Mat Foundation Question 2

[sach4u1]

I have a mat foundation below 3 piers located in a triangular pattern. I need to find minimum depth and steel required for the loading shown in the sketch. Any help would be great help. Thank you.

 

[Ron]

Pardon my open aggressiveness, but....

Are you nuts? If you are a structural engineer, this is your job. Don't ask for the answers in a technical forum. If you don't know how to do this, why would you trust the answers from total strangers who have nothing but a sketch?

If you need guidance on the approach to solving an issue or if you want some interpretative guidance, there are thousands of us in this forum who are willing to help. However, we are not here to do your work for you.

 

[hokie66]

Agree with Ron. This sounds like homework to me. If you are a student, posting classwork assignments is not allowed on this site.

 

[MikeDB]

Here is a link to where you can download bulletin 1724E-300 "Design Guide for Rural Substations". Beginning on page 483 is a design method for spread footings subject to moments. I think it will address some of your concerns and perhaps clarify it for your.

http://www.usda.gov/rus/electric/bulletins.htm

 

[sach4u1]

Ron,

Just by posting a thread saying you are not allowed and all that crap, if you know how to solve this problem then let me know. You are not someone who tells others what to post.
I am a structural engineer and the only thing I deal with is steel. I hardly do any foundations. This one was critical and thought people here will give me some approach like Mike DB did.

hokie66,
If I were a student I would ask my professor.

Does this make sense to you both?

MikeDB,

Thanks for your reply. I hope it will take me somewhere.

 

[Ron]

sach4u1...you are new to the forums, but yes, all members have the right and expectation to tell others what they can post...it's part of the process of participation in the forums. It's also one of the reasons the Red Flag link is available. I chose not to use that for your post.

I did not consider your post to be a homework problem. From your wording of the post, it appeared like so many others we've seen in the forums....just give me the answer and I'll go away. My apologies if I misunderstood your intent.

As I think you understand, the forums are for professional interaction...an opportunity to share information, to bounce ideas around, to validate assumptions, to agree, to disagree, and the ultimate end result is to grow professionally and enhance our profession.

Now that I understand your intent a bit better, I'll offer my opinion on the approach to solving your problem....

If you do a lot of steel design, you probably have a finite element analysis program available to you. You can model the mat as a concrete plate element. You can model the soil as springs. Another piece of information you'll need that I don't believe was shown in your sketch is the modulus of subgrade reaction (K).

Another method you can use is to do your analysis upside down...your piers or columns become supports for a concrete beam, uniformly loaded by the soil and the other imposed loads. This will be good to use as a check for any other method you might employ.

Since you are in the US, use ACI 318 as your basis for the factors required in the analysis. It is an LRFD method, similar to what you've used in steel design.

Many foundation design texts offer reasonably good treatment and examples for design application. (Check "Principles of Foundation Engineering" by Braja Das, for example).

Mat foundation analysis can be detailed and difficult or treated simplistically and conservatively (not always right to do this!). In any case, I would suggest that once you've done your analysis and think that you have your design established, that you run it by a more experienced engineer, locally, to check the things that we as a group would not have the time nor the information to do properly.

Good luck and welcome to the forums.

 

[SlideRuleEra]

sach4u1 - I'll add a couple of my observations to the excellent outline that Ron has presented:

For a heavily loaded mat foundation making the mat "rigid" can be just as important making it "strong". Stiffness in needed so that the load is t applied uniformly to the soil. If the mat is NOT rigid then soil under the columns will have higher loading than the area at the perimeter.

Almost always the most cost effective way to make a plate more rigid is to make it thicker (than it has to be to meet strength requirements alone). A general-purpose software analysis can probably indicate this, but the correct interpretation of the results may not be obvious. IMHO, this is why a conservative design approach can still be considered a good check on a precise solution or even a viable design itself.

Also, if this is for an industrial project cost issues may not be as critical as they are on commercial jobs. This is for a variety of reasons:

1. Industrial loads may not be as well defined, then a conservative approach is warranted.
2. Use of an industrial structure often changes over time resulting in additional loads not originally known.
3. Industrial structure are often built with the INTENT that the original Owner will BE the Owner for many decades. Since the bulk of my experience has been as an Industrial Owner's Representative, I can assure you it is comfortable feeling to know, as time goes by, that unexpected events / loads will NOT cause problems at an inopportune moment.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[csd72]

also check for uneven loads - maximum loads is not always critical in these situations.

You will get a much better response if you indicate that you have at least tried to research it yourself. This forum is much better for specific questions rather than general ones.

 

[oldestguy]

My two cents.

The two above (Ron and SlideRuleEra) have demonstrated on this forum from what I have observed that their advice is based upon years of experience and is very practical.

They gave great answers in my opinion. You found two of the best. Leave it at that.