ANALYTICAL DETERMINATION OF EQUIVALENT MODAL DAMPING RATIOS OF A COMPOSITE TOWER IN WIND-INDUCED VIBRATIONS

We present two methods to determine the equivalent modal damping ratio s of a composite tower under wind-induced vibrations. Different dampin g characteristics arise from the construction of the tower with two ma terials; for the lower main part, say of reinforced concrete and an up per part, say of steel. The first method employs a detailed numerical integration procedure in which the static condensed stiffness matrices are established by the transfer matrix method. The damping matrices o f the upper and the lower substructures are modelled with the Rayleigh damping formulation. The second method makes use of a simplified appr oximation of two lumped masses. Computation using the two methods is d emonstrated with a numerical example for a composite T.V. tower of a t otal height 459 m. Results show that, while the detailed method gives good accuracy but demands intensive computation, the approximate metho d involves simple calculations and produces data of enough precision f or engineering applications.