Determine Live Load Distribution Factor from Finite Element Analysis

[edward1]

I have modeled about 40 simple reinforced concrete slab bridges using finite element software for a state dot and am now required to rate these on a two dimension program, Virtis, which defaultes to a swag formula that only uses span length and width for the live load distribution factor. It does not account for skew angle. What I did was model the 3 dimension slab with 2 lines of wheel loads plus impact for a one lane bridge for maximum moment and shear. A wheel line was always placed 2 feet from bridge edge, another wheel line placed 6 feet laterally, another wheel line 4 feet laterally, and another wheel line 6 feet laterally for a two lane bridge. The second and forth wheel loads are not always on positions of maximum moment or shear due to the skew angle. After I got maximum moment from finite element analysis which was always within the 2 feet edge beam, I used a 2 dimension bridge girder program which requires a distribution factor for the 2 wheel lines and does a moving load analysis for maximum moment and shear. The distribution factor always iterated around 0.10 for no skew angle. With skew angle of 45 degree this was .0791. Does this work?

 

[TheMoonlitKnight]

AASHTO LRFD 1998 Ed. 4.6.2.3-3 has a reduction of longitudinal effects equation for slab bridges. At 45 deg. it turns out to be 80% of non-skewed. Your numerical value is 79% - close.

It seems like a waste of time to model these in a FEM program. You could have reduced the DF by hand in a hot second.

Generally, you can only spread out LL effects appreciably for certain girder bridges using FEM software.