var fecWalls = new FilteredElementCollector(doc)
.WhereElementIsNotElementType()
.OfCategory(BuiltInCategory.OST_Walls);
PrintCollector(fecWalls, "Walls");
var fecColumns = new FilteredElementCollector(doc)
.WhereElementIsNotElementType()
.OfCategory(BuiltInCategory.OST_StructuralColumns);
PrintCollector(fecColumns, "Columns");
fecWalls.UnionWith(fecColumns);
PrintCollector(fecWalls, "Result");