06-02-2013, 04:33 PM
VIBRATION ANALYSIS AND DESIGN OF BLOCK-TYPE MACHINE
FOUNDATIONS INTERACTING WITH SOIL
VIBRATION ANALYSIS AND DESIGN.pdf (Size: 979.88 KB / Downloads: 390)
General requirements of machine foundation
The following requirements should be fulfilled from the design point of view of machine foundations
(a) The foundation should be able to carry the superimposed loads without causing shear or crushing
failure of the underlying soil.
(b) The settlement should be within the permissible limits.
© The combined center of gravity of machine and the foundation should be on the vertical line passing
through the center of gravity of the base plane.
(d) There should be no resonance; that is the natural frequency of the foundation-soil system should be
either too large or too small compared to the operating frequency of the machines. For low-speed
machine, the natural frequency should be high and vice-versa.
(e) The amplitude of motion at operating frequencies should not exceed the limiting amplitude, which is
generally specified by machine manufacturers. If the computed amplitude is within tolerable limit, but is
close to resonance, it is important that this situation be avoided.
(f) Where possible the foundation should be planned in such a manner as to permit a subsequent
alteration of natural frequency by changing base area or mass of the foundation as may subsequently be
found necessary.
From the practical point of view, the following requirements should be fulfilled [7]
(a) The ground-water level should be as low as possible and ground-water level should be at least deeper
by one-forth of the width of foundation below the base plane. This limits the vibration propagations,
ground water being a good conductor of vibration waves, especially P-waves
(b) Machine foundations should be separated from adjacent building components by means of expansion
joints.
© Any steam or hot air pipes, embedded in the foundation must be properly isolated
(d) Machine foundation should be taken to a level lower than the level of the foundations of adjacent
buildings.
(e) The foundation must be protected from machine oil by means of acid resistant coating or suitable
chemical treatment.
Design Data
The specific data required for design vary depending upon the type of machine. The general
requirements of data for the design of machine foundations are, however, as follows:
(i) Loading diagram showing the magnitude and position of static and dynamic loads exerted by the
machine on its foundation.
(ii) Power of engine and the operating speed.
(iii) Diagram showing the embedded parts, openings, grooves for foundation bolts etc.
(iv) Nature of soil and its static and dynamic properties.
Permissible Vibration Amplitudes
The manufacturers of the machinery generally define the permissible amplitudes. The permissible
amplitude of a machine foundation is governed by the relative importance of the machine and the
sensitivity of neighboring structures to vibration.
If manufacturer's data do not contain the permissible amplitude, the values shown in Figure 3.6 are
generally taken as a guide for various limits of frequency and amplitude.
DESIGN PARAMETERS
General
In the design of machine foundation, the geometric properties of the foundation block and the physical
properties of the underlying soil are the basic properties, which should be determined first.
The geometric properties of the block include center of gravity, moments of inertia and mass moments
of inertia. The physical properties of the soil include damping and stiffness characteristics of the
supporting soil formation.
Evaluation of Soil parameters
In order to evaluate the spring stiffness and the corresponding damping ratios required for analysis of
dynamic structure-soil interaction problems, it is necessary to determine relevant values for the soil
parameters including dynamic sheer modulus, G, Poisson’s ratio, , and mass density, .
Shear modulus (G)
Unbalanced loads in vibrating machinery produce shear strains in the supporting soil that are usually of
a much smaller magnitude than the strains produced by static loading. The mechanism governing the
stress-strain behavior of soils at small strains involves mainly the stress-deformation characteristics of
the soil particle contacts and is not controlled by the relative slippage of particle associated with longer
strains. As a consequence, the stress-deformation behavior of soil is much stiffer at very small strain
levels than at usual static strain levels. It is therefore inappropriate to obtain a shear modulus directly
from a static stress-deformation test, such as a laboratory triaxial compression test or a field platebearing
test, unless the stress and strains in the soil can be measured accurately for very small values of
strain [5].