Torsional Effect Analysis And Control Of Irregular Isolated Structures Based On The Degradation Of Bearing Stiffness

In recent years,the base isolation system is more and more used in irregular structures.Under horizontal load,even small amounts of torsion will cause large displacement in the side support of the structure,which will affect the safety of the superstructure and the isolation support.The rubber bearing is the most commonly used isolation element in base isolation system.Due to the different degradation mechanism of the stiffness of isolation bearings,the horizontal stiffness of the lead rubber bearing decreases significantly with the increase of the shear deformation of the bearing.The torsional calculation of structures is closely related to the horizontal stiffness of isolation bearings.The stiffness degradation effect of the bearing in the rare earthquake is not conducive to the structural isolation layer and the torsional effect control of the superstructure.Therefore,this paper studies the torsional effect of irregular isolation structures caused by different degradation mechanisms of bearing stiffness,and also studies the torsional optimization design method of irregular isolation structures.In order to calculate the equivalent horizontal stiffness of lead rubber bearing under rare earthquake for structural torsion effect analysis,a method for calculating the equivalent horizontal stiffness is proposed in this paper,which is based on the double-line mechanical model of lead-core rubber bearings and existing national codes.It provides a basis for the analysis of torsional effect of irregular structures.Two engineering examples of irregular isolation structures are established by Etabs software,and the torsional effects of structures under rare earthquakes caused by different degradation mechanisms of bearing stiffness are analyzed.The results show that the torsional effects of irregular isolation structures under rare earthquakes will be increased by the degradation of bearing stiffness.In order to more intuitively study the damage of beams and columns under rare earthquakes,the finite element model of multi-layer irregular isolation structure was established by using Perform-3D software,and the elastoplastic analysis under rare earthquakes was carried out.The performance of isolation structure is evaluated from the whole structure and component level.The results show that under rare earthquakes,because of the torsion effect,the beam and column around the base of the irregular isolation structure will first enter the yielding state,resulting in large plastic deformation.Therefore,in the torsion optimization design,it is necessary to focus on the control measures to limit the torsion of the beam and column around the isolation layer.For multi-storey irregular structures,the torsional optimization design scheme of adjusting the bearing arrangement of isolation layer is adopted,and for high-rise irregular structures,the torsional optimization design scheme of installing viscous dampers in isolation layer is adopted to compare the torsional effect control effect of structures under rare earthquakes.The analysis results show that the measures to adjust the layout of isolation bearings are suitable for irregular isolation structures with multi-storey and redundant adjustments.The scheme of installing viscous dampers in the isolation layer is more effective for irregular isolation structures with high flexibility.