原田 幸博

Buckling restrained braces are among the most effective seismic-resisting structural members. This study focused on the feasibility of using buckling restrained braces for base-isolated steel buildings with long-span girders. A series of analyses were conducted to characterize the seismic response of base-isolated steel building structures reinforced by concentratedly placed K-bracings with ordinary or buckling restrained braces. Buckling restrained braces were shown to reduce not only the lateral deformation of the buildings but also the tensile force acting on rubber bearings of isolators.

The purpose of the present study is to investigate how longitudinal stiffener plates can improve plastic deformation capacity of the H-section steel beams with a thinner web. Efficiency of the retrofit can be quantitatively estimated by an accumulated plastic deformation obtained from cyclic loading tests and numerical simulation. The quantitative estimate method of prevention effect of local buckling by the theory of elastic buckling of plates is proposed. The design procedure of stiffeners by prediction formula of plastic deformation capacity and increasing rate in strength is proposed.

This study describes fracture strength of electro-slag welded (ESW) connections in beam-to-welded box column connection. The discussions are based on subassemblage connection test results of ESW joints and finite element analysis (FEA) results of corresponding analytical models. Case studies are also carried out making the gap between an interior diaphragm and a beam flange main analytical parametes. Results are summarized as follows; 1) The subassemblage connection test results show roughly positive correlation between the fracture toughness and the fracture strengths of ESW joints. 2) The subassemblage connection test results and the corresponding FEA results correlate the fracture toughness with calculated equivalent maximum principal stress. 3) The fracture behavior of ESW joints shows good correlation between subassemblage connection test and cruciform model test.

Shear-bending cyclic loading tests were performed to investigate structural behavior of shear-yield-precedence short-span beam. Main test parameters are the following: beam-end connection detail (only welded or both welded and bolted connection), design principle (designed only for bending or for bending and shear), and beam web detail (stiffened by web stiffeners or with reinforced web openings). Beam-end weld connections do not fracture in the all specimens, and the all test specimens showed good plastic deformation capacity because shear yielding of the web occurred prior to shear buckling; all the test results support feasibility of shear-yield-precedence design of short-span beam.

This study describes fracture strength of electro-slag welded (ESW) connections under biaxial loading condition. The discussions are based on model test results of ESW joints and finite element analysis (FEA) results with corresponding analytical models. Case studies are also made where an axial force ratio and a penetration shape are the analytical parameters. Results are summarized as follows; 1) The model test results show roughly positive correlation between the fracture toughness and the fracture strengths of ESW joints; 2) The model test results and the corresponding FEA results correlate the fracture toughness with calculated equivalent maximum principal stress(MPA); 3) In the results of the case studies, the calculated MPA in a heat affected zone highly depends on an ESW fusion line position.

The strength of an H-section column-to-beam weak axis connection with split-tee stubs is dominated by out-of-plane deformation of web plate of the column, especially when transverse stiffeners are omitted to simplify fabrication process of a beam-to-column connection. The main purpose of the present study is to analytically derive general yield and maximum strengths of the bolted column web-to-split-tee stub connection by yield line analysis, including the case where reinforcing plates or angles are attached at the web. The validity of the analytical strengths of the connection is investigated by comparing with tensile test results of the connections.

In order to study structural beheavious of H-section weak axis column to H-section beam connection with high-strength bolts, cyclic loading tests of subassemblage specimens with this connection type are performed. The obtained load-drift angle relationship of each specimen is stable spindle type with satisfactory deformation capacity. The connection strengths of the subassamblage are analyzed by the yield line theory, giving good correspondence with the test results. The effect of the moment-local rotational angle of the connection on the stiffness and deformation capacity of the subassemblages is then discussed.

In order to design H-section column-to-H-section beam connection with split-T stubs, out-of-plane deformation of flange plate of H-section column should be considered, which causes semi-rigidity of the connection, especially in the connection detail without transverse stiffener. Estimation of yield and ultimate strengths of the connection is derived from plastic analysis by assuming yield and ultimate mechanisms concerning out-of-plane deformation of the flange plate of H-section column and stretch of high-strength bolts. The assumed mechanisms and analytical estimation of strength of the connection are verified by comparing with the results of experimental studies by tensile tests of the connection.

In order to design RHS (rectangular hollow section) column-to-H beam connection with split-T stubs, out-of-plane deformation of column plate should be considered, which causes semi-rigidity of the connection. Estimation of yield and ultimate strengths of the connection is derived from plastic analysis by assuming yield and collapse mechanisms concerning out-of-plane deformation of the column skin plate and stretch of the high-strength bolts. The assumed mechanisms and analytical estimation of strengths of the connection are verified by finite element analysis. The analytical estimation of strengths is also verified by comparing with the results of previous experimental studies by tensile tests of the connection.

Many damaged steel buildings were repaired after Northridge earthquake in 1994 and Hyogoken-Nanbu earthquake in 1995, but the efficiency of the repair for beam-to-column connection has not been examined sufficiently. The purpose of the present study is to experimentally investigate the efficiency of the repair and modification for beam-to-column connection of steel buildings. Subassemblage specimens for this experimental study is prepared by repairing damaged subassemblage specimens which is obtained by previous experimental study. In this study, the maximum strength of the connections in repaired or modified subassemblage specimens are estimated, with indicating resisting mechanism in the connections.

Brittle fractures in beam-to-column connections of steel buildings were widely observed under Northridge earthquake and Hyogoken-Nanbu earthquake. Many studies after these earthquakes have revealed that the brittle fracture is complexly affected by various factors. The purpose of the present study is to investigate influence of connection detail and material properties upon the brittle fracture. In order to estimate maximum strength and plastic deformation capacity of the connection, loading tests using subassemblage specimens and numerical analysis are performed. In this study, performance of the connection is discussed mainly in the following subjects: influence of connection detail and yield ratio upon strain distribution, and correlation between fracture toughness and initial fracture.