Heat Induction Technique for Seismic Retrofit of Steel Beam to Column Connections

The concrete slab in existing buildings presents a problem for economic considerations in seismic retrofit projects. Unless the concrete slab is removed, it is impossible to modify the top flange and its welded joint. Meanwhile, as the majority of reported damages occurred in the bottom flange of the beam during the past earthquakes, it is anticipated that modifying only the bottom flange may be sufficient to significantly improve the performance of the steel frame connections. Making a ductile fuse in the beam section through weakening and gaining the most possible plastic behavior from the beam can be a suitable solution. In current research, a new and practical rehabilitation scheme based on heat induction to the bottom flange of the beam was developed and experimentally validated. Accordingly, three large-scale steel moment frame connections containing one reference (Pre-Northridge) and two retrofitted connections were tested under cyclic loads. The experimental results showed near weld fracture in the reference specimen at story drift of 5.5 % with no qualified plastic behavior for special moment frames.In connections retrofitted through heat induction (annealing of the beam material), plastic hinge occurred at 6% story drift in weakened section far enough from the column face. The main advantage of this technique was low stress demands in near-weld region. Meanwhile, as the beam was heated with no material removal, the out of plane buckling resistance was similar to that of the reference specimen. Strength degradation of the retrofitted connections occurred gradually with no brittle failure as opposed to the reference specimen. The retrofit technique can be easily achieved through a handmade torch and a laser thermometer that simplifies its application in situ.