Student: Kenneth Kleissl
Supervisors: Christos Georgakis, Holger Koss
Testing performed at: FORCE Technology, wind tunnel laboratory
Through improved bridge monitoring and a greater openness between bridge owners and engineers, it has become increasingly apparent that a large number of the world's long-span cable-supported bridges suffer from some form of cable vibration. These vibrations have the potential to lead to long-term fatigue damage and economic loss, through a reduction of consumer confidence. Examples of bridges with a history of cable vibrations include the First and Second Severn Crossings, Øresund Bridge, Storebælt East Bridge, Humber Bridge, and the Fred Hartman Bridge.
Attempts to eliminate or dampen these vibrations have been met with varying degree of success. To date no ultimately successful cable vibration control system has been devised for all types of cables under all conditions. This is most probably due to the fact that the observed vibrations are a result of varying excitation mechanisms, several of which may need differing control strategies to combat. Tested control systems have included cable-ties, viscous/magnetoreological dampers, tuned mass dampers, spiral strands and dimples.
Figure 1: Test setup to investigate the different cable shapes.
The objective of this project was to examine the viability of modifying cable shape and surface, for the elimination of not only cable galloping, but also vibrations due to vortex shedding and buffeting - always under the prevalent meteorological conditions in Scandinavia. Several of these passive modifications can be seen in Figure 2 and include a wavy cylinder, faceted cylinder and shrouded cylinder.
Figure 2: (from left to right) plain cylinder, wavy cylinder, faceted cylinder and shrouded cylinder.
Experimental work in a high-speed wind tunnel facility have been undertaken for varying flow conditions, angles of attack and cable inclination, using a static test rig. The theoretical work focus on the application of instability models based on the quasi-steady theory of moving bodies in a flow. Results of such aerodynamic instability evaluation can be seen in Figure 3 and include both the detuned and perfectly tuned systems.
Figure 3: (2 plots on left) Aerodynamic instability of plain circular cylinder – negative aerodynamic damping,
(2 plots on right) Aerodynamic stability of shrouded cylinder – positive aerodynamic damping.
Initially, the goal of this project was to investigate, if it is possible to introduce an effective uniform damping along the cable by injecting a viscous material between the strands and the polyethylene tube. It is believed that introducing a uniform damping will be the most effective way to dampen a cable, because of the low bending stiffness and structural damping. During the process literature were found applying the exact same analytical approach of complex stiffness, showing that even with a fictive viscous material with the perfect stiffness-damping ratio, the damping ratio obtained was too low.