how to apply snow drift in STAAD pro?

I have tried plate load (which allows me to do nonuniform load) but it does't consider load with tributary area like floor load does, it seems to me "plate load" only distribute load to nodes not to the beam member.  I am wondering is there a way to apply a CHANGING LOAD (load changes from one end to the other by a certain slope) in STAAD pro and at the same time CONSIDER TRIBUTARY AREA. I know both "surface load" and "plate load" allows you to do trapezoidal loads but they don't seem to give the right answer (or I could be wrong). As shown in the figure below, I applied 10kn/m2 load on the plate and the result on this beam is very different.