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Silo Foundation
2

Silo Foundation

Silo Foundation

(OP)
I was recently asked to design a silo foundation. I have never done this before. The silo is an elevated cylinder that is supported by a steel platform, similar to the image below. It seems to me that whoever designed the steel platform should provide signed/sealed drawings for the platform, as well as reactions for the foundation design (similar to the process for designing metal building foundations). Is that right?

RE: Silo Foundation

Yes, that has been my experience with similar structures. I've done grain silos for breweries, material transfer system towers, and other industrial projects that involved "off the shelf" structures like these. They always came with an engineer package including reactions, anchor bolt size and layout, etc.

RE: Silo Foundation

Though for what it's worth I've always checked behind them to make sure I agreed. Nothing too rigorous, but at least a "I know most buildings around here have a 25psf design wind pressure, so what do I get when apply that" and see if it's close. If so, I go with their stuff, if they're really light I'll look a little deeper.

RE: Silo Foundation

(OP)
pharm - have you ever seen drawings from Phoenix Industries? I Googled the company. It looks like they are out of India. I am in the USA.

RE: Silo Foundation

Quote (Ben29)

I was recently asked to design a silo foundation.
Ah! Welcome to my weld. Ask me all the questions you need. I've designed plenty of outloading structures like the one pictured.

Quote (Ben29)

I have never done this before. The silo is an elevated cylinder that is supported by a steel platform, similar to the image below. It seems to me that whoever designed the steel platform should provide signed/sealed drawings for the platform, as well as reactions for the foundation design (similar to the process for designing metal building foundations). Is that right?
Ah! You seem to be assuming a perfect world. "Signed/sealed" isn't a thing in all jurisdictions and it certainly isn't a thing if the item is being imported (which it possibly is).

Yes, you should be chasing down the reaction for the foundation design if they are available. But if they are not you can compute them yourself if you know the masses involved, the seismic conditions and the wind. (I have to do this a fair bit myself.)

RE: Silo Foundation

(OP)
Nevermind, they are out of Arizona.

RE: Silo Foundation

I caution against using reactions that you have derived by some means. Even if you had the weights and geometry you would have to assume the structural configuration (where are the pins and fixitys) and also for seismic you would have to assume the R and Ω factors used for the design of the frame.

RE: Silo Foundation

You should be able to get those reactions... but sometimes for the amount of time that you could be spinning your wheels waiting on those, you could knock out your own reactions for it pretty quick.

Interesting - never seen a drive through structure with a truss perpendicular to the traffic side. Usually see extended end plates, or knee braces.

RE: Silo Foundation

(OP)
Right now I don't have any information except the height/weight of the silo and the x/y coordinates of the columns. And they want to pour footings tomorrow. bigsmile

If they didn't provide signed/sealed structural drawings for the silo structure, would they even be able to install it in the state of Virginia? I bet this client isn't even going to get a permit. He said it is going on private property.

RE: Silo Foundation

Quote (Ben29)

It looks like they are out of India. I am in the USA.
Well that confirms it. Even if you DID get reaction loads there is no way I'd trust them without checking them base on local codes and conditions. For a 4 column one way moment frame like this that shouldn't be too hard. The bulk of the load is product load in the silo. The critical design aspects is generally global overturning in wind. Generally I'd expect a raft slab or grade beam design unless you have particularly poor soils.

Quote ((Ben29))

Right now I don't have any information except the height/weight of the silo and the x/y coordinates of the columns.
You really don't need much else apart from the volume of storage and the material. Oh and I'd check the fixity of those columns. I presume they are pinned, but some designs use moment connections at the base.

RE: Silo Foundation

Bern - Phoenix doesn't ring a bell, sorry.

As for wanting to pour the footings tomorrow when they haven't even ordered the silo...well they're silly. The way I've handled that in the past is that I'll generate preliminary reactions and provide a preliminary drawing they can use for pricing - not for construction. If they choose to pour it without a final drawing it's at their own risk, and I explain that 1) the preliminary reactions will be based on assumptions that may or may not reflect the actual design of the silo (see diftLimiter's comment), 2) changes to the silo design between now and production could alter the foundation requirements, 3) anchor bolt layouts from these manufacturers are often not compatible with post installed anchors. I've had towers with anchor bolts that only work when cast in. The spacing was too tight to work with epoxy and violated minimum spacing. It was only after I detailed them to be cast in and developed with the foundation reinforcing that I got the anchorage to work.

RE: Silo Foundation

Yeah, I would tell them no to placing foundations tomorrow. As phamENG pointed out, you NEED that A.B. layout.

Something we have done with prelim drawings for foundations (for clients that seem to think "NOT FOR CONSTRUCTION" is only a suggestion) is to completely leave off the reinforcing call outs.

RE: Silo Foundation

As pham noted... I usually do a quick calculation for wind (not in a seismic area) and dead load to see if my reactions and those of the supplier are similar. Something similar, I've done a lot of 'dead end' structures for switching stations... and I still run the suppliers numbers through my spreadsheet as a quick check. My loads of generally about 10% greater, and I've not encountered one where the loading was significantly less.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

RE: Silo Foundation

Quote (And they want to pour footings tomorrow.)


