Intermediate-story Collapse of Reinforced Concrete Building Structures

在1995年兵库县南部地震中，许多混凝土建筑物的中间层完全倒塌。这种倒塌被认为是由于在某个中间层的柱遭受严重的剪切破坏，最终无法承受轴向载荷。为了研究这一点，剪切失败RC柱标本进行了测试。对样本施加横向载荷，直至其无法承受轴向载荷。轴向载荷比和加载路径为试验参数。研究结果表明：1）当剪力减小到零附近时，结构发生破坏; 2）轴向荷载水平对结构的破坏起决定性作用。当轴向应力为0.27fc时，倒塌时的侧向位移和竖向变形分别约为3 ~ 5%和1%;当轴向应力为0.18fc时，倒塌时的侧向位移和竖向变形分别约为10 ~ 20%和2 ~ 5%。3）荷载历史对倒塌行为有显著影响，包括破坏模式。竖向变形增量与侧向位移增量的比值随加载的进行而增大，当未来发生强震时，许多按旧规范设计的建筑物极有可能遭受类似中层倒塌的严重破坏。因此，必须对其进行改造。因此，对旧建筑和墙体改造建筑的地震位移要求进行了分析和比较。研究了10层和5层模型建筑。主要发现如下：1）如果旧建筑物以其底部抗剪强度大于或等于总重量的0.5的方式进行改造，则每个层间位移需求保持在层高的1%以内。2）增加的墙壁强度的微小差异偶尔会导致位移需求的较大差异。

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.