Why there are significant axial forces in the bridge girders but zero axial force for the entire bridge section?
The axial forces in the beams are the net effect of stresses acting on the beams. Remember that the bridge modeler deck section is a composite cross-section. The bending moments applied to the composite section are resisted by longitudinal stresses and the beam axial force you are seeing is the resultant of the stresses that act on the beam. If your bridge is not longitudinally restrained, combining the slab longitudinal forces with the girder axial forces will result in zero axial forces for the bridge deck cross-section.
