【作者】 黄仕香；

【导师】 吴波；

【作者基本信息】 华南理工大学 ， 结构工程， 2011， 博士

【摘要】 以往人们评估结构的抗倒塌性能多采用突然移除某一主要竖向构件的做法,较少涉及掉落构件(或上部结构)对下部结构的碰撞作用。结构发生落层倒塌(即结构某层或该层绝大部分突然整体下挫,该层楼盖与下层楼盖发生大面积接触碰撞的倒塌形式)时,上部结构撞击下部结构产生巨大的碰撞荷载,只有当下部结构足以承受该碰撞荷载时才能防止连续倒塌。本文从模型试验、数值模拟、简化评估方法等方面,对混凝土结构落层倒塌碰撞问题进行了初步探讨,主要工作和结论如下:1.开展了3个单层空间混凝土框剪结构模型的抗震试验,考察了框剪结构在楼板参与作用下的破坏过程,并与弹塑性分析结果进行了对比,初步考察了楼板对抗侧力构件剪力分配的影响。研究表明:①水平荷载作用下剪力墙附近楼板损伤较大,该处楼板钢筋承受较大拉应力;②考虑实际楼板和忽略楼板作用相比,前者情况下框剪结构所能承受的最大水平荷载有所增大,同时剪力墙在所有抗侧力构件中的贡献比例也有所提高;③具有相同横截面积和配筋的不同位置框架柱所承受的剪力总体上相差不大。2.在前述抗震试验的基础上,进行了该3个混凝土框剪结构模型的落层倒塌碰撞试验,考察了模型结构的落层倒塌模式和相应的碰撞荷载时程及其平面分布。研究表明:①剪力墙面外折断和柱轴压比增大是诱发落层倒塌的重要原因;②落层倒塌碰撞是一个多体接触的复杂动态过程,碰撞荷载时程包含两组波动;③碰撞荷载的大小与碰撞双方材质密切相关,混凝土构件撞击钢板产生的总碰撞荷载最大值是倒塌层及其以上总重力荷载的7.90倍,而混凝土构件撞击钢-混凝土组合板产生的总碰撞荷载最大值仅为倒塌层及其以上总重力荷载的3.58倍。3.开展了混凝土框架梁-楼板组合体3个不同碰撞高度共9次(每个高度3次)自由落体碰撞试验,考察了相应的碰撞荷载时程,提出了等效碰撞荷载的近似估算方法并给出了相关参数的初步参考取值,同时对自由落体碰撞试验进行了数值模拟。研究表明:①若忽略混凝土框架梁—楼板组合体在每次碰撞中的累积损伤,总碰撞荷载最大值与碰撞高度近似成正比关系;②由于剪力墙和框架柱等竖向构件的存在,落层倒塌碰撞时总碰撞荷载最大值与下落体总重力荷载之比仅为自由落体碰撞时相应比值的16%左右。4.基于能量平衡原理提出了一种评估楼板落层倒塌碰撞效应的简化方法,该方法包括两个步骤:一是进行被撞击楼板(下板)的非线性静力分析,得到该楼板的荷载—板中心点挠度曲线;二是根据下板中心点在静荷载作用下的最大允许位移,以及碰撞时掉落楼板(上板)动能转移至下板的动能转移百分比(简称下板动能转移百分比),估算上板的最大允许重力荷载,将此最大允许重力荷载与上板实际重力荷载进行对比即可评估下板的碰撞安全性。针对两种碰撞极限状态——完全塑性碰撞和完全刚性碰撞,理论推导了相应的下板动能转移百分比,考察了相关因素对该百分比的影响,同时采用ABAQUS有限元软件对两种碰撞极限状态进行了下板动能转移百分比的数值计算。研究表明:①上、下板质量比等于1时,完全塑性碰撞后的总动能转移百分比和下板动能转移百分比分别为33.3%和16.7%;②完全刚性碰撞后的下板动能转移百分比与碰撞前上板速度、碰撞后上板周边反弹速度等参数有关,上、下板质量比等于1时下板动能转移百分比介于44.4%～97.1%之间;③计算得到的两种极限状态对应的下板动能转移百分比,总体上与理论分析结果较为接近。5.采用上面所提楼板碰撞效应简化评估方法,考察了下板边界约束条件、钢筋强化、下板动能转移百分比等因素对上板最大允许重力荷载的影响,比较了两种措施增强下板抗碰撞能力的效果。研究表明:①考虑钢筋强化效应后,四边固结板的最大允许位移相比未考虑强化效应时增大近50%;②对应不同的下板动能转移百分比,四边简支板可承受的上板最大允许重力荷载是四边固结板的2.3~2.6倍;考虑钢筋强化时四边固结板可承受的上板最大允许重力荷载约为未考虑钢筋强化时的1.8倍;③增大板厚可显著增强下板的抗碰撞能力,提高配筋率虽然也可在一定程度上增强下板的抗碰撞能力,但增强幅度有限。更多还原

【Abstract】 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.更多还原