Abstract
In the 1995 Kobe earthquake, many steel structures suffered damage at beam-to-column connections. In those damaged buildings, fractures of beam flange at beam-end connections were observed. After that, a remarkable number of studies have been conducted to prevent early fracture at beam-end connections and enhance the plastic deformation capacity of steel beams. In these research studies, it was pointed out that low joint efficiency of beam web, which is caused by out-of-plane deformation of skin plate of rectangular hollow section (RHS) column used for moment resisting frames in Japan, decreases the plastic rotation capacity of beam-end connections governed by beam flange. On the other hands, in high-rise buildings of Japan, the on-site full penetration welding of beam flanges has to be achieved after connecting beam web with high-tension bolts (WBFW type connection). The bolted beam-web joint has the same problem due to its poor joint efficiency. Recently, cyclic deformation capacity of the WBFW connections subjected to long-duration ground motion with many small amplitude cycles emerged as a very important issue for structural engineers in Japan. The cyclic deformation capacity of the WBFW type beam-end connections is evaluated using its low cycle fatigue characteristics described by fracture life vs. ductility factor relations, it is difficult to evaluate several test results with various conditions such as steel grade and beam span.
In this paper, cyclic loading tests focusing on establishing an appropriate evaluation method of cyclic deformation capacity of WBFW type beam-end connections were carried out. Three groups of specimens were tested. Each group of specimens has similar joint efficiency and connection details. The 400MPa conventional strength steel was used for the first group of specimens, and the 490MPa conventional strength steel and the 590MPa high strength steel were used for the second and third one, respectively.
The test results can be summarized as follows:(1) the beam-end connection made of the 590MPa steel fractured during the 14th loading cycle with constant rotation angle of 1.5θp, although that of the 400MPa steel showed sufficient plastic deformation capacity (more than 100 cycles) under the same ductility factor; (2) cyclic deformation capacity of the 590MPa steel was almost the same as that of other steel grades regarding the evaluation of low cycle fatigue characteristics based on the maximum rotation angle θmax; (3) finally, the conversion method of ductility factor for various steel grades was proposed.
