LARS can calculate LRFD LLDF's, and will auto-generate the ranges over which the varying distribution factors will govern. For a continuous member, it appears as if it calculates these ranges such that 20% of the span length on either side of an interior support will be governed by the negative moment distribution factor. In other words; for a 3 span continuous member with different span lengths, LARS calculates a positive bending distribution factor for each of the following ranges: 1.0-1.8, 2.2-2.8, 3.2-4.0 and a negative bending distribution factor for each of the following ranges: 1.8-2.0, 2.0-2.2, 2.8-3.0, 3.0-3.2. If my assumption about how LARS is calculating positive/negative bending factors is incorrect, please let me know. It definitely generates the ranges in that manner, but my independent calculations don't exactly match the factors LARS calculates so my interpretation may be flawed.
If my interpretation is correct though, where does that 20% assumption come from? When calculating LRFD LLDF's by hand and inputting them into LARS rather than relying on the values generate by LARS, what are the proper ranges to input? I have dimensions for top and bottom flange tension zones, as well as the DL point of contraflexure.