How to determine the thickness of mat foundation with only walls on top

[Graceli]

Hi, all

I'm designing a mat foundation for a building with only concrete walls (without columns).
I know we need to check beam shear (one-way shear), but how to check it? the width is the midline of the two walls?
Can you recommend any books that talk about the mat foundation design?

Thank you very much!

Graceli

 

[JedClampett]

It all depends on your wall loads vs. your soil allowables. I've got good soil and low buildings, so most of my mats are pretty trivial design. But if your wall load is 10 kips per foot and your soil can only take 800 psf, you need to have a mat stiff enough to distribute that load over 12 feet. As a rule, I would say most mats need to be able in take positive and negative bending, so that two layers of reinforcing is required. And usually that means a 12 inch slab.
I looked at a bunch of books and mat foundations are kind of neglected. I found a chapter on them (very technical with all kind of soil stiffness interactions) in "Foundation Engineering Handbook" by Fang and Winterkorn. Good luck in finding that as it's been out of print for about 40 years.
I guessing what most people do is to overdesign the mat thinkness, so that the soil loads are evenly distributed and design it as a beam, with the load upwards and the walls acting as the supports.

 

[Graceli]

Thank you very much for your reply!

 

[RFreund]

I realize I'm late on the this so I'll keep it brief:
The approach of most of the older foundation books is to use conservatism i.e. a thick mat, then assume that the Mat is rigid in comparison to the soils. Then in this case you will get uniform loads and it simplifies the structural analysis / design. However now we realize that the mat really isn't rigid and there would be higher pressures at columns (or walls). Basically the pressure depends on the deflection of the mat. Well there were / are several ways to model this but I'd say the most common is to use the modulus of subgrade reaction which is modeled as a spring (actually many springs below your mat). However the modulus of subgrade reaction really depends on the applied load and location of interest. So the modulus of subgrade is typically varied (i.e. lower value toward the center than at the edges).

References:
For new modeling descriptions:
Foundation Design Principles and Practice by Coduto

Older references:
Foundation analysis and design by Bowles
Substructure analysis and design by Anderson
Foundation Engineering Handbook by Fang and Winterkorn

EIT

http://www.HowToEngineer.com

 

[Graceli]

I agree with you. Thank you very much for your reply, RFreund!