Braces buried in bulk storage bins

[Schambach]

On a bulk storage fertilizer containment building with 16' tall concrete walls I'd like to brace the tops of the concrete walls to not have to design pure cantilevered walls. The material will actually be stored HIGHER than the walls so a brace at 16' would be embedded in the stored material. My concern is not the corrosion aspects of what fertilizer against this brace would be (though that's an issue) but rather the "fluid" pressures that would act on this brace as it is an obstruction to free-flowing material as the piles are created and/or removed from the bin. Anybody have experience with this? Round tubes better than square shapes? Can I look at the forces on these braces as a uniform load equal to the height and density of material above it? Any other suggestions or concerns?

Thanks.

 

[CANPRO]

What is the shape and dimensions of this building in plan?

 

[Schambach]

This is a rectangular shaped building that is 80' wide and 90' long with five bins that are open on one narrow end and closed at the other. Each bin is 18' wide x 60' deep and 16' tall. This leaves a 20' loading area under roof. On top of the concrete walls will be a wood framed/wood sheathed wall that will allow the raw material to be stacked higher than 16'. That is the reason for my concern.

 

[ajh1]

IF there is to be vehicular access into the bins to fill or empty them, be sure that your braces will not interfere with the equipment. Once the material is in the bin, nobody is going to be able to see the brace to avoid running it with the equipment.

 

[WARose]

I'd pick a angle of repose and consider the brace laterally/vertically loaded on one side only from that active pressure (along with your [axial] brace load). Not sure what that angle would be though.

 

[jayrod12]

Personally I would probably treat it as a true fluid, it would have pressure from above and the side equal to the depth above the brace. It would be overly conservative, but it's quick and I know the brace would be plenty strong enough when all said and done.

I could see an argument for designing the brace for almost no out-of-plane loading as well. So this likely would be a judgement call.

 

[Schambach]

Thanks. Braces will be above the equipment so that won't be an issue.

Angle of repose is around 30 degrees for the material I'm using so I can figure that active pressure Ka.

The reason I'm concerned is that the owner of the firm I worked for years ago had used cables on a really large grain storage facility thinking that the grain would just "flow" around the cables as the "bin" was filled up and he found out that it didn't. Tore that building apart............and I'd like to not repeat the same mistakes that he did. Owner has been out of the engineering world for a long time now and I'm not sure he can answer this sort of question.

In the forum's professional opinion, does a smaller diameter rod that will not have any strength to resist lateral load (but allow for better flowage) be better or worse than a tube perhaps that could be designed to handle the lateral loads (but has a larger tributary area?)

 

[WARose]

In the forum's professional opinion, does a smaller diameter rod that will not have any strength to resist lateral load (but allow for better flowage) be better or worse than a tube perhaps that could be designed to handle the lateral loads (but has a larger tributary area?)

Only thing about that scenario that worries me is the P-Delta effect. If it's flexible enough to be easily bent....and it still has to handle axial load.....that could become an issue.

To me, a decent sized HSS is probably the answer.

 

[Schambach]

Thanks WARose. I'm looking at the load being carried only in tension on these braces. If one bin is full and the bins immediately beside it aren't, then the bin braces where the material is container are in tension. It could be argued in the empty bin beside this one that brace could go into compression, but there would be no material by which to load it laterally. Am I thinking about that right?

 

[WARose]

I'm looking at the load being carried only in tension on these braces. If one bin is full and the bins immediately beside it aren't, then the bin braces where the material is container are in tension. It could be argued in the empty bin beside this one that brace could go into compression, but there would be no material by which to load it laterally. Am I thinking about that right?

I don't know that I'd count on that scenario. I've done storage bins (adjacent to each other) and when you look at them in practice.....you see all sorts of scenarios that could hit you here. I could see the brace being loaded vertically/laterally in spots....yet not putting much pressure on the adjacent wall. (I.e. a pile/pyramid shape on top of the brace (or part of it).) And then (maybe) on the other side of that wall....it gets full active pressure.

Ergo you wind up with your brace being vertically/laterally loaded [red]and[/red] getting compression. I'd just design for the worst possible case.

 

[SAIL3]

I have not designed a problem similar to this, but, here are a few thoughts that come to mind.....
at rest...vert and lateral load on brace combined with axial load in brace...
dynamic....dynamic impact load during flow...any chance of vibration?...fatigue issues?..
erosion of brace wall...depending on the type of fertilizer material...
corrosion as mentioned...
round pipe should offer less obstruction than square....
a sketch would help...

