Investigating Seismic Performance of Steel Moment Frames Equipped with Self-Centering Connections

The occurrence of the Northridge earthquake in 1994 and its effects on structures and infrastructures drew attention to the philosophy of damage-avoidance design (DAD). One of the most damaged lateral load-resisting systems during that earthquake was the steel moment-resisting frame which had more than 130 cases with connection failures. In lots of cases, brittle fractures were started in the connection at low levels of plastic demand, and in some cases while the structures remained elastic. After earthquakes like Northridge and Kobe, many efforts have been made to improve the ductile behavior of the connections. Since that time, different details have been proposed. The beam sections near the column face are either reduced or enhanced with various components, such as cover plates, side plates, slit dampers, annular stiffeners, ribs and diaphragms. However, permanent deformations and residual drifts can be seen after an indestructible earthquake, which is difficult to investigate and repair. To eliminate or reduce inelastic deformations as a DAD, self-centering (SC) mechanism has been considered seriously for moment-resisting frame connections. These types of connections are supposed to be composed of columns, beams, energy dissipation elements, and post-tensioned cables or bars to return the structure to its initial position and generate SC behavior. Different types of details have been suggested since the start of investigations. However, one of the most used details comprises bolted top and seat angles and high-strength steel strands that are post-tensioned after installation energy dissipators. Contact stresses develop at the interface of the beam and column under the action of strand force and the resulting surface provides notable moment resistance. Strands are tied up at the outer surface of the column. The shim plates and reinforcement plates are also used in the connection area. The resulting connection is not required for field welding and has initial stiffness almost identical to a common welded connection. An important phase in the performance-based design of moment frames with SC connection is to assess the structural performance and obtain engineering demand parameters EDPs by conducting nonlinear time history analysis. When it comes to the decision-making phase, compression between the results of EDPs for the conventional welded moment frames and frames with SC connections can be meaning full and important for the final SC connection details. Thus, in this study, two designed four- and eight-story steel moment frames are considered, and their connections are modeled with both SC and welded details. The lateral resistance of both structures were supposed to equal. Nonlinear time history based on 22 far-filed records which are scaled based on codes are conducted and results are compared. Values of the median for maximum drifts in stories are increased for first and second floors in, four-story SC model by 2.10, 2.44 and decreased by 0.31 and 9.00 percentage, respectively, for third and fourth stories; also for 8 story SC frame, results are shown, decreasing values for first, second, fourth, seventh and eighth stories by 6.65, 1.52, 9.90, 12.58 and 12.97, while for third, fifth and sixth stories increased by 3.69, 12.71 and 3.11 percentage, respectively. For story acceleration, it can be seen an increase of median values for both 4 and 8 story SC models which are 7.73, 24.40, 24.34 respectively for stories 1 to 3 of four-story SC model, and 20.65, 13.91, 32.81, 12.91, 18.65, 23.28, 7.05 percentage respectively for stories 1 to 7 of 8 story model and only for roof stories of 4 and 8 story models, it decreases by 7.21 and 10.20 percent, respectively. Finally, significant decreases are shown for median values of maximum residual drift for both in the 4 and 8-story SC models, which are 71.98, 91.47, 72.54, and 84.35, respectively, for stories 1 to 4 of the four-story SC model, and 55.65, 87.00, 93.13, 87.11, 87.23, 83.54,73.12 and 61.63 respectively for stories 1 to 8 of 8 story SC frame. Also, results of the nonlinear static analysis show that frames with SC connections have the same strength compared to welded moment resisting frames.