Researches on load-carrying capacity and crack width estimation of composite bridges are reviewed, and future directions for developing the next-step limit state design method are stated. In particular, the stress is placed on design issues, which is significant to develop the limit state design specifications allowing inelastic behavior of composite bridges. It is still necessary to develop new design procedures to deal with recent composite bridges, such as composite I-girder bridges without diaphragms and laterals, and composite sections with partial shear connection using precast prestress concrete decks.

 In the method of bonding steel or FRP plate to recover of the performance of members, the evaluation of debonding of adhesive bonded joints in design and verification is focused. In particular, standards for fatigue design have not established because the data of debonding strength under cyclic stress is scarce. In this study, a specimen was prepared by bonding a steel patch to steel plate using epoxy resin Konishi E258R and E250 observed debonding under cyclic bending stress. 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 bending test. Also, the result of bending test and those of tensile test can be seen similar tendency. It was suggested that we can evaluate fatigue durability independent of the working force.

 The purpose of this study was to determine the effect of joint conditions on the slipping behavior of high-strength bolted GFRP (Glass Fiber Reinforced Polymer) frictional joints with hot-dip galvanized splice-plates. In this study, slip experiment and contact pressure test were conducted using the strip-type specimens, and the slip surfaces were observed using an electron microscope on the joint surfaces after the slip experiment. The joint conditions of the slip specimens were determined by the bolt axial force, bolt hole diameter, and surface treatment of the splice-plates and GFRP plates. Slip experiment results showed that phosphate treatment, roughening the surface properties of the splice-plates, and applying a fluoropolymer coating to the GFRP surface improved the slip resistance. The results of the contact pressure test and the observation of the slip surfaces indicated that the use of a low-hardness material for the surface treatment of the base metal improves the adhesion between the base metal and the splice-plate, which may increase the slipping behavior.

 GFRP pultrusion-molded materials are widely used as civil engineering structural members, but they are not widely used as main structural members due to the lack of data after long-term exposure and unclear durability. In this study, static four-point bending tests were conducted to clarify the residual performance of GFRP pultrusion-molded members after long-term exposure. Specimen used in the tests were GFRP pultruded square pipes exposed outdoors for approximately 20 years and on GFRP pultruded square pipes stored indoors for the same period. As a result, authors revealed that the residual stiffness and bending load capacity of these members decreased after the exposure. It was also confirmed that the deformation behavior could be generally evaluated based on Timoshenko's beam theory even after long-term exposure. The obtained bending load capacities were compared with the calculated load capacities using the local buckling stress and elementary beam theory, and it was shown that the deterioration of the corners due to exposure may accelerate the progress of corner failure with loading and reduce the effect of rotational restraint, resulting in a smaller local buckling load capacity.

 The use of carbon-fiber-reinforced polymer (CFRP) as a strengthening material for aging-steel-structural members in seismic retrofitting has become an increasingly important area of interest. This study conducted monotonic tension loading and cyclic loading tests on tension diagonal members of steel truss bridges to evaluate the effectiveness of CFRP-retrofitting methods. The CFRP-retrofitting methods involved varying the bonding range of the CFRP sheets on the inside and outside of the flanges of the specimens. The test results indicated that retrofitting methods which bonded the CFRP sheets on both sides of the flanges significantly improved the load-carrying capacity and yield strength of the retrofitted diagonal members. However, the load-carrying capacity of the retrofitted diagonal members suddenly reduced after the local buckling of the flanges and rupture failure of the CFRP sheets occurred.

 The joint strength of GFRP plates depends on the bearing strength and shear strength of GFRP materials. Since these strengths are relatively small compared to steel, the joint strength of GFRP plates is also small, which is a structural design issue. Therefore, as a method to improve the joint strength between GFRP and steel plates, a method of joining GFRP plates reinforced by bonded steel plates with steel splice plates was devised. The number of bolt rows (one or two) was experimentally investigated as a parameter. As a result, it was confirmed that the proposed method is effective and that by bonding steel plates to GFRP plates, it is possible to prevent the prior bearing stress failure in GFRP plates and increase the joint strength. It was also found that the debonding of the adhesive joint between GFRP and steel plates is dominant for the joint strength, and it does not depend on the number of bolt rows.

 It is desirable that emergency bridges used in large-scale disasters are light in weight so that they can be mounted on vehicles and can be erected quickly. The Acquisition, Technology and Logistics Agency is conducting has been undertaking research on the application of CFRP to the emergency bridges for the Self-Defense Forces with the aim of improving the performance of emergency bridges. Through this study, the strength characteristics of the emergency bridges have been acquired by applying CFRP on the outline design of the assumed bridge, and manufacturing the prototype for the strength examination.

 As a result, it was confirmed that the designed assumed bridge can sufficiently withstand the assumed load. In addition, it is affirmed that the weight can be reduced approximately by 26% compared to the conventional aluminum structure.

 In the process of pursuing rationality, various composite structural forms have been developed and applied to real structures. In these structures, the volume ratio of steel to concrete and the method of transmitting forces and deformations between them have changed significantly compared to the conventional RC and PC structures. To identify problems related to shrinkage and creep of concrete in steel-concrete composite structures, conduct academic and technical studies, compile findings, and reflect them in design methods, a “Subcommittee on Effects of Shrinkage and Creep of Concrete in Hybrid Structures Committee on Hybrid Structures” was established to conduct research activities for five years from 2017. This paper reports the summary of the committee's activities.