Study On Vibration Characteristics Of Footbridge Based On Pedestrian-structure Interaction

With the use of high strength materials and the improvement of construction level as well as people’s aesthetic requirements,modern footbridges are becoming large-span and slender.The footbridges characterized by light weight,low frequency and small damping easily suffer from vibration problems due to pedestrian walking excitation.The intrinsic dynamic characteristics and vibration response characteristics of pedestrian-structure coupled system were studied by use of theoretical analysis,numerical simulation and experimental test.The main novelties and contributions are as follows.(1)A new walking force test method was proposed to simulate the pedestrian’s continuous walking with natural walking gait,which can obtain the time history and spatial position of walking force at the same time.A set of walkway test system was designed and produced,which can overcome the shortcomings of single force plate and treadmill,that the single force plate can capture only one footstep and the treadmill can only walk in situ.Typical Chinese walking characteristics were measured using the walkway test system,and the normal distribution models of step frequency and step length were fitted.Based on the Fourier-series walking load model,the linear model of dynamic load factors(DLF)and the uniform distribution model of phase angles were fitted.Compared with the results of foreign pedestrians,the mean step frequency,mean step length and mean first order DLF of Chinese pedestrians are slightly smaller.It indicates that there exist some differences in the walking characteristics between Chinese and westerners.The proposed models of step frequency,step length and walking load have filled in the blank of data for the walking characteristic of Chinese pedestrians.(2)Based on the spring-mass-damper(SMD)model of pedestrian,the pedestrian/crowd-structure interaction models were established.The time-varying characteristics of fundamental frequency and damping ratio of the coupled system were analyzed.The influences of a range of input parameters on the coupling frequency and coupling damping ratio were analyzed,such as motion form of human body,structure frequency,structure mass per unit length,walking speed,pedestrian number,pedestrian space interval and pedestrian SMD model parameters.Based on the Monte-Carlo simulation,the random characteristics of coupling frequency and coupling damping ratio were analyzed when the footbridge were occupied by a crowd of pedestrians under different crowd densities.(3)Based on the pedestrian SMD model,the calculation method of vibration response of pedestrian/crowd-structure coupled system under pedestrian walking dynamic loads were proposed,and this method was compared with the method that the action of pedestrians is considered as forces applied to the structure.The influences of a range of input parameters on the results of the two methods were analyzed,such as step frequency,step length,pedestrian SMD model parameters,structure mass per unit length,pedestrian number and pedestrian space interval.The random vibration responses of the footbridge due to single pedestrian as well as a crowd of pedestrians under different crowd densities were calculated.The enhance factors of crowd dynamic effect were calculated based on the response of single pedestrian.Compared with the enhance factors in the available literature and design guidelines,it is suggested that the maximum accelerations will be overestimated when neglecting the pedestrian-structure interaction and the influence of crowd density on the walking parameters.(4)Considering the randomness of pedestrian weight,step frequency,step length,initial position and initial phase angle,a random crowd walking load model in the two-dimensional plane was established,and a method of applying the random crowd walking loads using ANSYS was proposed.The vibration responses of Beijing KY footbridge under several working condition were calculated.The results were compared with the results of pedestrian SMD model,measured response and equivalent pedestrian number models,which further validate the reliability of the random crowd load model and finite element simulation method.