Evaluation of strong column-weak beam criterion in spliced columns of steel moment frames

In the seismic design of steel special moment frames, it is necessary to ensure that columns are generally stronger than beams. This reduces the probability of a weak story failure mechanism of the frame and ensures the formation of beams' plastic hinges earlier than the columns'. This criterion is known as strong column-weak beam (SCWB) in seismic design codes and is checked by a formula in the form of a ratio of total flexural strengths of columns to beams framing at each joint. It is common practice to ignore the column section change at the splice location and to use the flexural strength of the larger column section in evaluating this ratio. In this paper, several steel special moment frames were optimally designed by the genetic algorithm, using constraints on the SCWB ratios and controlling the amount of sectional change in the columns of a certain story. Then, the specified story's behavior and the sequence of plastic hinge formations in the beams and columns were examined using nonlinear time history analyses. The results indicate that satisfying the SCWB ratio with the larger column section results in the earlier formation of plastic hinges in columns and using the smaller column section's flexural strength prevents this phenomenon. Moreover, a significant column section change adversely affects the nonlinear behavior of the frame. It is recommended to limit the amount of column section change at splices and to use the properties of the smaller column section in the SCWB formula.