Computer-aided free vibration analysis of guyed towers

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Militano, Giovanni
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Guyed towers are used for communication purposes and are frequently designed to heights of 300 meters (1000 feet). A guyed tower is a non-linear structure in which the mast, typically consisting of multiple truss members is supported laterally at several points by inclined guy cables. The guy cables are anchored to a foundation and are pre-tensioned. Wind induced vibrations may result in a fatigue failure of a guy anchor linkage or a cable, ultimately causing the collapse of an entire tower. In addition, excessive deflections or vibrations may interfere with communications and control systems resulting in serviceability failure. According to design standards, a basic understanding of the dynamic characteristics of guyed towers is important. This study develops an easy to use software package, aimed at practicing engineers, to determine free-vibration characteristics (natural frequencies and mode shapes) of guyed towers. The analysis presented herein employs the finite element method to determine the naturalfrequencies and mode shapes of guyed towers. The structural system is broken down into two main components, the tower mast and guy cables. Utilizing an equivalent beam-column analysis that take into consideration different lacing patterns, the mast is modelled as a beam-column with equivalent properties. A three-dimensional cable finite element following a catenary or parabolic profile is used to model guy cables. Several comparisons are made to ensure the accuracy of the cable element and the guyed tower model. Selected numerical results are presented for natural frequencies and mode shapes of a few representative towers. The influence of cable tension, lacing pattern of the mast and configuration of guy cables is also examined. The software package can be extended to include forced vibration response through the modal superposition method.