Page 140 - Dynamic Loading and Design of Structures

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Strong motion records
Accelerograms of strong motions (i.e. time histories of acceleration) are recorded by
accelerographs; these instruments record simultaneously the three components of the motion,
two perpendicular horizontal (longitudinal, for instance N–S, and transverse, E–W), and one
vertical. Before being used for ‘engineering’ purposes (e.g. for deriving response spectra, see
Section 4.3.2), the records are corrected to remove frequency dependent instrument response
and ambient noise. An example of corrected accelerogram from the 1971 San Fernando (S.
California) earthquake is shown in Figure 4.3. Although this is not really a typical record, the
observed difference in frequency content between the vertical and the horizontal components
is indeed quite typical.
Accelerograms are arguably the most valuable information for deriving design seismic
loads and it is fortunate that nowadays a very large number (several tens of thousands) of
accelerograms are available; on the other hand, though, there are seismic areas for which the
number of records is very low or even zero. Databanks of accelerograms have been compiled
in many regions, particularly in the United States, Japan and Europe. One of the largest
collections containing over 15,000 digitized and processed accelerograph records from all
over the world (but mainly from the US), dating from 1933–1994, is available from the
National Geophysical Data Centre in Boulder, Colorado. A number of American records can
be downloaded directly from the web sites of the Strong Motion Data Centre of the US
Department of Conservation, and from NISEE (National Information Service on Earthquake
Engineering, University of California, Berkeley). In Europe, accelerograms are available from
organizations such as the Institute of Engineering Seismology and Earthquake Engineering
(ITSAK), Thessaloniki, Greece, and Servizio Sismico Nazionale (SSN), Rome, Italy.
The main purpose of using accelerograms is to characterize the strong ground motion, with
a view to defining appropriate design loads. In this respect, the Peak Ground Acceleration
(PGA, or simply A) (i.e. the highest value of the acceleration time history), is a parameter that
has been extensively used in seismic hazard assessment (see Section 4.2.5). It is worth
pointing out, though, that this is mainly due to its convenience, because otherwise the PGA is
often a rather poor indicator of the destructiveness of the ground motion. The Peak ground
Velocity (PGV, or simply V) and/or the Peak Ground Displacement (PGD or D) are better
indicators of damage potential and have been used in some studies. Velocity and displacement
time histories of the ground motion can be calculated by integration of the acceleration time
history, but they are quite sensitive to the filtering procedure used in correcting the
accelerograms. Hence, by far the most useful information that can be extracted from
accelerograms is the response spectra, discussed in Section 4.3.2. One factor, though, that is
not reflected in the spectra (which are plots of peak response) is the duration of the motion.
This can be quite critical in certain cases, such as structures susceptible to strength
degradation under reversed cyclic loading (i.e. change of the sign of the applied force or
moment).