Incorporating Lower-Bound Capacities into LRFD Codes for Pile Foundations: Fid-Bau Portal

Incorporating Lower-Bound Capacities into LRFD Codes for Pile Foundations

Gilbert, R. B. / Najjar, S. S. / Choi, Y. J.

There is generally a physical limit to the smallest possible capacity for a deep foundation. However, a lower bound on the capacity has rarely been accounted for in performing reliability analyses and developing reliability-based design codes. The objectives of this paper are to (1) investigate the possibility of a lower-bound capacity using pile load test databases; (2) study how a lower-bound capacity could affect the reliability for a pile foundation; and (3) propose a Load and Resistance Factor Design (LRFD) design-checking format that includes information on the lower-bound capacity in addition to the conventional design information. Databases with driven pipe piles show clear evidence for the existence of a lower-bound capacity in both clays and sands. This lower-bound capacity is a physical variable that can be calculated based on mechanics with site-specific soil properties. For driven pipe piles, the calculated lower-bound capacity typically ranges from 0.5 to 0.9 times the calculated predicted capacity. Reliability analyses illustrate that the presence of a lower-bound capacity can have a significant effect on increasing the reliability of a deep foundation. Its effect increases as the coefficient of variation for the capacity increases and as the target reliability index increases. Finally, it is shown that information about the lower-bound capacity can be incorporated into LRFD codes using two alternative formats that would not require substantive changes to existing codes. Incorporation of a lower-bound capacity into design is expected to provide a more realistic quantification of reliability for decision-making purposes and therefore a more rational basis for design.