Unit-2 [Dynamic Equipment Foundation].pdf

UNIT-II
DYNAMIC EQUIPMENT FOUNDATIONS
By
Dr. V. Vignesh,
Assistant Professor,
Sanjivani College of Engineering, Kopargaon

Introduction
• Machine foundations are subjected to the dynamic forces caused by the machine.
• These dynamic forces are transmitted to the foundation supporting the machine.
This produces an oscillating force.
• Basically, there are three types of machines:
(i) Machines which produce a periodic unbalanced force, such as reciprocating
engines and compressors. The speed of such machines is generally less than
600 r.p.m. In these machines, the rotary motion of the crank is converted into
the translatory motion. The unbalanced force varies sinusoidally.

(ii) Machines which produce impact loads, such as forge hammers and punch presses. In these
machines, the dynamic force attains a peak value in a very short time and then dies out gradually.
The response is a pulsating curve. It vanishes before the next pulse. The speed is usually between
60 to 150 blows per minute.
(iii) High speed machines, such as turbines and rotary compressors. The speed of such
machines is very high; sometimes, it is even more than 3000 r.p.m.

TYPES OF MACHINE FOUNDATIONS
The following 4 types of machine foundations are commonly used.
(1) Block Type. This type of machine foundation consists of a pedestal resting on a footing. The
foundation has a large mass and a small natural frequency.
(2) Box Type. The foundation consists of a hollow concrete block. The mass of the foundation is
less than that in the block type and the natural frequency is increased.

(3) Wall Type. A wall type of foundation consists of a pair of walls having a top slab. The
machine rests on the top slab.
(4) Framed Type. This type of foundation consists of vertical columns having a horizontal frame
at their tops. The machine is supported on the frame.

SUITABILITY OF VARIOUS TYPES.
1. Machine producing Impulse: Block type foundation
Example: hammer, Presses etc
2. Rotating type machine with low to medium frequency: Block type foundation with large
contact area.
Example: Large reciprocating engine, Compressor, large blower
3. Rotating type machine with medium to high frequency: Block type foundation resting
on suitable elastic pad or spring.
Example: Medium sized reciprocating engine, diesel engine and gas engine
4. Rotating type with very high frequency: Framed foundation or massive block with
minimum contact area.
Example: internal combustion engine, electric motors, turbo-generators.
Some machines which induce very little dynamic forces, such as lathes, need not be
provided with a machine foundation. Such machines may be directly bolted to the floor.

IS-CODES FOR MACHINE FOUNDATIONS:
• IS 2974 Code of practice for design and construction of machine foundations:
1. Part 1: 1982 Foundations for reciprocating type machines
2. Part 2: 1980 Foundations for impact type machines ( hammer foundations )
3. Part 3: 1992 Foundations for rotary type machines (medium and High frequency)
4. Part 4: 1979 Foundations for rotary type machines of low frequency
5. Part 5: 1987 Foundations for impact type of machines other than hammers (forging and
stamping press, drop crusher and jolter )

BASIC CRITERIA FOR THE DESIGN OF MACHINE FOUNDATIONS:
• These foundations should be well design to take care of the static loads coming
on the foundation.
• No shear or bearing capacity failure should occur. That means, the bearing
capacity of the foundation against shear failure is to be checked.
• No excessive settlement, that is amount of settlement as calculated under static
load and that has to be compared with different codal guidelines.
• Under the dynamic loading condition, the foundation should not resonate.
• The natural frequency of foundation soil must be far away from machine to avoid
the resonance.
• Dynamic displacement amplitude must not exceed the permissible limit.
• Vibration of the machine and foundation system together must not be annoying
to the person working in the environment and it should not damage the adjacent
structures.

METHODS OFANALYSIS
1. Linear elastic weightless spring MSD model.
2. Linear elastic theory.
3. Indian standard design code: IS 2974, Part 1 provisions.
As per IS codal provision the machine foundation has to be designed by checking
three criteria.
• Dimensional criteria,
• Vibration criteria,
• Displacement criteria.

Method of Analysis by IS Code method:
1. Check the Dimension:
• For a block type of foundation, the criteria given that the size of the foundation block must be
larger than the base plate of the machine .
• The second criterion says minimum all-around clearance of 150 mm must be provided as per
IS codal provision.
• A third criterion is that the foundation block should be placed deep enough on good bearing
strata.
• The combined centre of gravity of the machine plus foundation block should be as far
below the top of the foundation as possible.
2. Vibration Check:
• Foundation which is having natural frequency either much higher or lower than the operating
frequency of the machine, is called under tuned or over tuned respectively.
• If the ratio of operating frequency to the natural frequency is less than or equal to 0.5 that can be
designed as Under tuned criterion.

