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Pier/footing design

Pier/footing design

Pier/footing design

(OP)
Hello,

This seems like a dumb question to me but I'm trying to minimize the reinforcing in this pier beam design. The government client has set the parameters of what you see in the attached section I've drawn. There will be a rigid frame column sitting on this beam and I've included the worst load combination for thrust at the top of this beam. I understand how to check stability against overturning. I understand that the beam is being twisted to the left thus producing tension in the right face etc. I typically design true piers to spread footings and this is a hybrid for me and want to make sure I'm attacking it correctly. There is no soils report but I'm assuming 2000psf allowable bearing. Also not sure how minimum steel requirements per ACI affect this structural element? So, in summary, how would you design this member?

Thanks,

Dwayne

RE: Pier/footing design

There must be more to the story here. Is there a slab on grade that can resist the thrust? Maybe piles or footings in the foreground? What you've got in the sketch, on its own, does't look promising.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Pier/footing design

Run a tie beam to the other end of the frame. Otherwise you'll end up with a monster of a footing. Run a few #6 bars with a healthy splice to counteract the thrust.

If designing the pier without a tie beam. It may act as both a column and a beam, adjust phi factors accordingly. Minimum steel will vary as well.

RE: Pier/footing design

(OP)
Hey guys,

This is the boiler plate section this U.S. State uses for all its salt/storage facilities. There are no left out elements, this is what they use. They have dozens of buildings like this and we are trying to break into the market there. I'm not holding out on you, this is what they use and my unfamiliarity with this type of foundation prompted the question.

RE: Pier/footing design

This must be a state where they never get snow loads, and winds are light. In other words, they have never had a situation where any significant thrust was developed. First thing is to worry about the stability of the system, not the reinforcing. Good luck getting this to work. Not a good idea to repeat someone else's mistake. Ask them to justify their detail with supporting calculations from a previous project.

RE: Pier/footing design

Ok. What are the dimensions of the shed? What is the frame spacing? Are you sure that the frame isn't stiff enough to be viable without utilizing foundation thrust? I'm imagining something small here.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Pier/footing design

(OP)
This building is 120' wide and has a bay spacing of 20'. This is in snow country and has 90mph winds. They pushed this beam down 3' to frost.

RE: Pier/footing design

A tie beam would be the best if possible. How much of the footing to the left and right of the column are you depending on to get your bearing to work?

RE: Pier/footing design

(OP)
I did a quick calculation on this footing utilizing about 8' of the length at each column and it is still unstable. I'm not comfortable using more than that because the beam is only 7'-6" tall, even though the building columns are 20' o.c. I guess what I'm hearing from you guys is this is a bad situation?

RE: Pier/footing design

Yes. If you are comfortable using an 8' long footing, then determine how wide it needs to be to be stable. I dont see why you need a 4' wide footing all around this building, design a pad footing at the column, then perhaps a 18" wall around between the pad footings.

RE: Pier/footing design

I just calculated out a no-tie footing for a 120' wide, 30' bay spacing building in the upper Midwest last week. Ended up with a 12' long (direction of thrust) x 7' wide x 3' thick block of concrete to resist the forces. Even with that much mass, I still had to include dead weight from the continuous footing for stability.

Going by that, I really see no way that the pier that you've shown can possibly hope to resist the imposed forces.

Also, the 16.2kips of thrust looks light for a 100' span with 20' bays - this, however, is just my opinion.

RE: Pier/footing design

(OP)
ztenguy - The shape of this footing has been dictated to us by a DOT department. I don't have the option to change the width of the footing, it was their requirement. The 4'x7'-6" footing/beam is continuous all the way around the building. It is their design.

RE: Pier/footing design

I could see someone (not me) pulling out all the stops and convincing themselves it could work if not for the 4'-6" grade step. Just looks like a bad idea, made worse with that step.

RE: Pier/footing design

(OP)
Winelandy- I guess I getting the picture that no one likes this footing. The reactions are accurate, we have Metal Building Software that calculates these forces and this is due to DL+CL+SL, the reduced for slope snow load is 14.3 psf.

