Parameter Analysis For Base Isolated Buildings On Soft Soil

After years of development,building isolation technology has been gradually well established,which has not only been used for the design of new buildings but also provides an effective way for the seismic strengthening of old buildings.Through the isolation layer at certain structural elevation,the natural period of the structure can be elongated and the transmission of seismic energy can be mitigated,so that the accessory structure and the building itself will not be damaged.However,base-isolated buildings often require good soil conditions,as in the Class ?,? and ? construction sites,while it is not recommended to use seismic isolation on soft soil(Class ?).In order to study the isolation effect of the isolated structure on the Class ? soil and influence of various parameters on the isolation effect,the following investigations are conducted.(1)The finite element software PERFORM-3D,ETABS,PKPM are used to model a seven-story frame building and the seismic design of the frame is carried out.The seismic structure and the seismic isolation structure of the same frame are analyzed through modal analysis and response spectrum analysis.Seismic waves representing different soil conditions are selected from the PEER database to make elasto-plastic time-history analysis of the frame.The peak acceleration was adjusted to represent earthquakes with various intensities,and the seismic-reduction coefficients corresponding to different construction sites and various seismic intensities are compared.The results show that the earthquake-reduction coefficient decreases with the increase of the intensity of frequent earthquake.Due to yielding of some components of the non-isolated frame under the design basis earthquake(DBE)and maximum considered earthquakes(MCE),the seismic-reduction coefficients increase slowly or have insignificant change with the increase of seismic intensities.(2)The seismic isolation effect with different site types and under the same earthquakes is studied,and the elasto-plasticity analysis of the frame is carried out under the frequent,DBE and MCE earthquakes,with the seismic intensity of 8 degree(0.2g).The results show that the structure has obvious isolation effect on the Class ? construction site;however,the seismic isolation coefficient is the largest and the isolation effect is the worst.Under the MCE,the frame on the Class ? site has the largest displacement,followed by the Class ? site,and the Class ? has the smallest displacement.Under the MCE(400gal),displacements of the isolation layer are still within the threshold value.The elastic-plastic energy dissipation of the structure under the frequent,DBE and MCE earthquakes is mainly provided by the rubber bearings,and the upper beam and column are in the elastic state,showing that the upper structure can also be effectively protected from damage on the Class ? site.(3)Influence of the yield to weight coefficients,stiffness of the upper structure and the post-yielding stiffness ratio ? on the seismic-reduction coefficient,the roof acceleration and the displacement of isolation layer is studied.As the yield to weight coefficient increases,the seismic-reduction coefficient decreases first and increase gradually,and the roof acceleration always increases.The optimal yield ratio is from 2%to 6%,and it is recommended to take 4%in design.The influence of interlayer stiffness ratio on the seismic-reduction coefficient of the isolated structure is insignificant.The roof acceleration decreases with the increase of the interlinear stiffness,and the change in displacement of the isolation layer is not obvious as the interlinear stiffness changes.Therefore,it is not economic to achieve optimal isolation effect by changing the stiffness of the upper structure.When y is small,the damping coefficient is large and the seismic-reduction effect is not good.When the y is from 2 to 10,the damping coefficient is significantly reduced;and when y is greater than 10,the damping coefficient is still reduced,however,its marginal reduction is small,and the effect of y on the damping coefficient is insensitive.Under the DBE and MCE earthquakes,the peak acceleration decreases monotonically,and when y is less than 10,the reduction is more obvious,while the displacement of the isolation layer increases first and then increases with the increase of y.Based on the seismic-reduction effect,the peak acceleration and the displacement of the bearing,it is suggested that y should be more than 10 on the soft soil.It is concluded that it is possible to build base-isolated buildings on soft soils with perfect isolation effect and have the displacement of the bearing under control through reasonable bearing arrangement and parameter selection.The presented studies provide some references to the isolation design of buildings on soft soils.