone can support above calculated load.

if it can, then the 1/4" shell plate only serves as bracing member, and you can cut a hole as large as you want then replace back a cross bracing member with corresponding bracing capacity.

if it can not, then the 1/4" shell plate not only serves as bracing member but also provides some protion of axial load capacity. then after you cut a hole on the 1/4" shell plate, not only you need replace back a cross bracing member, but also need add some column with corresponding axial capacity.

 

[dhengr]

Chrislaope:
I think you are kinda mixing apples and oranges here as you apply your bracing load rules of thumb. I would like to see that more detailed analysis, since 40 to 80 kips is a very large range for any type of load. I am not doubting that the hopper plate might offer some bracing, but I doubt that a curved plate (Dmitriy’s cone shaped hopper) offers the same kind of bracing that a truly planar plate welded to four boundary members on its sides might, that is surrounding structure for a shear panel or tension field. The curved plate will buckle in an unacceptable way, and lose it’s bracing capacity. Your statements and thought process would likely be more true if you were dealing with a rectangular tank structure, truly plane plate surfaces, and straight stiffeners and compression struts, etc. I think things start to fall apart (maybe I mean buckle) when any of these elements are curved, out of plane. Maybe you can FEA this structure to the nth degree and prove that it will actually check, but you will end up with out of plane stresses and moments at every joint in the structure which will be very difficult to deal with. The biaxial and triaxial stress fields will kill you at every joint and at every weld detail.

Furthermore, it’s one thing to ask what bracing force do I need to keep this column or strut from buckling under an axial load, your 2% rule, give or take a couple percent; and quite another to ask what bracing force do I need to keep this 106' tall x 32.5' dia. tank structure standing on some 50' tall legs, under gravity loads, plus any lateral loads from wind and earthquakes. The tank design, above the compression ring, does involve some rule of thumb thinking, such as your 2% rule, as we think of the shell plate bracing the stiffeners/compression members, or is it the other way around, the stiffeners brace the shell plate?

 

[dmitriy555]

what sort of cross bracing? a horizontal brace from column to column? What if the opening will have a stiffener ring perpendicular to the tank plate? Will it serve as a load transfer mechanism?

 

[dhengr]

Dmitriy:
You finally get to the real question, maybe a sketch would have helped us get there quicker. I was trying to tell you about how I would look at this design from scratch, but you are looking at an existing condition and want to do something specific. Since the 22 columns and 1/4" shell plate are there, they will be loaded along with the moment frames at the drive-through openings, you just don’t know exactly how, without doing a complete analysis. Keep the observation hole as small as possible, stay away from the moment frames, don’t cut any of the smaller columns, radius the corners of the shell plate cutout and put stiffeners around the new opening to more than compensate for the lost material. Whatever load is in that shell plate from above, and you probably don’t know exactly, you want to provide a load path around the new opening and back into the plate below, while stiffening the edge of the opening cutout. Chrislaope’s approach sounds about right, short of a full blown analysis.

 

[BAretired]

Diameter = 32.5', so circumference = 102'. You have 22 columns spaced at about 4.6' around the circumference. I don't think you mentioned how the plate is attached at the bottom. Does it attach to a foundation?

I think the purpose of the 1/4" plate is twofold. It helps to support the weight of the stored material and it prevents torsional buckling of the structure. It is an essential part of the structure.

BA

 

[JStephen]

You could also assume that the shell carried the entire load and that the beams were there just to keep the shell from buckling. That would actually make for a simpler design, and it's hard to say how the original designers approached it. Regardless of how it was figured, if the beams are welded to the plate, then they ought to carry similar stresses when loaded, and you'd want to reinforce the opening one way either way.

There's not a lot of standardization for silos like that, or for the wind or seismic loading on them, so it wouldn't be too surprising if applying modern codes and methods gave you quite a bit different results than the original design.

 

[paddingtongreen]

If I was designing the columns to only prevent buckling, I would not run them down to the foundation. If they have baseplates and anchor bolts then they carry or share the load.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

paddington,

I don't think anyone is arguing with that.

BA

 

[chrislaope]

dhengr:

Please see attached document where my 40 kips minimum braced capacity come from. with the assumption that the 1/4" shell plate is fully welded (or bolted) to W-shape, the W-shape can be considered as fully braced in weak axis. The control factor will be in strong axis of the W-shape.

 

[paddingtongreen]

What has been worrying me is that I have never seen the columns go up inside the bin. I guess it was to maximize the volume in a given space but I would expect it to play hell with the flow characteristics. Inside the bin, the columns do nothing to ease the circumferential tension so I assume that it was to start transferring the vertical load into the columns. If the columns go down to the foundations, it should be possible to get the ratio of column and skirt bearing on the foundation as one estimate and see how it compares with the ratio of the actual steel areas.

I didn't realize that it was to be a small opening, I missed that. If it is only a foot or so diameter, I would cut it and simply weld some strips of steel from column to column, above and below the opening, enough to replace the cut out cross-section, and the same vertical between the horizontal strips and leave it at that.

Relatively small round holes in large tubes usually have no practical effect.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

chrislaope,

Your conclusion that the cylindrical shell does not contribute to the axial load carrying capabilities does not agree with "Theory of Elastic Stability" by Timoshenko and Gere. According to Equation 11-9 on p. 465 of that reference:

[σ]_cr = Eh/a(3(1 - [ν]²))^1/2

with h = 1/4" and a = 195" I find [σ]_cr to be about 22,500 psi.

BA

 

[chrislaope]

Hi, BAretired:

Your formula seems can not represent axial load carrying capability, because the axial load carrying capability has to be related to the 50' effective length. In your formula there is no parameter which reflects the 50' effective length.