Study On Torsion Effect And Failure Modes Under Strong Earthquake Of Structure Supported By Foundations With Different Elevations

Due to the fact that the structure supported by foundations with different elevations has multiple grounding terminals with unequal heights,the vertical rigidity is uneven,and its center of mass and rigidity do not coincide,and the torsion effect of structure supported by foundations with different elevations is significant.When the actual engineering site has a large height difference,taking into account factors such as large excavation works,slope stability,environmental impact,etc.,three or more grounding terminals are often present.At present,there have been researches on the dropped-layer frame structure with two grounding terminals,and it is necessary to carry out research on the torsion effect of the dropped-layer frame structure with three grounding terminals.This paper uses finite element analysis software to establish a number of models of dropped-layer frame structures with two grounding terminals and three grounding terminals.The torsion effect under elastic and elastoplastic conditions and the failure mode under strong earthquake of a dropped-layer frame structure with three grounding terminals was studied,investigate the influence of the total number of layers dropped,the number of spans of the middle ground terminal,the number of layers lost at the middle ground,and the number of spans of the upper ground terminal on the torsion effect and failure mode of the dropped-layer frame structure,compare the torsion effect and failure mode of the dropped-layer frame structure with two grounding terminals and three grounding terminals.On this basic,it is proposed to adopt torsion measures such as strengthening the side and middle grounding pillars,installing horizontal grounding members,adding steel supports,and adding viscous dampers to reduce the torsion effect of the dropped-layer frame structure with three grounding terminals.The main conclusions of this paper are as follows:(1)Comparing the setting of two grounding terminals with three grounding terminals in the dropped-layer frame structure,under the elastic state,the dropped-layer frame structure with three grounding terminals can reduce the torsion angle and drift angle angle of the floors below the middle ground layer(including)in the dropped-layer frame structure with two grounding terminals.The torsional displacement ratio of the droppedlayer frame structure with three grounding terminals reaches the maximum at the middle ground layer and the upper ground layer,and the torsional displacement ratio of the the dropped-layer frame structure with two grounding terminals reaches the maximum value at the upper ground layer,about 1.7,which is already greater than the specification limit.(2)The structural torsion evaluation index interlayer torsion angle and torsion displacement ratio can reflect the torsion effect in the layer-dropped frame structure with three ground terminals,but the period ratio does not play a controlling role in the layerdropped frame structure.The torsion effect of the dropped-layer frame structure with three grounding terminals is most significant on the floors near the upper and middle ground layers.(3)In the elastoplastic state,the maximum inter-story torsion angle of the flexible side of the dropped-layer frame structure is obtained at the upper layer of the lower ground layer.Regarding the story drift angle,the maximum story drift angle under rare earthquakes is still far from the specification limit of 0.02,and the maximum value under super earthquakes is close to or exceeds 0.02.From the perspective of the proportion of plastic rotations angle of the beam and column members,the installation of the three grounding terminals can improve the degree of plasticity of the structural members of the two grounding terminals.(4)For the dropped-layer frame structure with two or three grounding terminals designed according to the current specifications,when the number of X-direction and Ydirection spans of the layer-drop structure is set more,the torsion effect does not necessarily increase with the increase of the total number of layers.(5)In the elasto-plastic state,the shear force of this floor borne by the ground terminal posts of the upper ground layer and the middle ground layer is 70% to 80%.The beam hinges and column hinges of the dropped-layer frame structure with three grounding terminals mainly appear in the two layers above the middle ground terminal and the two layers above the upper ground terminal,and the beam and column have the highest degree of plasticity in the entire structure.Under rare earthquakes,the upper grounding terminal beams and columns have greater plasticity than the middle grounding terminal beams and columns,under super earthquake,the upper grounding terminal beam is more severely damaged than the middle grounding terminal beam,and the middle grounding terminal column is more severely damaged than the upper grounding terminal column.The failure mode of the dropped-layer frame structure with three grounding terminals is "beam-column hybrid hinge" mode.(6)The anti-seismic and torsion measures such as strengthening the side and middle grounding pillars,installing horizontal grounding members,adding steel supports,and adding viscous dampers can reduce the torsion effect of the dropped-layer frame structure with three grounding terminals.The reasonable lateral stiffness ratio between the steel brace and the dropped-layer frame structure is 0.5≤ k ≤2.