DESIGN OF CONCRETE MEMBERS SUBJECTED TO UNIAXIAL BENDING AND COMPRESSION USING REINFORCEMENT SIZING DIAGRAMS

Traditional approaches to the design of reinforced concrete members for flexural moment and axial load have emphasized analysis to determine the location of the neutral axis for a given configuration of longitudinal reinforcement and axial load. An alternate approach is described in this paper, wherein the reinforcement required to resist the specified design moment and axial load is determined as a function of the neutral axis depth. A range of neutral axis depths is considered, resulting in a number of potential design solutions consisting of different combinations of top and bottom reinforcement. These potential design solutions are portrayed graphically using a new diagram termed a reinforcement sizing diagram (RSD). The comprehensive set of solutions displayed by the RSD provides a basis for the design engineer to select a specific combination of top and bottom reinforcement to be used in a particular design. Reinforcement may be selected to minimize total reinforcement area or to accommodate nominal amounts of compression reinforcement as may be required for convenience in construction. Several examples are presented, illustrating the technique according to the assumptions adopted in the provisions of ACI 318 and Eurocode 2. The effects of optimal reinforcement on the axial load-moment interaction diagram are illustrated for one example.