shapes and frequencies in the Time-History analysis in determining the seismic response of the coupled tower system. A linear transient time-history analysis ...
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Proceedings, 3 International Earthquake Symposium, Bangladesh, Dhaka, March.5-6 2010,
DYNAMIC ANALYSIS OF TRANSMISSION TOWERS UNDER STRONG GROUND MOTION
Gopiram Addala1, D.Neelima Satyam2 and Ramancharla Pradeep Kumar3
ABSTRACT Transmission tower lines are one of most important life-line structures. It is necessary that these structures are designed for seismicity. The dynamic behavior of a single transmission tower and transmission tower system that are linked by conductors has been studied in this research work. The dynamic analysis is performed on both the single transmission tower and the tower line system. The wind force acting on the tower is considered as an equivalent static force. The seismic analysis of the tower has been performed in SAP2000 Nonlinear computer program (SAP2000, Computers and Structures). The tower elements are modeled using 3 D frame elements. The tower has been subjected to North-ridge strong ground motion (1994) and the Koyna strong ground motion (1967) to study the dynamic behavior as described in the present work. Parametric study has been carried out by studying the influence of cable on the tower when the force in the cable is applied at different angles to the cross arms. The non-linear material behavior of the leg members of the tower has been studied when it is subjected to the two strong ground motions considered. To understand the non-linear behavior of the material bi-linear model has been considered and it is analyzed in the present work.
Introduction Transmission towers are necessary for the purpose of supplying electricity to various regions of the nation. Seismic design of transmission towers is important in the earthquake vulnerable areas (Ying and Chien, 2005 ). Transmission towers are classified based on their usage and based on the number of circuits. The transmission towers are mainly designed for the forces due to wind, ice and other loading conditions but not for the seismic forces. Since more than 60% of the Indian Sub-continent is prone to moderate to severe earthquakes it has become more vital to design the life line systems for seismic safety. During the Northridge earthquake (1994) there are instances of damages to power lines, transformers and power stations etc. In spite of the level of importance attached to these structures a very few countries are following guidelines for seismic design of towers. In addition to the single tower, the behavior of transmission tower line system which is 350m apart is considered in the present study to understand the response of these life line structures under strong ground motions. 1
Graduatestudent, Earthquake Engineering Research Centre, IIIT Hyderabad, Hyderabad-32, India Assistant Professor, Earthquake Engineering Research Centre, IIIT Hyderabad, Hyderabad-32, India 3 Associate Professor, Earthquake Engineering Research Centre, IIIT Hyderabad, Hyderabad-32, India 2
Gopi A, Neelima Satyam D and Ramancharla Pradeep Kumar
Modeling The tower selected was angle tower. Angle towers sometimes called as semi-anchor towers are used where the line makes a horizontal angle greater than 20 .The model of the tower bracings is offset or staggered bracing system. This model of tower has been selected because it is one of the most widely used models of the tower. The sections used for the tower are all mild steel angles conforming to IS-2062 (Grade A). The transmission tower has been modeled and designed for the wind and other loading conditions. The tower is modeled in STAAD PRO (STAAD PRO, Bentley Corporation) and then imported to SAP 2000 (SAP2000, Computers and Structures). This program was chosen because of its dynamic analysis capabilities. Once the tower has been imported the material properties were assigned to the members of the tower. Then the tower was replicated in the y-direction at a distance of 350m. The transmission tower is fixed at the base. These two towers are connected by cables made of Aluminum Centered Steel Reinforced (ACSR). The height of the tower is 35.54m and the base width of the tower is 7m. The angles used were ISA 110x110x15mm, ISA 110x110x8 mm, ISA 100x100x8mm, ISA 60x60x4mm and ISA 45x45x4mm (IS 802, 1995 and. IS 800, 1984). The properties of the tower and conductors are shown in Table 1.
Table 1. Properties of the Transmission Tower System Parameter
Tower
Conductor
Type
132Kv Double Circuit
20mm dia 7strands, ACSR
Weight
340kN
148kg/km
Height
35.54m
350m
Analysis
The tower that has been modeled in STAAD PRO is imported into SAP2000 for analysis. The loadings considered on the tower are the self weight of the tower and the wind load, as a equivalent static load acting on it. The model was analyzed with SAP2000 using both an Eigenvector and Time-History analysis. The Eigenvector analysis was used to find the un-damped free-vibration mode shapes and frequencies of the tower system. The program used these mode shapes and frequencies in the Time-History analysis in determining the seismic response of the coupled tower system. A linear transient time-history analysis was chosen to find the seismic response of the coupled-transmission tower system (Faisal Abdullah,1999). The directions of the ground acceleration load were chosen to act both transverse (y-direction) and longitudinal (xdirection) to the alignment of the conductor .In present work the forces that were developed in the leg members of the transmission tower were studied. The reason for selecting leg members was if one of the legs fails the entire structure collapses (Mohamed Mohsen , 1998). Parametric study is carried out on the tower by applying the tension developed in the conductor at three angels 5 0, 100 and 150 to the cross arms (in this the mass of the wire is neglected). The time periods of the tower and tower system are as shown in Table 2. Figure 3 shows the mode shapes of single transmission tower.
