ConcreteBucklingCalculator was added

StructureHelperLogics/NdmCalculations/Buckling/IConcretePhiLLogic.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace StructureHelperLogics.NdmCalculations.Buckling
+{
+    public interface IConcretePhiLLogic
+    {
+        double GetPhil();
+    }
+}