Determining resultant e for soil pressure on a combined footing with lateral loads?

[cijuanni]

I am trying to verify a software programs results and unable to determine if e for the resultant load for soil pressure is calculated correctly.
I have been unable to find a single example calc on a combined footing with a moment on one column, a moment on both columns, a shear load on the base of the column(s) or anything close.
Just lots of combined footings w/o moments etc....

So to cover the basics, a column #1 with an axial and moment load, is resolved to a axial load only using e1 = M1/P1, ditto column #2.
Summing moments about a corner of the footing base and dividing by the sum of the axial forces gives the moment arm for resultant from the footing corner. Determine the dimension of the resultant to the center of the footing, e.

If there are lateral shear loads on the base of the columns they are treated as moments V*t (the footing thickness) and added to the moment about the footing corner to determine the location of the resultant.

So where am I going wrong? [URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1555613869/tips/Image_oeiy2w.tiff[/url]

 

[BAretired]

Total moment about left end of footing M = M1 + M2 +10P1 +30P2 + 2(V1+V2)

x = M/(P1 +P2)

BA

 

[jayrod12]

You're summing your moments wrong. When summing moments with the column loads and moments in their actual places (not their resultant places, i.e. P1 @ e1) you don't bring the eccentricity into it at all. The easiest way would be to write your units and follow them. In your summation the first two terms would be in lb-ft^2, not lb-ft.

So Sum moments about O = P1*10 + M1 + P2*30 + M2 + (V1+V2)*2

If you want to use the resultant locations then you actually need to move your load locations to the eccentric point and then sum moments not including M1 or M2 in the summation at all (because they're addressed via moving P1 and P2 to their eccentric locations).

Damn, BA beat me to it.

 

[BAretired]

Sorry jayrod!

BA

 

[cijuanni]

Thanks fellas, think I got tangled up in the forest and trees.