Civil Engineering Reference
In-Depth Information
shaking has ceased, the pendulum will tend to return to a stable position, and thus could
indicate false ground movement. Therefore a pendulum damping system is required so that
the ground displacements recorded on the chart will produce a record that is closer to the
actual ground movement.
Much more sophisticated seismographs are presently in use. For example, the engineer
is often most interested in the peak ground acceleration
a
max
during the earthquake. An
accelerograph
is defined as a low-magnification seismograph that is specially designed to
record the ground acceleration during the earthquake. Most modern accelerographs use an
electronic transducer that produces an output voltage which is proportional to the acceler-
ation. This output voltage is recorded and then converted to acceleration and plotted versus
time, such as shown in Fig. 2.14. Note that the velocity and displacement plots in Fig. 2.14
were produced by integrating the acceleration.
The data in Fig. 2.14 were recorded during the February 9, 1971, San Fernando earth-
quake. The three plots indicate the following:
1.
Acceleration versus time:
The acceleration was measured in the horizontal direc-
tion. In Fig. 2.14, the maximum value of the horizontal acceleration
a
max
, which is also com-
monly referred to as the
peak ground acceleration,
is equal to 250 cm/s
2
(8.2 ft/s
2
). The
peak ground acceleration for this earthquake occurs at a time of about 13 s after the start of
the record.
Since the acceleration due to earth's gravity
g
is 981 cm/s
2
, the peak ground accelera-
tion can be converted to a fraction of earth's gravity. This calculation is performed by divid-
ing 250 cm/s
2
by 981 cm/s
2
; or the peak ground acceleration
a
max
is equal to 0.255
g.
2.
Velocity versus time:
By integrating the horizontal acceleration, the horizontal
velocity versus time was obtained. In Fig. 2.14, the maximum horizontal velocity at ground
surface
v
max
is equal to 30 cm/s (1.0 ft/s). The maximum velocity at ground surface for this
earthquake occurs at a time of about 10 s after the start of the record.
3.
Displacement versus time:
The third plot in Fig. 2.14 shows the horizontal dis-
placement at ground surface versus time. This plot was obtained by integrating the hori-
zontal velocity data. In Fig. 2.14, the maximum horizontal displacement at ground surface
is 14.9 cm (5.9 in). The maximum displacement at ground surface for this earthquake
occurs at a time of about 10 s after the start of the record.
2.3 SEISMIC WAVES
The acceleration of the ground surface, such as indicated by the plot shown in Fig. 2.14, is
due to various seismic waves generated by the fault rupture. There are two basic types of
seismic waves: body waves and surface waves. P and S waves are both called body waves
because they can pass through the interior of the earth. Surface waves are only observed
close to the surface of the earth, and they are subdivided into Love waves and Rayleigh
waves. Surface waves result from the interaction between body waves and the surficial
earth materials. The four types of seismic waves are further discussed below:
1.
P wave (body wave):
The P wave is also known as the primary wave, compres-
sional wave, or longitudinal wave. It is a seismic wave that causes a series of compressions
and dilations of the materials through which it travels. The P wave is the fastest wave and
is the first to arrive at a site. Being a compression-dilation type of wave, P waves can travel
through both solids and liquids. Because soil and rock are relatively resistant to compres-
sion-dilation effects, the P wave usually has the least impact on ground surface movements.
