Bridges are structures often constructed for the main purpose of offering transit between two locations separated by a geological feature. Several types of bridges are available and selection of the type of bridge depends on numerous factors such as economy, design feasibility and existing geological layout. Cable-stayed type is suitable for bridges having medium to long span range. However, due to low inherent damping and flexibility, stay cables on cable-stayed bridges are susceptible to dynamic excitations. In the past, either aerodynamic countermeasures or mechanical solutions have been developed to mitigate excessive cable vibrations, the latter of which mainly includes the attachment of an external damper to the vulnerable cable to increase its damping or using cross-tie(s) to connect the vulnerable cable to its neighbouring ones to improve its in-plane stiffness. However, the effectiveness of the damper solution is limited by the damper installation location, whereas cross-tie cannot help to directly dissipate system energy. Combination of these two solutions gives rise to the hybrid system which offers better control on cable vibration. Unfortunately, little has been published with regard to the dynamic response of such hybrid systems. Worse off, much of the little that is published is not applicable to a generalized case since it was developed for a hybrid system with specific layout.
Recently, Professor Shaohong Cheng and Dr. Faouzi Ghrib at University of Windsor in Canada, in collaboration with Dr. Javaid Ahmad at National University of Computer and Emerging Sciences in Pakistan, developed a generalized approach for formulating an analytical model of hybrid systems which will allow the development of an analytical model of a complex hybrid system based on a relatively simple parent system. The researchers expected that their proposed model would greatly save modelling effort and provide a convenient and useful tool for evaluating and comparing the effectiveness of different hybrid system layouts in the preliminary design stage. Their work is currently published in the research journal, Journal of Engineering Mechanics.
In brief, the research method employed commenced with the consideration of a typical basic hybrid system consisting of a vulnerable cable connected to a neighbouring cable by a transverse linear flexible cross-tie and equipped with a linear viscous damper close to one supporting end. Next, the researchers assessed and validated the proposed model by verifying it using a number of cable systems with various configurations, already analytically derived in literature. Lastly, the developed system was applied to a more intricate system comprising of two damped main cables interconnected with a cross-tie extended to and anchored on the ground or the bridge deck.
The authors observed that the proposed approach was a reliable tool and it allowed for straightforward formulation of an analytical model of a complex hybrid system from a simpler parent system. In addition, they noted that their system resulted in faster modelling hence much time saving. They also found out that the quicker modelling allowed for more evaluation and comparison of the effectiveness of several hybrid systems with different layouts during the preliminary design stage.
In a nutshell, the study presented the derivation of a generalized approach to formulating analytical models of hybrid systems used for bridge stay cable vibration control. In general, the results obtained from application of the developed approach showed that if an additional connector was to be placed at the existing nodal point of a main cable in the parent system, it would only affect the global modes of the parent system, leaving the local modes unchanged. Altogether, the researchers showed that if the cross-tie is not located at the nodal point of the main cables, it is beneficial to increase its flexibility to suppress the local modes.
Javaid Ahmad, Shaohong Cheng, Faouzi Ghrib. Generalized Approach for the Formulation of Analytical Model of Hybrid Cable Networks. Journal of Engineering Mechanics, 2018, volume 144(6): 04018035.Go To Journal of Engineering Mechanics