Study On Characteristics Of Suspended Structure Under Vertical Seismic Action

China is an earthquake-prone country.A large number of records on earthquake damage show that vertical seismic action has a significant impact on structures,and is closely related to the damage of some structures and components in high-intensity areas.The suspended structure,as a novel structure type,with the advantages of small footprint,flexible building space and excellent seismic resistance effect,has now been increasingly recognized in engineering.However,there are relatively few studies on the characteristics of suspended structures under vertical seismic action.Therefore,this paper systematically studies the characteristics of vertical ground motion and the response properties of suspended structures under vertical seismic action.The main contents are as follows:（1）The characteristic parameters of vertical ground motion are statistically analyzed in this paper.Numerical statistics are conducted on 512 groups of ground motion records from 20 earthquake events in the past 5 years to study the effects of magnitude,epicentral distance,site category and design basic seismic acceleration on the peak acceleration of vertical and horizontal ground motion and ratio of response spectrum.The results show that epicentral distance and site type have essential effects on the vertical ground motion characteristics,and the regulation is relatively not accurate that the vertical earthquake peak effect coefficient is taken as 0.65 times of the corresponding value on the horizontal direction.The curves of the ratio of vertical and horizontal acceleration response spectrum varies with the structure vibration period.The influence of vertical seismic action should be fully taken into consideration on short-period structures on weak sites near the epicenter,and medium-period and longperiod structures on hard sites.In seismic design of structures,the recommended values of spectral ratio given in this paper can be properly referred to.（2）The second-order resonance vibration and vertical seismic action of the suspended structure are analyzed in this paper.A finite element model of the intensive care center building of a hospital is built to judge the possibility of second-order resonance vibration under vertical seismic action and discuss the availability of the calculation method of vertical seismic effects on suspended structures in the regulation.The results show that there is not second-order resonance vibration under vertical seismic action of the research model.In real seismic design,it is not necessarily accurate to calculate the vertical seismic action of suspended structures by static method and response spectrum method,but the modified method of vertical mode response spectrum based on the recommended values of spectral ratio is more suitable.It is supposed to use time-history analysis method to supplement and check the structure,and the three-direction data of ground motion should be selected as far as possible and adjusted according to the original proportion,so that the calculated results can be more consistent with the actual demand of earthquake damage.（3）The response characteristics of the suspended structure under near-fault earthquakes is analyzed in this paper.The differences between the displacement and internal force of the suspended structure under mid-to-far fault and near-fault earthquakes are studied,and the influences of vertical effects of near-fault earthquakes on the structure are assessed.The incremental dynamic analysis method is applied to study the efficiency of the inter-story displacement angle as the overall index of the suspended structure under vertical seismic action,and the influence of the structure caused by the order of three-direction peak of near-fault earthquakes are analyzed.The results show that only horizontal seismic effects can be considered in the incremental dynamic analysis generally,but the vertical effect of near-fault earthquakes has obvious amplification effect on the node vertical displacement and suspender axial force,consequently,when carrying out incremental dynamic analysis of suspended structures in the near-fault,the maximum inter-story displacement angle cannot effectively reflect the damage degree of the structure,and the important performance change points of the structure will be ignored if the effect of near-field horizonal earthquake is considered only.