Evaluation of design equations for out-of-plane shear strength of deep concrete sections in nuclear power plant buildings

Reinforced concrete walls in nuclear power plant buildings, which are generally more than one-meter thick, are typically designed to resist out-of-plane shearing force using concrete resistance only, namely, through-thickness shear reinforcement is not provided. Standard ASCE 43, which addresses design of safety-related nuclear facilities in the United States, references ACI 349 for design of reinforced concrete nuclear structures and components, with exceptions as deemed necessary. The code ACI 349–13, assumes, identically to ACI 318–08 and ACI 318–14, that the nominal shear stress capacity of longitudinally reinforced concrete sections is independent of cross-section depth: an assumption not supported by data. In the last revision cycle of ASCE 43, the ACI 349–13 design equation for concrete shear stress capacity was evaluated for conformance with the performance target of the standard, namely, a 98% or higher probability of exceedance, and found to be inadequate. A correction to the ACI 349–13 equation was developed and implemented in ASCE 43–19 to account for the effects of effective depth and longitudinal reinforcement ratio on concrete shear stress capacity. In parallel with the revision of shear provisions in ASCE 43, a size-effect modification factor, λ_s, was introduced in ACI 318–19 to account for the reduction in concrete shear stress capacity with increasing effective depth. In this paper, the technical basis for the correction factor included in ASCE 43–19 is developed and the ACI 318–19 equation is assessed to judge whether it achieves the ASCE 43 performance target. The assessment utilized the test data of longitudinally reinforced beams assembled by Reineck and his co-workers for slender sections (ratio of shear span-to-effective depth greater than or equal to 2.4) and non-slender sections (ratio of shear span-to-effective depth less than 2.4). For slender sections, the probability of exceedance of the ASCE 43–19 (ACI 318–19) design strength is greater than 98% (99%). For non-slender sections, the exceedance probabilities are greater than 99%. Because the ACI 318–19 equation meets the ASCE 43 performance target for both slender and non-slender sections, the correction invoked in ASCE 43–19 could be retired once ACI 349 adopts the shear provisions of ACI 318–19 for longitudinally reinforced concrete.