To clarify the contribution of martensite increase to work-hardening during transformation-induced plasticity (TRIP), the changes in local strain distribution with tensile deformation were investigated for dualphase steels with different volume fractions of martensite in ferrite using the precise marker method. Particular attention was paid to the changes in strain and stress bearing by martensite and ferrite with tensile deformation. The precise marker method is especially useful for local strain analysis of multiphase materials. Three types of steels with volume fractions of 25%, 50%, and 75% martensite were subjected to local strain analysis at several stages of deformation. The important results are as follows: (1) in steels with a large volume fraction of martensite, the contribution of the plastic deformation of martensite to the overall tensile strain is large from the beginning of the tensile deformation, and (2) the difference in strain bearing by martensite and ferrite increases through tensile deformation in both 25% and 50% martensitic steels. Using the constitutive equations for the stress-strain response, the strain distributions in each phase were translated into stress distributions. Then, the stress-strain response was numerically estimated by applying the general rule of mixtures using the average values of strain and stress, and compared with the experimental results. The relationship between the controlled martensitic transformation and tensile deformation behavior in TRIP steel is also discussed.

Steel column testing in which a compressive axial force and a cyclic biaxial bending moment were applied simultaneously was conducted in order to clarify the structural performance of the column. Although the AIJ beam‐column design formula provided overly conservative results, the correlation equation for full plasticity evaluated the results with reasonable accuracy. The loading direction was not crucial to the plastic deformation capacity. As a result, the limitation applied to columns in order to guarantee sufficient plastic deformation under a loading direction of 0‐degree can also be used for columns with a loading direction of 45‐degree.

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The square hollow section (SHS) is commonly used for the column of the steel building structures in Japan. The moment‐resisting frame will resist the lateral force with the flexural manner of the columns. Under the seismic event, the bending moment will be significant, and the plastic hinge may be formed in the columns. It is crucial to understand the structural behavior at the component level to capture the structure system's ultimate limit state by computational simulation. Understanding the strength and the ductility is one of the essential aspects, but it is also crucial to know the structural elasto‐plastic behavior until the ultimate limit state. It is worthwhile to conduct the testing of the members under combined loading conditions. In this paper, the test results obtained from the different width‐thickness ratio SHS were reported. The forces subjected to the specimen were axial force and cyclic bending moment; therefore, the member was under combined loading condition. Collapse mode, maximum bending moment, and plastic deformation capacity were evaluated in consideration of the effect of the width‐thickness ratio; comparison study between the current design recommendation was also shown.

Steel column testing in which compressive axial force and cyclic bending moment were applied simultaneously was conducted to clarify the columns' structural performances. AIJ Recommendation specifies requirements for a column to guarantee sufficient strength and ductility. Due to the lateraltorsional buckling, some specimens could not achieve the structural performance stipulated in AIJ Recommendation even though the specimens fulfilled the requirements. Comparison between the proposed method per researchers also showed that adequate results could not be found. In order to compose the design formulas for H-shaped columns, a consideration of the index that includes the sensitivity of lateral-torsional buckling is needed.