Study Of Fatigue Properties For Tuned Mass Dampers Based On Long-span Floors Under High Cycle Fatigue Load

The pedestrian-induced vibration of long-span flexible floors is a common problem in modern structural system.Tuned mass damper(TMD)is widely used in the vibration control of long-span flexible floors due to its simple structure,economical and obvious vibration damping effect.The damping effect of TMD is mainly related to its mass ratio,tuning frequency and damping coefficient.The change of TMD parameters will affect the vibration damping effect of the main structure under fatigue load.The fatigue performance study of TMD can provide some suggestions for the modification of TMD design parameters and the service life of TMD in practical engineering.The main concerns and conclusions are as follows:1.Study on the vibration reduction performance of TMD.By comparing tests before and after vibration reduction under different frequencies of walking load and jumping load on the floor,it is concluded that the TMD have better vibration reduction effect on the load of1.9Hz~2.1Hz and the maximum vibration reduction rate is 52.07%.The finite element model of the floor was established,and the results of finite element calculation are the same as the measured ones,which indicates that the test TMD is in normal working state.Contrast tests were carried out on the floor under different vibration levels before and after vibration reduction.It is concluded that the greater the vibration level is,the greater the vibration damping effect of TMD will be.2.Study on the fatigue performance of TMD.Based on the reliability design theory of the fatigue strength of the cylindrical spiral spring considering the normal distribution of the strength and stress,the reliability of the spring used in TMD is calculated to be equal to 1 and no fatigue failure occurs.One hundred thousand fatigue tests were performed on TMD.After the test,the stiffness of TMD changed from 691.27 kN/m to 591.26 kN/m which decreased by14.47%,the TMD damping ratio and the modal properties of floor had not changed.The peak acceleration of the floor changed from 190.88mm/s~2 to 223.70 mm/s~2 which decreased by12.43%and the TMD works well in its design life.3.Study on the pedestrian-induced vibration reduction design of a long span footbridge and the reliability of TMD-structure.The pedestrian-induced vibration of the footbridge is studied with finite element software.The optimal parameter design theory of damped main structure is used to design TMD parameters.The vibration response of the bridge meets the limit requirement after TMD is installed.Compared with the uncontrolled structure,the reliability of the controlled structure is obviously improved by using the stochastic model of human-induced load.Based on the fatigue test results of TMD,it is calculated that the TMD vibration reduction effect decreases with the increase of service life.The farther the TMD frequency deviates from the optimal value,the faster vibration reduction effect decreases.But the vibration response of the footbridge still meets the limit requirement when the TMD service life is reached.The influence of TMD damping ratio on the vibration reduction effect is analyzed,the change of TMD damping ratio has less influence on the effect of vibration reduction than the change of TMD stiffness.At the same time,based on the stochastic model of human-induced load,the failure probability of TMD-structure increases in the design service life,but the reliability requirement of TMD-structure is satisfied.It is suggested that the design life of TMD should be 25 years,the frequency error of TMD in practical engineering should be within 2%,the damping error within 15%and the adjustable frequency TMD should be adopted.