ConcreteBucklingCalculator was added

StructureHelperLogics/NdmCalculations/Buckling/RCStiffnessLogicSP63.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace StructureHelperLogics.NdmCalculations.Buckling
+{
+    internal class RCStiffnessLogicSP63 : IRCStiffnessLogic
+    {
+        IConcretePhiLLogic phiLLogic { get; }
+        IConcreteDeltaELogic deltaELogic { get; }
+
+        public RCStiffnessLogicSP63() : this(new ConstPhiLLogic(), new ConstDeltaELogic()) { }
+
+        public RCStiffnessLogicSP63(IConcretePhiLLogic phiLLogic, IConcreteDeltaELogic deltaELogic)
+        {
+            this.phiLLogic = phiLLogic;
+            this.deltaELogic = deltaELogic;
+        }
+
+        public (double Kc, double Ks) GetStiffnessCoeffitients()
+        {
+            const double initialKs = 0.7d;
+            const double initialKc = 0.15d;
+            const double deltaEAddition = 0.3d;
+            double phiL = phiLLogic.GetPhil();
+            double deltaE = deltaELogic.GetDeltaE();
+            double kc = initialKc / (phiL * (deltaEAddition + deltaE));
+            return (kc, initialKs);
+        }
+    }
+}