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Spread Footing Bearing Resistance — AASHTO LRFD §10.6.3.1.2a (U.S. units)

Gross nominal bearing resistance q_n with shape factors, ground-water correction factors C_wq, C_wγ and effective footing dimensions from load eccentricity; factored resistance q_R = φ_bq_n and an allowable value for building practice.

Soil

φ (drained; 0 for undrained) deg

c or s_u psf

Total unit weight γ pcf

Ground water below grade D_w ft

Footing and loads

B ft

L ft

Embedment D_f ft

Gross vertical load P kip

Moment about axis ∥ L (e along B) kip-ft

Moment about axis ∥ B (e along L) kip-ft

Resistance factor φ_b

FS for allowable (building practice)

Results

N_c · N_q · N_γ—

B′ × L′ · s_c, s_q, s_γ—

C_wq · C_wγ—

Gross nominal q_n (c · q · γ terms)—

Factored q_R = φ_bq_n vs demand P/(B′L′)—

Allowable q_n/FS (gross)—

AASHTO LRFD§10.6.3.1.2a (Eq. 10.6.3.1.2a-1; Tables 10.6.3.1.2a-2 ground water and -3 shape factors), Table 10.5.5.2.2-1 (φ_b). Factors N_q, N_c (Prandtl–Reissner) and N_γ (Vesić). Clause numbers only.

VerifiedCaltrans Geotechnical Manual, Shallow Foundations for Bridges (2021), Appendix 2: preliminary, strength and extreme-event cases reproduced to the example's rounding. Depth and inclination factors deliberately omitted, as in the example.

Preliminary calculation tool. Final design requires the full text of the referenced standard (see the sources on this page) and review by a licensed PE.