Ground Motions and Structures - Earthquake Engineering for Structural Design - page 312

312 Earthquake Engineering for Structural Design

So, it is very clear that the ground motion is dominated by the very short segment of 2
seconds, the other segments having just a modest contribution.
Another conception regarding the structural response, valuable especially for near-
source ground motions characterized by pulse velocity time history and acting on tall
and regular buildings, is the so-called wave-propagation approach , developed by Iwan
(1996, 1997). During an earthquake, a fault initiates seismic waves which are
transmitted outward in all directions through the Earth's layers. The waves, after
reaching the surface, are reflected back into the Earth when the surface is free. When
there is a structure on the surface, however, the waves continue to propagate into the
structure, causing the structure to vibrate. In other words, the vibration of a structure
during an earthquake is caused by the propagation of seismic waves into the structure
itself (Safak, 1999). Theoretically, the vibration and wave-propagation approaches
represent two alternative solutions for the same problem. Vibration concept, such as the
modal representation of structure movements, has been the standard approach in code
provisions. Wave-propagation concept is useful when the structure can be modeled as a
continuous medium. This concept, used for directly determining the interstory drifts,
avoids the complex analysis of modal resonance. For structures subjected to near-
source ground motions, the coherent pulse propagates through the structure as a single
harmonically oscillating wave , causing large localized inter-storey drifts (Fig. 8.15).
The structure is modeled as a shear-beam with regular stiffness and mass. The coherent
pulse-like nature of near-source ground motion time history may cause the maximum
response of the structure before a resonant mode-like response. This concept allows
considering the effect of localized yielding, which produces softening in the affected
stories. In this way, it is easy to see that localized yielding may lead to substantially
larger interstory drifts. This method has the advantage to localize the yielding at
different stories and to determine the most suitable position of the collapsed story,
some times being the middle one. An example of wave-propagation in a framed
structure is presented in Figure 8.16 (Kohler et al, 2007), showing that a pulse ground
motion is transferred into the structure as a pulse story drift with a very high velocity
(about 200m/sec).
Therefore, in far-source areas the concept of superposition of modal vibrations is the
best way to understand the structural response, but in near-source areas the concept of
wave propagation seems to be the most proper and simple to describe the structural
response.
Concluding from the above presented aspects, the determination of the structural
response involves the description of the structure movements at every time within the
considered period, characterized by accelerations, velocities and displacements. The
key of a proper determination of the structure response is related to the consideration of
the actual characteristics of ground motions, very different in function of the source
type: crustal interplate and intraplate earthquakes, and deep intraslab earthquakes.
Differences come from propagation of waves, different natural periods, pulse or cyclical
ground motions, earthquake duration, acceleration level, etc.