9.1 Piling for machinery foundations
9.1.1 General principles
The foundations of machinery installations have the combined function of transmitting the
dead loading from the machinery to the supporting soil and of absorbing or transmitting to
the soil in an attenuated form the vibrations caused by impacting, reciprocating, or rotating
machinery. In the case of impacting machinery or equipment such as forging hammers or
presses, and reciprocating machines, piston compressors and diesel engines, the dynamic
loads transmitted to the soil take the form of thrusts in a vertical, horizontal or inclined
direction. Rotating machinery, such as gas and steam turbines, creates a torque on the shaft,
resulting in lateral loads or moments applied to the foundation block. Rock crushers and
metal shredders produce random dynamic loads as a result of rotating imbalances depending
on the particular operation. Dynamic loading from hammers or presses, or from low-speed
reciprocating engines has a comparatively low frequency of application, but the vibrations
resulting from out-of-balance components in high-speed rotating machinery can have a
high frequency.
The higher the frequency of dynamic loading, the less is the amplitude which can be
permitted before damage to the machinery occurs, or before damage to nearby structures,
and noise and discomfort to people in the vicinity becomes intolerable. When the frequency
of vibration of a machine and its foundations approaches the natural frequency of the
supporting soil, resonance occurs and the resulting increased amplitude may result in
damage to the plant and excessive settlement of the soil. The latter is particularly liable to
happen when the vibrations are transmitted to loose or medium-dense coarse soils.
When the mass of the machine and its foundations and vibration characteristics of the
soils are known, it is possible to calculate the resonant frequency of the combined
machine–foundation–soil system. In order to avoid resonance, the frequency of the applied
dynamic loading should ideally not exceed 50% of the resonant frequency for most impact
hammers or reciprocating machinery. In the case of high-speed rotating machinery it is
probable that the applied frequency will be higher than the resonant frequency of the
machine–foundation–soil system. For this condition the aim should be to ensure that
the applied frequency is at least 1.5 times the resonant frequency. The need for the wide
divergency is to allow for the starting-up and shutting-down periods when the frequency
of the machine passes through the resonant stage. If the applied frequency is too close to
the resonant frequency the stage of resonance at the acceleration or slowing down of the
machine might be too protracted.
Chapter 9