P-M CHARACTERISTICS OF REINFORCED CONCRETE SECTIONS

This manuscript presents two parametric studies which were performed to evaluate a provision which originated in the 1994 Uniform Building Code (UBC). This provision states that if gravity-induced axial loads carried by a reinforced concrete member are greater than 35% of the pure axial load-carrying capacity (Po) of that member, then the member cannot be used as part of the lateral force resisting system (LRFS). Along with the UBC provision, the Structural Engineers Association of California\u27s (SEAOC) Blue Book states that the value of 0.35Po corresponds to the level of axial load representing a balanced state of strain, i.e., the balanced point of the axial load-moment (P-M) interaction. It is also generally accepted that the balanced point on a P-M interaction is located at the point of maximum moment. To evaluate these assertions, two parametric studies were performed. In the first study, square column cross-sections were analyzed while the second study considered rectangular wall cross-sections. Concrete compressive strength and reinforcement ratios were considered as variables. Two noteworthy observations are drawn from the results of the studies: depending on reinforcing schemes and reinforcement ratios; (1) the balanced point on a P-M is not always located at a value of 0.35Po; (2) the balanced point is not necessarily located at the point of maximum moment - it may lay above or below the point of maximum moment, or may be located approximately at the point of maximum moment depending on cross sectional and material properties, as well as reinforcement scheme