Force redistribution of steel moment-resisting frame retrofitted with a minimal disturbance arm damper

This paper presents an experimental investigation of a multi-span steel frame retrofitted with a minimal disturbance arm damper (MDAD). The primary objective of the paper is to analyze and verify the retrofit mechanism in terms of force redistribution. An elastic design of the MDAD, targeting the reduction of the positive bending moment at beam ends was first developed for the seismic rehabilitation of steel frames having a beam-collapse mechanism. Here, the composite beams under positive bending sustain tensile force at the bottom flanges. Its effectiveness was examined experimentally and numerically. This study also explored the effect of the MDAD on delaying the fracture of the beam ends. Particularly, the behavior of the retrofitted steel frame after the fracture of beam ends was majorly focused. By comparing the test results of specimens with and without MDAD, it was found that the MDAD successfully reduced the positive bending moment of the beam ends as intended in the proposed design equation and delayed the fracture. The retrofitted substructure did not exhibit strength deterioration even after beam end fracture because of the backup load resistance provided by the MDAD. Finally, enhancement of the collapse limit by the MDAD is discussed through numerical simulations for a moment-resisting steel frame.