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

PPT Presentation

Pdf Summary

The presentation examines inconsistencies in the AASHTO LRFD Bridge Design Specifications for retaining-wall design. The issue emerged during value engineering: designing a permanent wall under AASHTO LRFD requirements resulted in longer piles than an Allowable Stress Design (ASD) approach.<br /><br />ASD and LRFD are intended to produce comparable designs, but the presenters argue that AASHTO’s prescribed load and resistance factors can lead to different outcomes in geotechnical applications. Geotechnical engineers have limited control over structural loads and must account for uncertainty in soil properties and behavior. Applying uniform load factors across different loads may not adequately reflect that variability.<br /><br />The study compares required pile lengths for cantilever walls, considering both discontinuous and continuous wall configurations. Its analyses examine how pile length changes with soil friction angle, compare results with a base model, and assess the ratio of AASHTO LRFD pile lengths to ASD lengths. Lateral pressures are also compared by wall elevation. The presentation reports that AASHTO’s inconsistencies may extend beyond the granular-soil cantilever walls studied, including to braced walls, cofferdams, and clay profiles.<br /><br />Longer pile requirements can increase project costs, even when the added toe length is modest. The presenters recommend refining AASHTO factors through calibration—using approaches similar to those applied for pile driving—and linking resistance factors to testing and finite-element analysis.

Keywords

AASHTO LRFD

bridge design

retaining wall

value engineering

geotechnical engineering

ASD

pile length

load factors

resistance factors

lateral pressure