Design of machine foundation

1. Design of Machine Foundation

2. Vibration Isolation
Vibrations may cause harmful effects on adjoining
structures, machines and the persons working in the
area around the machine. However, if the frequency
ratio is kept outside the critical range of 0.4 and 1.50,
and the amplitude is within the permissible limits, the
harmful effects are considerably reduced, especially if
the system is damped.
Transmission of vibrations can be controlled and the
damaging effects considerably reduced by isolating
either the source (active isolation) or by protecting the
receiver (passive isolation). The following measures are
generally adopted.

3. Vibration Isolation
The following measures are generally adopted.
(i) The machine foundation should be located away from
the adjoining structures known as geometric isolation.
(ii) Additional masses known as dampers are attached to
the foundations of high-frequency machines to make it
multiple-degree freedom systems and to change the
natural frequency. (In reciprocating machines, the
vibrations are considerably reduced by counterbalancing
the exciting forces by attaching counterweights to the sides
of the crank.)
(iii) Vibrations are considerably reduced by placing
absorbers, such as rubber mountings, felts and corks
between the machine and the base.

4. Vibration Isolation
(iv) If an auxiliary mass with a spring is attached to the
machine foundation, the system becomes a two-degree-
freedom system. The method is especially effective when
the system is in resonance.
(v) If the strength of the soil is increased by chemical or
cement stabilization, it increases the natural frequency of
the system. The method is useful for machines of low
operating frequency.
(vi) The natural frequency of the system is modified by
making structural changes in the foundation, such as
connecting the adjoining foundations, changing the base
area or mass of the foundation, or use of attached slabs.
(vii) The propagation of waves can be reduced by providing
sheet piles, screens or trenches.

5. Block foundation consists of a massive block of concrete
resting directly on soil or supported on piles or a pedestal
resting on a footing. It has a large mass and, therefore, a
smaller natural frequency.
Box or Caisson type foundation is provided to increase the
natural frequency as the mass of the foundation is less.

6. Plate foundation consists of a continuous plate made of
concrete resting directly on soil or supported on piles. The
plate has high static stiffness and is easy to construct.
Spring foundation consists of a concrete slab or a steel
frame on which the machine rests. The slab/frame is
supported on a set of springs. This type is used, if the
subsoil is weak or we need to isolate the subsoil from the
transmitted vibrations.

7. Information Needed for Design
1. Static weight of the machine and accessories.
2. Magnitude and characteristics of dynamic loads
imposed by the machine operation and their point of
application
3. The soil profile of the site and dynamic soil properties
such as dynamic shear modulus and damping
4. Trial dimensions of the foundation. These are generally
supplied by the manufacturer. This will give the total
static weight.
5. An acceptable method of analysis i.e., a mathematical
model to determine the response of the foundation-soil
system
6. A criteria for adequate design.

8. Soil Profile and Dynamic Soil Properties
Satisfactory design of a machine foundation needs
information on soil profile, depth of different layers,
physical properties of soil and ground water level. This
information can be obtained by usual sub-surface
exploration techniques. In addition, one must determine
dynamic shear modulus, material damping, poisons ratio
and mass density of soil for dynamic analysis of the
machine foundation.

9. Soil Profile and Dynamic Soil Properties
Dynamic shear modulus of a soil is generally determined
from laboratory or field tests.
Material damping can be determined from vibration tests
on soil columns in the laboratory.
The values of dynamic shear modulii and damping may
be estimated from empirical estimations for preliminary
design purposes. Geometrical damping is estimated from
elastic half-space theory and appropriate analogs.

10. Design Criteria for Reciprocating Machine
The machine foundation should be isolated at all levels
from the adjoining foundations.
The natural frequency of the foundation–soil system
should be higher than the highest disturbing frequency
and the frequency ratio should be less than 0.4, however
if not possible then the frequency ratio should not be less
than 1.50.
The amplitude of vibration should be within the
permissible limits or any specified permissible limits. For
most soils, another criterion, amplitude for low-speed
machines is usually taken as 0.2 mm.
According to another criterion, the amplitude in mm is
limited to 4/f frequencies less than 30 hertz and 125/f2.
For higher frequencies, where f is frequencies in hertz
(cycles/sec).

11. Design Criteria for Reciprocating Machine
Concrete block foundations should be used. However,
when the soil is not suitable to support block foundation,
cellular foundation may be used.
The size of the block in plan should be larger than the bed
plate of the machine. There should be a minimum all
round clearance of 15 mm.
The total width of the foundation measured at right
angles to the shaft should be at least equal to the
distance between the center of the shaft and the bottom
of the foundation.
The eccentricity of the foundation system along X-X and
Y-Y axes should not exceed not 5% of the length of the
corresponding side of the contact area.

12. Design Criteria for Reciprocating Machine
The combined center of gravity of machine and
foundation should be much below the top of foundation.
The depth of foundation should be sufficient to provide
the required bearing capacity and to ensure stability
against rotation in the vertical plane.
The stresses in the soil below the foundation should not
excess 80% of the allowable stresses under static loads.
The base pressure is limited to half the normal allowable
pressure qna in extreme cases.
Where it is not practicable to design a foundation to give
a satisfactory dynamic response, the transmitted
vibrations may be reduced by providing anti–vibration
mounting either between the machine and foundation or
between the foundation and the supporting system.

13. Design Criteria for Reciprocating Machine
The machine should be anchored to the foundation block
using a base plate and anchor blots. Bolt holes should be
backfilled with concrete and the space below the plate
should be filled with 1: 2 cement mortars.
A number of similar machines can be erected on
individual pedestals on a common raft.
The analysis for such machines can be made assuming
that each foundation acts independently up the raft into
sections corresponding to separate machines.

15. Design Procedure for Impact Machines
The hammer system usually consists of a frame that
guides a falling, weight, known as the tup, that strikes
down on the anvil that is supported on the foundation
block.
Typical tup weights range between 2.5kN and 100kN and
the drop height is in the range of 0.3 m to 2 m or more.
The anvil gets repeated blows from the tup so that the
piece of metal held by the anvil gets forged to the desired
shape or is broken.
High impact energy is transmitted to the anvil from the
falling tup. A part of the energy is used in forging and the
rest gets transmitted to the soil below.

16. Design Procedure for Impact Machines
To avoid breakage of the concrete foundation due to
impact stresses, an elastic pad made of felt, cork or
rubber is placed between anvil and foundation block.
To ensure that the compressive stresses induced in the
pad do not exceed the specified value. This is an
additional design criterion for block foundations for a
hammer.
In the design of these foundations, the damping in the
soil is neglected since the period of impact is very small
compared to the period of natural frequency of vibration
of machine-foundation-soil system.
The rest of the design procedure is the same as that of
block foundation for reciprocating machines.