Research On Progressive Collapse Behavior Of RC Frame Structures

The integrity security of the building structures has always been a main problem in the structural design and already a highlight in the engineering fields. When it suffers from the casual extreme load caused all of a sudden by the events such as explosion,impact and fire,the structure will take on a disproportional collapse locally or on the large area,which results in massive casualty and economic loss and bad social influence,therefore,in addition to ensuring the security and reliability of the structure in strength,rigidness and stability when it bears the normally design load according to the normal design methods,the structure should be ensured to be able to resist the above collapse caused by the casual extreme load, However, there is no operational references to ensure it for structural engineers.1. A pseudo-static test was adopted here to test the collapse-resistant behavior of 3-story and 4-bay one-third scale model plane RC frame structure where the frame collapsed owing to the steel bars'rupture at a vertical unloading displacement of 456mm, Based on the test results,the mechanical behavior,failing process and load behavior of the frame structure during the collapse were studied and the forced mechanism and the conversion mechanism of the load during the collapse were explored. A dynamic time-history analysis was used to study the dynamic response during the collapse with the help of the load-resistant curve obtained in the pseudo-static test and numerical simulation analysis was performed in considering the column failure rate with the software of Ls-Dyna. The dynamic effect during the collapse was studied by converting the load-resistant curve into the capacity curve according to energy equivalence,The necessary conditions that the final collapse didn't occur were inferred with the collapse-resistance energy equation obtained on the basis of energy principle. The capacity-demand curve was put down here to evaluate the collapse-resistance behavior of the structure during the collapse-resistant design and assessment.2. The estimation of the catenary action effect was an essential step in the collapse-resistant design. In order to study the catenary action mechanism, destructive tests of 6 axially restrained beams were done in considering of change of the reinforcing bar ratio,strength grade of steel bar, anchoring mode and loading rate where the tested specimen's bar ruptured which results in a rapidly declined load-carrying capacity except that the tested specimen B1 didn't rupture during the loading because of excessive deformation of the specimen. The load-carrying capacity of the restrained beams in the limit collapse state was about twice as much as that at plastic phase and its deformation about twenty times on amount of the catemary action phase. A simplified calculation was presented here to estimate the two design parameters,maximum deformation and catenary load,on the basis of test data.3. A collapse test was carried out that the side-and-mid column and corner column were rapidly removed respectively with the charge explosion so as to study the collapse-resistant behavior of the spatial frame with the distribution load 6kN/m2, where the frame only took on elastic deformation after the failure of the columns with maximum vertical displacements 8.5mm and 4.3mm respectively. A static collapse destructive test was done to the frame after the bottom columns' failure by continuously loading with a hydraulic jack, The test showed that the frame structure took on plasticity during collapse owing to the membrane action of the slab and the spatial effects of the beam which played a big role in relieving and constraining the collapse of the structure, The action of longitudinal and horizontal vierendeel action was the main forced mechanism of the structure during the load redistribution after the column failure.4.The idea of comprehensive disaster resistance design is presented here, and the collapse-resistant design should be performed for the structural system after the normal design. The collapse-resistant behavior of the structure will be increased by improving the structural system, strengthen design of the support element,frame beam,cast-in-place slab and connection,ensuring the continuity and redundant structural measures and sufficiently ultimazing the catenary action.