When to use: Compute soil bearing pressure beneath a rectangular spread footing carrying an axial load P and a moment M. When the eccentricity e = M/P stays inside the kern (e ≤ L/6) the footing remains in full contact; outside it the heel lifts off and a triangular partial-bearing distribution governs. Results check the peak pressure against the allowable bearing q_allow.
Compute soil bearing pressure under a rectangular spread footing subjected to axial load and overturning moment. Checks the eccentricity against the kern limit (L/6) to determine whether full contact or partial triangular bearing governs, and compares peak pressure against the allowable bearing capacity.
Enter the axial column load P, applied moment M, footing dimensions L (in the moment direction) × B, and the allowable bearing q_allow. Eccentricity e = M/P is compared to the kern limit L/6. When e ≤ L/6 (full contact): q = P/A ± M/S where S = BL²/6. When e > L/6 (partial bearing): q_max = 2P/(3B(L/2 − e)), q_min = 0 (heel lifts off).
Area A = L×B. Eccentricity e = M/P. Kern: e ≤ L/6 for full contact. Full contact bearing: q = P/A ± Mc/I = P/(LB) ± M/(BL²/6). Partial bearing (triangular): q_max = 2P/(3B(L/2 − e)). Factor of safety (ASD) = q_allow / q_max (typically FS ≥ 2.5 to 3.0 for foundations).
Use for preliminary design of isolated spread footings, combined footings, and mat foundations under columns with eccentric loading (gravity + wind or seismic moment). Note that actual allowable bearing capacity from a geotechnical report accounts for shear failure per Terzaghi/Meyerhof theory — this tool checks contact pressure, not ultimate bearing capacity.
The kern (or middle third rule) is the zone within L/6 of the centroid where the resultant must fall to keep the entire footing base in compression. When eccentricity e > L/6, the heel of the footing loses contact with the soil, creating a triangular bearing stress distribution.
Allowable bearing capacity q_allow is the maximum soil pressure permissible under service loads, typically established by a geotechnical engineer from a site investigation. It incorporates the ultimate bearing capacity (Terzaghi: q_ult = cNc + γDNq + 0.5γBNγ) divided by a factor of safety of 2.5–3.0.
When e > L/6, the assumption of full bearing is violated. Only a reduced contact length 3(L/2 − e) is active. This concentrates bearing pressure near the toe and significantly increases q_max. ASCE 7 and IBC often limit eccentricity to keep the footing in full contact under service loads.
No — this tool only checks bearing pressure under the footing. Footing thickness must also be checked for two-way (punching) shear per ACI 318 §22.6 and one-way shear per §22.5, as well as flexure for the footing cantilever beyond the column face.