Ground Motions and Structures - Earthquake Engineering for Structural Design - page 310

310 Earthquake Engineering for Structural Design

The best example is the recorded accelerogram during the 1977 Vrancea earthquake.
The main duration of 17 seconds was divided in four segments of time: the first of 5
seconds, the second of 2 seconds, the third of 5 seconds and the last of 5 seconds (Fig.
8.14a). The first segment corresponds to the beginning of the earthquake with moderate
ground motions (arrival of P and S waves). The second segment, with the duration of
only 2 seconds, marks the arrival of the surface waves, which occur under form of
acceleration pulse with long period due to very bad soil conditions. The last two
segments represent the ground motion as the consequence of the main shock.
Considering each interval as an independent earthquake, Figure 8.14b shows the
domination of the second segment corresponding to pulse acceleration.

Figure 8.13 Amplification factors for different structure natural periods
(Gioncu and Mazzolani, 2006)