Lattice discrete particle model simulations of energetic size effect and its implications for shear design specifications of reinforced concrete squat walls

Reinforced concrete low-rise, or squat, walls are frequently used for shear resistance in safety-related nuclear facilities and seismic-sensitive conventional structures. While common building components, there are numerous examples in the literature of failure under less-than-design loading or in unexpected failure modes. One potential cause of such phenomena is energetic size effect, the reduction of structural strength with increased geometric size due to energy release. In developing such walls, current design provisions, including the common ACI 318 and ASCE 43, do not account for size effects. Further, poor design methodology and inappropriate application of design formulae are often cited as contributing factors to mismatch of performance in the field versus in laboratory settings. To examine the implications of energetic size effect on squat walls in a controlled environment, the Lattice Discrete Particle Model (LDPM) – a robust discrete model formulated at the scale of concrete aggregate — was adopted for validation and prediction exercises. In this study, LDPM was calibrated to an experimental concrete mix, and finite element model (FEM) properties calibrated to steel rebar. Combined LDPM-FEM simulations of various squat wall geometries were developed and validated against existing experimental tests in the literature. Following validation, the squat wall geometries were varied in scale from 30 centimeters to nearly 5 meters in size to examine the consequence and application of size effect. This study revealed a clear change in the behavior of the examined squat shear walls with size for geometrically similar structures. Larger walls exhibited a small reduction in strength but a significant decrease in ductility and energy dissipation capacity. When these results are examined through the lens of traditional shear wall design provisions, a pronounced disconnect between theory and reality becomes evident.