Eccentric Footing Bearing Pressure and Kern Check
The trapezoidal-or-triangular bearing pressure under a column with axial load plus moment, generalizing the wall check to any footing: while e = M/P stays in the middle-third kern (e <= B/6) the pressure is q = (P/BL)(1 +/- 6e/B); past it the heel lifts to a triangle over the front 3(B/2 - e). A 60 kip load on an 8 ft footing at e 1 ft runs 0.23 to 1.64 ksf full-bearing; push e to 2 ft and the toe spikes to 2.5 ksf with a third of the footing in no contact - the reason columns are kept concentric. Uniaxial, rigid footing. A design aid; the engineer of record governs.
Formula and source
e = M/P; e <= B/6: q = (P/BL)(1 +/- 6e/B); e > B/6: q_max = 2P/(3L(B/2 - e)), q_min = 0, bearing length = 3(B/2 - e).
The eccentric spread-footing bearing-pressure relations - the middle-third (kern) trapezoidal form and the resultant-outside-kern triangular form - a standard foundation-engineering result, by name.
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