As a “prospect paper” supposed to review the state of arts of current technology for systemization and the future technology development regarding hybrid structures, the suitable issue was deliberated carefully and eventually the tentative title of the prospect paper was decided such as Forensic Structural Engineering or Clinical Structural Engineering in the keyword so as to summarize what I had been thinking about for the past several decades. For the enormous stock of important civil infrastructures that have been maintained continuously in order to improve the quality of life, related to design, fabrication, erection and maintenance of civil infrastructures, “technology quality assurance/sustainability”, “accident (disaster) investigation”, “acceptability and risk of new technology” must be dealt with across various fields by not only researchers and practitioners but also administrative experts or legal practitioners; that is, “Forensic Engineering” is becoming very important. I hope that the content of this manuscript will help structural engineers to widen a new and cross sectoral research field based on their specialization.

 Research on the use of FRP for construction materials began in the 1980s, and research and development of the FRP sheet bonding method for concrete members of existing structures became active from the 1990s, and since the 2000s it has been widely used for repairing and strengthening concrete members. Because FRP is a relatively new material as a construction material, in order to effectively utilize it for the maintenance of concrete structures, it is necessary to conduct research and development on both materials and structures. This paper reviews the history of research and development on materials, FRP molding methods, bonding methods to concrete, and performance evaluation of reinforced members, focusing on flexural strengthening that is widely used to reinforce concrete members by FRP bonding method. Finally, recent research results on the improvement of adhesion characteristics, which is an important issue for reinforcing concrete members by FRP adhesion are shown, and prospects are described.

 This study conducted compression tests and numerical analyses of pultruded hybrid FRP angle section which has been developed to be used for replacement member and reinforcement member for corroded steel bridges, in order to understand global and local buckling characteristics and to obtain compressive strength data. As a result, the angle section with a large slenderness ratio when the rotation around the strong axis is free shows a flexural-torsional buckling mode with a twisting of section, and compressive strength of flexural-torsional buckling is closer to the flexural buckling strength about the weak axis than that about the strong axis. Since the angle section with a small slenderness ratio is controlled by the local buckling, the compressive strength of the angle section in this study may be determined based on the local buckling strength and the global buckling strength about the weak axis.

 The purpose of this study is to develop a method to recover performance by integrating CFRP and steel members with sectional loss using the vacuum assisted resin transfer molding (VaRTM) technique. The effect of arrangement of CFRP on the recovery of bending strength has been experimentally investigated for steel members with trapezoidal sectional loss near the weld joint of stiffeners. The three-point bending tests have been conducted. Three cases of repair method have been prepared and examined. As a result, by attaching resin to the corner and then bonding the multi-layered CFRP, when CFRP is in a bending tensile state, it was found that it is possible to recover to maximum load of sound.

 The Research and Investigation Committee on Hybrid Structures introduces the history and examples of design, construction, maintenance, and research of concrete-filled steel tubes. The research committee organized examples of applications to railway structures, road structures and buildings. Trial calculations based on these criteria were performed and compared. In addition, the maintenance of concrete-filled steel tube members in service was described. Furthermore, an example of nonlinear finite element analysis is shown. The research committee organizes these contents and provides useful information to design and construction engineers and researchers.