Experimental Investigation of Size Effect on Shear Strength of Reinforced Concrete Pile Caps

Engineers design structures based on physics, experiments, and experience. But due to increasing demands, structures today are being built at scales that far exceed experience and experimental testing. Bazant summarized the problem well in 1984: "Most laboratory tests are carried out on a reduced scale, from which generalizations must be made for much larger real structures." Tall structures often require deep foundations with thick reinforced concrete pile caps. In the United States, pile-cap strength is calculated according to ACI 318 provisions, which now include a size effect factor for concrete shear strength. This factor reduces concrete unit shear strength in proportion to effective depth for sections built without minimum shear reinforcement. This strength reduction forces engineers seeking to eliminate vertical ties to either increase concrete strength or deepen pile caps. But there is a gap in knowledge. This strength reduction is calibrated to databases for which tests do not vary across a large scale, and for which key unitless ratios are not always controlled. The present study fills this gap by quantifying the strength reduction due to size effect while controlling other key ratios using new laboratory tests. Experimental tests of tripod pile caps with effective depths of 250, 500, and 1000 mm (9.84, 19.68, and 39.37 in.) are presented. Results showed a reduction of 13% in shear strength from 250 to 500 mm and a total reduction of 14% from 250 to 1000 mm. The findings indicate the new ACI 318-19 size effect factor may underestimate strength for deep pile caps.