How Strong Are Deep Concrete Pile Caps?

A paper by Exponent's Lucas Laughery, Ph.D., P.E., and co-authors, titled "Experimental Investigation of Size Effect on Shear Strength of Reinforced Concrete Pile Caps," published in the ACI Structural Journal, is being awarded the American Concrete Institute's Wason Medal for Most Meritorious Paper. This work was a collaboration between Nagoya Institute of Technology in Japan, where Dr. Laughery worked before joining Exponent, Tokyo Institute of Technology in Japan, the National Center for Research on Earthquake Engineering (NCREE), and National Cheng Kung University in Taiwan.

The size effect factor refers to the reduction in concrete unit shear strength associated with deep structural elements. The study included large-scale structural testing of pile caps in which key dimensional parameters were deliberately controlled to study size effect in two-way shear. Test specimens had effective depths as large as 1 m (39.4 in.) and were loaded to failure at the state-of-the-art NCREE facility in Tainan, Taiwan. They were subsequently sawn to study crack patterns and understand failure modes.

The research showed that when other variables are controlled and tested at scale, the reduction in two-way shear strength is not as large as expected or as administered in current American Concrete Institute Building Code Requirements for Structural Concrete. Overall, the results indicate that the current approach may underestimate the two-way shear strength of pile caps.

From the publication: "The findings indicate the new ACI 318-19 size effect factor may underestimate strength for deep pile caps."