ConcreteBucklingCalculator was added

StructureHelperLogics/NdmCalculations/Buckling/EilerCriticalForceLogic.cs

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+using StructureHelperCommon.Infrastructures.Exceptions;
+using StructureHelperCommon.Infrastructures.Strings;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace StructureHelperLogics.NdmCalculations.Buckling
+{
+    internal class EilerCriticalForceLogic : IEilerCriticalForceLogic
+    {
+        public double LongForce { get; set; }
+        public double StiffnessEI { get; set; }
+        public double DesignLength { get; set; }
+
+        public double GetCriticalForce()
+        {
+            double Ncr = - Math.Pow(Math.PI, 2) * StiffnessEI / (Math.Pow(DesignLength, 2));
+            return Ncr;
+        }
+
+        public double GetEtaFactor()
+        {
+            if (LongForce >= 0d) return 1d;
+            var Ncr = GetCriticalForce();
+            if (LongForce <= Ncr)
+            {
+                throw new StructureHelperException(ErrorStrings.LongitudinalForceMustBeLessThanCriticalForce);
+            }
+            double eta = 1 / (1 - LongForce / Ncr);
+            return eta;
+        }
+    }
+}