Evaluation of the strong column-weak beam criterion in strengthened steel columns

Abstract

In seismic design of special steel moment frames (SMF), the design codes require the satisfaction of the strong column-weak beam (SCWB) criterion to prevent the formation of weak stories and to ensure that plastic hinges form in beams instead of columns. Adhering to this criterion often results in larger column sections, which is not economically or architecturally favorable. Therefore, the idea of strengthening columns only adjacent to the beam connection—rather than along the entire length of the column, as previously proposed for eliminating continuity plates and doubler plates in steel box columns—is investigated here. This study determines the height of strengthening and the amount of strengthening required for each column. In addition, the research examines the effectiveness of this idea in achieving the goals of the SCWB criterion and evaluates the factors influencing it. For this purpose, multi-story moment frames with varying number of stories, spans, and plans were designed to meet all the code requirements except the SCWB criterion. Then, by locally strengthening the columns and performing nonlinear time history analyses at the maximum considered earthquake (MCE) level, the behavior of the structures in the original and strengthened states were compared and the impact of local strengthening was assessed. The results indicate that using this method can shift plastic hinges from columns to beams, meeting the code acceptance criteria with a high safety margin. Additionally, with the assumption of an appropriate nonlinear mechanism for the structures strengthened near the connection zone, a relationship for controlling the SCWB criterion for these types of frames has been proposed which can be used by designers.