Semi-active systems for the wind effects mitigation of a suspension bridge

The world today is facing a growing need of control of the increasing number of large structures, as suspension bridges. The modern design of complex structures must be in line with the definition and evaluation of performance, while safety must be assessed under different conditions. Structural control solutions can give an important contribution so as to satisfy the high standard of performance, feasibility and safety. Dynamic loading from interaction with the wind is regarded as one of the most aggressive external excitation for long-span flexible structures in terms of displacements and internal actions. The increased frequency of extreme events like hurricanes in many parts of the world suggests for new design the analysis, and for existing structures the re-analysis, of the response in various configurations. In this paper a suspension bridge model is accounted as a case study for performing numerical simulations. The wind loading is considered the main dynamic excitation and it is applied on the towers, the cables and the deck of the suspension bridge under study. The wind load, is simulated as a spatially correlated process and acts in the horizontal direction, transversal to the deck. The multipurpose ANSYS finite element (F.E.) code is used as the work frame and the wind intensity is tuned at different levels so as to investigate the bridge response with sufficient generality. A preliminary study of the optimal passive control strategy for the suspension bridge allows to determine the reference control parameters for developing the semi-active approach in a decentralized configuration. This type of control systems enjoys the same positive qualities of passive ones requiring a reduced amount of active external power which is intended to modify the rheology of the semi-active structural devices and not to feed themselves, as in the active case. Additionally it presents robustness qualities in presence of a device failure due to the fact each device works independently from the others. From a structural point of view semi-active systems are able to modify the stiffness and damping characteristics of the bridge without introducing mechanical energy. This aspect ensures that the controlled system behaves in a stable manner during the response to a dynamic excitation. Several semi-active control law are implemented for managing the semi-active devices and their effectiveness is pointed out. They should be able to adapt the characteristics of the devices within the same level of excitement and even in cases where the level of excitement is overall more intense. In other words, the control laws modulate the level of energy dissipation both during an event when the average wind speed remains constant and in cases the average wind speed increases considerably in intensity. Finally the proposed solutions are compared, establishing their relative performance