Tips to Solve Convergence Problems

Hereby a complete list of the messages that are output by the SeismoBuild solver in the case if divergence is provided, together with possible measures that the user can take, in order to make the analysis converge.

Hereby a number of steps to follow for solving convergence problems are proposed. Users are advised to:

  • Apply the automatic adaptation of the norms in the Convergence criteria tab of the program’s Advanced Settings (Analysis Parameters>Advanced Settings).
  • Select to show Convergence problems in the post-processor through the Analysis Parameters>Advanced Settings> Convergence criteria tab. The visualisation of the locations of the structure (elements or nodes), where the convergence difficulties arise, provides significant feedback for the identification of the reasons for divergence (e.g. under-reinforced beams that cannot sustain the gravity loads, elements with very high deformations demand, such as short columns or coupling beams, etc.).
  • Uncheck the ‘Do not allow unbalanced forces in case of elm_Ite’ for both the force-based (infrmFB & infrmFBPH) element types in the Element Iterative Strategy tab of the Advanced Settings.
  • Reduce the Maximum Interstorey Drift value in the Analysis tab of the Code Requirements. This value should not exceed 1.00 or 1.20% for tall buildings and for stiff buildings with large shear walls.
  • Assign 100 pushover analysis steps in the Analysis tab of the Code Requirements. This value should be further increased in the cases, where significant loading is expected.
  • Select the ‘Apply Displacement Based Frame Elements To All Members With Length (m) <’ in the Advanced Building Modelling tab of the program’s Advanced Settings (Analysis Parameters>Advanced Settings), in order to use the infrmDB element type for short members. This change typically leads to improved convergence.
  • Increase the maximum number of iterations to 70, the number of stiffness updates to 60 and the divergence iteration to 60 in the Iterative Strategy tab of the Advanced Settings (Analysis Parameters>Advanced Settings).
  • Use the elastic frame element type for the coupling beams that cause convergence problems. In such cases the elements’ moment releases should released by selecting the relevant checkboxes for the M2a, M3a, M2b and M3b degrees-of–freedom, through the Advanced Member Modelling Parameters of the member in the Building Modeller, in order to account for the formation of plastic hinges at the ends of the coupling beams.
  • Increase the values of the convergence norms from the Convergence Criteria tab of the program’s Advanced Settings (Analysis Parameters>Advanced Settings).
  • Increase the rigidity of the rigid diaphragms to 1.0E+13 through the Constraints tab of the Advanced Settings.
  • Select the Control Node to be in the side of the building with the larger deformation demand, through the Structural Modelling tab of the Building Modelling Settings inside the Building Modeller.
  • If the divergence messages of the analysis are mostly Max_Tol or fbd_tol, increase the Maximum Tolerance value to 1e40 in the Iterative Strategy tab of the Advanced Settings (Analysis Parameters>Advanced Settings).
  • Increase the number of fibers for the walls in the Modelling Parameters of the members inside the Building Modeller.
  • For taller buildings uncheck the Include Geometric Nonlinearities checkbox in the Analysis tab of the Advanced Settings.

Moreover:

  • users are advised to check the last or the 2-3 last steps of the analysis with convergence problems in order to understand and resolve the reasons for divergence. In such cases the Convergence Problems page of the post-processor should be advised. Furthermore, running an Eigenvalue analysis with the same model might offer valuable insight to the problem (e.g. identify a beam that is close to, but unconnected, to an adjacent column, and behaves as a cantilever, not being able to sustain the gravity load);
  • it is noted that elements that cause divergence problems are not necessarily the ones that withstand significant loading. They are the ones that at the current step face increased tangential change of the deformation state/internal force re-distribution. Hence, sometimes failed elements can increase significantly the load sustained by adjacent elements, thus leading them to convergence difficulties, contrary to the failed elements themselves, which converge easily;
  • the removal of the effective width of beams should also be considered by unchecking the ‘Include Effective Width’ checkbox in the Structural Modelling tab of the Building Modelling Settings inside the Building Modeller.