The study on introduction of Limit State Design method (LSD) or Reliability-based design method have been carried out for more than 40 years in Japan. However, its usefulness or importance have not necessarily been recognized, and they have not been employed in Japan. On the other hand, the design codes of AASHTO LRFD and EC are based on LSD. In this paper, the importance of shifting to LSD is explained. In addition, through LSD of composite girders, in which the ultimate flexure strength is defined to be plastic moment for verifying the safety performance, the possibility of change of the structural dimension is explained. As an actual applied example, Kanayago viaduct bridge design and construction is introduced. It is also explained, on designing hybrid bridges such as hybrid girder bridges and double composite girder bridges, that their applicability and competitiveness has a close relation to the design method. Finally, Limit State Design method for maintenance being employed in AASHTO LRFR is introduced, and the importance of developing the design method based on LSD for the maintenance is also emphasized.

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 A hybrid structure is originally a composite structure in which structural steel and concrete have been compounded in its cross-section and then is expanded to a mixed structure in which a steel member and a reinforced concrete member has been joined. Moreover, various kinds of materials such as FRP recently have come into use instead of structural steel. Continuous developments of technical standards on design and execution contribute the expansion of hybrid structures. Still there are active research program to improve or to more rationalize hybrid structures. In addition to this, as a new approach, sustainability considerations will be emphasized in this paper. The author provides perspective of research and technology developments on hybrid structure from the sustainability point of view.

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 This study was conducted to find out the behavior up to failure and deformation capacity of concrete filled tubular steel (CFT) columns with rectangular section for railway structure. The experiments were conducted with the specimens in which concrete was casted fully and ratio of long side to short side is 2. In addition, the effect of stiffeners was also investigated.
 From experimental results, the damage process of the specimens was similar to that of CFT columns with square section or circular section. However, the strains of steel tube embedded in concrete footing and length of local buckling etc. were different as a function of loading direction. It was confirmed that the maximum horizontal loadings and deformation capacities were increased by effect of stiffeners.

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 In the general design of the steel concrete hybrid structures, the relative slip between the steel part and concrete part is usually ignored. However, the numerical analysis which simulate the behavior up to the ultimate stage of hybrid structures is becoming possible by setting appropriately the shear force-relative slip relation to the shear connector element. In this study, the perfobond strip without penetrating rebar was focused, and its shear force-relative slip relations obtained by push-out tests, was compared to the standard specifications for hybrid structures of JSCE. After that, the relational expression of shear force-relative slip was reconsidered to increase the correlation with the experimental value.

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 This study aims at the development of construction method using a vacuum assisted resin transfer molding (VaRTM) for repairing and reinforcing of steel structures with section loss by externally bonded CFRP. Recovery and improvement of load carrying capacity by CFRP members were investigated for the girder ends with section loss in steel plate girder bridges. In this study, experimental and analytical study was conducted on the effect of repair and strengthening by VaRTM technique. As a result of study, it was confirmed that installation of the CFRP member is effective on the tensile side of the web, and the optimum strenghtning mehod has been proposed by the appropriate arrangement of CF sheets.

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 In this paper, toward the establishment of repair or strengthening method of steel structures by externally bonded patch plates, the static tensile tests and fatigue tests of adhesively bonded joints are carried out and the fatigue durability is evaluated. Two types of epoxy resin, Konishi E250 and Konishi E258R and two types of patch plates, steel and CFRP laminates are considered varying the thickness of patch plates and the applied stress ranges. The result shows that the fatigue life of adhesively bonded joints can be evaluated in the function of principal stress ratio, the ratio of principal stress range against principal stress at debonding by tensile test, independently to either the thickness/type of patch plates or type of adhesives under these two types of epoxy resin used in this study. Also, the fatigue limit is confirmed to be approximately at 30% of principal stress at debonding by tensile test.

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 The objective in this study is to develop a method of strengthening steel structures using externally bonded CFRP by VaRTM technique. As a fundamental study, the effect of strengthening of steel plates using proposed method under compressive force has been investigated experimentally and analytically. Buckling test has been conducted using steel plates with pinned ends varying the number of carbon fiber (CF) sheets. And the workability of VaRTM was also investigated in multi-layered CF sheets. Moreover, compression test has been conducted in order to improve the load carrying capacity of column members with section loss. As a column members of girder end, the patial models with section loss were fabricated and repaired using externally bonded CF sheets by VaRTM technique.

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 If a sequence of the yielding of steel beam and the compressive failure of concrete slab can be controlled arbitrarily in verifying the bending moment resistance capacity of the steel-concrete composite beam, more rational proportion of the composite beam can be potentially designed using constitutive material performance. In this research, we attempt to clarify the influence of the arrangement of shear connectors, the yield strength of steel and the compressive strength of concrete on the load carrying behavior of the composite beam by using the RBSM analysis with nonlinear material properties. As a result, we clarify that the load carrying behavior of the composite beam is characterized at four load stages based on the damage of steel beam and concrete slab, and that those stages depend on the arrangement of shear connectors and the yield strength of steel.

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 The hybrid structures have been expected as the reasonable structural type which complements: steel and concrete, are reinforces mutually. Research Committee on Load Carrying Mechanism of Hybrid Structures (H212), being belong to Committee on Hybrid Structures, has performed research activities on proposing the evaluation method of performance of hybrid structures. The committee focused on “damage events” of steel and concrete to explain the load carrying mechanism of the structures. This paper explained the load carrying mechanism of hybrid structures and joints based on the occurrence order of damage events, in order to develop universal design method.

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