Keywords: buckling, differential settlement, dynamic analysis, transmission line, steel tower, earthquake.

Since the early 1900s when steel towers were adopted, a trunk line network of electric power transmission has been constructed, along with generating higher voltages and expanding the transmission distance of electric power. However in September 1999 when the Ji-Ji earthquake hit Taiwan, many transmission towers collapsed. This was an unprecedented quake disaster of transmission towers in Japan. Studies [1,2,3] conducted in the past focused on the fact that towers collapsed even though Taiwan's design specification was slightly stricter than that of Japan and that the steel towers examined have unequal length legs. The importance of seismic performance evaluation of steel towers with unequal length legs was pointed out.

This study focused on 220kV steel towers with equal length legs and carried out a fundamental study of their quake resistance performance. Two analytical models of the single transmission line tower were prepared. Evaluating the impact of ground differential settlement in an earthquake, two boundary conditions are presumptively set, one is a four-point support and the other is a three-point support which is the most unfavourable condition. Natural periods and vibration modes of the two models are compared and the records observed in the 1995 South Hyogo Prefecture earthquake were applied to the two models.

To evaluate the dynamic response of two models, acceleration time history and displacement time history of the tower top are compared. In the case of Jmakobe NS, response acceleration and response displacement are gradually decreased in the four-point support model, but the decrease of the response is relatively small in the three-point support model with respect to the transversal input. In the case of Takatori EW, occurrence times of maximum response are different for each input. Double the response displacement occurred in the three-point support model. In the case of the Higashi transversal, the response displacement of the three-point support model increased by about 40% for the longitudinal input. An increase of the response in the three-point model is predominant in the case of the Takatori EW and responses are relatively large with respect to the longitudinal input.

Simplified single tower models for 220kV steel towers considering the effects of transmission lines and ground differential settlement were proposed. The models can accurately simulate the dynamic behavior of the steel tower during the earthquake. Vibration modes of the four-point support model and the three-point support model obtained from eigenvalue analysis were confirmed. Dynamic responses of the three-point support model were significantly increased. The three-point support structure of the steel towers is greatly affected by differential settlement, which influences the seismic performance of the steel towers at the time of earthquakes.

1

Japan Society of Civil Engineers, "The 1999 Ji-Ji Earthquake, Taiwan -Investigation into Damage to Civil Engineering Structures", 1999.

2

T. Mazda, H. Otsuka, H. Uchida, S. Ikeda, "A study on earthquake responses of steel tower with additional damping", ASME PVP-Vol.428-2, Seismic Engineering, 43-48, July 2001.

3

T. Mazda, H. Otsuka, S. Ikeda, K. Uno, "A study on damage of transmission steel tower with unequal legs in the chji-chi earthquake Taiwan", Proceedings of 13th World Conference of Earthquake Engineering, 739, 2004.

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