Eriksson Culvert calculates the ASTM M-dimension in 2 ways:
The Method of Equivalent Capacities: This method determines the location in the top slab, from the wall, where the flexural capacity of the member is greater-than or equal to the demand. The M-dimension is this location plus the splice length.
The Method of Contraflexure: This method determines where the location of 0 moment is in the top slab, from the wall. Th M-dimension is this location plus the splice length. This method usually results in a larger M-dimension than using the method of equivalent capacities. Additionally, not all culvert designs will have a location of contraflexure in the top slab, when this occurs Eriksson Culvert will not display the M-dimension computed by this method.