Finding eccentricity from pressure distribution

[schmertdude]

I'm designing a 185-ft by 250-ft mat foundation. I know the area outside the mat is subjected to a -6.0 ksf pressure and the edge of the mat is subjected to a 9.5 ksf pressure for hurricane loading (P/A + Mc/I analysis). How do I find the eccentricity of the load on the mat? Thanks!

 

[JedClampett]

When you say a minus 6.0 ksf is that tension? Or are they both in the same direction (pressure)?

 

[schmertdude]

It's the surcharge of the soil outside the mat. If the mat is pushing the soil under it down, the soil outside it tends up (here, 6.0 ksf).