• If the operating frequency is much higher than the natural frequency and frequency ratio, that
machine design criteria is Over tuned.
β = 𝜔/𝜔𝑛> 2 for important machine and
β = 𝜔/𝜔𝑛> 1.5 for less important machine
• To design machine for which operating frequency is very high, over tuned criteria is used.
• To design machine for which operating frequency is low, under tuned criteria is used.
• However for the range of say 1000 rpm or even in the range of 600 rpm, it is always better to
check your design for both over tuned and under tuned.

3. Displacement Criteria
• The amplitude of permissible dynamic displacement should be less than or equals to 0.2 mm.
If it exceeds, foundation is to be redesigned.
• The permissible displacement should be checked using Richart’s chart. So, that it should not
become annoying to the workers or adjacent structures.
• Y-axis of the chart shows dynamic displacement amplitude and X axis represents operating
frequency of the machine.

DEGREE OF FREEDOM OF A BLOCK FOUNDATION
A rigid block foundation has 6 degree of freedom :
(1) Translation along X-axis, (2) Translation along Y-axis, (3) Translation along Z-axis, (4)
Rotation about X-axis (Pitching), (5) Rotation about Y-axis (Rocking), (6) Rotation about Z-axis
(Yawing).

GUIDELINES FOR INITIAL TRIAL SIZING OF BLOCK FOUNDATION:
1.The bottom of the block foundation should be above the water table when
possible. It should not be resting on previously backfilled soil nor on a sensitive (to
vibration) soil.
2. Block‐type foundations resting directly on soil:
a. A rigid block‐type foundation resting on soil should have a mass of two to three
times the mass of the supported machine for centrifugal machines.
However, when the machine is reciprocating, the mass of the foundation should be
three to five times the mass of the machine.

b. The top of the block is usually kept 1ft (30 cm) above the finished floor or
pavement elevation to prevent damage from surface water runoff.
c. The vertical thickness of the block should not be less than 2 ft (60 cm), or as
dictated by the length of anchor bolts used. The thickness is seldom less than one
fifth (1/5B) the least dimension or one tenth (1/10L) the largest dimension.
d. The foundation should be wide to increase damping in the rocking mode. The
width should be at least 1 to 1.5 times the vertical distance from the base to the
machine centerline.
e. The length (L) is determined according to (a) above, provided that sufficient plan
area is available to support the machine plus 1‐ft (30 cm) clearance from the edge
of the machine base to the edge of the block for maintenance purposes.
f. The length (L) and width (B) of the foundation are adjusted so that the center of
gravity of the machine plus equipment coincides with the center of gravity of
the foundation.

(g) For large reciprocating machines, it may be desirable to increase the embedded
depth in soil such that 50 to 80% of the depth is soil embedded. This will increase
the lateral restraint and the damping ratios for all modes of vibration.
(h) The mass of the foundation is increased or decreased so that, generally, the
modified structure is overturned or under‐turned for reciprocating and centrifugal
machines; respectively.

Unit-2 [Dynamic Equipment Foundation].pdf

3. Block foundations supported on piles:
a. The pile cap mass should be 1.5 to 2.5 times and 2.5 to 4 times the mass of the
machine for centrifugal and reciprocating machines, respectively.
b. The thickness, width, and length of the block are selected as in 2(b) through 2(f).
c. The number and size of piles are selected such that no single element carries
over one half of its allowable design load.
d. The piles are arranged so that the centroid of the pile group coincides with the
center of gravity of the combined structure and machine loads.
e. Piles are battered away from the pile cap to carry any transverse and longitudinal
unbalanced forces. Vertical piles provide small resistance to horizontal loads, and
the batter piles are usually designed to carry all such horizontal forces as axial
loads.
f. If resonance conditions are predicted to occur, modifications are necessary as
described in 2(h) above.
g. Piles and piers must be properly anchored to the slab for adequate rigidity and
for meeting the design conditions assumed during the analysis phase.

GUIDELINES FOR INITIAL TRIAL SIZING OF FRAMED FOUNDATION:
1. The designer should carefully analyze equipment size and clearance requirements to assure
that sufficient space is allocated to equipment, anchor bolts, piping, and clearance for
installation, maintenance and operation, that is, physical space limits and requirements should
be clearly identified and considered.
2. The bottom of the foundation mat should be placed no higher than the minimum founding
depth recommended by the soil consultant. This generally includes considering the location of
adequate bearing strata, water table, depth of frost penetration, paving elevation, and special
local soil conditions. However, in very poor soils, the geotechnical consultant may
recommend the use of piles.

