Investigation of the Seismic Behavior of the Rehabilitated Steel Frames through Asymmetrically Weakening of the Beam

After the Northridge earthquake in 1994, numerous reports of failures due to brittle fractures of the weld joint for steel structures were presented in various Codes. One of the ideas to solve this problem was to promote the plastic hinge away from the weld through the intentional weakening of the beam at a certain distance from the column. In this idea, by weakening the top and bottom flanges of the beam at a certain distance from the column, the ductility of the connection is increased and brittle failure in the weld area is prevented. The existing steel frame buildings that are designed according to Pre-Northridge seismic provisions need to be rehabilitated to prevent the connections from experiencing brittle fracture at their welds. The presence of concrete slab in existing steel buildings imposes economic problems in retrofit projects. Asymmetrically weakening the beam is considered as an appropriate method for seismic rehabilitation of steel frame connections in which the rehabilitation action is conducted through intentional weakening the bottom flange of the beam and without the difficulty of removing concrete slab. Two techniques “reduction” and “heat induction” are among suggested methods for asymmetric weakening of the beam. In the “reduction” technique, the weakening action is conducted by cutting some parts of the beam bottom flange at a certain distance from the connection. In the “heat induction” technique, the weakening action is conducted by applying a special process of heating to the bottom flange of the beam at a certain distance from the column. In this heating process, which reduces the yield and ultimate strength by 35% and 25%, respectively, in other words the steel is annealed. This drop in strength in the heated area causes the plastic hinge to move over the beam.
The main purpose of this study is to investigate and compare the seismic behavior of low-, medium-, and high-rise 2-D steel frames improved through these two techniques “reduction” and “heat induction” under far-field and near-field earthquakes, numerically.
Two types of verification are conducted to ensure the accuracy of numerical modeling. First, 2-D rehabilitated connections through two “reduction” and “heat induction” techniques are verified with experimental results. Then, three 2-D frames are verified with Gupta & Krawinkler results.
The results of the frames analysis showed that inter-story drift and total rotation of rehabilitated frames by “reduction” technique were on average 15 percent more than rehabilitated frames by “heat induction” technique. This indicates the defect of the improved connection through the “reduction” technique in low elastic stiffness and torsional-lateral instability compared to the "heat application" technique. In addition, it was determined, as the height of the frame increases, the effectiveness of far-field earthquakes decreases and near field earthquakes show their effects on the structure more. As the ratio of the earthquake pulse period to the main period of the structure increases, the imposed deformation on the frames increase. So that, if this ratio is equal to one, the maximum relative drift for the frames is estimated.