Hi Michel,
The company that I work for uses AutoCAD 2010 for production, how ever I can try IGES export after work hours. I am aware about SAT and have used it quite often.
Unfortunately creating solids from bridge geometry is not best option if that is only to be used as a reference or source data  points and lines/polylines are most efficient way. With bridges software must handle xyplane curvature but also curvature in z/lenghtpane (sorry about clumsy terms). When we add changing slopes in both sides of this previous centerline and different widths from it to bridge eges geometry is quite hard. Almost forgot that cuts along center line are straight to zaxis and for that simple extrude is not an option. If this geometry is to be modeled in AutoCAD (as a solid) the result is a series of separate curved solids or solids that have planar faces. There is ways to find exact (evenly distributed) points for example centerline and with Moi3D you can create smooth curved surface that is very accurate. It really is faster to handle points and lines/polylines than surfaces or solids. If You ever make dxf import to Moi3D, one cool tool could be to change 3dpolylines to Freeform curves that go through 3dpolylines vertex points :)
I will test IGES, STEP and SAT. Thank You for quick answer.
 JP
