In many tall steel structures having heavy members and high strenght steel, welding joints have come to be adopted in beam-to-column connection. It is considered that the brittle fracture occurs, when structures are received repetive large strain due to a strong earthquake. It may be that main factors of the brittle fracture are the stress level in the column, a welding method and the stiffness of column web with doubler plates. For such joints having heavy steel members, it remains as important problem how to design beam-to-column connections. This paper describes the influence of the stress level in column on the charactristics of fracture and the structural design method of beam-to-column connections. 10 full scale specimens of sub-assemblage including beam-to-column connections are experimented applying axial force and bending moment. A constant axial force of -0.45, -0.2, +0.1 (axial load/yielding axial load of column) was applied at the column and cyclic vertical force was loaded at the end of beam. The results obtained from the above experiment are as follows, 1) In early stage, the brittle fracture of the specimen applied a tention axial force at a column occure and a crack propagats in the direction of the through-thickness of column flange. 2) The specimens reinforced with doubler plates at column web within connection are more fragile then non reinforced.
