This paper presents the results of large-scale tests on two pile cap specimens subjected to concentric loading: one with a square plan and the other with a rectangular plan. During the tests, the bottom reinforcement in both specimens experienced high strain levels, either approaching or exceeding the yield strain. A series of arch-shaped cracks also developed on the side faces of the specimens. These observations confirm the presence of internal force flow that originates from curved-bar nodes located between adjacent supports. To evaluate the effect of the curved-bar nodes in the behavior of pile caps, the required bend radii for curved-bar nodes were calculated based on the concrete strength and the yield strength of the reinforcement. These values were then compared with the standard 90° hooked bend radii applied in previously tested large-scale pile cap specimens, including those in this study. Findings suggest that insufficient bend radii can affect the failure surface, causing premature punching shear-type failure in the central region. A refined three-dimensional strut-and-tie modeling approach is proposed to reflect this behavior. The proposed model includes curved-bar nodes and subdivides the conventional system to better capture crack patterns and failure mechanisms. Additional horizontal bottom ties are introduced, enabling independent design for the reinforcement placed between piles. Identification of side face strut-and-tie panels further supports the necessity of side face reinforcement by explaining its role in force redistribution and crack control. Model validation was conducted by comparing estimated and measured bottom bar stresses at the ultimate load. Bars were categorized based on their location within or outside the bandwidth defined from the bearing frustum. The results confirm that the proposed model can reasonably estimate bottom bar stresses, even when failure was not governed by yielding.
Yi et al. (Sun,) studied this question.