On the characteristics of ground motion rotational components using Chiba dense array data

Rotational motions (torsional and rocking) induced by seismic waves have been essentially
ignored for a long time, first because rotational effects were thought to be small for man-made
structures [1], and second because sensitive measuring devices were not available until quite recently.
The benefits of the determination of rotational motion in seismology and engineering are
still under investigation (e.g. [2, 3]). In seismology, rotational motions can provide accurate data
for arrival times of SH waves and, in the near-source distance range, rotational motions might
provide more detailed information on the rupture processes of earthquakes [3]. Rotational motions
could also be used to better estimate the static displacement from seismic recordings, identifying
translational signals caused by rotation [2].
In engineering, dynamic response estimation of structures subjected to earthquake-induced base
excitations is often simplified by ignoring the rotational components. This has been a widely
accepted practice in engineering community, mainly caused by the lack of recorded strong motion
accelerograms for these motions. Many structural failures and the damage caused by earthquakes
can be linked to differential and rotational ground motions. Torsional responses of tall buildings in
Los Angeles, during the San Fernando earthquake in 1971, could be ascribed to torsional excitation,
while rotational and longitudinal differential motions may have caused the collapse of bridges
during San Fernando (1971), Miyagi-ken-Oki (1978) [4] and Northridge (1994) [5] earthquakes.
For the first time, Newmark [6] established a simple relationship between translational and torsional
components of the ground motion. He presented a deterministic procedure for estimating the
increase in displacement of symmetric-plan buildings caused by rotational ground motions at the
base due to horizontal propagation of plane waves with a constant velocity and further explored
in the other studies [7, 8]. Several studies have shown the importance of torsional components
in seismic analysis and design of structures [6, 9–13]. The seismic design codes also prescribe
‘Accidental Eccentricity’ in design force calculations to account for the unknown torsional inputs
and unpredictable eccentricities [14, 15]. Since then, many researchers have studied the dynamic
and accidental eccentricities of structures [12, 13, 16, 17]. The significance of rocking excitations
for continuous [18] and for base-isolated structures [19] is emphasized. Furthermore, the effects of
rocking motions on dynamic response of multistorey building have been analytically investigated
and the results revealed that stiff structures, such as nuclear power plants, having short vibration
periods, might be influenced more by this component in typical earthquake excitations [20].
Although some theoretical studies [20–22] have been carried out to estimate effects of rocking
components on response of structures, no provisions are made in design codes to account for the
effect of ground rocking motion.
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