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Active control in civil engineering

Deliverables

Bridges are required for the transportation infrastructure of any country. Cable-supported structures and particularly long-span suspension or cable-stayed bridges are amongst the most important structures of the last fifty years. Improvements in materials and computational technology have led to the construction of progressively longer, structurally more efficient and slender bridges. Consequently, the structures are more and more flexible. The mitigation of the vibrations has become a major issue in cable-supported structure design. It is a difficult problem to assess because of the highly non-linear behaviour of cables with sag. Avoiding significant levels of wind excited oscillations, and in the worst case flutter instability, is a new challenge for the designers. In the long term, there is a potential for serious fatigue damage. In the short term, excessive levels of vibration hamper the traffic and reduce the end-user comfort. The aim of this research was to bring the laboratory development to an industrial level and to implement it into complex civil structures. The main thrust of the research programme was to produce active tendon devices. Consequently two types of actuators were developed and tested: -A large magnetostrictive actuator, as an extension of the devices used with the small-scale mock-up, but of course a very innovative but previously unexplored solution due to the high forces required in civil engineering applications. -An hydraulic actuator, more ‘classical’ in the field of civil engineering cable-supported structures, but more sensitive to phase lag and response speed requirements. The design and construction methods employed for the magnetostrictive actuators represents a step forward in the state-of-the-art for this technology; they are at present the largest manufactured in Europe. Although their performance will not meet the overall project objective of damping the whole mock-up structure by the incorporation of the actuators in series with the main stay cables, useful damping of the cables has been achieved with a parallel system. In addition, the practical, application-specific, information arising from the mock-up tests, together with general advances in the technology anticipated in the relatively near future, has provided the basis for further exploitation of magnetostrictive technology in this and a range of other market sectors.
To minimise cable vibrations, passive damping devices for cable-supported structures have already been developed and used, especially dashpot dampers, cable-ties and viscous elastic systems. These devices are generally efficient enough to reduce the vibrations of the cables, but they cannot reduce the vibrations of the complete structure. To minimise structure vibrations, Tuned Mass Dampers have also been studied. Their efficiency is often limited by geometrical constraints of the deck cross-section. In addition, all these passive devices are tuned on the results of theoretical simulation, which may not fully define all applications, and rely on data obtained from previously defined scenarios. Deck streamline profiles have also been developed in order to take advantage of aerodynamic damping. Their efficiency is determined by the specific wind conditions and subjected to the knowledge of the aerodynamic behaviour of the structure. Another way to reduce the vibrations of the complete structure is the use of an active damping strategy. The aim of the active control system is to upgrade the structural damping of the complete structure and consequently to mitigate the induced vibrations of both the cables and the cable-supported structure. The methodology considered for this research programme is based on producing an active tendon including an actuator that is collocated with a force sensor. The active damping is driven by the displacement control of the cable anchor point. This technique developed at Université Libre de Bruxelles has a strong physical support and its effectiveness has already been confirmed experimentally by tests performed on small-scale laboratory mock-ups. (a) Université Libre de Bruxelles has developed an approximate linear theory assuming that the dynamics of the active cables can be neglected and that their interaction with the structure is restricted to the tension in the cables. The experimental campaign has shown that this theory could be extended to complex civil structures. (b) Analytical design models have been developed regarding the actuator behaviours. (c) Manufacturing procedures have been developed by the device manufacturers. (d) Application of the active control strategy to real structures by the civil engineering experts including technical and economic aspects has exhibited the efficiency of the system and the large field of application to cable-supported structures.

Exploitable results

The design and construction methods employed for the magnetostrictive actuators represents a step forward in the state-of-the-art for this technology; they are at present the largest manufactured in Europe. Although their performance will not meet the overall project objective of damping the whole mock-up structure by the incorporation of the actuators in series with the main stay cables, useful damping of the cables has been achieved with a parallel system. In addition, the practical, application specific, information arising from the mock-up tests, together with general advances in the technology anticipated in the relatively near future, has provided the basis for further exploitation of magnetostrictive technology in this and a range of other market sectors.
-Universite Libre de Bruxelles has developed an approximate linear theory assuming that the dynamics of the active cables can be neglected and that their interaction with the structure is restricted to the tension in the cables. The experimental campaign has shown that this theory could be extended to complex civil structures. -Analytical design models have been developed regarding the actuator behaviours. -Manufacturing procedures have been developed by the device manufacturers. -Application of the active control strategy to real structures by the civil engineering experts including technical and economic aspects has exhibited the efficiency of the system and the large field of application to cable-supported structures.

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