Environmental Engineering Reference
In-Depth Information
Accelerographs (Strong-Motion Seismographs) (Accelerometers)
The
purpose
of the strong-motion seismograph is to provide ground-response criteria in an
area of interest for the dynamic design of structures. Modern accelerographs measure and
record the three components of absolute ground acceleration (one vertical, two horizontal)
and are able to resolve peak accelerations to 0.1 cm/sec
2
or smaller. People at rest are able
to feel motions as small as 1 cm/sec
2
. In moderate magnitude earthquakes, damage to
poorly designed structures occurs at accelerations of about 100 cm/sec
2
(10%
g
).
The
instrument
does not operate continuously, but is rather designed to begin operating
and recording when affected by a small horizontal movement. The sensor is typically a
damped spring-mass system. Through the 1970s the recording medium was usually photo-
graphic film and recording was slow. Modern designs convert an electrical signal into a dig-
ital format, which is recorded in a digital memory within the unit providing for rapid
recording of accelerations. Accurate time recording is achieved by connection with satellites.
Location
of most accelerographs is on the ground surface, and not necessarily on rock:
therefore, data correlations between sites are difficult unless subsurface conditions are
known for each. Many accelerographs are also located in buildings.
Accelerograms
are the records obtained of ground accelerations
g
as illustrated in Figure
11.7. Ground motion displacements and velocities are then computed from the accelera-
tion records by the integration of Equations 11.7 and 11.8.
The
peak horizontal ground acceleration
(PHGA) is the most common index of the intensity
of strong ground motion at a site (Munfakh et al., 1998). The PHGA is directly related to
0.3
0.2
0.1
0
−
0.1
−
0.2
−
0.2
ÿ
m
= 0.32 g =
a
m
Ground acceleration,
ÿ
8
0
−
8
ÿ
m
= 13.7 in/s =
y
m
Ground velocity,
y
−
16
y
m
= 8.3 in. =
d
m
8
4
0
4
−
Ground displacement,
y
0
5
10
15
20
25
Times, s
FIGURE 11.7
Strong ground-motion record of the N-S component from E1 Centro, California, earthquake of May 18, 1940.
Ground acceleration is integrated to obtain velocity and displacement. (From USAEC,
Soil Behavior under
Earthquake Loading Conditions
, National Technical Information Service TID-25953, U.S. Department of Commerce,
Oak Ridge National Laboratory, Oak Ridge, Tennesse, January 1972. With permission.)
