Foundation Natural Frequency Calculator

GMRC and ACI 351.3R require every foundation natural frequency (vertical, horizontal, rocking, torsional) to be at least 20% away from operating speed and from its first 4 harmonics. Inside the avoidance band the dynamic amplification can exceed 5×, leading to chronic vibration and structural fatigue.

For typical 600–1200 RPM reciprocating compressors (10–20 Hz operating frequency), keeping horizontal modes well below 10 Hz and vertical/rocking modes well above 20 Hz puts both ends of the natural-frequency spectrum outside the operating band. This sub-critical/super-critical split is the GMRC industry guideline for high-speed packages.

Natural frequency scales with √G — doubling soil stiffness raises every mode by 41%. Because field G can vary by ±50% across a site, ACI 351.3R recommends running upper- and lower-bound G to prove the foundation stays out of the avoidance band under all credible soil conditions.

With VFD- or engine-driven units, the operating frequency sweeps a range, so each harmonic becomes a band rather than a line. This calculator checks whether any natural frequency falls inside that wider band — variable-speed packages typically need either a wider separation margin or a sub-critical foundation that stays below the lowest operating speed.

Sub-critical (foundation stiffer than the machine) avoids passing through resonance during start-up and gives the lowest steady-state amplitude — preferred for fixed-speed reciprocating compressors. Super-critical (foundation softer than the machine) requires more mass and adequate damping to ride through resonance on start-up; it suits high-speed centrifugal or variable-speed units that start and stop infrequently.