ETL 1110-2-533
30 Sep 94
ground vibrations. The vibration of the ground will
induce vibration in the structures and equipment
resting on the ground. In general, the severity of the
ground shaking decreases as the distance from the
source increases; however, local soil conditions can
significantly change the character of the ground
motion and increase its damaging effects. As the
depth and softness of the soil at a site increase, the
low frequency content of the ground motion is ampli-
fied and the high frequency content tends to be atten-
uated. Ground vibration levels may be amplified by
soil conditions by a factor of three or more. Equip-
ment supported in structures may experience an addi-
tional amplification due to the response of the
structure. Soil and structure amplification tend to
attenuate high frequencies and amplify lower frequen-
cies, with the boundary between high and low fre-
quencies depending on the characteristic frequency of
the soils and the lower natural frequencies of the
structure.
(2) Earthquake excitations can be characterized
by the amplitude of the shaking, its frequency con-
tent, and its duration. The frequency content of
earthquake ground motions often coincides with lower
Figure B-1. Motion of unanchored transformer
natural frequencies of a significant portion of lifeline
facilities and equipment. The effect of earthquake-
(Richter magnitude 6.4). The motion of the trans-
induced vibration is the major cause of lifeline equip-
former can cause bushings and surge arrestors to be
ment damage.
damaged, particularly if rigid bus is used. Control
cables can also be damaged.
(3) The response of ground-mounted structures
and equipment is not only determined by the ampli-
tude of the ground motion but also by the degree to
b.
Soil liquefaction.
which the frequency content of the ground motion
matches the natural frequencies of the items being
(1) Under certain conditions, when saturated
excited. Likewise, the response of equipment
soils experience vibrations, shear strength decreases
mounted in structures is also influenced by the match
and soil liquefaction can occur. Liquefied soil has
between the frequency content of their input and their
been observed to flow on 1 percent grades. Surface-
natural frequencies. Thus, equipment mounted in
supported structures have settled several feet below
structures may experience a dynamic response much
grade, and buried tanks have floated to the surface.
larger than the ground motion.
Lateral spreading associated with liquefaction can
cause large horizontal motions, often several feet.
(4) Vibration-induced sloshing of oil in trans-
Even when a rail-mounted transformer (Figure B-2) is
formers and motion of transformers frequently cause
secured by chocks, the chocks can slip, or the trans-
sudden pressure relays to trip, causing circuit breakers
former can tip over damaging bushings, radiators, bus
to isolate transformers. As a result, in moderate and
work, and internal parts and connections. In Fig-
strong earthquakes, busses are often de-energized so
ure B-3, the transformer pushed the chock off of the
that there is a loss of offsite power. Massive objects,
end of the rail and allowed the transformer to fall
such as power transformers, require substantial
from its pedestal.
anchorage to prevent movement relative to support
structures when ground motions are imposed, as in
earthquakes. The transformer shown in Figure B-1
moved over 3 ft in the 1971 San Fernando earthquake
B-2
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