STUDIES OF DYNAMIC RESPONSE OF A GUYED TOWER
DARBRE, GEORGES ROBERT
Doctor of Philosophy
The objective of this investigation was to contribute to improved understanding of the dynamic response of offshore guyed towers. This was accomplished through studies of the following interrelated problems: (1) Effects of cable and foundation constraints on the natural frequencies and modes of vibration of representative structural systems; (2) Low-amplitude harmonic stiffness and free vibrational characteristics of inclined parabolic cables; (3) Effects of cable-tower interaction on the dynamic response of the tower; and (4) Response of representative guyed tower models to simulated wave loadings. Both the force method and the displacement method are applied to the analysis of the first problem, and the interrelationship of the two methods is elucidated. A study is made of the factors that affect the natural frequencies and modes of vibration of guyed towers, and comprehensive numerical data are presented for a proposed structure which illustrate the sensitivity of the results. Closed-form expressions are presented for the low-amplitude harmonic stiffness of a viscously damped, inclined parabolic cable, and extensive numerical data are included to illustrate the effects of the various parameters involved. Both a complete and an approximate solution are considered, and their interrelationship is examined. In addition, a single-degree-of-freedom model is proposed which reproduces with good accuracy the salient features of the response of the system over a wide range of conditions. The free-vibrational characteristics of simply supported cables are re-examined, and a simple procedure is presented for evaluating the complete spectrum of natural frequencies. Useful closed-form expressions are includedl for certain infinite series involving integrals of the natural modes of vibration of cables. Also described is a finite method of analysis for the problem. The effects of cable-structure interaction are investigated for a harmonic condition of loading using mainly a single-degree-of-freedom representation for the tower. The implications of the results for more complex representations of the tower and for transient excitations are also considered. The nonlinear response of the guyed tower is evaluated by application of the pseudo-force method in combination with the modal method of analysis. Flow charts are included for a computer program developed to implement the analysis. The response of representative structures is evaluated for simulated wave loadings obtained from a Pierson-Moskowitz wave spectrum and Morison's equation for wave forces, and the results are analyzed and discusssed.