// Starting the transaction, using CYBORG's FabricationTransaction class
using (FabricationTransaction trans = new FabricationTransaction(doc, false, "Create shortening"))
{
   Reference eRef = activeDoc.Selection.PickObject(ObjectType.Element, "Pick a beam to add shortening on it");
   Element elem = null;

   if (eRef != null && eRef.ElementId != ElementId.InvalidElementId)
   {
      elem = (doc.GetElement(eRef.ElementId));
   }

   if (null == elem)
   {
      return Result.Failed;
   }

   // adding fabrication information, if the element doesn't already have it.
   SteelElementProperties cell = SteelElementProperties.GetSteelElementProperties(elem);
   if (null == cell)
   {
      List<ElementId> elemsIds = new List<ElementId>();
      elemsIds.Add(elem.Id);
      SteelElementProperties.AddFabricationInformationForRevitElements(doc, elemsIds);
   }


   // Create the modifier using AdvanceSteel API.
   // For more details, please consult http://www.autodesk.com/adv-steel-api-walkthroughs-2019-enu
   FilerObject filerObj = Utilities.Functions.GetFilerObject(doc, eRef);
   if (null != filerObj)
   {
      if(!(filerObj is Beam))
      {
         return Result.Failed;
      }
      Beam beam = filerObj as Beam;
      Beam.eEnd end = Utilities.Functions.CalculateBeamEnd(beam, new XYZ(10, 20, 20));
      BeamShortening beamShort = new BeamShortening(end, 150.0);
      beam.AddFeature(beamShort);
   }
}

public static FilerObject GetFilerObject(Autodesk.Revit.DB.Document doc, Reference eRef)
{
   FilerObject filerObject = null;
   Autodesk.AdvanceSteel.DocumentManagement.Document curDocAS = DocumentManager.GetCurrentDocument();
   if (null != curDocAS)
   {
      OpenDatabase currentDatabase = curDocAS.CurrentDatabase;
      if (null != currentDatabase)
      {
         Guid uid = SteelElementProperties.GetFabricationUniqueID(doc, eRef);
         string asHandle = currentDatabase.getUidDictionary().GetHandle(uid);
         filerObject = FilerObject.GetFilerObjectByHandle(asHandle);
      }
   }
   return filerObject;
}