within a building where maximum vibration isolation is
required, the following methods provide vibration
isolation between the generator mounting foundation
and the structure.
17-19. The foundation should be separated from the
floor slab and the subsoil so vibrations will not be
transmitted to the structure (see figure 17-2). Water
resistant, processed natural cork has been found to be
one of the most effective organic materials for isolating
the foundation from the subsoil. By lining the foundation
pit and its perimeter with cork plates before pouring the
concrete foundation, it is usually possible to reduce the
units vibration to a satisfactory degree.
17-20. Another method of isolating a foundation is to
use 8 or 10 inches (20 or 25 cm) of wet gravel or sand
as a bed in the foundation pit. Tests have shown sand
and gravel are capable of reducing the amount of unit
vibration transmitted by as much as one-third to one-
half. The isolating value of gravel is somewhat greater
than that of sand and does not vary appreciably with its
depth. To minimize settlement of the concrete
foundation from the floor slab, expansion joints should
be incorporated between the slab and the foundation.
17-21. If the floor slab has sufficient strength to support
the weight of the unit, and a separate foundation is not
used, a satisfactory mounting can be accomplished by
the use of vibration isolators between the floor slab and
the unit. To limit vibration effects to an absolute
minimum, the unit should be isolated from its foundation
by flexible mounts and the foundation isolated, as
described, from the floor.
17-22. To facilitate maintenance on the unit once it is
installed, the foundation should be constructed so as to
protrude above floor level by 6 to 8 inches (15 to 20 cm)
as illustrated in figure 17-2 and extend 8 inches (20 cm)
wider beyond the front and sides of skid and 18 inches
(20 cm) beyond the rear skid as shown in figure 17-3.
17-23. A minimum clearance of 5 feet (15 m) between
an installed set and adjacent walls should be maintained
on three sides of the unit. A minimum clearance of 8
feet (24 m) should be maintained at the rear of the unit
to facilitate the removal of the generator should it
become necessary. See table 17-1 for generator set
weight and dimensions.
Table 17-1. Weight and Dimensions