The AASHTO LRFD [Load and Resistance Factor Design] Bridge Design Specifications (BDS) recognizes that piles in settling soil experience downdrag forces from negative skin friction and may also move downward with the adjacent soil. Many bridges are constructed in areas where there is compressible soil or liquefiable soil, which necessitates the con- sideration of downdrag forces and associated settlement. Downdrag for static and seismic conditions has increasingly placed greater demands on existing and new foundations and also led to higher construction costs. Therefore, there was an immediate need to review the fundamental behavior of piles subject to downdrag and propose design requirements for the implementation of the neutral plane method for the evaluation of downdrag and the static and seismic design of piles.

Under NCHRP Project 12-116A, “Proposed AASHTO Specifications for Design of Piles for Downdrag,” the University of Arkansas was asked to (1) propose modifications to the bridge design requirements for the static and seismic design of piles for downdrag and

(2) develop design examples to demonstrate the application of the proposed design require- ments. The research team proposed draft language for consideration by AASHTO in the next update of the LRFD BDS and provided analysis and design examples to illustrate the proposed updates.

NCHRP Research Report 1112: Design of Piles for Downdrag presents procedures for determining downdrag loads used in the design of bridge piles. The procedures were developed based on an extensive review of the state of practice and a collection of existing field test data. The research findings will be used by geotechnical engineers responsible for designing bridge foundations for state departments of transportation.

In addition to this report, NCHRP Web-Only Document 398: Pile Design for Downdrag: Examples and Supporting Material is available on the National Academies Press website (nap.nationalacademies.org) by searching for NCHRP Web-Only Document 398.