This paper investigates the design load for deeply buried concrete box culverts. The current AASHTO LRFD and Standard Specifications for Highway Bridges stipulate the computation of the design load on the top flange of the box culvert primarily based on research work by Marston and Spangler. Depending on the construction method, trench installations or embankment installations, the effective density or the soil-structure interaction factor on concrete box culverts can be substantially different as the settlement of the backfill is affected by the vertical shear between the adjacent soil column. Although the AASHTO procedure may be applied conservatively for most ordinary culverts, an opportunity exists to evaluate the more realistic soil-structure interaction behavior based on the modern finite element analysis. The study presents results that are based on two-dimensional finite element analyses, linear elastic analysis by ABAQUS, CANDE-89, and nonlinear analysis by ISBILD considering the construction layers. A number of hypothetical deeply buried concrete box culverts have been analyzed. The backfill heights are varied from 50 to 200 feet for the embankment condition and 50 to 150 feet for the trench condition. In the case of trench condition, the distance between the concrete box culvert wall and the adjacent existing trench soil column is also varied. The soil properties used are selected to be representative of the typical upper ranges and lower ranges.
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