Evaluation of Overstrength Factors for Capacity Design of Reinforced Concrete Moment Frames

Accurate estimation of flexural overstrength is crucial for the capacity design of reinforced concrete (RC) structures, ensuring effective resistance to seismic forces while preventing brittle failure. A key parameter in determining flexural overstrength is the professional factor (PF), defined as the ratio of measured to code-predicted strength while accounting for modeling uncertainties. This study evaluates the flexural overstrength of RC beams and columns designed according to ACI 318-19 by determining PF values based on a large database of experimental tests and nonlinear section analyses, incorporating material and geometry uncertainties through updated statistical models. The beam database comprises 110 RC specimens tested between 1999 and 2020 from multiple experimental programs, The column database is derived from the Pacific Earthquake Engineering Research Center (PEER) database and includes 195 rectangular columns with hoop reinforcement and 67 circular columns with spiral reinforcement tested from 1979 to 2001. The statistical assessment indicates that the mean PF values for beams range from 1.12 to 1.31 depending on section shape and reinforcement ratio, whereas they vary between 1.10 and 1.44 for columns, with higher values observed for compression-controlled sections. The effects of various design parameters, including section shape, reinforcement ratio, axial force ratio, and material properties, on flexural overstrength are examined. Based on stochastic analysis results at a 90% confidence level, the proposed flexural overstrength factors for rectangular, T-shaped, and L-shaped beams are typically from 1.20 to 1.50. Flexural overstrength is strongly influenced by section shape, strain in longitudinal tension reinforcement, reinforcement ratio, axial force ratio, concrete compressive strength, and reinforcement yield strength. Proposed flexural overstrength factors range from 1.25 to 1.8 for rectangular columns and 1.4 to 2.2 for circular columns at a 90% confidence level. A comparative analysis of the seismic design of a multistory RC frame using ACI 318-19 provisions and the proposed flexural overstrength factors suggests that beams and columns in critical regions may require increased transverse shear reinforcement. Whereas this analysis highlights the potential for refining existing design provisions, further case studies are recommended to generalize these findings.