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

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The paper examines discrepancies between Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD) for retaining walls under the AASHTO Bridge Design Specifications. Although the methods are generally calibrated to produce comparable results, the authors find that AASHTO LRFD can require substantially greater toe embedment than traditional ASD for geostructural applications.<br /><br />Using limit-equilibrium analyses of 15-foot cantilever walls, the study compares continuous sheet-pile walls and discontinuous soldier-pile-and-lagging walls across soil friction angles of 26°–36°. It evaluates basic conditions and the effects of wall friction, surcharge, and groundwater. Across the scenarios, LRFD designs require walls approximately 7–26% longer for continuous walls and 6–13% longer for discontinuous walls. The difference generally diminishes as soil friction angle increases, but remains significant. Combined site conditions also produce greater driving pressures and reduced passive resistance under LRFD factors, increasing required embedment.<br /><br />The authors attribute the discrepancies partly to applying linear load factors to nonlinear soil-resistance behavior, particularly passive earth pressure, and partly to factors that do not reflect variations in soil type, wall type, groundwater, or surcharge. They argue that these differences can increase construction costs and may imply instability in existing ASD-designed walls despite their successful performance.<br /><br />The paper recommends recalibrating AASHTO LRFD load and resistance factors using broader wall-performance data and accounting for site-specific conditions. Pending further research and correction of the discrepancies, the authors recommend suspending LRFD use for retaining-wall design in favor of ASD. Their analyses do not address structural wall-element design, cohesive soils under undrained conditions, or all wall types.

Keywords

Allowable Stress Design

LRFD

retaining walls

AASHTO Bridge Design Specifications

cantilever walls

sheet-pile walls

soldier-pile-and-lagging walls

geostructural design

passive earth pressure

wall embedment