Make sure that they do so at their own peril... pipe

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

RE: Silo Foundation

(OP)
OK - So I was being a little sarcastic when I said that they want to pour footings tomorrow. I mean, they do, but they can't pour footings until they get an engineer to actually provide signed/sealed foundation drawings. :) The concrete contractor is demanding it, and rightfully so.

RE: Silo Foundation

Usually, the silo designer would need to furnish those loads.
It's possible there are already two designers involved, one for silo and one for structural steel.
The design might have included partial fixity of the column bases, or might require seismic loading based on yield in the cross-bracing, etc., so not necessarily something to just jump into. They might or might not have included compression in those x-braces.
I think it's been pointed out elsewhere that a design approach may vary, but has to be consistent, so if you assume pinned bases and the silo guy assumed fixed bases, you may get some incompatibility.
A building permit is not always required, that may vary by jurisdiction. But it needs to be designed and built right regardless.

RE: Silo Foundation

I'm not sure there's a requirement in most construction contracts that drawings that they be sealed. They just have to be clearly identifiable. ponder

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

RE: Silo Foundation

Yeah, the desire to build the foundations before you know final loading is a strong one!

The company I worked for out of college specialized in "fast track" industrial projects. Which meant we were often putting foundations in the ground before final equipment / loading was known. If this was a "lump sum" contract, then the clients had to be willing to pay a big premium for this because the cost of re-work could be high. But, the key was that they'd get their industrial plant running 3 to 18 months earlier than if we waited for all the final information before building.

There are lots of tricks you do on the engineering side to make this work. You can use embed plates with nelson studs at the tops of your piers and then weld the steel base plates directly. You can make your piers a little oversized then used post-installed anchors.

My first project, we had already poured a large mat foundation when the operations people decided they needed a full stairway (rather than just ladders) to access the equipment. So, we doweled into the existing slab and poured an adjacent slab right next to it. A little inefficient as far as construction costs goes, but it saved a ton of time in the overall schedule.

RE: Silo Foundation

Yes. For projects I work on foundation are often poured before final loadings are known. So I'm used to calculating loads based on uncertain final designs.

It happened to me two weeks ago, I was on the other side of this situation. The client and foundation engineers were gearing up to start forming up the foundation for a silo when they only had approximate loads that were tender/quotation documentation. I had to supply loads for a silo whose design was not finished yet. I have done enough with silos that I have a spreadsheet for all this.

Quote (JStephen)

The design might have included partial fixity of the column bases, or might require seismic loading based on yield in the cross-bracing, etc., so not necessarily something to just jump into. They might or might not have included compression in those x-braces.
I think it's been pointed out elsewhere that a design approach may vary, but has to be consistent, so if you assume pinned bases and the silo guy assumed fixed bases, you may get some incompatibility.
Most/all of those unknowns can be readily surmounted IMO. Take the conservative choices. The only one I'd really try to get an answer for is the fixity of the bases. But that should be able to be determined by the base plate bolting pattern which should be much more easily obtainable. Even from inspection of that drawing (if it is accurate) I'd be comfortable calling it pinned. However you could always go ahead and design the foundation to take moment anyway. Like I said unless you have extremely poor soils you are going with ground beams or a raft slab so sorting out a stiff foundation isn't hard.

RE: Silo Foundation

Make sure to watch for uplift condition when the silo is empty. It may not be clear where the vendor included material weight so be sure they are clear on material weight.

RE: Silo Foundation

If you anticipate a large loading it is beneficial add supplement anchor reinforcing. Here is the reasoning. This is conservative and helps ensure you can "develop" the anchor forces at the anchor location. Secondly, this makes the contractor think a bit about where the anchors are located especially if you have cast in place anchors. Third, piece of mind since a couple #5 u-bars cost nothing compared to the equipment cost.

RE: Silo Foundation

Clients like to turn-key the structural supports for this kind of equipment because they think it will be easier to purchase the platform along with the silo. Simple, right?

We often find that the silo manufacturer does not have the expertise to design the structure, so either you get garbage loading from a mechanical designer using some off-brand program he doesn't understand or you have to wait for the sub-consultant the manufacturer hired to give you the loads. You are lucky to get anything meaningful in terms of wind or seismic loading. The equipment people say things like "sure we can design the structure - what seismic zone is this in?" I explain that seismic "zones" want away with the 1999 UBC and receive blank stares from the equipment guys.

I had an bin guy send me what he thought was a swell finite-element model of a bin support frame with a 20 kip column load. I explained to him that you can't just model one cell of a continuous frame multi-cell structure with shared columns and call it good. His column loads were at least 40 kips each but he simply didn't understand the concept of tributary area. Getting seismic calcs from him was absolutely out of the question.

In the end I told the Owner that his supplier didn't know what he was doing, did my own calcs for the column reactions, and designed the slab on grade accordingly.

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