RemoveUnderlying property for primitives was added

StructureHelperCommon/Models/Forces/IForceCombinationByFactor.cs

@@ -0,0 +1,16 @@
+using StructureHelperCommon.Models.Shapes;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace StructureHelperCommon.Models.Forces
+{
+    public interface IForceCombinationByFactor : IForceAction
+    {
+        IForceTuple FullSLSForces { get; }
+        double ULSFactor { get; set; }
+        double LongTermFactor { get; set; }
+    }
+}