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When the user selects the 6th Edition (or newer) of LRFD, the calculation of each load rating follows the procedure outlined in The Manual for Bridge Evaluation (MBE), Second Edition/2010, by AASHTO (including all interims up through 2014).  Support for culverts was added with the 2013 Interims in Article 6A.5.12.4.  The new load rating equation, 6A.5.12.4-1 (shown below), is equivalent to the procedure currently contained in Eriksson Culvert, with one exception.  The new MBE states that max/min effects for horizontal earth loads shall be produced via the ratio of the maximum and minimum equivalent fluid pressures (basically the method followed in STND).

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In summary, when the governing specification is LRFD 5th Edition, the load factors for EH are used in the load combinations and rating equations.  When the governing specification is LRFD 6th Edition (or newer), the ratio of the maximum and minimum fluid pressures are used for EH instead.

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