Geotechnical Engineering

DHARM

ELEMENTS OF SOIL DYNAMICS AND MACHINE FOUNDATIONS 841

The relevant aspects with regard to the design of machine foundations will be dealt

with in the subsections that follow.

20.4.1Types of Machines and Machine Foundations
Machines may be classified as follows, based on their dynamic effects and the design criteria:
(i) Those producing periodical forces—reciprocating machines or engines, such as com-
pressors.

(ii) Those producing impact forces—forge hammers and presses.
(iii) High speed machines such as turbines and rotary compressors.
(iv) Other miscellaneous kinds of machines.
Based on their operating frequency, machines may be divided into three categories:
(a) Low to medium frequency machines up to 500 rpm:
Large reciprocating engines, compressors, and blowers fall in this category. Usual
operating frequencies range from 50 to 250 rpm.

(b) Medium to high frequency machines—300 to 1000 rpm.
Medium-sized reciprocating engines such as diesel and gas engines come under this
category.

(c) Very high frequency machines-greater than 1000 rpm:
High-speed internal combustion engines, electric motors, and turbo-generators fall in
the category.

Machine foundations are generally classified as follows, based on their structural form:
I–Block-type foundations, consisting of a pedestal of concrete on which the machine
rests (Fig. 20.26 (a)). Reciprocating machinery falling into category (a) above are supported on
block-type foundations with large contact area with the soil.

Reciprocating machinery of category (b) above may be also supported on block-type foun-
dations, but these are made to rest on springs or suitable elastic pads to reduce their natural
frequencies.

High-speed machinery of category (c) above may also be supported on massive block
foundations; small contact surfaces with suitable isolation pads are desirable to reduce the
natural frequencies.

II–Box or caisson type foundations, consisting of a hollow concrete block (Fig. 20.26 (b)).
III–Wall-type foundations, consisting of a pair of walls which support the machinery on
their top (Fig. 20.26 (c))

IV–Framed-type foundations, consisting of vertical columns supporting on their top a
horizontal frame work which forms the seat of essential machinery (Fig. 20.26 (d)).

Turbomachinery requires this type of foundations, which accommodate the necessary

auxiliary equipment between the columns.

Some machines such as lathes, which induce very little dynamic forces do not need any

foundations; such machines may be directly bolted to the floor.