Add stirrup update strategy

StructureHelperCommon/Models/Forces/IBeamSpanLoad.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace StructureHelperCommon.Models.Forces
+{
+    /// <summary>
+    /// Implement properties for shear beam load 
+    /// </summary>
+    public interface IBeamSpanLoad : IAction
+    {
+        /// <summary>
+        /// Value of level where action is applyied at, 0.5 is top surface of beam, -0.5 is bottom surface of beam
+        /// </summary>
+        double RelativeLoadLevel { get; set; }
+        /// <summary>
+        /// Ratio of substraction load from total shear force 
+        /// </summary>
+        double LoadRatio { get; set; }
+    }
+}