In TBC when you elevate Lines and Arcs we do not Chord the lines, we only chord the surface model that is derived from the lines, the lines are still curved. If we output 3D lines to DXF for the field, DXF cannot handle 3D Arcs so we have to chord the output lines if you send them out as 3D lines, however you do have options when you output a DXF file to output them as 2D lines in which case the Arcs remain as ARCs and are output as 2D lines.
If you elevate an Arc using Change Elevation command using a Surface to define the elevation, the arc is still not chorded - it adds VPIs to the Linestring where the Arc crosses the surface triangle sides, so there should be no chording there either.
In a corridor model, however when you use a 2D line as an Offset reference, we do chord the surface model, as it is only interpreted every xx feet, and if you then explode the corridor model then the lines created are from the surface model so they will be chorded at the intervals or densification locations - if you want 3D lines that are not chorded, then you would have to take your original 2D curved lines and use the Change Elevation command to elevate them using the corridor model - then they will be true 3D curvilinear lines.
In TBC we try not to destroy source line Geometry, it is exports to DXF / DWG that require us to do that because they cannot handle the 3D curvilinear Geometry that you have in TBC, and also in Corridor Surface Models because those are interpolated outputs of the true corridor (which is 100% accurate everywhere you query it, however to output a surface we have to generate nodes at Intervals and key locations and through Densification to get a surface fit for purpose. If and when Siteworks and the Machine Control Systems can work off a True VCL file of a true corridor, then all this would be a thing of the past as then they could truly compute from the curvilinear linework.
Alan