Machine Foundations

Machine foundations

Machines such as compressors, pumps, turbines, generators, crushers, forging hammers and presses transmit dynamic (vibratory or impact) loads to their foundations. An ordinary foundation designed only for static load may vibrate excessively, damaging the machine, the building and nearby structures, and causing discomfort. Indian practice follows IS 2974 (design and construction of machine foundations, several parts) and IS 5249 (determination of dynamic properties of soil).

Types of machines

Types of foundations

• Block foundation — massive concrete block on which the machine rests; large mass and contact area; common for reciprocating machines and hammers.
• Box / caisson foundation — hollow block (box) — less mass, higher frequency.
• Wall foundation — pair of walls supporting the machine on top.
• Framed foundation — table top supported by columns and base raft (steel or RCC); common for turbo-generators, provides space for piping and condensers below.
• Foundations on piles where soil is weak.

Design criteria

1. Static criteria — bearing pressure within allowable (commonly reduced, e.g. to about 80% or less of the normal allowable value for vibratory loads) and uniform settlement (centre of gravity of machine + foundation and the centroid of the base area on the same vertical line; eccentricity limited to a few per cent of base dimension).
2. Dynamic criteria
   • No resonance — the natural frequencies of the machine–foundation–soil system should be well away from the operating frequency. Foundations are high-tuned (natural frequency above operating) or low-tuned (natural frequency below operating); a frequency ratio of about 0.5 or less, or 1.5–2 or more, is commonly sought, with at least about ±20% separation.
   • Amplitude of vibration within permissible limits (depend on machine type and speed; typically fractions of a millimetre).
   • Vibrations should not be harmful or annoying to people, sensitive equipment and nearby structures.
3. Structural criteria — strength for dynamic forces, fatigue, reinforcement and construction joints (generally cast monolithically without joints).

Basics of vibration

A foundation block on soil is modelled as a mass–spring–dashpot system.

Degrees of freedom: a rigid block has six modes — vertical translation, two horizontal translations (sliding), rocking about two horizontal axes, and yawing (torsion about the vertical axis). Sliding and rocking are usually coupled.

Soil stiffness

Barkan's coefficient of elastic uniform compression $C_u$ relates uniform pressure to elastic settlement of a rigid block:

Related coefficients (approximate relations, Barkan):

• Elastic uniform shear: $C_{\tau }\approx 0.5C_u$ → $k_x=C_{\tau }A$
• Elastic non-uniform compression (rocking): $C_{\varphi }\approx 2C_u$ → $k_{\varphi }=C_{\varphi }I$
• Elastic non-uniform shear (yawing): $C_{\psi }\approx 0.75C_u$ → $k_{\psi }=C_{\psi }J$

$C_u$ decreases as the contact area increases (it is inversely proportional to $\sqrt{A}$ for the same soil). Elastic half-space theory gives alternative spring constants in terms of shear modulus $G$ and Poisson's ratio.