open access

https://doi.org/10.14943/doctoral.k16125

https://hdl.handle.net/2115/93640

Large-Deformation Behavior and Failure Process of Steel Moment-Resisting Frames Examined by a Shake-Table Test

英語

2024-09-25

Hokkaido University

viii, 239p.

Steel moment-resisting frames are popular structural systems in seismic areas. However, seismic excitations produce large deformations that eventually lead to member damage, degradation of strength and stiffness, and collapse. The structural damage changes the vibration properties and therefore the response structural response. Very few studies have been conducted in the past to examine the behavior of steel moment-resisting frames to very large deformations, i.e., story drift ratios greater than 0.10 rad. Furthermore, ductile moment-resisting beam-to-column connections are important to achieve good performance against earthquakes. This research studies the behavior of steel moment-resisting frames through cyclic static tests on beam-to column moment connection subassemblages (component-level response) and dynamic shake-table tests of a scaled moment-resisting frame (system-level response). The beam-to-column connections of the subassemblages and shake-table tests use the current detailing of the Japanese construction practice. Six beam-to-column moment connections that used an I-section column were tested under cyclic loading. I-section columns were chosen instead of the typical square box-section columns to examine the effect of the current Japanese welding details and fabrication in moment connections to the flange and to the web of the column. The T-configuration of the specimen represented an external beam-to-column subassembly of two to four-story steel moment frame buildings in Japan. The specimens to cyclic loading according to the protocol specified in Section K2 of AISC Seismic Provisions. Three specimens were for moment connection to the column flange and three for moment connection to the column web. Complete Joint Penetration (CJP) groove welds were used to connect the beam flanges to the column. In all cases, the CJP welds were completed with no weld access holes. The tests showed that beam-tocolumn web connections were prone to fracture of continuity plates initiating at the corner between the beam flange and the continuity plate. This failure mode was promoted by yielding in the continuity plates. The shop-welded, no-access-hole detail for beam-to-column moment-resisting connections, which is commonly adopted in Japan, is a detail not included in the ANSI/AISC 358 but met ductility requirements for the highly ductile Special Moment Frames per the AISC Seismic Provisions. Furthermore, the quality of the welding process, in combination with having the beam web directly welded to the column web and the use of a stronger continuity plate were important factors in achieving a good seismic performance of the minor-axis moment connections even when continuity plates flush to the column flanges were used. The same no-weld access hole detail was used in the shake-table test specimen, but with the connection to square-HSS columns. Shake-table tests of a steel moment-resisting frame were conducted to obtain detailed data on the occurrence and propagation of damage leading to the collapse of steel structures. A 2/5-scale, 4-story, 2-bay, planar specimen was subjected to in-plane, unidirectional ground motions. The specimen was intended to sustain large deformations, or story drift ratio greater than 0.10 rad, and avoid a first-story collapse mechanism. The specimen comprised I-section beams and square-HSS columns. The CJP groove welds between the beam flanges and column diaphragm plates were completed with no weldaccess-hole. The ground motions included the JMA Kobe NS record scaled from 10 to 100%, and white noise excitations to trace the change in vibration properties between primary excitations. The 100%- scale record was repeated four times to produce as much deformation and damage as possible. The frame deformation response, change in internal load distribution, and acceleration were measured with a combination of displacement transducers, strain gages and accelerometers, respectively. Each of the 100% motions induced significant yielding of the beams, columns, and some column panels, which resulted in story drift of +0.08 and -0.02 rad, and added a residual story drift of +0.02 rad. As expected by design, a sidesway mechanism involving the first three stories formed during the first 100% excitation. Beam fracture occurred at two locations on the second floor, and the maximum story drift reached +0.15 rad during the last 100% excitation. The fundamental period remained constant until beam fracture occurred. Despite the widespread damage and fracture of the second-floor beams, the specimen did not collapse due to the high strength of the columns, which had 1.8 times the nominal strength. Three supplementary cyclic-loading tests were conducted on beam-column subassemblages constructed from the same materials and fabrication process as the shake-table specimen. The subassemblages represented one interior and two exterior connections of the shake-table test specimen. The loading protocol followed the AISC loading protocol for beam-to-column moment connections up to 0.05-rad story drift, but it was changed to a ratchetting protocol up to 0.125 rad afterward to emulate the observed beam-to-column large deformation response in the shake-table tests. However, the subassemblages exhibited more rapid development of local buckling deformation and associated strength degradation. Comparison of the shake-table and component test results showed good agreement in strength capacity at the beam-to-column connection level but differed in terms of degradation. The difference was attributed to axial deformation restraint in the shake-table test beams, evidenced by smaller beam contractions than in the component test beams. Additionally, the inertia forces imposed on the beams of the shake-table test specimen axial loads that were not present in the component tests. The axial loads synchronized with the bending moments to increase the demands in the beam bottom flange. For this reason, fracture occurred in the bottom flange of the shake-table test beams. The subassemblage test beams, which were subject to axial-free bending moment, fractured at either the top or bottom flange. The size effect of the shake-table test was examined by comparing the subassemblage test beam response with full-scale beam-to-column connection tests that followed a similar fabrication practice and load protocol. The weld detail and fabrication used in this study was found to not affect the test outcome. The larger length-to-depth ratio of the scaled specimen, however, reduced the maximum bending strength of the beam with respect to full-scale specimens. The major contributions of this research are: first, the obtention of seismic response data of a steel moment-resisting frame that experienced deformations up to 0.15-rad, without premature collapse. The data can be used to develop analysis schemes for large-deformation behavior prediction. Second, the comparison with subassemblage tests showed that in order to predict the global behavior with component tests, the axial restraint and moment redistribution present in the steel moment frame must be addressed. Third, even though small cracks are likely to happen, the current Japanese fabrication of welded beam-to-column moment connections ensures ductile behavior in either beam-and-column subassemblages or complete moment-resisting frames.

2024-09-25

甲第16125号

博士(工学)

(主査) 教授 岡崎 太一郎, 教授 菊地 優, 准教授 高井 伸雄

工学院（建築都市空間デザイン専攻）

博士論文

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