Study On The Progressive Collapse Of The Base-Isolated Mountain Step-Terrace Structure

Compared with other structural systems,the general flat-ground seismic isolated system has been widely researched and applied around the world because of the small stiffness of the seismic isolated layer and the weak constraint,which makes the seismic isolated structure perform well under the earthquake.It is often used in lifelines and important buildings such as hospitals,command posts,government units,schools,etc.However,once the continuous collapse and damage caused by unexpected effects occur,the impact is very serious.At present,China’s social economy is developing at a high speed,rapid urbanization,the supply of land resources for construction is increasing,and the application of mountain building structures is increasing.In addition to the current situation of frequent earthquake disasters in China and the weak earthquake resistance of Step-terrace structure,seismic isolated technology is gradually being developed.However,due to the particularity of the Step-terrace structure itself,its failure mode and force distribution are very different from the general flat-isolated structure system.The progressive collapse failure research It always focuses on the common structural system of,so this paper analyzes and studies the anti-continuous collapse performance of the base-isolated Step-terrace structure.The main work of this paper are as follows:(1)The design criteria,research methods and failure evaluation indexes of progressive collapse analysis at home and abroad are summarized,and the Do D2010 collapse failure criterion of the United States is selected as the evaluation index to provide a theoretical basis for the analysis of progressive collapse about the base-isolated Step-terrace structure.(2)The general FEA software ANSYS / LSDYNA was used to simulate and analyze the collapse test of a 4-span and 3-layer plane frame to verify the applicability and accuracy of the LSDYNA fiber element model and software program.(3)According to China’s specifications,nine base-isolated Step-terrace structures of different spans and layers are designed and a general flat-isolated frame structure also.Based the LSDYNA computing platform,a one-span plane frame was selected to establish a FEA analysis model.(4)The fourth chapter analyzed the progressive collapse resistance of the-isolated frame structure by the dynamic nonlinear analysis method which is based on the alternate path method;Chapter 5 added the horizontal seismic action on the basis of the the alternate path method,and analyzed and studied the anti-continuous collapse performance of the base-isolated Step-terrace structure under two-direction dynamic coupled excitation.And the conclusions shows:(1)Under the condition that the total number of layers is the same,the number of droped layers increases,and the anti-continuous collapse performance of the base-isolated Step-terrace structure weakens;Under the condition that the total number of spans is the same,the number of spans under the ground increases,and the anti-continuous collapse performance of the base-isolated Step-terrace structure increases.(2)In the case of accidental failure of the corner bearing,the continuous collapse resistance of the ordinary base-isolated structure is weaker than the upper-ground corner bearing of the base-isolated Step-terrace structure and stronger than the lower-ground corner bearing;In the case of unexpected failure of the corner column,the ordinary base-isolated structure is more resistant to continuous collapse than the base-isolated Step-terrace structure.(3)The resistance mechanism of the bottom corner support and the bottom corner column is only the beam mechanism,and the resistance mechanism of the inner column and inner support failure includes the beam mechanism(compression arch effect),mechanism conversion and catenary mechanism.And the resistance mechanism of the failure span of the bottom support(bottom column)adjacent to the sill lags behind the resistance mechanism of the upper ground span.(4)The degree of the effect of the failure time on the dynamic effect of the remaining structure is related to the natural vibration period of the remaining structure.Generally speaking,the shorter the failure time,the greater the impact effect of the structure and the greater the internal force of the structural member.(5)The continuity collapse resistance of the remaining system after the failure of different parts of the base isolation frame structure is as follows: the inner support is the strongest,the upper grounding angle support is second,and the lower grounding angle support is the weakest;the bottom inner column The strongest,the upper grounding and lower grounding corner posts are the weakest;the failure of the upper grounding isolation bearing is stronger than the bottom failure condition,and the failure of the lower grounding isolation bearing is weaker than the bottom working condition.(6)The failure of the isolation bearing(bottom column)under the bidirectional dynamic coupling excitation has a stronger influence on the continuous collapse of the falling-layer isolation structure than the vertical working condition that only considers the unexpected failure of the component.Under the bidirectional working condition,the collapse resistance of the bottom column of the flat isolation structure is stronger than that of the base isolation structure.For the bidirectional working condition of the isolation support,the flat isolation structure is stronger than the base isolation.The corresponding support grounded under the layer structure is weaker than the support grounded below.(7)The displacement of the failure point of the isolated step-terrace structure bearing(bottom column)under the long-period ground vibration is significantly greater than the normal ground vibration,and is likely to cause the seismic isolation support to fail,reducing the resistance of the remaining structure to progressive collapse.