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Proceedings, 3 International Earthquake Symposium, Bangladesh, Dhaka, March.5-6 2010,
5.6m
5.2m 5.3m
35.4m
19.27m
7m
Figure 1.
Model of Single Transmission Tower
150m 350m 150m
Figure 2.
Transmission Tower Line System
(Not to Scale)
Gopi A, Neelima Satyam D and Ramancharla Pradeep Kumar
Figure 3.
Mode Shapes of the Single Transmission Tower
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Proceedings, 3 International Earthquake Symposium, Bangladesh, Dhaka, March.5-6 2010,
Table 2.
Time Periods of Tower and Tower System
Mode Number
Time Period Time Period (Single (Tower system) tower) seconds seconds
1
0.2920
0.2926
2
0.2890
0.2920
3
0.1183
0.2902
4
0.1180
0.2896
5
0.1106
0.1183
6
0.1065
0.1183
7
0.1065
0.1180
8
0.0933
0.1180
9
0.0884
0.1135
10
0.0857
0.1139
11
0.0791
0.1132
12
0.7868
0.1132
Results and Discussion In the present work, dynamic analysis is performed on a single transmission tower and transmission tower line by subjecting them to two ground motions viz., Northridge (1994) and Koyna (1967) strong ground motions. The force developed in the leg members of the single tower under Northridge (1994) strong ground motion in the present parametric study by applying the tension in the cable at three different angles 50,100 and 150 to the cross arms of the transmission tower was around 430kN. The leg members of the tower were designed for a force of 330KN, which is less than the compressive force developed in the leg members 229 and 5. Permissible stress from IS 800-1984 for leg member number 229 is 136N/mm2. But the stress developed in the leg member number 229 due to force developed exceeds this permissible stress (136N/mm2). This clearly shows that the member is moving into a state of Geometric non-linearity. The forces developed in the leg members in present the parametric study, when subjected to Northridge strong ground motion are as given in Table 3. The stress-strain graph of the leg members 5 and 229, if the material goes into a state of non-linearity is as shown in Fig. 4. From the analysis it is observed that the forces developed in the transmission tower line systems when it was subjected to Northridge (1994) strong ground motion were less than the forces in the single tower. It was also observed from the analysis that the forces developed when the Northridge (1994) ground motion is applied in direction of cables are more when compared to forces developed when the ground motion is applied perpendicular to the conductors as shown in Table 4. To understand the complete behavior of transmission tower line it is necessary to model the cables. The present work can be extended in the future by modeling the cable members and performing the dynamic nonlinear analysis.
Gopi A, Neelima Satyam D and Ramancharla Pradeep Kumar
Table 3.
Compressive Forces developed in Leg members when tension is applied at various angles to cross arms
S.No
Leg Member
Compressive Force(kN) at 50 to cross arm
Compressive Force(kN) at 100 to cross arm
Compressive Force(kN) at 150 to cross arm
1
5
378
371
384
2
229
430
423
436
(a)
Figure 4.
(b)
Graphs displaying the Non-Linear Behavior of Leg Member (a) 229 and (b) 5
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Proceedings, 3 International Earthquake Symposium, Bangladesh, Dhaka, March.5-6 2010,
Table 4: Forces developed in leg members when strong ground motion is applied parallel and perpendicular to transmission tower system
S.No
Leg Member
Force(kN) when ground motion is Parallel to conductors
Force(kN) when ground motion is Perpendicular to conductors
1
5
275
186
2
171
299
193
3
203
336
251
4
229
327
272
References
SAP2000, Version 12, Computers and Structures, Inc., Berkeley, California STAAD PRO, Bentley Corporation. Ying-Hui Lei and Yu-Lin Chien, (2005) ”Seismic Analysis of Transmission Towers Considering both Geometric and Material Nonlinearities”, Tamkang Journal of Science and Engineering, 8 (1), 29-42 . Faisal Abdullah Al-Mashary (1999) “Non-Linear Analysis of Transmission Towers”, J.King Saud Univ, 11 (1), 19-32. Mohamed Mohsen El-Attar (1998) ”Non-linear dynamics and seismic response of Power Transmission Lines”, PhD Thesis, McMaster University, Canada IS 802(Part 1/Section 1)-1995, Bureau of Indian Standards. IS 800-1984, Bureau of Indian Standards.
