Title:
Experimental Investigation of Size Effect on Shear Strength of Reinforced Concrete Pile Caps
Author(s):
Lucas Laughery, Toshikatsu Ichinose, Kazuhiko Kasai, Srinivas Mogili, and Shyh-Jiann Hwang
Publication:
Structural Journal
Volume:
121
Issue:
1
Appears on pages(s):
105-117
Keywords:
experimental investigation; pile caps; reinforced concrete; shear strength; size effect; two-way shear
DOI:
10.14359/51739188
Date:
1/1/2024
Abstract:
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. Bažant 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.