A Discussion of Inconsistencies Between LRFD and ASD Methods for Geo-Structural Design Applications of the AASHTO Bridge Design Specifications-Video

Video Transcription

Video Summary

The presenters compared allowable stress design (ASD) with AASHTO load and resistance factor design (LRFD) for temporary retaining walls. Using cantilever-wall models in granular soils, they found LRFD generally required longer pile embedments, with the difference increasing when groundwater, surcharge, or other site conditions were included. In some continuous-wall cases with groundwater, LRFD produced embedments up to 25% longer than ASD; across modeled cases, increases varied substantially. The effect arose because LRFD reduced passive resistance more than active pressure, pushing the force and moment balance deeper. The presenters emphasized that this was not a criticism of LRFD: its factors are intended to reflect uncertainty, but retaining-wall design currently offers limited opportunity to account for investigation quality, soil type, construction methods, or testing. Cofferdams may show the opposite trend. Longer walls can increase costs and schedules, so the presenters recommended calibrating and refining retaining-wall factors, potentially using construction and load-testing data and more advanced analyses.

Keywords

allowable stress design

AASHTO LRFD

temporary retaining walls

pile embedment

groundwater effects