Convergence and Divergence flags

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.

Converg: This message means that the analysis has converged in the current loading step, and is proceeding to the next step.

Max_Ite: This message is output, if the maximum number of iterations has been reached in the current loading step, and convergence has not been achieved yet. In such cases, either increase the maximum number of iterations (Global Iterative Strategy), increase the convergence criteria values (Convergence Criteria), employ a less stringent type of convergence check (e.g. Displacement/Rotation based only scheme instead of Displacement/ Rotation based AND Force/Moment based).

Prd_Ite: This flag is similar to the Max_Ite message, the difference being that the solver does not wait until the maximum number iterations have been reached. Instead, it makes a prediction of the number of iterations that are expected to be needed for convergence, based on how the iterative solution is converging (i.e. size of out-of-balance forces, and how fast the convergence tolerance is being reached). If the predicted iterations is larger than the maximum iterations specified by the user, the Prd_Ite flag is output and the analysis diverges. In such cases, either increase the Maximum number of iterations (Global Iterative Strategy), choose a looser convergence criteria scheme with larger convergence tolerances (Convergence Criteria), or decrease the loading step, as with the Max_Ite message. It is noted that Prd_Ite is the most common divergence flag.

Diverge: This flag is output when the iterative process in the current step is diverging, instead of converging to the solution. It is noted that the check for diverging solutions is always carried out after the Divergence Iteration that is specified by the user in Global Iterative Strategy. This is done because in general the solution procedures are unstable at the initial 3-4 steps, before they get stable and gradually converge to the solution. Users are advised to either increase the Divergence Iteration and the Maximum number of iterations from the Global Iterative Strategy page, choose looser convergence criteria from the Convergence Criteria page, or decrease the loading step.

elm_Ite: This message appears when the maximum number of iterations is reached in the internal element loop of the elements that require iterations on the element level (infrmFB and infrmFBPH), without internal equilibrium having been achieved. Users are advised to either increase the number of iterations or increase the convergence tolerance from the Element Iterative Strategy tab. Alternatively, the ‘Do not allow element unbalanced forces in case of elm_ite’ option may be unchecked. Finally, measure on the global level may be taken, for instance the analysis load step can be decreased (by increasing the load factor increments or the time-steps), and the global convergence criteria can be increased. Users are advised to refer to the specific documentation [e.g. Spacone et al. 1996; Neuenhofer and Filippou 1997] for a better understanding of the internal loops of the force-based elements.

elm_Inv: This message appears when the stiffness matrix of an  element that employs internal iterations cannot be inverted during the internal element loops. In such cases, users are advised to increase the elements' convergence tolerance from the  Element Iterative Strategy tab, to decrease the global analysis load step or to increase the global convergence tolerance values.

fbd_Tol: This message appears when the maximum tolerance value, as specified in the Global Iterative Strategy page, has been exceeded during the internal element loops of the force-based elements. Similar actions with the fbd_Inv flag should be taken.

Max_Tol: This flag signifies solutions that become very unstable numerically with out-of-balance forces larger than the Maximum Tolerance (default=1.0E+20) that is specified in the Global Iterative Strategy  page. Users are advised to decrease the load increment, or to adopt looser convergence criteria. Alternatively, the Maximum Tolerance may be increased, but its value should never exceed values of 1.0E+35 or 1.0E+40, whilst it is noted that in very few cases the latter will lead to stable solutions.

Sol_Prb: This message means that a solution of the analysis equations in the current iteration could not be found. There are numerous reasons for this behaviour, such as extreme values of out-of-balance forces or zero diagonal stiffness values. Similar measures to those suggested for the case of Max_Tol flag should be taken.