Static & Dynamic Loading · Industry Guidelines
Unbalanced forces from reciprocating compressor
Soil bearing analysis, dynamic loading effects, and foundation settlement
Soil bearing pressure is the load per unit area that a compressor foundation exerts on the underlying soil under combined static (equipment + concrete weight) and dynamic (unbalanced primary forces) loads.
Industry practice (ACI 351.3R) targets ≤1,500 psf to limit settlement on dynamic equipment, with 2,000 psf as a maximum for typical soils. Site-specific allowable should come from a geotechnical report.
The unbalanced force times CG height gives the overturning moment M. Eccentricity e = M/W is checked against the kern (Lshort/6). Inside the kern, edge stress = 6M/(Llong·Lshort²) for the worst-case orientation. Outside the kern (heel lift-off), qmax = 2W/[3·Llong·(Lshort/2 − e)].
Per ACI 351.3R guidance for dynamic equipment, qallow/qmax ≥ 2.0 is recommended. This effectively reduces the static allowable by 50% to account for cyclic dynamic loads. Note this differs from the geotechnical FS = 2.5–3.0 (qult/qallow).
The kern is the central region of a footing where applied eccentricity keeps the entire base in compression. For a rectangular footing, the kern limit is L/6 in each direction. If e > L/6, the heel of the footing lifts off the soil and bearing distribution becomes triangular — undesirable for vibrating machinery.