combined footing, biaxial bending

[Yt.]

Hi,

I'm designing a small structure on clay soil. Two columns are close together (about 1,5 ft face two face) so i'm planning combined footing. Each column supports beams with very differents loads turning out in biaxial bending. I have some concerns about bending.
They are acting on opposite direction for each axis, i mean... column 1: +Mz, +My; column 2: -Mz,-My. Considering that columns are aligned in z axis, is reliable to design that foundation? Will torsion be an issue due -Mx, +Mx? How to manage it? is reliable to apply some warping between moments to get soil pressure? My loads are not really high.

Any guide, thx

Note: Also posted in foundation topics but no replies already.

 

[KootK]

I'm happy to help but I'm not willing to invest the time without a sketch showing:

- plan view.
- axes
- moments and shears
- torsion

I think that I know what you're getting at but I'll not waste my time guessing.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Yt.]

xD, please Kookt, I wasn't atacking you

 

[KootK]

I wasn't feeling attacked. I was feeling like this might be an interesting thread to help out on if additional information were supplied.y

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

So is this two columns on one combined footing or a bunch of column pairs on a larger mat footing?

For the design of the soil pressure, bending, and one way shear, I would think that it would be sensible to treat it as a single point load imposing combined, net actions on the footing. The tricky bit is evaluating the mechanism by which opposing actions cancel one another out locally, near the columns. There would be a torsion in the footing concrete between the two columns that traditional methods would not address. Strut and tie would be appropriate but 3D models are time consuming. I'd ensure that each column to footing joint works in punching shear when considered alone. And I'd make sure that punching shear also works with the combined, net loads on a combined punching perimeter. I feel that would be sufficient in this case.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Yt.]

Thanks Kootk,

I would be able to manage the loads combination, but only by assuming rigid behaviour of the footing. It may be the case because my loads are small at this point of the structure, but torsional cracks must be prevented because they may change the assumed stress distribution.

Some thing like this is also said here:
Link

I'm thinking to place a strap beam between columns to trace a path between opposite moments, but i'm not sure if it will be rigid enough to "unload" the footing and carry all the torsion. I'll give it a tough

 

[rapt]

Ytyus,

Can you please stop posting the same thread on multiple forums.

Sounds to me like there will be a lot of torsion involved if you combine the footings. Is it possible to combine the columns!