Seismic Performance of a Steel Transmission Tower During the 2011 Great East Japan Earthquake

Abstract eng:
The Central Research Institute of Electric Power Industry (CRIEPI) has launched a field observation study on an ultrahigh voltage steel transmission tower approximately 140m in height to evaluate the soundness of aged steel transmission towers under natural environmental conditions, including earthquakes and strong winds. In order to study earthquake and wind resistance we set up a variety of instruments to observe ground motion, acceleration and axial forces The instruments recorded ground motion and the dynamic response of the tower during the 9.0 Mw Tohoku Earthquake. The observation provided rare data on the earthquake resistance capabilities of steel transmission towers. We examined earthquake-induced dynamic response characteristics based on axial forces and acceleration time histories of members and we performed frequency analyses. Horizontal peak ground surface accelerations of about 0.5G were amplified roughly five or six times at the top of the tower. The amplification was governed by the second stage of the tower, because the ground motion involved relatively higher frequency components. We applied acceleration data taken from the top of the tower and story shear force data from some of the panels to estimate damping ratios for the first and second modes. Using the half-power method, we estimated all of the ratios to be approximately 1%. Some residual axial force extended to the primary members, but peak axial forces remained below the buckling strength for those members. In addition, visual data obtained by remote industrial television cameras showed that the earthquake induced vibrations simultaneously at the redundant members and along the entirety of the tower. This indicates that there was a slight dislocation among members at the connection panels during the earthquake.

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