How to connect two cable trays in bottom Vertical Justification by Revit API?

Hi, 

When I use Revit ribbon drawing function, it is easy to connect two cable trays with bottom Vertical Justification, but I can't do it by Revit API.

I know the RBS_CURVE_VERT_OFFSET_PARAM parameter. After I create the two cable tray, I set the value of the RBS_CURVE_VERT_OFFSET_PARAM parameter to bottom, and then create a Transition Fitting by NewTransitionFitting Method, but it doesn't work.

Best Regards,

Zhangzg