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.
So then are the AASHTO labels in Table 4.6.2.2.1-2 for "Positive Moment" and "Negative Moment" are kind of... misleading? They are intended to be applied over the DL positive/negative ranges, rather than governing for LL's that induce a positive or negative moment... due to the fact that a live load at any location will induce both positive and negative moments in different sections. Or should LARS allow the option to define both a positive and a negative moment factor for overlapping ranges, if the intent of the AASHTO code is to define LLDF's with respect to live load force effect, rather than with respect to location as constrained by DL force effect.
Also, I can confirm that LARS does not calculate negative moment LLDF's using the length as defined by AASHTO LRFD in Table 4.6.2.2.1-2. I mocked up a sample bridge, 2 span continuous. With equal length spans, the LLDF moment factors for all 4 ranges (1.0-1.8, 1.8-2.0, 2.0-2.2, 2.2-3.0) were identical. Changing the length of span 2 so that now the spans were unequal, resulted in a changed LLDF for only ranges 2.0-2.2 and 2.2-3.0. What should have happened (based on my interpretation of the aforementioned AASHTO table) is that range 1.0-1.8 should stay the same, range 1.8-2.0 and 2.0-2.2 should have both changed and be equal to each other, and range 2.2-3.0 should have changed and not be equal to any of the other three.
Am I doing/interpeting something incorrectly? Or should LARS be calculating these factors differently?
Your interpretation of the 80%-20% is correct. For your ranges, the DL point of contra flexure will be fine.