Seismic Ground Motion - Dynamic Behavior of Concrete and Seismic Engineering - page 98

3.2.3. Accelerometer data banks
The rate at which new recordings are collected is constantly increasing, and a
cursory inspection of recent strong motion seismological data demonstrates they
have all been caused by “astonishing” recordings, the analysis and interpretation of
which has forced engineers to re-evaluate certain hypotheses in the field.
Whereas this data was spread confidentially until the 1980s, the advent of the
Web has forced a state of mind change thanks to the volume now available online.
Several sites are listed at the end of this chapter, but the system evolves so rapidly
that many of these may already be obsolete.
Two Internet site categories are given, some presenting recordings from specific
networks and others gathering data from quite varied sources to compile as
databases.
3.3. Quantitative characterization of seismic movements
Real accelerograms possess different forms of time, amplitude and frequency
content. In most cases, para-seismic dimensioning is not carried out on complete
accelerograms (used only in non-linear dynamic analysis cases), but is performed
using a restricted number of reducing quantities which are representative of the
damage a structure might receive.
Only those most commonly used are referred to; more elaborate descriptions are
given in [BET 03], [JEN 83] and [KRA 96].
3.3.1. Time maximum values
The most direct and simple quantity is the acceleration maximum, often referred
to as the “PGA” (peak ground acceleration). For a long time, when only analog
recordings were available, this was the only quantity available. As we will see later,
it remains in use because it links directly to the spectral characterization that para-
seismic engineers prefer, i.e. the acceleration response spectra.
For destructive earthquakes, PGA data has sensitivity better than 1 m/s². Even if
until the 1970s it was generally thought that ground acceleration could never be
more than 1 g, observations have since indicated values in excess of 20 m/s², even
along the vertical component (which may explain some reported discrepancies in
construction works with spans resting on their supports).