mat footing
 

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