Tips to Solve Convergence Problems

Hereby a complete list of the messages that are output by the SeismoStruct 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 Project Settings
Select to show Convergence problems in the post-processor through the Project 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 force-based (infrmFB & infrmFBPH) and the masonry element types in the Element Iterative Strategy tab of the Project Settings.
Reduce the maximum pushover displacement to 2% in general. This value should not exceed 1.00 or 1.20% for tall buildings and for stiff buildings with large shear walls.
Assign 50 to 100 pushover analysis steps in the general case. This value should be increased in cases of demanding loading.
Change the fracture/buckling strain for the steel materials to a very large value (e.g. 1) in the Materials module. When the fracture/buckling strain for the steel materials is reached, the rebar is deactivated and leads to a sudden drop of the building strength and convergence difficulties. here are cases, such as coupling beams, where the deformations are enormous. It is very likely that, if this strain level is reached, the rebars are deactivated (assumed as fractured) and the beams cannot sustain any gravity load and convergence cannot be reached.
Go to the Element Class module and change the element type of all short elements (e.g. short
columns, and coupling beams) from infrmFBPH or infrmFB to infrmDB. It is noted that the infrmDB element type generally provides acceptable accuracy only for short members only. Hence, if applied to short members, this does not affect the analytical results, whilst typically it leads to significantly improvements in the convergence and overall stability of the analysis.
Increase the Maximum Number of Iterations to 70, the Number of Stiffness Updates to 60 and the Divergence Iterations to 60 in Iterative Strategy tab of the Project 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 element’s Properties window within the Element Connectivity module, 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 Project Settings.
Increase the rigidity of the rigid diaphragms to 1.0E+13 through the Constraints tab of the Project Settings.
If the divergence messages of the analysis are mostly Max_Tol or elm_tol, increase the Maximum Tolerance value to 1e40 in the Iterative Strategy tab of the Project Settings.
Increase the number of fibers for the walls in the Element Class Properties window of the members within the Element Classes module.
For taller buildings uncheck the Include Geometric Nonlinearities checkbox in the Analysis tab of the Project 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 accidentally not connected to the 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. 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. The introduction of rectangular, rather than T-shaped, beams sometimes leads to more stable solutions, however it should be noted that removing the slab effective width weakens the beams and it could render lightly reinforced beams unable to sustain the gravity loads, hence leading to new convergence problems.