girder_buckling: bug fix for negative sqrt

Author: theScriptingEngineer

anystruct/calc_structure_classes/dnv_buckling.py

@@ -637,7 +637,10 @@ def girder_buckling(self, optmizing = False) -> dict:
         sysd = derived_stress_values.sysd
         sxsd = derived_stress_values.sxsd
 
-        Cys = 0.5 * (math.sqrt(4 - 3 * math.pow(sysd / fy, 2)) + sysd / fy)
+        if 4 - 3 * math.pow(sysd / fy, 2) > 0:
+            Cys = 0.5 * (math.sqrt(4 - 3 * math.pow(sysd / fy, 2)) + sysd / fy)
+        else:
+            Cys = 0.0
 
         lambda_p = 0 if thickness*E == 0 else 0.525 * (spacing / thickness) * math.sqrt(fy / E)  # reduced plate slenderness, checked not calculated with ex
         Cxs = (lambda_p - 0.22) / math.pow(lambda_p, 2) if lambda_p > 0.673 else 1