Ahmed,
I would create a custom railing using columns for the posts and walls or beams for the hand rail. Place the columns as you wish along the sides of the sloped ramp and use the gravity tool (see attached) so that each column will fall to the surface of the ramp. Gravity tool will only work on slabs, roofs and meshes though, so your bridge will need to be created with one of these tools for this method to work.
Then using the elevation, as a reference of what you have created so far, create a profile for the hand rail that will sit atop the posts. I would use the wall tool as it will heal better where the curvature of the rail is more intense.
Here is a great article by LaszloNagy on the ArchiCAD Wiki that explains how to use a beam or the curved ramp tool to make a curved beam (in this case, a spiral):
http://www.archicadwiki.com/SpiralBeam.
HTH
Nicholas Cornia