Intermediate-story Collapse of Reinforced Concrete Building Structures

在1995年的小古肯-南部地震中，许多混凝土建筑在中间层完全倒塌。这种倒塌被认为是由于某中间层的柱发生了严重的剪切破坏，最终无法承受轴向荷载。为此，对钢筋混凝土柱试件进行了抗剪破坏试验。试件被横向加载，直到它无法承受轴向载荷。轴向加载比和加载路径为试验参数。主要研究结果如下：1)剪切力减小至零左右时发生坍塌。2)轴向荷载水平对破坏行为起决定性作用。轴向应力为0.27fc时，崩塌时的侧向漂移和垂直变形分别为3 ~ 5%和1%，而轴向应力为0.18fc时，崩塌时的侧向漂移和垂直变形分别为10 ~ 20%和2 ~ 5%。3)加载历史对包括破坏模式在内的倒塌行为有显著影响。(4)竖向变形增量与侧向位移增量的比值随着加载的进行而增大。当未来发生强烈地震时，许多按照旧规范设计的建筑物很可能遭受类似中层倒塌的严重破坏。因此，它们必须进行改造。因此，对旧建筑和墙体改造的抗震漂移需求进行了分析和比较。研究了10层和5层的模型建筑。主要研究结果如下：1)如果旧建筑的改造方式使其基础抗剪强度大于或等于总重量的0.5，则每层间位移需求保持在层高的1%以内。2)加墙强度的微小差异有时会导致漂移需求的较大差异。

During the 1995 Hyogoken-Nanbu earthquake, many concrete buildings suffered complete collapse at an intermediate story. Such collapse is believed to be due to that columns at a certain intermediate story underwent severe shear failure and eventually came to be unable to sustain axial load. To study this, shear-failing RC column specimens were tested. The specimens were laterally loaded until it came to be unable to sustain axial load. Axial load ratio and loading path were test parameters. The major findings are as follows:1) Collapse occurs when shear force decreases to about zero.2) Axial load level is decisive for the collapse behavior. Lateral drift and vertical deformation at the collapse were about 3 to 5% and 1% for axial stress of 0.27fc while they were about 10 to 20% and 2 to 5% for axial stress of 0.18fc.3) Loading history has a significant effect on the collapse behavior including a failure mode.4) A ration of vertical deformation increment vs. lateral drift increment increases as the loading proceeds.When strong earthquakes occur in the future, many buildings designed according to the old code are very likely to suffer severe damages similar to intermediate-story collapse. Hence, they have to be retrofitted. Therefore, seismic drift demands of old buildings and those retrofitted using walls were analyzed and compared. Ten-story and five-story model buildings were studied. The major findings are as follows:1) If old buildings are retrofitted in such manner that their base shear strength is more than or equal to 0.5 of the total weight, each inter-story drift demand remains within 1 % of the story height.2) A slight difference in the strength of added walls may occasionally result in large difference in the drift demand.