This paper proposes a method to improve plastic deformation capacity of beam-to-column connections in existing steel buildings using supplemental H-section haunches jointed by high-strength bolts and welding which expected to secure construction quality. Cyclic loading tests were performed with eight specimens which have various connection types and haunch lengths to verify the seismic retrofit effects of proposed method. By attaching supplemental haunches to upper and lower flanges of the beam-ends, the critical section was moved from the beam-ends to the beam cross-section corresponding to the tip of the supplemental haunch. As a result, it could prevent the fracture of beam flange initiated from toe of weld access hole. FEA were conducted to discuss stress transfer mechanism of beamto-column connection retrofitted by haunches and identify forces transferred by supplemental haunches. From the FEA results, required overdesign factor ζ taking into account strain hardning of plastic hinge was estimated to secure the seismic retrofit performance according to proposed design guideline.
