Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 246

earthquakes are considered to be small and partly because no instruments exist for
directly measuring the absolute rotation (Igel et al, 2005).
The translationalmotions consider two horizontal components in perpendicular
direction, (NS and EW or arbitrary) and one vertical component. If there are no
problems regarding the direction for the vertical motions, for the horizontal ones it
is not sure that the two selected perpendicular directions provide the maximum
effects produced by an earthquake. Therefore, using the recorded values, it is very
important to know the direction which yields the largest value. This value is
measured by varying the orientation of an elastic oscillator in different directions
(Erkay and Karaesmen, 1996, 1998, 2002). The ground accelerations are
determined into two perpendicular directions, (1) and (2), corresponding to the
recorded acceleration directions. By composing these values, one can plot a set of
oriented values. The obtained three-dimensional representative diagrams are
presented in Figure 7.25, which allows determining the upper value of oscillator
response. The example of this methodology refers to the records of the 1985 Vina
del Mar (Chile) earthquake (Vasquez, 1996). One can see that the spectrum is
strongly influenced by the recorded direction (in the range of 0.6 to 1.0 periods),
the largest acceleration being obtained for an orientation of 15 degrees from the
recorded direction. Contrary, for degrees between 45 and 75, the largest
accelerations occur in the range of 0.2 to 0.4 sec periods.
The rotational component of ground motions has been basically ignored in the
past decades, compared to the substantial research in the field of translation
components. Even the torsional oscillations of the Earth were predicted in the 19

th
Century, by examining the rotation of some objects on the ground surface (i.e. the
rotation effects at different heights of some obelisks, Teisseyre, 2006). During the
last decades, numerous observations pointed to rotation effects at the ground
surface, so a new direction of research can be framed into the so-called Rotational
Seismology (Lee et al, 2007). In 2007, the First International Workshop on
Rotational Seismology and Engineering Application was organized in California-
USA (USGS, 2007). Three main directions of research can be noted:
(i) Development of fundamental theory.
(ii) Providing additional information to Earthquake Engineering.
(iii) Developing some special instruments to measure these Earth's
rotations.
From the Structural Engineering point of view, the interesting aspects are
related to the effect at free surface, rotation and tilt, produced by Love and
Rayleigh waves. It seems that in the near-source of an earthquake the tilt effects
are not always negligible compared to the effects of translation motions.
Therefore, in these areas it is necessary to measure and to consider in the design all
the six components of the motion. Contrary, in far-field areas these effects can be
neglected (Graizer, 2005).