Use AutoCAD's "Extrude" and "Revolve" commands to turn 2-D designs into 3-D models. "Extrude" is how 3-D modelers to refer to the technique of stretching a 2-D shape into 3-D space.

The next step is to create a 3D model of the bridge from your sketch. You can use the Extrude, Sweep, Loft, and Revolve commands to transform your 2D shapes into 3D solids.