RE: LARS LRFD live load distribution factors and ranges

James Meisterich — Wed, 10 Sep 2014 20:11:39 GMT

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?

RE: LARS LRFD live load distribution factors and ranges

RALBrIM — Wed, 10 Sep 2014 19:41:48 GMT

Your interpretation of the 80%-20% is correct. For your ranges, the DL point of contra flexure will be fine.