Dynamic Response · Unbalanced Forces · GMRC Guidelines
From compressor OEM data or pulsation study
Vibration theory, dynamic response, damping, and acceptance criteria
The resonance zone is r = 0.7 to 1.3, where r = operating frequency / natural frequency. ACI 351.3R recommends r < 0.5 (under-tuned, massive foundation) or r > 1.5 (over-tuned, stiff foundation) so the dynamic magnification factor stays below 1.33. The 2x harmonic must also be checked.
API 618 and GMRC guidelines target less than 2.0 mils peak (≈4 mils peak-to-peak) at the foundation top. ACI 351.3R uses an inverse-frequency criterion Xallow = 150/fCPM mils peak. Peak velocity should stay below 0.15 in/s (ISO 10816 Zone B for medium machines on rigid foundations).
DMF = 1 / √[(1 − r²)² + (2ζr)²], where r is the frequency ratio and ζ is the damping ratio. At r = 1 (resonance) the DMF reduces to 1/(2ζ); for ζ = 0.05 this gives DMF = 10, a tenfold amplification of the static deflection F₀/k.
Primary unbalanced forces oscillate at 1× crankshaft speed; secondary forces oscillate at 2× crankshaft speed (from connecting-rod kinematics). Each excites the foundation at a different frequency ratio, producing different DMFs and amplitudes. The peak response is combined by SRSS for screening, and both 1× and 2× frequency ratios must avoid the resonance zone.