 

[WARose]

@Schambach: In my last post, I did not adequately address your point (as I understand it) about the braces never being in compression. See the attached pic. In "Scenario 1", you've got braces on both sides.....so theoretically the brace in the right bin will go into tension right (and take all the load)? Well, I wouldn't be 100% sure. What about if the slab uplifts, the connection of the brace to the wall/slab displaces a bit (through prying action), the concrete anchors give a bit (in the slab/wall concrete), etc, etc?

In such cases, you will wind up with the brace in the left bay going into compression whether you want it to or not. And then with that pile on it.....you've got some moment to consider as well.

Probably the best way is to do it like "Scenario 2". And note no material under that brace in "Scenario 2". (Which is how I should have drawn it in "Scenario 1".)

Not sure how you were envisioning doing the braces.....but this is the first thing that came to mind.

 

[patswfc]

Schambach, a quick sketch would help to clarify your proposal. From reading the thread i was thinking that your plan is to tie the tops of the walls together with horizontal braces, opposed to WARoses sketch with diagonal props down to slab level.

 

[Schambach]

Here is a sketch as to what I'm envisioning. The fertilizer of course could be in every bin or every other or any other combination and at various levels in each. I'm contending that the braces either go into tension OR if the building was completely full, the forces on the walls each cancel each other out with the exception of the outside walls.............in which case the brace will be in tension. I recognize that I have external suction pressures on the wall as well due to wind loads.

Thanks.

 

[WARose]

Where is that brace on the right side going? To another wall (off paper)? Assuming it's going to a stiff support.....I still see the chance that brace could get put into compression. You could get it (while filling) more heavily loaded on that right hand wall than the left.

Outside of some thermal contraction or lateral event....I see what you are saying about it (i.e. the brace on the left hand side) never being in compression and having lateral load on it simultaneously. But I wouldn't do it that way .

 

[Schambach]

All braces are connected to tops of walls. There is nothing that they are connected to that we could say is perfectly pinned. My rationale though is that as a bin is loaded, the forces are equal and opposite in both directions. I know that wind load will affect the end bays and that will be accounted for. Therefore, I don't think I care whether or not the braces are pinned to an immovable object (like the floor for example.)

I believe, and I stand to be corrected, that regardless of the multiple combinations of filled bins that are possible on this 5-bin project, it doesn't matter which is filled or empty. If they're filled, the forces are equal and opposite and will counteract each other. In other words, regardless of how the bins are filled, the net lateral forces on the entire structure are zero from a bulk fertilizer standpoint. If every other bin was filled, the two intermediate ones could experience some compression in their braces equivalent to the amount of strain in the tensioned braces. But at that point, there is no fertilizer to load that brace out of plane.

Am I making sense WARose? I appreciate your dialogue.

 

[WARose]

I think I am following you now. I still think there is a possibility of one of them being thrown into compression. Perhaps if a few in the interior were loaded unequally (on one side; the same side) the end bay brace could get thrown into compression? Not sure I'd take that chance.

Furthermore, what would you really be getting by doing this (even if it did work)? You'd cut down on some of the reinforcing steel at the bottom of the wall.....but the shears would still put you into having just about the same wall thickness.....and you are likely driving up the necessary horizontal steel.

Me personally, I'd just do it without the braces.

 

[WARose]

I guess my curiosity got the better of me.....so I made a crude model of this in STAAD....with about 3 braces per top of wall. (See below for a pic.)

One thing I tried was to load 3 of the interior walls on one side and in only one direction (with the other walls unloaded). And what happened was the braces in several bays went into compression as the wall shared the load based on flexural stiffness. I didn't bother modeling the side walls because the results will probably be the same. (Especially for the middle brace.)

So in conclusion: I'd just forget about a tension only brace.

 

[Schambach]

WARose,

Thanks for challenging my assumptions. With an angle of repose 28-33 degrees, it IS possible to fill an 18' wide bin with some one-sided loading. The bins are loaded out from the ends so a really lop-sided pile configuration could only occur during the loading of the bin. I'll have to check on how that is done.

Worst case you could have about 10' higher on one wall than the other given an 18' bin width. That amounts to around a 3,200plf load differential in one direction..............so..........if even one bin was loaded as such, the adjacent brace would go into compression.

I'm thankful you've taken this thread in a different direction as you're exposing to me a load case that I hadn't considered.

I appreciate your help.

 

[WARose]

I appreciate your help.

Anytime.