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EFFECT OF SEISMIC SOIL-STRUCTURE-INTERACTION ON TRANSMISSION TOWERAuthor: Anshu Singh Rajpoot Date: 2020-05-20 Report no: IIIT/TH/2020/38 Advisor:Pradeep Kumar Ramancharla AbstractTransmission tower is a 3-D truss and indeterminate structure, which plays key role in the operation of a reliable electrical power system. Transmission line tower has to support conductors carrying heavy electrical power and more than one ground wires at appropriate distancesfrom each other above the ground level. Also transmission tower are more commonly used in telecommunication for broadcasting purpose. The total cost of transmission line towers is about 35 to 45 % of the total cost of the transmission line. Apart from conductor load it has to sustain all kinds of natural disaster. So transmission tower designing is an important engineering aspect where knowledge of various engineering fields viz, civil, mechanical and electrical engineering is required. Depending on the purpose and location, transmission towers are designed by varying the shape and size of the tower member.The change in shape and size of diagonal bracesof transmission tower leads to unconventional results like buckling and uplifting.These effects are significant when soil -structure –interactions are considered. Several past studies have shown that large number of structures mainly tall structures damages has been occurred during earthquake due to behaviour of underlying soil and foundation because of geotechnical properties of different landscape units will change dynamic state and greatly influence on the response super structure.Large number of transmission tower and telephonic tower were failed during landers earthquake(1992), Kobe earthquake (1995),chi-chi earthquake(1999),Wenchuan earthquake(2008) etc. In all these earthquake there is one thing common apart from other parameters like earthquake magnitude and frequency, design of structure as per conventional design, ductility of structure and construction quality, there is great influence of interaction between soft and medium soils with the structure which greatly affect the behaviour of the transmissions tower. Therefore it is essential to study the effect soil-structure-interaction on Transmissions tower. Numerical analysis has been performed by two methods i.e., sub-structure (using soil springs at the base) and other one direct method for analysis of SSI. In direct method integrated structure-foundation-soil system is analysed by finite element software ABAQUS. Numerical simulations have been carried out on transmission tower with fixed base, isolated footing, isolated pile foundation, raft foundation and raft pile foundation on soft and medium soil conditions. The aim of this study is to find how soil and foundations behaviour influences the behaviour of transmission tower during earthquake loading. After considering SSI effects in to analysis, In case of soft soil, isolated foundation cannot be used because of excessive lateral deflection, instead of that raft foundation or isolated foundation should be used. Raft foundation would me more economical than the other foundation types. In case of medium soil (density > 1700kg/m3), no need to opt for deep foundation and raft foundation. Isolated foundation would be economical for transmission tower using SSI help. In other words, consideration of SSI in case of medium soil is beneficial as base shear stress reduced as compare to fixed base and also maximum lateral deflection is in the limit. Whereas in case of soft soil base shear stress is reduced but lateral deflection is problem, which leads to failure. So if SSI is not taken in to consideration accuracy in assessment of transmission tower safety could not be reliable. Full thesis: pdf Centre for Earthquake Engineering |
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