Ground Motions and Structures - Earthquake Engineering for Structural Design - page 309

Ground Motions and Structures 309

Figure 8.12 Vibration modes (after Kohler et al. 2007)

Looking to ground motions and structural response, one can see that the latter is
much larger that the input movements. The reason of this amplification is due to the
phenomenon of resonance , which is maximum when one of the frequencies of the
ground motion oscillations is equal to the one of the natural frequency of the structure.
In this situation, there is a very important amplification of the structural response, as a
function of damping effects.
For instance, in some cases, especially for interplate earthquakes, when the main
frequency of ground motions corresponds to the frequency of the first vibration mode
of the structure, the resonance increases the amplitude of this mode. But in some cases,
mainly for interplate earthquakes, when the main ground motion frequency
corresponds to the frequencies of the second or the third structural mode, only these
modes will be amplified by resonance (Fig. 8.13) (Gioncu and Mazzolani, 2006). This
aspect gives rise to a very complex problem, which cannot be ignored during a proper
seismic design process. It is a very important observation, because in case of resonance
with superior mode of vibrations, the most affected part is the top of the structure,
while in case of resonance with the first mode, the main damage is concentrated at the
first structure levels (Gyorgyi et al, 2006). The main resonance with the superior modes
is characteristic for intraplate crustal earthquakes, producing ground motions with very
short natural periods of vibration, while for interplate crustal and intraslab earthquakes,
with medium and large ground motion natural periods, the main resonance occurs with
the first mode.
Fortunately, due to the very chaotic nature of ground motions, these resonance and
amplification phenomena take place for a short time only, the ground motion acting as a
damper for the structure motions during the remainder time. Unfortunately, in
exchange, during the earthquakes, there are multiple resonance phenomena, when the
superior structural vibration modes interact with the corresponding vibration modes.