| In recent decades there has been a trend towards increasing use of new light and high strength structural materials and longer spans of structures,accompanied with aesthetic requirements of structural appearance.It has enabled engineers to design more flexible,longer and low-damping structures,such as modern glued-laminated timber(glulam)footbridges.The modern glulam bridge have overcome the defects of traditional wooden structures by incorporating new and high technologies for wood processing.They integrate the design and construction with the aesthetic feeling,functionality and cost-effectiveness,and have the advantages of resource recycling,green and environmental protection,high strength-to-weight ratio,aesthetic appearance,and convenient construction(especially for alpine regions)and so on.They are also seismic and wind-resistant,moistureproof,anticorrosion,fireproof,antitermite and durable.Although the static performance of these pedestrian bridges,such as bearing capacity and deformation,can meet the requirements of structural design and normal use,they become more susceptible to vibrations when subjected to dynamic loads.The large-amplitude vibration induced by crowd excitation has become one of the key factors for the design of modern glulam bridges and attracted great public attention.To study the human-induced vibration performance,comfort assessment and optimal control of the glulam footbridge,an experimental and finite element simulation analysis with ANSYS of human-induced vibration tests was conducted on a large-scale arch model.The analysis result well predicts the damping efficiency of multiple tuned mass dampers(MTMD),which is in a good agreement with experimental results and can be applied to the optimal vibration control of the glulam footbridge.The main research contents and achievements are introduced as follows:1.On the basis of systematically summarizing the latest research results of pedestrian loads at home and abroad,a mathematical model of the human-induced load suitable for our country is established through statistical analysis of pedestrian characteristic parameters of Chinese people.Combined with the corresponding vibration theory,the human-induced vibration responses of footbridges are calculated in time domain and frequency domain respectively.2.Based on the standard values of crowd loads in China,the limits of human perception of vibrations are studied,and the vibration comfort assessment method suitable for timber pedestrian bridges is proposed.This paper briefly introduces the problems of ergonomics related to the evaluation of human exposure to whole-body vibration,the research history and the famous vibration comfort experiment,in order to clarify the basic method of vibration comfort assessment.The research achievements of vibration comfort assessment are summarized from the aspects of evaluation method,evaluation index and relevant standards and specifications,which provides theoretical background for the vibration comfort assessment of footbridges.By comparing and analyzing the characteristics of comfort assessment methods in floor vibration and wind vibration of high-rise buildings,it provides an effective reference for the vibration comfort assessment of footbridge.3.A structural vibration reduction optimization design method based on genetic algorithm(GA)is proposed by solving the multi-parameter optimization problem of MTMDs systems.After studying the STMD's parameter optimization method for undamped main structures,a numerical method of optimization search based on genetic algorithm is proposed for optimization objective.The correctness of this method is proved by comparison with analytical solution.The method is applied to the optimization calculation of STMD's parameters for damped main structures,and good results are achieved.Taking the dynamic magnification factor(DMF)of the main structure as the objective function,the optimal expression of MTMDs with arbitrary damping ratio and frequency ratio is presented.When the genetic algorithm is used to solve the MTMD's multi-parameter optimization problem,the calculation efficiency is higher and the damping effect is remarkable.In addition,considering the uncertainty of structural parameter identification,the mass,stiffness and damping coefficient of the main structure may deviate.The robustness of STMD and MTMDs to structural parameter perturbation is further studied.4.The reliability and practicability of the vibration serviceability design methods of timber pedestrian bridges proposed in this paper are verified by the human-induced vibration test conducted on a glulam model bridge.The dynamic characteristics of a large-scale glulam arch model and the structural vibration response under human-induced excitation are calculated by using finite element simulation analysis with ANSYS,and the human-induced vibration comfort is evaluated.According to the narrow-band random characteristics of walking forces,3-MTMDs were installed to reduce vibrations,and genetic algorithm was used to optimize the structural parameters of 3-MTMDs.The analysis results well predict the damping efficiency of 3-MTMDs,and are in a good agreement with experimental results.5.A domestic glulam arch bridge is used as an example to demonstrate vibration serviceability design procedures of the modern glulam footbridge.The vertical fundamental frequency of pedestrian bridge is less than 3 Hz without load and lateral fundamental frequency is more than 1.2 Hz with full loads,which indicates that the lateral vibration comfort of this bridge meets the requirements.It is only necessary to check whether the peak vertical acceleration is less than the allowable value of comfort under single and crowd loads respectively.According to the calculation results of human-induced vibration responses,it can be concluded that the vertical vibration comfort of this bridge under single load meets the requirements,but the vertical vibration comfort of the bridge under crowd loads does not meet the requirements.The damping absorber is considered to be installed on the bridge to reduce excessive vibrations of the structure.The results show that the dynamic response of the glulam footbridge in the narrow frequency near the resonance can be controlled within the allowable range when the parameters of 3-MTMDs are optimized by genetic algorithm. |
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