Study On Progressive Collapse Resistance Of RC Frame Structure Under Impact Load

In today’s world, with the rapid development of urban construction and urban traffic system,the accident of vehicles hit buildings happen now and then. Especially, transnational connections and racial tensions became increasingly comp lex and intense. So that, in recent years, a growing number of terrorist attacks happened. They both increase the probability of structure progressive collapse under impacting. So that structure progressive collapse caused by impacting becomes a major threat to public safety. Universally, the current design method for preventing structure progressive collapse is not specific to a certain disaster load. Besides, the design method specified to a certain disaster load is extraordinary difficulty and complicated, because of disaster loads’ strong indeterminacy. At present, there is no design code make any specific provisions about that. The existing research on structure progressive collapse specified to a certain disaster load is mainly focus on explosion and fire, the aspect of impacting is almost empty. So that, carrying out the research work on analytical method of structure progressive collapse under impacting has profound significance.The paper takes reinforced concrete frame as research object. The major research contents and corresponding results show as follows:(1) Summarize the existing analysis method for structure progressive collapse under impacting, that is alternate path method and direct simulation method. Using ANSYS/LS-DYNA software to conduct numerical analysis on specific examples and comparing these two methods. Results show that alternate path method overvalue structure’s resisting progressive collapse capacity since it ignores adjoining members’ initial damage, initial velocity and initial acceleration caused by removed key member during its failing progress. While the direct simulation method is more precise, but impact load’s value is indeterminacy, the process is complicated and time-consuming. Besides, this method has a higher request for users’ technical level. So that, direct simulation method is not suitable for engineering application.(2) Reinforced concrete columns’ dynamic response and failure mode under impacting are studied. The parameter analysis results show: column s’ section size, mass of impactor, velocity of impactor can significantly influence columns’ dynamic response and damage degree; column s’ section size, columns’ heights, longitudinal reinforcement ratio have a big influence on columns’ failure mode, and with the increase of these parameters’ value, columns’ failure mode change from direct shear failure into oblique shear failure.(3) The failure criterion, failure time and dynamic response at fa ilure time of RC columns are studied. In order to simplify analysis, take the factors which significantly influence columns’ dynamic response as parameters, and consider other factors safely, statistical analyses are conducted and obtain that: ①The law curve of RC columns’ minimum impact speed. ②The law curve of RC columns’ failure time.③The law curve of RC columns’ initial velocity(vertical) at failure time. Besides, a fitting is made to these law curves, the formulas to calculate minimum impact velocity which can lead columns’ failure and the formulas to calculate RC columns’ initial velocity(vertical) at failure time are obtained.(4) A melioration is conducted on the deficiency of traditional alternate path method. A new analysis method for RC frame structure progressive collapse under impacting is come forward, which takes member ’s non-zero initial conditions into consideration. The determination of critical impact load’s value and member ’s non- zero initial conditions is also carried out. Finally using ANSYS/LS-DYNA analyzes progressive collapse resistance of a certain RC frame structure under impact load with different methods. By comparing the results of new method, traditional alternate path method and direct simulation method, that can verify new method’s accuracy and reliability. The results show that, compared with alternate path method, new method considered member ’s non- zero initial conditions, that makes this method is more precise and reliable. Compared with direct simulation method, it is more convenient and practical.