3. All columns should be stressed almost equally when subjected to vertical load. The columns
should be capable of carrying six times the vertical load. Column spacing should
preferably be less than 12 ft (3.5m). The intermediate columns should be located preferably
under the couplings or the gear box.
4. The beam depth should be a minimum of one fifth (1/5) of the clear span, and the beam
width is normally equal to the width of the column consistent with anchor bolt requirements
for spacing, embedded depth, and edge distance. The beams should not deflect over 0.5mm
when subjected to static loads.
5. Flexural stiffness of the beams should be atleast twice the Flexural stiffness of the
columns.
6. Mstructure + Mat > 3Mmachine for centrifugal machines
Mstructure + Mat > 5Mmachine for reciprocating type machines
7. The mass of the top half of the structure should not be less than the mass of the machine.

8. The maximum static‐bearing pressure for soil‐supported foundations should not exceed one
half of the allowable soil pressure. For pile‐supported foundations, the heaviest loaded pile
should not carry over one half of its allowable load.
9. The column moments of inertia should be "balanced" about the centroid of the
equipment.
10. The center of column resistance should coincide with the center of gravity of the
equipment plus the top half of the structure loads in the longitudinal as well as the transverse
directions.
11. All the columns should deflect equally in the vertical, lateral, and longitudinal directions when
subjected to equivalent static machine loads acting in those directions.
12. The columns and beams should be checked for individual member resonance with the
machine acting frequency.

Construction Materials of Machine Foundations:
• Apart from the normal requirements of reinforced concrete construction as given in relevant
codes of practice, a few additional points especially applicable to the construction of machine
foundations are discussed here.
1. Concrete
• M 150 concrete should be used for block foundations and M 200 concrete for framed
foundations.
• The concreting should preferably be done in a single operation.
• The location of construction joints should be judiciously chosen. Proper treatment of the
joints with a suitable number of dowels and shear keys is required.
• Cement grout with non-shrinkable additive should be used under the machine bed-plate and
for pockets of anchor-bolts.

2. Reinforcement
• Reinforcement should be used on all surfaces, openings, cavities, etc., required to be provided in
the machine foundation.
• In block-type foundation, reinforcements should be used in the three directions.
• The minimum reinforcement should be 250 N/cum of concrete.
• The reinforcement usually consists of 16 to 25 mm bars kept at 200 to 300 mm spacing in both
directions, and also on the lateral faces.
• The concrete cover should be a minimum of 75 mm at bottom and 50 mm on sides and at top.
• Around all openings, steel reinforcement equal to 0.50 to 0.75% of cross-sectional area of the
opening shall be provided, in the form of a cage.
3. Expansion Joints
• Machine foundations should be separated from adjoining structural elements by expansion joints
to prevent transmission of vibration.

4. Connecting Elements
• Base plates and anchor bolts are used to fix machines to the foundation.
• For this purpose, concreting should be stopped at the level of the base plate. This gap will be
filled later by cement mortar.
• A 150 mm × 150 mm hole is generally sufficient for bolt holes.
• A minimum clearance of 80 mm should be provided from the edge of the bolt hole to the nearest
edge of the foundation.
• The length of a bolt to be concreted is generally 30 to 40 times the diameter. Bolt holes should
be invariably filled with concrete.
• Concreting the spaces under the machines should be done with extreme care using 1:2 mortar
mix.
• Machines should not be operated for at least 15 days after under-filling, since vibrations are
harmful to fresh mortar. The edges of the foundation should be protected by providing a border
of steel angles.

5. Spring Absorbers
Spring absorbers are commonly used for providing isolation in machine foundations. These can be
installed by using either ‘supported system’ or ‘suspended system’.
In the former, the springs are placed directly under the machine or the foundation; in the latter, the
foundation is suspended from springs located at or close to the floor level.
In the suspended system, access to the springs becomes easy for future maintenance or
replacement.
For well-balanced machines, relatively smaller springs are adequate; in such cases, the supported
system may be used. For machines with large exciting forces, heavy springs will be required; in
this case, the suspended system is preferred.

LINEAR ELASTIC WEIGHTLESS SPRING MSD MODEL

Unit-2 [Dynamic Equipment Foundation].pdf

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