I am laying out a 3 span continuous bridge with reverse curves and am working on verifying the girder lengths produced by OBM. This verification was done in a separate linework model that took the alignment and offset it to represent the centerline of girders. I then took the support lines in the OBM model, overlaid it on the linework model so I could get exact locations. I offset for the location of the end of girder and trimmed the centerline of girder to begin and end at the actual begin and end of girders. These match up exactly (visually) on the linework model. I then broke up the centerline of girder at the centerline of bent supports for the two interior piers so I could get exact lengths of girders to compare with the OBM model.

The following are the results that I got for Girder 1 (left exterior girder):

Span A: 114.351’ (OBM) / 114.559’ (Linework) / 0.208’ (Delta)

Span B: 156.272’ (OBM) / 156.479’ (Linework) / 0.207’ (Delta)

Span C: 160.723’ (OBM) / 160.723’ (Linework) / 0.000’ (Delta)

Total Girder 1 Length: 431.346’ (OBM) / 431.761’ (Linework) / 0.415’ (Delta)

As you can see, for Girder 1, the total is off by almost 5”. Now, what is interesting are the results that I got for Girder 4 (right exterior girder):

Span A: 119.447’ (OBM) / 119.447’ (Linework) / 0.000’ (Delta)

Span B: 160.944’ (OBM) / 160.945’ (Linework) / 0.001’ (Delta)

Span C: 159.294’ (OBM) / 159.295’ (Linework) / 0.001’ (Delta)

Total Girder 4 Length: 437.685’ (OBM) / 439.687’ (Linework) / 0.002’ (Delta)

Girder 4 is basically exact (with a bit of rounding). Girders 2 and 3 delta are 0.121’ and 0.001’ (respectively). On the second unit in this bridge, the delta between both programs increased to 0.572’. I did at least verify that the beam layout in OBM and the beam definition table in OBM provide the same lengths of girder.

What could be causing this discrepancy in girder lengths and what would be the solution to resolve it? I was wondering if somehow the length of girder was taking in to account the vertical profile, however, the OBM output is always less than the projected plan length that was generated in the linework model.