Investigation and comparison of some techniques in asymmetrically retrofitting of steel frame connections

Abstract

The presence of concrete slab in steel framing frame structures always makes retrofitting problems difficult. On the other hand, most of damages reported from Northridge earthquake (1994) has focused on the bottom flange of the beam. Therefore, in order to reduce the cost of retrofit, this operation can be done only at the bottom flange of the beam. In other words, by defining and applying a ductile fuse in the beam, by weakening the bottom flange of the bottom, it is possible to reduce the concentration of tension in the near-weld regions in connection and fracture failure in these areas. In this paper, a steel frame connection was analyzed numerically and after ensuring the modeling accuracy, it was under rehabilitation. Retrofit techniques have been made on the bottom flange of the beam. Two general "cut-off" and "heat" methods were considered to weaken the beam. In this research, incomplete penetration of the weld was placed in the bottom flange connection to the column as a deficiency in the joint. The modeling results showed that the connection with incomplete weld, underwent 4/4%, suffering a deflection in the weld area in the flange. In the specimens which were retrofitted asymmetrically, the possibility of brittle fracture in the weld region was decreased due to relocation of the plastic hinge in the weakened area of the beam. In the method of reducing the bottom flange, lateral buckling led to sudden decrease in flexural strength at 2.5 % inter story drift. In the heat-induction method accompanied with steel annealing, the out of buckling occurred at 5% drift. The retrofitted connection with this method showed a post elastic displacements of 3% and could be expected to be suitable for special moment frames.

Keywords

• moment frame connection
• seismic retrofit
• heat treated beam section (hbs)
• reduced beam section (rbs)
• finite element method