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Recently, ultra high strength steel plate (in this study, over 780N/mm 2 class steel is called 'ultra') such as 1000N/mm 2 class strength is focused as material of various type of steel structure. 1000N/mm 2 class steel has already applied in hydropower plant in order to enjoy its effect on cost reduction. However, in building construction field, high strength steel remains unfamiliar material. There are only a few example of application of 780N/mm 2 class strength steel in building field. On the other hand, safety issue in a huge earthquake is more focused recently. Very large extent of elastic region of ultra high strength steel come to the front. Authors started the investigation of application of 1000N/mm 2 class steel plate for buildings. When ultra high strength steel is investigated, welding always become an issue. However, authors think welding is absolutely necessary in assembling steel material for buildings because welding is the most common and economic way of joining the materials at the moment. In this report, welding material, which can match 1000N/mm 2 class steel plate and can optimize the properties of welded joint, has been developed. Especially wide range of inter-pass temperature and heat input is tried to be available. By controlling carbon equivalent and oxygen content of deposit metal, finally new welding material of GMAW and SAW for building has been developed.

Recently, ultra high strength steel plate such as 1000N (950N/mm 2 Class) class strength is focused as material of various type of steel structure. 1000N class steel has already applied in hydropower plant in order to enjoy its effect on weight and cost reduction. However, in building construction field, high strength steel remains unfamiliar material. There are only a few example of application of 780N/mm 2 class strength steel in building field. Authors have already developed 1000N class steel plate and welding consumables for building. However, from viewpoint of actual application, structural performance under massive earthquake must be evaluated. Local area of column-to-beam connection seems be subject to large amount of strain when seismic event. So, in this report, low cycle fatigue properties of 1000N class steel and its weld was evaluated and structural experiments were conducted. As a result, it is revealed that Low fatigue property of 1000N class steel is equivalent to that of 780N class and can maintain the loading capacity under 26~51 times of strain amplitude at c p . Furthermore, it is also shown that structural performance can be estimated by test results of small specimen with strain calculation by FE analysis and finishing of edge of beam is effective for extension of lifetime through decrease of strain.

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