RE: Pier/footing design

14.3 psf?

I doubt that shed is heated. Ct = ?

If it's government, they may just want it this way. Design a big ass footing.

RE: Pier/footing design

I think everyone is leery because that 3 feet of backfill on the left side is doing nothing to help. By the time the footing were to move enough to engage the resistance of the soil it would allow for too much lateral movement at the top of the wall.

Given the loading and geometry shown (assuming the whole vertical reaction can be used to resist overturning) I figure you need approximately 8' of footing length to provide enough overturning resistance (and that doesn't include any safety factor). Are you comfortable saying that 8-10' of the footing length will contribute to overturning resistance before the top of footing moves laterally? I don't think I am.

RE: Pier/footing design

I would change the reinforcing in that massive beam a bit, but that 4.5'x7.5' beam can do a lot of good. I did some scratch calcs and sliding and overturning do not look as bad as I thought they would when I make use of that beam. But once I add in the soil loads from the grade step it blows up on me, and that doesn't account for any surcharge in the building.

RE: Pier/footing design

For effective width, I'd be willing to go 15'. I still don't like it though. Same gut feeling as Jayrod.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Pier/footing design

(OP)
dcarr82775-

What I have is Movert=16200x7'-6"=121,500ft-lbs. disregarding soil pressure and surcharge loads like you stated.
Mresis=23,200 x 4'/2 + 4'x7'-6"x8'x150x4'/2=118,400 ft-lbs. (used 8' of beam)
F.S.=118400/121500=0.97

Thus factor of safety is not even 1.0 and I was shooting for 1.50.

RE: Pier/footing design

And the surcharge loading due to the salt/sand being stored in the building is a considerable number. and the machinery to move it etc.

Ask the State's reviewing engineer's to seal it for you. And then demand calculations that back up their design.

RE: Pier/footing design

(OP)
jayrod12-

I guess I will go back to the salesman and tell him the footing is wholly inadequate for this application and let him decide what he wants me to do.
Thanks guys

RE: Pier/footing design

If its the states deisgn, why are you designing it again? Put on the drawings footings by others, and move on. Write a letter with your drawings saying you dont feel its adequate.

RE: Pier/footing design

If you reinforce that beam better I don't see any reason you can't use the full 20-ft width (or 15ft for that matter). At 15ft I get dead on 1.5 F.S. for overturning. Don't know enough about the soil to wager a guess at friction but it looks like it could be shown to work. But again, once you add in the grade step it blows up.

RE: Pier/footing design

(OP)
To give you more of the picture; we are the building supplier who very often design footings because the owners are too lazy to hire separate SE's to do the foundation design. I am an SE and get pulled into designing these foundations more often than I would like. This section was put out by this DOT with no reinforcing shown and they wanted a full package bid, footings and building, therefore, once again I'm pulled into designing a foundation when I'm only interested in selling the building. Not that I don't care if the footing is inadequate I do, it just that there are dozens of consulting firms in their area who could nail this down for them.

RE: Pier/footing design

(OP)
Here is another section of that footing I drew with more detail. I didn't send this one earlier as we SE's tend to get distracted by all the questions I'm not asking. :) This would eliminate surcharge loading but I still would tend to put some soil pressure against the footing on the right side, still a no go as far as I'm concerned. I really don't want to rely on the footing being effective more than 5' either way down the sidewall from the point loads. Even the 12" slab with the reinforcing was dictated by the DOT.

RE: Pier/footing design

All the above comments make sense, except doesn't it bother anyone that there is no soil info for this serious building. To me, the fact that everyone's assuming some tiny 2000 psf and coming up with huge footings is most probably a very expensive assumption. I would recommend to his govt client that they perform four borings, and based on on what is found, then design the footing. BtW, if the surface soils are poor, the proposed tie-beam, or what I'd call a sub-grade strut might settle and induce unanticipated moments, movements etc.
also, the massive nature of the pier itself will probably cause excessive settlement and induce moments into the frame if there are in fact weak soils present. That's my two cents.

