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Pressure under a foundation 2

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XinLok

XinLok

Civil/Environmental
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If I have a foundation where the pressure under the foundation is p1, p2, p3 & p4.
Screen_Shot_2023-05-12_at_5.37.06_PM_fpiips.png


Can I consider the volume of the pressures under foundation (the edges are p1, p2, p3 & p4) equal to the vertical load?

If yes (I mean the volume of pressures equal to vertical load), and lets consider the p1 = -5 psi, and p2 = -10 psi, then to proceed with calculation and designing of the foundation, I should consider the p1= 0 and p2 = 0 to proceed with the design?

If I consider p1 = 0 and p2 = 0, the volume of pressure under foundation will not be equal to the vertical load !!!

Can anyone give a help please?
 
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You have to look at the fundamentals of foundation design.

1) The volume of the pressures is equal to the total vertical load + the resultant of the eccentric loads.
2) Soil cannot impart a negative pressure onto a foundation. If there is 'apparent' negative pressures then the equation you present for soil pressure is no longer true. the cross section of the footing in contact is not complete, and thus the soil distribution is not based on the complete contact area's moment of inertia (Ix and Iy) or extreme fiber distance (Ye and Xe).
 
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I am working for a combined foundation with Column C1 and Column C2
Applied loads and moments on Column C1: Fx1, Fy1, Fz1, Mx1, My1 and Mz1
Applied loads and moments on Column C2: Fx2, Fy2, Fz2, Mx2, My2 and Mz2

The resultant is equal to applied vertical load = Fy1 + Fy2, Correct?

Cause I was confused when mentioned the volume of pressure = Vertical loads (Fy1 + Fy2) + Resultant.


 
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You should review various threads regarding this topic in the Structural forum.
For example:
thread507-476601

Regarding your question, the sum of forces=0 in vertical direction supports your contention.
But,there is likely a variable pressure along each edge.
 
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