Returns thickness of flange measured in the 0.25 * width, sometimes used for sections with width <= 300 mm. 
   Namespace:   Autodesk.Revit.DB.Structure.StructuralSections  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 18.0.0.0 (18.2.0.0) 
  Since:  2018