RE: Pier/footing design

Sorry to be off topic but, I find it a little comical that you A) work for a metal building mfr and B) are being asked to design the foundation for this building. The thrust reactions at the base of these structures is sometimes difficult to resist. I sometimes get frustrated with the fact that the metal building mfr's don't really care what the reactions are at the base of the columns because after all, that is something someone else can worry about.

RE: Pier/footing design

(OP)
SteelPE-

Well, I sign both building drawings and foundation drawings so my concern is real. I can't say my concern is the same for a foundation by others but when my stamp is on the drawing, I do the work required.

RE: Pier/footing design

With that new detail I could see it working with the grade step....maybe

Seems pricy to buy all that concrete when it could be done with more efficient layout, but forming and multiple pours can easily push to costs up to where the mass concrete solution becomes workable. I also agree it seems stupid to do all this without a soils report of any kind.

RE: Pier/footing design

(OP)
Thus the frustration of working in a bid environment. There are enough guys who will assume something and propose something and get the project with little regard for the soil until the excavation starts to happen. I've been an SE for 30 years, 11 of it in consulting. I would say less than 50% of the foundation projects I've worked on ever had a soils report. The most important element in my opinion, the foundation, is the one owners most likely will make assumptions and cut as much cost from as possible.

RE: Pier/footing design

You were withholding information in your original post! Tsk..tsk..

Any way to tie the grade beam to the 12" slab on grade at the columns?

We could use a PEMB expert on this forum. I hope that you'll come back to help us out with our PEMB issues from time to time.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Pier/footing design

(OP)
ha ha
The precast panel is continuous, this DOT will never let tie beam penetrate the panel. I've been asked to come up with a new solution, crap, I'm a steel guy, not a concrete guy anymore.:) I don't even know if the DOT will accept my design as it varies from their standard detail and this is all just a bid. Some other SE, might think this works just fine as shown.

RE: Pier/footing design

Comes down to you have to sign and seal what you are comfortable with. Otherwise, here is our building, we are not designing the footings for this one. Sometimes its best to move onto another job.

RE: Pier/footing design

"Some other SE, might think this works just fine as shown"... not if he's got a brain in his head. Face it, this detail will not work if you get any kind of significant thrust. The only reason it's probably worked up to now is the buildings have never been loaded anywhere close to design loads. Same reason why I see architects "design" 3' or 4' square thickened slab footings under 100' clearspan frames that apparently "work"... oh, and no hairpins or tie rods to boot.

RE: Pier/footing design

KootK

There is a Butler Manufacturing Company expert who monitors this site and does respond on some PEMB issues and some Butler issues. (Not I).

He has been very helpful on several occasions with information about Butler products and Butler design.

Jim H

RE: Pier/footing design

Huh, great to know. Do you remember this fellow's handle? I've wanted a complete version of the old Butler engineering manual for years. Thanks Jim.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Pier/footing design

Spats,

If you take out the step in soil, it calcs out for the loadings specified(not enough known for sliding check). I am willing to bet the step is not what this typical detail was designed for. Not how I would do it, but I think it works under certain circumstances.

RE: Pier/footing design

@ Dcarr82775, would you feel comfortable signing it?

RE: Pier/footing design

I would for the non-step condition for which I suspect it was originally developed. It is a stupid amount of concrete oriented in an inefficient way. Not how I would do it if I had my choice, but I can see how it works. As I said earlier I would change the rebar a bit and would need to know a bit more about the soil, but without the step I can't see why it wouldn't work. The interior precast bins are an interesting wrinkle. Depending on their construction they could eliminate most, but not all of the soil load from the differential soil giving it a glimmer of hope for the condition shown. Would I stamp the as shown condition nope, but with some not so major changes I could see it working

RE: Pier/footing design

dcarr - the poster said earlier "They pushed this beam down 3' to frost". This leads me to believe they know there is an intended step down. However, it is still possible that it was not the original design intent. He also said "they have dozens of buildings like this...", which is a scary thought. The key may be "I guess I will go back to the SALESMAN and tell him the footing is wholly inadequate for this application". Are you talking only to a non-technical person at DOT dmoench01? That may be the problem.

