Simple Analysis of Rotational Ground Motions and Their Effects on Structure Displacement

A rotational ground motion is seismic wave that causes tipping or twisting at the base of a structure. These motions are non-translational. This differs from non-rotational motions which translate the base of a structure. Recent recordings of rotational earthquake ground motions have shown that rotational ground motions can be much greater than originally thought. While far-field ground motion rotations have agreed well with current theory, recent near-field measurements have shown that the magnitudes of these rotations can be 10 to 100 times greater than originally thought (Liu et al., 2009). This may possibly lead to a large increase in structure displacements and thus increased stress on these structures.;Using a small data set from Taiwan, a rigid stick was subjected to the recorded rotational and translational ground motions. Comparing the displacements between translational only ground motions to combined translational and rotational motions, it was found that a 15 to 60 percent increase in displacements occurred with the addition of rotational ground motions. However, the data used is only from one site and contains only 52 earthquakes, so results are highly skewed to this particular site. To look into how different sites and conditions would affect the results from Taiwan, a closed form solution of a seismic wave field was generated and the effects of changing different parameters was examined. This showed that the amount of displacement is highly dependent on the site conditions and other aspects specific to each earthquake. Finally, a set of synthetic three dimensional waves, generated from the seismology program "tk" for a site subjected to the Northridge Earthquake, were put into DEEPSOIL. By adjusting soil the soil dampening, a one dimensional site response was able to produce accelerograms that matched the three dimensional synthetic accelerograms. The matching of these accelerograms shows us that the modelling of a site using a one dimensional method does not mean that the motions are one dimensional. More analysis of empirical data is needed to fully understand these concepts and how much of an effect rotational motions have on displacements. This data is currently being collected by seismologists around the world.