| Most of previous studies have mainly concentrated on the potential for progressive collapse of structures resulting from instantaneous removal of a primary vertical support member,less study being conducted on the impact of the falling structure onto the structure below. However, it has long been recognized that when the pancake collapse (which denotes the whole upper structure sits down on the lower structure in this thesis) happen, the impact load of the upper structure onto the floor below is obviously much larger than the original static gravity load. Thus the impact load results in the development of considerable ductility demand for the lower floor (structure) to prevent progressive collapse. Therefore, through tests, numerical simulation, and simplified method based on energy balance, this thesis investigates the magnitude and influence factors of impact load resulted from pancake collapse, as well as its planar distribution being also observed and studied. Finally, a simplified methodology is proposed for progressive collapse assessment of floor within multi-storey buildings subjected to impact from upper failed floor. The main work and conclusions include as follows:1. Pseudo-static tests of three one-storey RC frame-shear wall model structures with floor slab were conducted to investigate their failure modes and hysteretic behaviors. The tests results are reported in this paper and compared with the pushover analysis results. It can be seen that: (a) damages of the floor slabs under lateral loading are significant in regions close to the shear wall, and tensile stresses of slabs’steel bars in these regions are much larger; (b) in comparison with the case that the floor slabs are neglected, the maximum lateral load that a frame-shear wall structure can bear is larger in the case that the floor slabs are taken into account, and also is the contribution ratio of the shear wall; and (c) shear forces carried by the frame columns with identical cross sections and reinforcement details but located at different positions are close to each other on the whole.2. After the Pseudo-static tests of the above three one-storey RC frame-shear wall model structures, the second stage pushover tests on pancake collapse then were conducted. The pancake collapse modes of the model structures and the time histories and planar distributions of the impact loads, which related to the second test stage, are reported in this paper. Test results show that: (a) the out-of-plane break of shear walls and increasing of axial load ratios of columns are main causes of the pancake collapse of frame-shear wall structures; (b) the impact process related to the pancake collapse involves complex and dynamic contacts of structural members, leading two groups of fluctuating in the time histories of impact loads; and (c) the impact loads are significantly affected by the materials of structural members, the maximum total impact load related to upper concrete members and lower steel plate is 7.90 times of the total weight over and including the collapse story (i.e., the slab and beams’weights, half of the shear wall and columns’weights, and all the additional weights), and the maximum total impact load related to upper concrete members and lower composite plate is only 3.58 times of the total weight over and including the collapse story.3. Free fall tests on the RC frame beam-slab combination at three different drop heights: 250mm, 500mm and 750mm were conducted (each test was repeated three times) to investigate the impact load time history. By integrating the results from pancake collapse test mentioned before, an approximation estimation method for the effective impact load was proposed and the reference value of relative parameter was initially given. The test results show that: (a) the maximum total impact load is nearly proportional to the free fall height if the accumulated damage is neglected, and (b) due to the effect from vertical structural components such as frame columns and shear walls, the maximum total impact load resulted from pancake collapse is only 16 percent of that resulted from free fall impact.4. Based on energy balance, a simplified evaluating method is proposed to assess the impacting effect between the failed and impacted slabs, including two stages: (a) nonlinear static analysis of the impacted floor slab, and (b) determination of the maximum acceptable gravity load of the failed floor slab using a simplified energy balance approach, then compared to the actual upper floor gravity load. In addition, kinetic energy transfer from an above failed floor slab to an impacted floor slab is theoretically analyzed for two extreme impact possibilities, namely fully plastic impact and fully rigid impact, and influence of a ratio of the failed slab’s mass to the impacted slab’s mass on the kinetic energy transfer is discussed. It can be seen that: (a) in the case that the failed slab’s mass is equal to the impacted slab’s mass, the total kinetic energy transfer percentage and the impacted slab’s kinetic energy transfer percentage related to fully plastic impact are, respectively, 33.3% and 16.7%; (b) the impacted slab’s kinetic energy transfer percentage related to fully rigid impact is affected by velocity of the failed slab before impacting and edge velocity of the failed slab after impacting, in the case that the failed slab’s mass is equal to the impacted slab’s mass, the impacted slab’s kinetic energy transfer percentage related to fully plastic impact is range from 44.4% to 97.1%; and (c) the calculated impacted slab’s kinetic energy transfer percentages related to the two extreme impact possibilities using ABAQUS are close to the theoretically analyzing results on the whole..5. The application of this proposed methodology above is demonstrated by means of a case study. The influences of several possibilities regarding the boundary conditions of the impacted floor slab, rebar hardening,energy transfer to the lower floor on the maximum allowed upper floor load are examined. The application study results show that: (a) the maximum allowable deflection of edge-fixed slab respect to the rebar strain hardening can increase 50% of that of those considering elastic-perfectly plastic rebar; (b) corresponding to various energy transfer percents, simply supported slab can carry 2.3 to 2.6 times maximum allowable upper floor load of the edge-fixed slabs. Additionally, the maximum allowable upper floor load of the edge-fixed slab considering rebar strain hardening is 1.8 times of that without regard to this favourable property of rebars; and (c) increasing thickness of slab can significantly improve the slab’s dynamic load-carrying capacity, though increasing rebar content also improve this capacity to some extent, the range is limited. |
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