Mat footing

[CROWN06]

I need to design a square mat footing. It has four concrete
pedestals symmitrical arrangement. Any idea how to find maximum moment and shear + other tips on what is needed to be done.

 

[Lion06]

Crown06-
This is my approach to a mat foundation like yours. I find the location of the resultant load from all columns with respect to the centroid of the mat foundation. From there, I use a P/A +/- Mx/Sx +/- My/Sy to ensure max brg isn't exceeded.
with Mx=the resultant load * the resultant ey; and
My=the resultant load * the resultant ex;
Sx=Section modulus of mat foundation in plan about horizontal axis;
Sy=Section modulus of mat foundation in plan about vertical axis.
Then I find the actual soil brg at ultimate (qu) at each column/pedastal location (using Pu/A +/- Mx*cy/Ix +/- My*cx/Iy). With that I would draw (4) beam diagrams, (2) in the horizontal direction and (2) in the vertical direction, with a trapezoidal load on the beam from the soil with supports at the columns/pedastals. I use the max moment/shear from the vertical "beams" to design reinforcing and thickness in that direction. Then do the same for the horizontal "beams". I check punching shear using the max column load (assuming of course that none of the columns are close enough to the edges to warrant a reduction in the bo dimension). Obviously, the thicker of the 2 values for shear would control.
Hope this helps.

 

[csd72]

This is basically just a flat plate upside down with the one exception that the load is not uniform.