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Designing A Rectangular Concrete Foundation 2ft deep

Designing A Rectangular Concrete Foundation 2ft deep

(OP)
Hello,  
  Looking for some help. I am designing a foundation that is 2' x 19'-9 1/2" x 11'-9". The structure that sits on the foundation is rectangular as well, 6'x 14'. I am having trouble developing the moment diagrams for the longitudinal and transverse sections since the only support is the soil pressure. I need the max moment value to find how much rebar i will need. The structure has a wind load of 2.5 kips at 16' above the foundation. It is perpendicular to the longer side of the structure and centered. The other load is a gravity load of 2.5 kips, which I centered since the structure is uniform. Oh, and the structure has 4 columns(4" std. pipe) which sits on the foundation. How could I simplify this to get my moment values?
    Thanks,

     I am a fresh out of college and just gaining experience, any advise will be helpful!

RE: Designing A Rectangular Concrete Foundation 2ft deep

This is a simple problem, but it's not easy to walk someone through it in a forum like this. Basically, you figure the reactions on the equipment supports, convert them to a soil pressure, and calculate shear and moments at critical sections. Soil pressures are likely to vary based on the layout and loads. It's a merger of the principles of foundation design and concrete design.
Is there a more experienced engineer there you can ask? I know it's a little embarrassing, but believe me, it's the way we all learn.

RE: Designing A Rectangular Concrete Foundation 2ft deep

(OP)
Hello Jed, thanks for the reply.

   Well one of the experienced engineers told me to assume a triangular soil pressure distribution, with that I took the moment about the edge of the foundation to find my values of soil pressure.I also compared the soil pressure I calculated to the allowable (1200psf). I also asked how to simulate the soil pressure as a support in order to develop the diagrams for shear and moment and he told me to put supports at 1 foot apart from each other throughout the width of the foundation. Wouldn't it make it like a indeterminate beam?  

RE: Designing A Rectangular Concrete Foundation 2ft deep


Since you have four supports I would treat it as a combined footing in both directions.  If your supports are centered on the foundation and the loads are equal you would have a uniform load distribution.  If they are unequal than you would have a linear varying distribution.

Find the c.g of you pier loads and that is the location of you soil reaction, than turn that single load at a location into a distributed load that would produced the same load and moment.

Like Jed explained it is hard to discussion on the forum, but any good soils book should have an example.

RE: Designing A Rectangular Concrete Foundation 2ft deep

(OP)
Hey guys, thanks again for the help. Found some information in a book I had.

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