Concrete Stress Block Method for Nuclear Containments

The design philosophy of nuclear concrete containments is to maintain a certain degree of leak tightness during abnormal/extreme environmental conditions. To achieve this performance target, stress and strains of concrete and reinforcing steel are required to meet the allowable stress and strain requirements of ACI 359-07 (or Section III, Division 2 of ASME Boiler and Pressure Vessel Code). Different levels of allowable stress and strains are specified depending on the load category, as their performance targets are different. Evaluating axial and flexural strengths for factored loads is a challenging task because the actual nonlinear distribution of concrete compressive stress needs to be considered in analyzing concrete sections. For service load conditions, a straight line of concrete stress-strain relationship is used to simplify the calculation for axial and flexural design. This paper proposes a concrete stress block method for designing nuclear containments. Stress block factors were developed based on ACI 359 requirements. With the use of the proposed stress block method, axial and flexural strengths of nuclear containments can be calculated in a manner similar to ACI 349. As such, a more consistent design approach can be implemented for both nuclear concrete containments and nuclear safety-related concrete structures. The accuracy of the proposed stress block method was examined using P-M interaction curves. Interaction curves from the proposed method were compared with results of the ACI 359 method. The comparison shows a good agreement between the proposed and ACI 359 methods.