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SEISMIC RESPONSE OF POWER TRANSMISSION TOWER-LINE SYSTEM UNDER MULTI-COMPONENT MULTI-SUPPORT EXCITATIONS

Vol. 6, No. 4 (2012)《Journal of Earthquake and Tsunami》

Abstract：The effect of multi-component multi-support excitations on the response of power transmission tower-line system is analyzed in this paper, using three-dimensional finite element time-stepping analysis of a transmission tower-line system based on an actual project. Multi-component multi-support earthquake input waves are generated based on the Code for Design of Seismic of Electrical Installations. Geometric non-linearity was considered in the analysis. An extensive parametric study was conducted to investigate the behavior of the transmission tower-line system under multi-component multisupport seismic excitations. The parameters include single-component multi-support ground motions, multi-component multi-support ground motions, the correlations among the three-component of multi-component multi-support ground motions, the spatial correlation of multi-component multi-support ground motions, the incident angle of multi-component multi-support seismic waves, the ratio of the peak values of the threecomponent of multi-component multi-support ground motions, and site condition with apparent wave velocity of multi-component multi-support ground motions.
Keywords：Power transmission tower-line system; geometric nonlinearity; multicomponent multi-support ground motions; apparent wave velocity.

Introduction
Power transmission tower-line system is an important facility of a power system.Its failure may lead to the outage of power supply. Until now, much of research effort has been focused on the actions of static load, impulsive load, and equivalent static wind load [American Society of Civil Engineerings Committee on Electrical Transmission Structures, 1982; American Society of Civil Engineerings, 1991;Mozer et al., 1977]. Only very few studies have dealt with the dynamic load. Design codes such as the code for Design of Seismic of Electrical Installations (GB 50260-96)[Ministry of China Electrical Industry, 1996] and the Design Regulations on 110 ∼500 kv Overhead Transmission Line (DL/T 5092-1999) [Huadong Electrical Power Design Institute, 1999] do not provide guidelines on how to consider the effects of lines in seismic analysis of transmission tower-line system.
There have been several cases of damage to transmission towers and lines during earthquakes. For instance, during the 1992 Landers earthquake, about 100 lines were broken [Hall et al., 1995] in the city of Los Angeles. During the 1995 Kobe earthquake, 38 transmission lines were damaged and 20 towers tilted as a result of foundation settlement [Shinozuka, 1995]. In the 1999 Chi–Chi earthquake, many lines were broken and some towers collapsed [Yin et al., 2005]. Figure 1 shows some pictures of damaged towers in Sichuan electric network during the 2008 Wenchuan earthquake in China.
In the past one or two decades, some research has been conducted to develop simplified analytical approaches for transmission tower-line system. For instance, Li et al. [Li et al., 2003, 2004, 2005] have completed a number of studies on seismic effects on transmission towers and have verified that the effect of lines in seismic design should not be neglected. Ghobarah et al. [1996] investigated the effects of multi-support excitations on the response of overhead power transmission lines. They modeled the transmission towers using space truss elements and the cables by straight two node elements, in which the system was subjected to spatially incoherent seismic ground motions. Tian et al. [2008] synthesized multi-support time histories of earthquake ground motion and analyzed the power transmission tower-line system under multi-support excitations considering traveling-wave and coherency effect, and the results showed that it was necessary to consider multiple support excitations in transmission tower-line system analysis. In all these studies,the transmission tower-line systems were assumed to be subjected to singlecomponent excitation.
Longitudinally extended structures such as lifeline systems are often multisupported and each support is subjected to multi-component ground motions during an earthquake. To date, seismic behavior of the transmission tower-line system subjected to multi-component excitations has not been investigated. This paper deals with such an analysis using three-dimensional finite element model. The multi-component multi-support earthquake input waves are generated based on the Code for Design of Seismic of Electrical Installations [Ministry of China Electrical Industry, 1996]. The time domain analysis takes into account geometric non-linearity due to finite deformation. The parameters studied include singlecomponent multi-support ground motions, multi-component multi-support ground motions, the correlations among the three-component of multi-component multisupport ground motions, the spatial correlation of multi-component multi-support ground motions, the incident angle of multi-component multi-support seismic waves, the ratio of the peak values of the three-component of multi-component multi-support ground motions, and site condition with apparent wave velocity of multi-component multi-support ground motions.

Conclusions
This paper study the seismic response of transmission tower-line system subjected to multi-component multi-support excitations. The effects of single-component multi-support ground motions, multi-component multi-support ground motions,correlations among the three-component of multi-component multi-support ground motions, the spatial correlation of multi-component multi-support ground motions, the incident angle of multi-component multi-support seismic waves, the ratio of the peak values of the three-component of ground motion, and site condition and apparent wave velocity of multi-component multi-support ground motions on the responses of the transmission tower-line system are investigated. The results of the parametric study lead to the following conclusions:
(1) The responses of the transmission tower-line system under multi-component multi-support excitations are obviously higher than those of system under single-component multi-support excitations. The responses of system are governed by the longitudinal component of ground motion.
(2) The higher is the correlations among three-component of ground motion, the larger is the system responses. The degree of the correlations among threecomponent of ground motion is carried out by changing the angle between the direction of major principal axis and the direction of wave propagation. The influence of the degree of the correlations among three-component of ground motion on the system responses is not obvious.
(3) The lower is the spatial correlation of ground motion, the larger is the structural responses. In order to obtain a representative analysis, the degree of the spatial coherency of multi-component multi-support ground motions is needed to consider.
(4) The incident angle of multi-component multi-support seismic waves has a significant effect on the responses of system. The direction of wave propagation is assumed to coincide with the longitudinal direction of the transmission towerline system could not obtain the maximum responses of transmission tower.
(5) The ratio of the peak values of the three-component of multi-component multisupport ground motions have considerable effects on the response of the system.The relative magnitude of the component which coincides with the longitudinal direction of the transmission tower-line system determines the magnitude of the dynamic responses in the tower and lines.
(6) Different site condition and apparent wave velocity of ground motion also have a significant influence on transmission tower-line system responses. Owing to the complexity of the large span structure, it is very difficult to give general conclusions from the researches on a single transmission tower-line system model. However, results from this study demonstrate the importance of considering the above parameters on seismic responses of transmission tower-line system. More studies are deemed to further investigate the multi-component multi-support effect on responses of the transmission tower-line system.

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