The US-Japan Cooperative Earthquake Research Program began in 1979 following the recommendations as outlined in the final report of the US-Japan Planning Group for the program ["Recommendations for a US-Japan Cooperative Research Program utilizing Large-Scale Testing Facilities", Report No. UCB/EERC 79-26, September 1979]. First four phases of the program have been implemented on 1) Reinforce Concrete, 2) Structural Steel, 3) Masonry, and 4) Precast / Prestressed Concrete. Phase 5 of the US-Japan Cooperative Research Program on Composite and Hybrid Structures was carried out from the fiscal year of 1993 as a five-year research program. In this phase of research program, the following four research topics were executed: 1) New materials, elements and systems, 2) Concrete-filled structural steel tube column system, 3) Reinforced concrete column and structural steel beam system, and 4) Reinforced concrete and steel reinforced concrete wall system.

In this paper, the technical research results conducted in the Japanese side research topic of the concrete-filled structural steel system is overviewed and summarized. It includes experimental results (columns, beam-columns and beam-to-column assemblies), proposed constitutive model for steel and concrete considering synergistic action, analytical results, and design implication study. Draft of the manuscripts was shared among the researchers directly involved in the research project, and it was summarized in this research paper by the editors.

As for the research results on the other three topics, summary paper has not yet been completed. The available technical papers, which cover a part of the research topics, are listed in the appendix for the reference to the readers.