Seismic Ground Motion - Dynamic Behavior of Concrete and Seismic Engineering - page 99

Obviously, the relationship of this quantity to the damage a seismic signal can
cause is somewhat obscure. It gives no information on the duration or spectral
content of the stress, both essential parameters for modeling non-linear oscillators
with several degrees of freedom, which lie at the root of paraseismic calculations.
Nevertheless PGA-macroseismic connections are often used quantitatively.
Everybody has to be aware that qualitative relationships are marred by huge
uncertainties and should never be used without explicitly taking into account the
associated standard deviation (a minimum factor 3). Most existing correlations show
that whenever intensity rises by 1 degree (MM or MSK), the PGA is multiplied by a
factor of 2 or 3.
To be able to take spectral content into account in an approximate way whilst
keeping the simple concept of a maximum value, we consider the ground velocity
(“PGV”) and displacement (“PGD”) maxima as well. Whereas the maximum
acceleration is mainly associated with the high-frequency spectral content (beyond 5
Hz), the maximum speed is associated with the intermediate frequencies (between 5
and a few Hz), and the maximum displacement is associated with low frequencies
(below 1 Hz).
For a destructive earthquake, values of PGV/PGD typically range from a few
cm/s to over 1 m/s, and from a few millimeters to a few meters. They are far more
sensitive to the size of the shock than the maximum acceleration, because size
influences the low frequency content far more than the high frequency content.
Nevertheless, neither of these two values shows a better correlation with macro-
seismic intensity. Furthermore, their estimation is less direct than that of the PGA,
since acceleration histories have to be integrated numerically. For analog or first
generation digital instruments, it is also less reliable because of the greater
sensitivity of the numerical process to low frequency noise.
However, it should be noted that knowledge of all three values gives a good idea
of the frequency level and content of a given accelerogram. Furthermore, the non-
dimensional ratio, PGA x PGD/(PGV) 2 , which relates to the width of the excited
frequency response, generally varies little from one recording to the next, with
values typically ranging from 1 to 10, and even from 2.5 to 7.5 [BET 03].
3.3.2. Spectral characterizations
Whilst characterizations that are more complex are often used in seismic
engineering, spectral characterizations convey information that is richer and more
consistent than mere maximum values. Unfortunately, two different spectral