Investigations On The Vibration Control Of Structures Based On “Column-in-Column”(CIC) Concept

With the rapid development of the construction technologies and the new materials,more and more tall slender engineering structures are built.In which,the concrete filled steel tubular(CFST)column is widely used owing to its advantages of good seismic performance,high loading capacity,favorable ductility,good fire resistance and easy construction.Considering that the CFST is mainly used as the vertical load-bearing member,in this dissertation,the concept of the TMD system for structural vibration control is introduced to extend the functions of the CFST.In other words,the CFST is converted into a new column-in-column(CIC)system with dual-functions of load-bearing and vibration mitigation,so as to further enhance the seismic performance of tall slender engineering structures.This novel CIC system basically has the same cross-sectional area as that of the traditional CFST,so the construction cost and the axial loading capacity of the CIC system are basically the same as the original CFST,while its seismic performance is better than the traditional CFST.By means of theoretical analysis,experimental validation and numerical simulation,a series of studies are carried out to investigate the static axial loading capacity,dynamic characteristics and vibration control performance of the proposed CIC system.The main research contents are as follow:(1)Based on the “unified theory” of the CFST,the formulae for calculating the ultimate axial loading capacity of the CIC system and its cross-sectional composite stiffness under compression are derived first,then the equivalent design method of the CIC system is proposed by keeping the axial compression stiffness of the CIC system equal to that of the original CFST.Finally,by theorectically “dividing” the traditional CFST into two columns to form the CIC system,the reliability and the accuracy of the equivalent design method of the CIC system is verified.(2)Using the developed design method of the CIC system,14 CIC specimens were designed and fabricated,then axial compression tests were carried out to investigate the compressive behaviours of the CIC system.The failure modes,load bearing capacity,sliding effect and deformation mechanism of the CIC system were analyzed,and the influences of height-to-diameter ratio and diameter-to-thickness ratio on the ultimate axial loading capacity of CIC system were discussed.Based on the tested results,the formula for calculating the ultimate axial loading capacity of the CIC system was proved to be accurate,which can be easily used in design analysis of CIC system.(3)The dynamic model of the CIC system is simplified as a double-beam system carrying a tip mass with rotary inertia.Based on the Euler-Bernoulli beam theory,the analytical solutions of free and forced vibration of the CIC system are obtained.The influences of the tip mass,rotary inertia,stiffness of the elastic layer,mass ratio of the secondary beam to primary beam and stiffness ratio of the secondary beam to primary beam on the natural vibration frequencies and dynamic response of CIC system were studied by numerical analyses.(4)The structural model of the sliding-type CIC system(SCIC system)is simplified as a2 DOF structure-TMD system.Based on the motion equations of the structure-TMD system,the displacement response transfer function of the primary structure is derived and the optimum design formulae of the TMD system is determined.Taking a tall slender tower as an example structure,sensitivity analyses are conducted on the SCIC system through MATLAB to study the influences of superstructure mass,damping and stiffness of the TMD system on the robustness of the SCIC system for vibration attenuation.The finite element(FE)models of the tower structures are developed in ABAQUS,and nonlinear time history analyses are carried out to demonstrate the control effectiveness of the SCIC system in reducing the seismic response of the structure.The influences of spring and damper arrangement scenario,superstructure mass,earthquake ground motion and pounding effect on the seismic induced response mitigation of the SCIC system are discussed in detail.(5)It is found that the control effectiveness of the SCIC system become less effective when a large superstructure acting on the system.To overcome this drawback,the SCIC system is modified by converting the inner column into a pendulum TMD(PTMD),i.e.,proposing a pendulum-type CIC(PCIC)system.The structural model of the PCIC system is simplified as a 2DOF structure-PTMD system.The displacement response transfer function of the primary structure is derived and the optimum design formulae of the PTMD system is determined by using the “fixed point theory”.Taking a tall slender tower as an example structure,sensitivity analyses are carried out via MATLAB to study the robustness of the PCIC system with respect to the superstructure mass,stiffness and damping of the PTMD.The FE models of the tower structures are developed by using the ABAQUS,and nonlinear time history analyses are conducted to verify the control effectiveness of the PCIC system for seismic induced vibration mitigation of the structure.The influences of superstructure mass,earthquake ground motion and pounding effect on the control performance of the PCIC system are discussed in detail.(6)Based on the studies of the performance under static axial loading,dynamic characteristics and vibration control performance of the CIC system,the design methodology of the CIC system for structural vibration control is established.Taking a typical multi-span continuous bridge as an application case,the PCIC system is used as bridge piers,which are designed and optimized to mitigate the bridge structure responses under earthquake ground motions.FE models of the bridge supported with the original hollow columns and the bridge supported with the PCIC system are developed by using ABAQUS software.Numerical simulations are carried out to study the seismic response of the two bridges subjected to spatially varying earthquake ground motions.The effectiveness of PCIC system on mitigating seismic responses of bridge is demonstrated by comparing the displacement responses,seismic induced forces and pounding forces of the two bridge models,and the influences of earthquake spatial variations on the bridge responses are discussed.