PSAD Civil Engineering Licensure Examination (CELE) Practice Test

In reinforced concrete design with both tension and compression reinforcement, the forces on the section must balance at ultimate. The tension reinforcement at the bottom carries a force equal to As2 times the steel yield strength fy. To maintain equilibrium, the compression side provides resistance through both the compression reinforcement (As' times f's) and the concrete in compression (about 0.85 f'c, the design value for concrete's compressive strength). Solving the force balance for the required area of compression reinforcement yields As' = As2 fy /(f's - 0.85 f'c). Here, the numerator represents the tensile force that must be resisted, and the denominator captures the net available compression capacity per unit area (the difference between the compression steel stress and the concrete’s design compressive stress). This is why that particular form is used: it directly ties As' to the tension reinforcement, steel strength, and the effective concrete strength. The other forms misrepresent the force balance or use incorrect signs or coefficients, so they don’t reflect the way the internal forces balance in such sections.