Page 465 - Industrial Power Engineering and Applications Handbook

Testing of metal-enclosed switchgear assemblies 14/439
Table 14.9 Likely energy released at different magnitudes of as a seismograph which amplifies and records small
an earthquake movements of the earth's surface and helps to identify
the epicentre and focal depth and determines the magnitude
M 5.0 6.0 6.5 7.0 7.5 8.0 of an earthquake.
E(1020 ergsa) 0.08 2.5 14.1 80 446 2500 More than a thousand earthquakes with a magnitude of
at least 2 (corresponding ground acceleration, < 0.01 g
"An erg is the unit of work in the cgs system and is equal to the to 0.02 g) occur daily. But earthquakes less than M = 5
energ required by a force of 1 dyne to move an object 1 cm (1 erg
= 10- Y. Joule). are considered minor, as they are generally harmless.
Earthquakes of the same magnitude may cause varying
amounts of damage at different locations, depending upon
The intensity of an earthquake is a subjective assessment the soil stratification and design considerations of the
of its effects on the primary systems and inhabitants in primary systems. Conventional seismograph are not
surrounding areas and is measured on the Mercalli scale. suitable for recording major ground movements and go
As noted above, this decreases with distance from the off-scale (and sometimes are even damaged) when severe
epicentre while the magnitude remains the same. For earthquakes occur in their vicinity. For recording signi-
details refer to DD ENV 1998. Generally, the magnitude ficant ground movements, strong motion accelerographs
and intensity of an earthquake at a location are interrelated. are used. Records of accelerographs are called accelero-
The energy so released propagates in the form of waves grams. They are basic requirements for seismic analysis,
and travels through the stratification of the earth's crust to design earthquake-resistant structures and buildings
in all directions, longitudinal (X axis), transverse (Y axis) and for other engineering applications. Large numbers
and vertical (Z axis) at the same instant, with varying of accelerograph stations have been established in
magnitudes subjecting to vibrations whatever stands in vulnerable locations throughout the world to record
its way on the earth's surface, such as buildings and seismic waves for further research and to take preventive
trees etc. These waves are recorded in the form of irregular measures by improving design practices. Geologists and
broad bands, Le., multi-frequency waveforms, composed scientists make use of these data to determine the
of many sine waves of different frequencies, similar to a magnitude of an earthquake and analyse the source
harmonic waveform, discussed in Section 23.5.2 and as mechanism to further their quest to predict an earthquake
shown in Figure 23.7. It is known as the time history of more accurately, while design engineers use them for
an earthquake. The normal practice to describe the motion developing earthquake-resistant systems, structures and
of an earthquake for a particular location, based on seismic equipment etc. So far these records have proved
studies and data obtained for and from nearby areas, is insufficient to provide required forecasts about an
in the form of a response spectrum (RS), as discussed later. earthquake's location, time of occurrence and magnitude.
Nevertheless, with continued efforts in this direction, it
is hoped that one day it will be possible to predict an
Duration of an earthquake earthquake more accurately.

An earthquake may last for 4-6 seconds only for M = Response spectrum (RS)
5.5 or less and for over 40 seconds for M > 1.5. The
greater the magnitude, longer will be the duration. An A response spectrum (RS) is analytically determined by
earthquake of M 2 6, for instance, may last for 15-30 calculating the peak response (also called the spectral
seconds and produce a maximum horizontal ground response) of a linear single degree of freedom system
acceleration of the order of 0.1 g to 0.6 g (98 cds2 to with different natural periods and dampening for a given
590 cm/s2) and higher, inflicting maximum damage in acceleration time history of ground movements recorded
the first 5-10 seconds only, and a frequency band between during an earthquake. It forms a part of seismic studies
1 and 33 Hz (IEEE 344). carried out for a particular area, and provides information
Figure 14.12(b) represents an actual time history about far responses of different types of structures during
(accelerogram) of the earthquake that occurred in Chamoli, an earthquake. It takes cognisance of the fact that an
India, on 29 March 1999. It had apeak ground acceleration of earthquake can be expressed indirectly in the form of a
nearly 0.15 g and a predominant frequency of about 2 Hz. response spectrum. This spectrum is the peak response
It is also accepted that after such an event, the ruptured of a linear single degree of freedom system on the
earth surfaces may try to settle down again. It is possible occurrence of an earthquake, as a function of its natural
that during the course of such a realignment there may frequency (periods) for different dampening. They are in
still remain pockets of energy between the two plates the shape of a curve, natural frequency or natural period
until they finally settle. These may develop into releases versus peak amplitude of vibrations of the system, as
of stresses once again, leading to occasional tremors or illustrated in Figure 14.13. They have a broad band as
earthquakes even for several days after a major earthquake noted later and can be expressed in any of the following
or volcanic eruption. The earthquakes in Turkey are forms for different dampening:
examples where two equally devastating earthquakes 1 Ground displacement response spectrum in terms of
occurred between September and November 1999.
spectral displacement versus natural period of oscilla-
tions (Figure 14.14).
14.6.2 Recording an earthquake 2 Ground velocity response spectrum in terms of spectral
velocity versus natural period of oscillations (Figure
This is carried out with the help of an instrument known 14.15).