Seismic Evaluation of Column-to-Beam Strength Ratios in Reinforced Concrete Frames

Previous research efforts have shown that the column-to-beam flexural strength ratio at joints in reinforced concrete frame buildings plays an important role in determining the building’s response during seismic loading. The current ACI 318 code requires a minimum column-to-beam strength ratio of 1.2 at joints for seismic design, whereas some building codes in other countries require higher ratios. The objective of this study was to analytically evaluate the seismic performance of frame buildings with various strength ratios using probabilistic measures. Two study buildings (three and six stories) with different strength ratios (ranging from 0.8 to 2.4) were assessed. Additionally, the influence of changing the column-to-beam stiffness ratio was investigated. Findings suggest that a minimum strength ratio of 2.0 is more appropriate to prevent the formation of a story mechanism under design seismic loading. Also, increasing the strength ratio alone is more effective than increasing the strength and stiffness ratios simultaneously.