Study On Dynamic Characteristic And Optimum Design Of Luffing Jib Tower Crane

Nowadays tower crane plays an important role in construction industry.With the increase of building density and the decrease of construction space,the luffing jib tower crane gets more and more attention owing to its small turning radius and large lifting height.Because the operating speed and the lifted load increase constantly in actual engineering construction,the dynamic load increases and the vibration problem intensifies accordingly.Elevating and lowering the jib can change the working range of the luffing jib tower crane,so the dynamic characteristics are complicated extremely.The research on the dynamic characteristics gets wide concern in the field of engineering machinery.In this paper,the dynamic characteristics of the luffing jib tower crane are deeply researched combining with engineering practice closely,and some important conclusions are obtained.(1)The statics of the luffing jib tower crane is studied.The influence of the wind direction and the jib's elevation angle on the strength,the stability and the static stiffness is analyzed.In the condition that the load mass and the wind speed don't change,the stress of the tower body is the biggest when the shadow of the jib in the horizontal plane and the diagonal of the body's cross section are coincident,and the biggest stress is irrelevant with the wind direction.The greater the elevation angle of the jib is,the greater the wind load is.The influence of “the back wind” on the static strength is greater in working state.The influence of “the lateral wind” on the static strength is greater in non-working state.The influence of “the lateral wind” on the stability and the static stiffness is greater in all state.(2)The influence of the movement and the structure of the luffing jib tower crane on the natural frequencies are analyzed.The dynamic models of the luffing jib tower crane are established by taking advantages of the less degree of freedom model method and Finite Element Method respectively and the dynamics calculation is carried out.The law about the influence of the elevation angle and the rotation angle of the jib on the natural frequencies and the vibration types is obtained.The influence of the elevation angle on the natural frequencies is greater but the influence of the rotation angle is smaller.When the elevation angle of the jib is changing,different natural frequencies have different variation law.The main vibration types corresponding to some natural frequencies can transform to each other when the elevation angle is small and large.When the jib is at different elevation angle,apply the harmonic force from different direction to the luffing jib tower crane,in which way the harmonic response analysis is finished.The natural frequency that affects the dynamic stiffness most is calculated.(3)The dynamic characteristics of the luffing jib tower crane in different working states are analyzed.The displacement response of the key nodes and the maximum stress response of the tower body and the jib are obtained.The vibration of the luffing jib tower crane is mainly in the lifting plane when the crane starts to lift a load or to brake as putting down a load.The greater the elevation angle of the jib is,the greater the influence of the dynamic load and the dynamic load coefficient of the tower body are.When the rotary mechanism is starting,the influence of the dynamic load caused by rotary acceleration on the jib is greater but on the tower body is smaller.(4)The time-history analysis and the response spectrum analysis of seismic waves are carried out when the jib is at different elevation angles.The results show that the weight load only affects the maximum value of the dynamic displacement and the dynamic stress and not affects the amplitude,when the luffing jib tower crane is vibrating caused by seismic waves.When the elevation angle of the jib is small,the "nod" movement of the luffing jib tower crane is greater,which causes the jib to break easily.When the elevation angle of the jib is large,the luffing movement is greater,which causes the tower body to break easily.The possibility of the luffing jib tower crane collapse is larger when it is in the lifting plane but smaller in lateral direction.The harm of the horizontal seismic wave to the luffing jib tower crane is greater than the vertical seismic wave.(5)The dynamic optimization design of the luffing jib tower crane is finished aiming at increasing the dynamic stiffness.Taking the natural frequency that affects the dynamic stiffness most as objective function,the sensitivity of the key components' size parameters is analyzed and the design variables are calculated.Taking the structural strength,the static stiffness,the maximum dynamic displacement,the mass and the single component stability as restraints,the mathematical model of the dynamic optimization design is established subsequently.The optimization results show that the low natural frequencies are greatly improved and the dynamic rigidity is obviously enhanced.Simultaneously the static displacement of the tower body is improved and the mass is reduced.The above achievements provide the theoretical basis for designing the luffing jib tower crane and the theoretical guide for making construction criterion.