Designing A Rectangular Concrete Foundation 2ft deep

[AlexGar]

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!

 

[JedClampett]

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.

 

[AlexGar]

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?

 

[ash060]

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.

 

[AlexGar]

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