Dynamic Stiffness And Modal Parameter Identification Of Cables

The dynamic stiffness of cables describes the load-deformation behavior of cables subjected to dynamic loading, which can represent the dynamic properties of cable structures accurately. The dynamic stiffness is basically defined as a ratio of response to excitation. From the viewpoint of modal analysis, this definition is identical to the frequency response function (FRF) that reflects the intrinsic characteristics of cables. The FRF can be synthesized using the measured excitation and response signals from cable modal tests. By fitting both cable dynamic stiffness and measured FRF, then, the FRF is cured fit to identify the modal parameters of cables. The main work of this thesis includes1. Assuming that the cable static equilibrium configuration is a parabolic profile and considering the damping, a closed-form expression of dynamic stiffness is derived based on the cable vibration differential equation. The derived cable dynamic stiffness has been compared with the cable static stiffness and the physical meaning of each item in the cable dynamic stiffness is clarified.2. Comprehensive numerical studies have been carried out on the closed-form expression of dynamic stiffness. Several simplified cable dynamic stiffness models are discussed and compared. Based on the numerical study results, a simplified dynamic stiffness model is presented for the purpose of cable modal parameter identification with a good accuracy.3. A series of cable modal tests have been conducted in the laboratory with different cable tension levels and corresponding FRFs are obtained. A number of issues related to the cable modal tests have been discussed such as sampling frequency, hammer toughness, excitation placement, accelerometer arrangement, frequency resolution, windowing and average technique. As a result, the FRFs are estimated with a relative accuracy form measured excitation and responses.4. Based on the proposed simplified cable dynamic stiffness and measured FRPs, the cable modal parameters (frequencies and damping ratio) have been successfully identified by using a least square curve fitting technique. Subsequently, a new method to estimate the cable modal parameters is established.