Pier/footing design

[dmoench01]

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

 

[dmoench01]

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

 

[ztengguy]

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.

 

[dcarr82775]

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.

 

[dmoench01]

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.

 

[dmoench01]

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.

 

[SolidPE]

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.

 

[SteelPE]

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.

 

[dmoench01]

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.

 

[dcarr82775]

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.

 

[dmoench01]

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.

 

[KootK]

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.

 

[dmoench01]

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.

 

[ztengguy]

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.

 

[spats]

"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.

 

[jimstructures]

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

 

[KootK]

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.

 

[dcarr82775]

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.

 

[ztengguy]

@ Dcarr82775, would you feel comfortable signing it?

 

[dcarr82775]

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

 

[spats]

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.