Transaction t = new Transaction(activeDoc, "Create Beam");
t.Start();
Line line = Line.CreateBound(start/ 304.797, end / 304.797);
FilteredElementCollector collector = new FilteredElementCollector(activeDoc);
collector.OfClass(typeof(FamilySymbol)).OfCategory(BuiltInCategory.OST_StructuralFraming);
FamilySymbol gotSymbol = collector.FirstElement() as FamilySymbol;
gotSymbol.Activate();
string filename = "C:\\ProgramData\\Autodesk\\RVT 2019\\Libraries\\Russia\\Каркас несущий\\Бетон\\Бетон-Балка прямоугольного сечения.rfa"
// пробовал ещё @"C:\ProgramData\Autodesk\RVT 2019\Libraries\Russia\Каркас несущий\Бетон\Бетон-Балка прямоугольного сечения.rfa"
activeDoc.LoadFamilySymbol(filename, "load"+gotSymbol.Id.ToString(), out gotSymbol);
FamilyInstance instance = activeDoc.Create.NewFamilyInstance(line, gotSymbol, level.level_Revit, Autodesk.Revit.DB.Structure.StructuralType.Beam);
instance.get_Parameter(BuiltInParameter.Z_OFFSET_VALUE).Set(elevation / 304.797);
t.Commit();