Force combination was added

StructureHelperCommon/Models/Forces/IDesignForceTuple.cs

@@ -0,0 +1,16 @@
+using StructureHelperCommon.Infrastructures.Enums;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace StructureHelperCommon.Models.Forces
+{
+    public interface IDesignForceTuple
+    {
+        LimitStates LimitState { get; set; }
+        CalcTerms CalcTerm { get; set; }
+        IForceTuple ForceTuple {get;}
+    }
+}