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In steel moment-resisting frames, concrete floor slab attached to steel beams contributes a larger stiffness and moment strength under positive bending. The composite action shifts up the section neutral axis and increases the risk of fracture at the bottom flanges of beam ends. A retrofit technique, named minimal disturbance arm damper (MDAD), is proposed as a solution to restrain the local deformation at beam ends and reduce the frame responses with moderate increase in stiffness, strength and damping. MDAD can retrofit steel frames rapidly by only using the bolt connections and preserve the open space of the frames. In this paper, the effectiveness of MDAD in improving the seismic performance of the steel frame was experimentally and numerically examined through the application to half-scaled multi-span specimens. The test and numerical results both showed significant benefits of MDAD in reducing the positive bending moment at beam ends, delaying the fractures at the bottom flange of beam ends and providing alternative load–carrying paths after fracture to prevent strength reduction.