The effect of confining concrete on strut capacity inside massive pile caps

Reinforced concrete pile caps in bridge engineering are often massive structures. Typically, the pile cap span to depth ratio is so small that it acts as a deep member, which in practice is usually de- signed by strut-and-tie models (STMs). The strength of a concrete strut in a STM depends on the state of stress and the orientation of the cracks. Design codes like Eurocode provide simplified and conservative values for the allowable compressive strength. Only limited research regarding the allowable strength of a strut is available in the literature, most of which is based on deep beams. In this paper it is investigated whether the codified efficiency factors for STM struts are also valid for a three-dimensional bulging strut in a pile cap. This has been done by comparison of the design approach usually used in practice with a range of experimental test results of pile caps found in the literature.