| In recent years,with the rise of tourism resources development projects throughout the country,more and more footbridges with larger span,lighter weight,thinner and more beautiful have been built in many scenic spots.This kind of footbridge structure,having lower natural frequency and smaller damping,is more susceptible to human-induced vibration.As the main factor causing structural vibration,people acting on the structure is an independent dynamic system and has their own dynamic characteristics.The possible interaction between them and the structure will change the dynamic characteristics of the structure in varying degrees,resulting in the overestimation of the dynamic response of the structure,thereby affecting the accuracy of vibration serviceability assessment.Therefore,in order to consider the pedestrian-footbridge interaction,it is very important to select a reasonable pedestrian load model in the process of evaluating the serviceability of human-induced vibration of long-span flexible footbridges.When the footbridge vibrates due to the action of pedestrians,the pedestrians acting on the footbridge are the most important,direct and obvious receivers of the vibration,but the sensitivity of different gender,age and posture to the vibration of different directions of the structure varies greatly.In the existing codes,the peak acceleration response limit of the structure under source-path is mostly used as an indirect criterion to evaluate the vibration comfort of people,but there is no direct definition of human comfort perception level,which has a greater subjectivity,leading to the evaluation value of vibration comfort of the footbridge is inconsistent with the human's actual perception level.In addition,most researchers only evaluate the vibration serviceability of footbridges by comparing the peak acceleration response of the structure under resonance with the upper limit of acceleration specified in the code.However,in fact,due to the randomness of pedestrian step frequency and the randomness of the receivers' distribution of footbridge vibration,the results of serviceability evaluation should be shown by the probability of vibration comfort within the range of pedestrian step frequency,rather than by the deterministic evaluation results of a specific position of footbridges under quasi-resonance conditions.At present,the excessive vibration problem of footbridges is mostly solved by installing some vibration control devices on the structure,which can improve the mechanicalproperties of the structure and resist external loads.As a common passive control device,tuned mass damper has been widely used in civil engineering.It can effectively control the dynamic response of structures with dispersed natural frequencies in narrow bands near resonance frequencies.However,at present,few researchers set different semi-rigid control devices at different structural supports in the design stage of footbridges to adjust the natural frequency of the structure,so as to keep it as far away from the pedestrian step frequency range as possible,thereby avoiding the occurrence of human-induced resonance to achieve the purpose of controlling the excessive vibration of the structure.Therefore,in the process of studying the above problems,choosing a reasonable pedestrian load model to consider the interaction between pedestrian and footbridge,and adopting the form of probability,the study of vibration serviceability assessment based on the full path of "source-path-receiver" was carried out.The tuned mass damper and semi-rigid device were respectively used to control excessive vibration,and the vibration reduction effects of the two methods are compared and analyzed.The main contents of this thesis include the following aspects:(1)The vibration governing equation of beam structure under pedestrian excitation is established,and the governing equation is discretized by the waved differential quadrature-integral quadrature(WDQ-IQ)method.Then the acceleration response calculated by this method and the mode superposition method is compared and analyzed to further verify the accuracy and reliability of the WDQ-IQ mixed method in solving the differential governing equation;(2)The governing equation of the pedestrian-footbridge coupled system under simply supported boundary condition is established,which is discretized by WDQ-IQ method.The dynamic response of the structure considering pedestrian-structure interaction is solved by Newmark method,and compared with the dynamic response of the structure under pedestrian excitation,which further illustrates the importance and necessity of pedestrian-structure interaction when evaluating the human-induced vibration serviceability.In addition,discussing the influence of pedestrian-structure interaction on the dynamic response of the structure in the pedestrian walking frequency range,the vibration serviceability is evaluated comprehensively by the form of cumulative probability;(3)The peak acceleration response of the structure calculated under different working conditions is compared with the peak acceleration limit selected in the existing code.Then a TMD is used to control the vibration of the structure when theserviceability limit is not satisfied,and the vibration reduction effect of the TMD under the quasi-resonance condition is evaluated.Finally,the cumulative probability form is used to show the overall vibration reduction effect of TMD in the pedestrian walking frequency range;(4)The semi-rigid device is used to control the vibration of the structure.The governing equations of pedestrian-footbridge coupled system under semi-rigid boundary conditions are established,and the corresponding dynamic responses of the structure with different stiffness ratios are solved,and are compared with those under simply supported,standard limits and TMD control.which can explain the influence of stiffness ratio in semi-rigid boundary conditions on the dynamic responses of structures and evaluate the effect of vibration reduction;(5)Firstly,the governing equation of the coupled system based on the full path of "source-path-receiver is established,dealt by the WDQ-IQ mixed method.The dynamic response of the structure and the receiver(the person standing in the middle of the structure)is solved by Newmark method and compared with each other.While respectively comparing them with the acceleration limit of the vibration serviceability specified in the code,TMD control and semi-rigid control were used for the working conditions that do not meet the serviceability limit requirement,and the dynamic responses of structure and receiver under different vibration reduction ways were respectively compared with that without vibration reduction control measures.Finally the vibration reduction effect was evaluated comprehensively. |
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