Seismic Overturning Stability Ratio at the Foundation

ScopeThe subtlety is that the dead load available to resist is NOT D: the same vertical acceleration that pushes the building sideways also lifts it, and ASCE 7 carries that as Ev = 0.2 SDS D acting UPWARD in the uplift combination, so LRFD combination 7 leaves (0.9 - 0.2 SDS) D and the ASD pair leaves (0.6 - 0.7 x 0.2 SDS) D. At SDS 1.0 that is 0.70 D, not 0.90 -- and skipping the term is the classic way an overturning check passes on paper and should not. A 5,422 kip-ft demand on a 20 ft footprint with 900 kips of dead load gives a 1.55 ratio after the 12.13.4 soil-interface reduction, yet the resultant lands 6.45 ft off center against a B/6 kern of 3.33, so the base is in PARTIAL UPLIFT even though global stability passes -- the thing a ratio alone hides. The 12.13.4 reduction does not carry up into the shear wall or its hold-downs. Global stability only; sliding, toe pressure, and hold-down forces are separate.

Formula or tableeffective dead-load coefficient = 0.9 - 0.2 SDS (LRFD combination 7) or 0.6 - 0.7 x 0.2 SDS (the ASD 0.6D + 0.7E pair); W_eff = coefficient x D; resisting moment = W_eff x lever arm (half the footprint unless entered); design overturning = M0 x 0.7 for ASD x 0.75 when the ASCE 7 12.13.4 soil-interface reduction is taken; ratio = resisting / demand; eccentricity = demand / W_eff, compared to the B/6 kern and to B/2.

Source and editionASCE 7 - the 0.9D + 1.0E uplift combination and the ASD 0.6D + 0.7E pair, with the vertical seismic effect Ev = 0.2 SDS D acting upward in the overturning case, and the reduction in overturning permitted at the soil-structure interface by Section 12.13.4: 25% for an equivalent lateral force design that is not an inverted pendulum or cantilevered column system, 10% for a modal design, none by default.

Free accessThe load-combination structure and the Ev = 0.2 SDS D definition are widely reproduced in free code commentary and university course material. The ASD effective coefficient is COMPOSED here from two separately stated pieces (the 0.6D combination and the 0.7 factor ASD applies to seismic) and the arithmetic is shown in the tile's note rather than presented as a memorized constant, so a user can audit it against their own edition.

What governsEstimate. AHJ and licensed professional govern.

API field namesoverturning_moment_kipft, dead_load_kip, footprint_width_ft, sds, design_method, apply_soil_reduction, required_ratio, dead_coeff, ev_coeff, w_eff_kip, arm_ft, m_resist_kipft, m_demand_kipft, ratio, eccentricity_ft, kern_ft

• LRFD effective dead load (0.9 - 0.2 SDS) D from combination 7 with Ev upwardASCE 7 load combinations
• ASD effective dead load (0.6 - 0.7 x 0.2 SDS) D, composed from the 0.6D combination and the 0.7 seismic factorcomposed; arithmetic shown in the note
• Soil-interface reduction optional: 25% (ELF, not an inverted pendulum or cantilevered column) or 10% (modal), at the interface only; off by defaultASCE 7 12.13.4
• Lever arm half the footprint by default; assumes a centered resultant dead loadstated assumption, overridable
• Kern check eccentricity against B/6 for full bearing and B/2 for a resultant off the baseelastic bearing geometry
• Scope global stability and resultant location; sliding, toe pressure, and hold-downs are separatestated scope limit