Progressive Collapse Analysis Of RC Frames Under Severe Ground Motions Considering Internal Contact-Impact Effects

In most domestic and international study, progressive collapse begins to happen after key components are removed artificially.The initial failure cannot be judged from the responses of the components, and the analysis model cannot be changed according to the responses of the structure in these studys. At present, there are mainly two kinds of methods to the handling the failure components, instantaneous unload method and instantaneous remove component method, to consider the dynamic effects when the components is failed. Various contact-impact effects will inevitably occur during the progressive collapse. Hence, if the contact-impact effects can be taken into account in the analysis, the progressive collapse procedure can be described more accurately.The similarities and differences of instantaneous unload method and instantaneous remove component method are compared. Through the mechanics modeling of instantaneous remove component method, it is proved that the acceleration of current node will changed abruptly after the removing of component, whether the structure is in a state of static equilibrium or dynamic equilibrium before removing. If the structure is in a static equilibrium state, the internal force of the removed component will completely translated to the the acceleration increment of current node. If the structure is in a dynamic equilibrium state, the internal force will be translated into two parts. The first part is the acceleration increment of current node. The second part is the displacement increment of current node and its adjacent nodes. The smaller the analysis step is, the higher proportion of the second part is.Impact behaviors due to a drop beam, side span collapse, and midspan collapse are studied here. Master-slave modeling technique is used to build the model. The model is modified by using the remove and equalDOF command, considering the dynamic effects after the components failure. Nonlinear Hertz-damp model is used in the mechanics modeling of the impact force. A Matlab program is called during OpenSees process to cumpute the impact force. The initial conditions in Matlab program are come from the OpenSees results, and the Matlab results will be used in subsequent analysis. The impact force is considered as a reaction force in the main structure. Therefore, the dynamic analysis is under the joint action of the impact force and the ground motion. Involved components need to be removed from the structure if mechanism is appeared, and the acting force from mechanism need to be added to the main structure, meanwhile.The horizontal displacement and rotation of the structure are increased under severe ground motions, with the consideration of beam-to-beam impact and gradual failure of components. The value of impact force is largely influenced by the mass of the active impact layer and the height of the failed column. The velocity of active impact layer increases, When these two factors increase.The impact force will also increase. Therefore, a larger impact effect is added on the passive impact beam.A weak catenary action will occur after the failure of side column, while a stronger catenary action occurs after the failure of middle column, for the frame structure system. The impact point for side span collapse is unique, under the assumptions of the strength of impact components are enough, without tangential slip and plastic collision. There are various kinds of impact cases for middle span collapse under the same assumptions. The impact pint for each case is different. The impact will not happen if the catenary action is strong enough.The impact case from chapter4, figure4.3(i) is study in detail. The impact between the upper structure and the main structure could happen four times.The parameters of these impact behaviors are discussed.And a example is designed to achieve this process.