ConcreteBucklingCalculator was added

StructureHelperLogics/NdmCalculations/Buckling/IRCStiffnessLogic.cs

@@ -0,0 +1,14 @@
+using StructureHelperLogics.NdmCalculations.Primitives;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace StructureHelperLogics.NdmCalculations.Buckling
+{
+    public interface IRCStiffnessLogic
+    {
+        (double Kc, double Ks) GetStiffnessCoeffitients();
+    }
+}