RE: Pier/footing design

KootK,

Do you mean the Butler/Computerized Structural Design, Inc. developed "Foundation Design and Construction Manual 2ed" from 1983/1984 or the Butler "Architectural & Engineering Manual" (much more recent but less engineering and more architectural) developed from the older "Product Reference Manual".

Jim H

RE: Pier/footing design

(OP)
Spats-

Actually our salesman finds projects like this to bid, in addition to the non-bid market we service. Most often these bids are building only but this was a foundation and building together as a bid package. Our salesman didn't dictate anything regarding the footing, it is just the approved DOT section they want to use. My final result was to redesign this section so it is workable and hope the DOT recognizes the shortcomings of their foundation section when everyone submits their bids. Our salesman is the point man on the project thus my reporting to him on the problem. Historically when we do things like this there is always at least one engineer involved on the DOT side. Whether they choose to take our suggestions, we will find out.

RE: Pier/footing design

Jim,

The Computerized Structural Design one from the 80's is what I had in mind. In 2004, someone emailed me a scan of an old fax. It contained eight, very useful, pages of the manual. Based on the page numbers, it appears that there are at least 66 pages out there somewhere to be had.

Like SlideRuleEra, I'm a bit of a curator of antiquated structural engineering reference material. The Butler Foundation manual has been on my bucket list for a long time. If you know how I might obtain a copy somehow, I would be grateful to hear about it.

KootK

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Pier/footing design

Sorry dmoench01... I thought you might be talking about DOT's salesman.

KootK & jimstructures: Regarding your little side discussion, I have a copy of "Foundation Design and Construction Manual", but it's the first edition. I was with Butler in the 70's, and got to meet with Jim Fisher when he was working on the manual... a little trip on the wayback machine.

RE: Pier/footing design

KootK,

About 125 to 130 double sided 8 1/2 x 11 pages. I do not have a scanned copy. 250 to 260 separate page sides in all.

We really need to move this to a separate thread.

Jim H

RE: Pier/footing design

If this is a salt storage shed, there will be lateral forces from the salt storage occurring above the floor.
  • Are the precast panels acting as retaining walls to contain the salt?
  • If so, are the panels relying on passive pressure from the earth outside the precast panels?
  • If not, what is the purpose of the precast panels?
  • Are the precast panels tied to the 12" slab?
  • What is the bulge at the bottom of the precast panels?
  • How do the precast panels terminate at the bottom?
  • Is there a concrete slab or footing under the precast panels?

  • The overall concept is not clear.

    BA

    RE: Pier/footing design

    Good observations BA. The rest of us are getting lost in our own side thoughts.

    RE: Pier/footing design

    (OP)
    To BAretired

    If this is a salt storage shed, there will be lateral forces from the salt storage occurring above the floor. yes absolutely, the panel projects another 12' above the floor
    •Are the precast panels acting as retaining walls to contain the salt? yes
    •If so, are the panels relying on passive pressure from the earth outside the precast panels? Normally when I see panels like this they are not embedded in the ground, they are completely above ground. They are self-stabilizing. Because this one is below ground it can't help but put surcharge load behind it because I know when they compact the soil behind the wall the loads will transfer through.
    •If not, what is the purpose of the precast panels? see previous
    •Are the precast panels tied to the 12" slab? no
    •What is the bulge at the bottom of the precast panels? Not sure what you mean here.
    •How do the precast panels terminate at the bottom? These are standard L or T interlocking panels you see in these types of buildings and grain or fertilizer storage facilities as well. They have a flat bottom.
    •Is there a concrete slab or footing under the precast panels? No, whether above or below grade, most often these type panels sit on grade and a poured slab is placed around them so there is no lip inside the building to catch the loader bucket on.


    The overall concept is not clear.

    I pulled this from a standard DOT detail for which they have dozens of these buildings. I understand what they are trying to accomplish, it just doesn't work at all on paper, thus I've proposed a different detail.

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