I'm working on a heavily skewed parallel girder bridge which requires the diaphragms to be considered primary members. A grillage analysis is being used. Under the "Cross Frame Forces" for the analysis, diaphragm end shears and moments are reported for the various dead and live load results. These results make sense to me.
I'm a little confused with the "Cross Frame Detailed Forces" report which is reporting some extremely large axial forces (over 1,000 kips under service live load with a cured deck). Service dead load axial forces are a little less than half of this. Under the old V-Load analysis of cross frames, axial loads were not part of the loads applied to the cross frame system (my references are back at the office so I'm not sure if diaphragms were addressed). Any insight to how LBS is generating the diaphragm axial forces would be appreciated. Thanks!
For FEM analysis :
We release the moment DOF at two ends for diaphragm, not the shear. If there is any shear force developed, it will be reported. We can offer options to make the ends are fixed (/welded). FEM analysis considers the Cross frames as primary members.
In the Grillage Model if the diaphragm flanges are not connected (typical), the moments really can’t be generated Also the grillage analysis does not consider the cross frames as primary members.