shridevi- types of machine foundation

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Machine foundation involves interaction of three domains viz.,

Structural engineering, Geotechnical engineering and Structural dynamics.

The machine foundations receive and transfer to soil

1. the static load due to weight of foundation, machine and accessories

and

2. the dynamic loads imposed as a result of imbalances in moving parts

during operation of the machine.

The dynamic loads, which occur during the operation of the machine,

result from forces generated by unbalance, inertia of moving parts or both,

and by flow of fluid and gases for some machines. The magnitude of thesedynamic loads depend upon the machine’s operating speed and the type,

size, weight and arrangement (position) of moving parts within the casing.

Dynamic loads due to operation of the machine are generally small

compared to the static weight of machine and the supporting foundation.

The dynamic load is applied repetitively over a very long period of time but

since its magnitude is small and therefore the soil behavior is essentially

elastic, or else deformation will increase with each cycle of loading and may

become unacceptable.

The main constituents of a typical machine-foundation system are

1. machine: rotary machines, reciprocating machines, impact machines;

2. foundation: block foundations, or frame foundations; and

3. support medium: soil continuum, or a soil-pile system, or a

substructure that, in turn, is supported over the soil continuum or soil-

pile system.

The important parameter to be determined in designing a machine

foundation is :

• amplitude of vibration of a machine at its operating frequency and



• natural frequency of a machine – foundation - soil system.

Depending on the nature of dynamic loads imposed, machines may be

divided into two:

1. those with periodic disturbing forces that produce forced vibrations in

the foundations and

2. those with percussive action that cause free vibrations in the

foundation

Some machines transfer both types of loads to foundations .

The first group includes machines with parts that exhibit uniform rotation ora reciprocating motion and second group includes machines having impact –

type working members (forging hammers, pile drivers) or components that

exhibit non-uniform motion (rolling mill, forging mill etc.)

Rotating machinery : This category includes turbines, turbo pumps

and compressors, fans motors, centrifuges etc. These machines have

rotating motion of Impellers or rotors.

Unbalanced forces in rotating machines are created when the mass centroid

of the rotating part does not coincide with the centre of rotation. This



dynamic force is a function of the shaft mass, speed of rotation and

magnitude of the offset. The offset should be minor under manufactured

conditions when machine is well balanced without wear & tear. Because

rotating machines normally trip and shutdown at some vibration limit, a

realistic continuous dynamic load on the foundation is that resulting from

vibration just below the trip level. The high speed machines may have speed

of more than 3000r/min and upto 12,000r/min.

Reciprocating machinery: For reciprocating machinery, such as

compressors and diesel engines, a piston moving in a cylinder interacts witha fluid through the kinematics of a slider crank mechanism driven by, or

driving , a rotating crankshaft. Individual inertia forces from each cylinder

and each throw are inherently unbalanced with dominant frequencies at one

and two times the rotational frequency. The operating speed are less than

600r/min. The unbalanced forces can be considered to vary sinusoidally.



Impulsive Machinery: Equipments, such as forging hammers and

some metal-forming presses, operate with regulated impacts or shocks

between different parts of the equipment. This shock loading is often

transmitted to the foundation system of the equipment and is a factor in the

design of the foundation. The pre-impact kinetic energy of the ram converts

to post–impact kinetic energy of the entire forging hammer. As the entire

hammer moves downward, it becomes a simple dynamic mass oscillating on

its supporting medium. This system should be well damped so that the

oscillations decay sufficiently before the next blow. Timing of blows range

from 40 to 100 blows per min. Ram weights vary from a few hundred pounds

to 35000 lb (156kN). Impact velocities is in the range of 7.6m/s.

Foundation Types

Block-Type foundation:- Dynamic machines are preferably located

close to grade to minimize the elevation difference between the machine

dynamic forces and the centre of gravity of the machine- foundation system.



The ability to use such a foundation primarily depends on the quality of near

surface soils. Block foundations are designed as rigid structures. The

dynamic response of a rigid block foundation depends only on the dynamic

load, foundation’s mass, dimensions and soil characteristics. A block

foundation has a large mass and hence a smaller natural frequency. For

compressors and reciprocating machines, a block foundation is generally

provided. These foundations consists of a pedestal resting on a footing.

Hammers may also be mounted on block foundation but their details are

different.

Combined block type foundation:- Combined blocks are used to

support closely spaced machines. Combined blocks are more difficult to

design due to combination of forces from two or more machines & due to

possible lack of stiffness of larger foundation mat.



Box or Caisson Type Foundation:- If a relatively lighter foundation

than the block foundation is desired, a box type foundation may be provided.

The mass of the foundation is reduced and its natural frequency increases.

Complex framed foundations:- It consists of a system of walls

,columns, beams and slabs. Each element of such a foundation is more

flexible as compared to a rigid block foundation or box & caisson foundation.

They are provided for large turbine driven equipment like electric

generators. Elevation allows for ducts, piping and ancillaries to be put below

the equipment.



Framed foundation with Isolators: Isolators (Springs and

Dampers) located at the top of supporting columns are sometimes used to

minimize the response to dynamic loading. The

The effectiveness of isolators depends on machine speed and natural

frequency of foundation

Spring Mounted Equipment : Pumps are at times mounted on

springs to minimize thermal forces connecting piping. The springs are then

supported on a block foundation. This arrangement has a dynamic effect

similar to that for tabletops with vibration isolators. Other types of

equipments are spring mounted to limit the transmission of dynamic forces.

Inertia block in structure: Dynamic equipment on a structure may

be relatively small compared to overall size of structure. In this situation,

dynamic machines are usually designed with a supporting inertia block to

alter natural frequencies away from machine operating speeds and resist

amplitudes by increasing the resisting inertia force.



Pile foundations: All the above foundations may be supported directly

on soil or on piles. Piles are generally used where soft ground conditions

result in low allowable

contact pressures and excessive settlement for a mat type foundation. Piles

may be end bearing or frictional. Transverse loads are resisted by soil

pressure bearing against the side of the pile cap or against the side of the

piles. Various types of piles are used.including drilled piers, auger cast piles,

and driven piles.

Machine foundation may be Undertuned or Overtuned. For Undertuned

machines the vertical frequency of vibration is less than operating frequency



of machine. This type of foundation is preferred for High and Medium

frequency machines. In Overtuned machines the vertical frequency of

vibration is more than operating frequency of the machine. Preferred for very

low to low speed machines.

In undertuned systems with impact machinery, the frequency shall be 30%

less than frequency of impact. For high frequency rotary machines the

natural frequency of the foundation should not preferably be within 20%

operating speed of machine/ frequency of dynamic loads and

moments. Foundation under low frequency machines should have their

natural frequency much higher than the operating speed of the machine.



Reference

Srinivasalu P. and Vaidyanathan C.V (1976) –Hand book of machine foundations.

Shamsher Prakash and Puri.V.K (2006)- “Foundation for VibratingMachines”– Journal of Structural Engineering, SERC, Madras,India , Special issue, April-May 2006.

James .P.L and others, “Foundation for Dynamic Equipments”,Report of ACI Committee



MACHINE-FOUNDATION SYSTEM

Dynamic forces are (i) internally generated forces by the machine itself, or (ii) externally applied forces

(that are applied directly to the machine, or transmitted through the support medium/foundation).

Figure 1 shows the schematic of dynamics between various elements of a machine-foundation

system. It is seen that all the three constituents of the machine-foundation system, viz., machine,

foundation and soil, contribute to the frequency of the system. This system, when subjected to dynamic

forces (whether internally generated, externally applied, or transmitted through the soil), results inresponse of the system

shridevi- types of machine foundation

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