The welding defect of the welding connection in steel structure causes the brittle fracture. Especially because the beam-to-column connection is a part by which the biggest stress is received, this defect gives fatal damage to the steel structure frame. In this research, the full scale experiment in the beam-to-column connection was done, and fracture behavior and the deformation capacity by which the welding defect was made a starting point were examined. The influence which the welding defect produces on the deformation capacity of the beam-to-column connection is large and decreases the possession performance. Moreover, it is possible to become an effective index to evaluate the deformation capacity that the crack characteristic size is shown by the penetration crack by WES-2805.
