Eccentric Footing Bearing Pressure and Kern Check
The trapezoidal-or-triangular bearing pressure under a column with axial load plus moment.
Example
You enter
- Vertical load P (kip) 60
- Moment about the B axis M (kip-ft) 60
- Footing width B, eccentricity direction (ft) 8
- Footing length L (ft) 8
You get
- E (ft) 1
- Maximum bearing pressure q_max 1.64 ksf
- Minimum bearing pressure q_min 0.234
Details, formula, and sources
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.
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.
The eccentric-footing pressure relations and the middle-third rule are public foundation-engineering results.
Estimate. AHJ and licensed professional govern.
Field names used by the API: p_kip, m_kft, b_ft, l_ft, e_ft, q_max_ksf, q_min_ksf
- Kern rule trapezoidal q = (P/BL)(1 +/- 6e/B) while e <= B/6 (full bearing)foundation engineering
- Outside kern heel lifts to a triangle over 3(B/2 - e) with q_max = 2P/(3L(B/2 - e))foundation engineering
- Scope uniaxial eccentricity, rigid footing; allowable bearing and settlement are separatescope of this tile