No convergence after nonlinear analysis

Problem description #

Nonlinear static analysis exits with an error message, with no convergence in maximum iterations note.

During the analysis, the Convergence tab of the Analysis process page displays that the fulfillment of the convergence criteria (displacement/work/force) for the tracked node does not happen in the frame of the set number of iterations.

Reason #

Based on the solution control parameters, there was no convergence within the maximum number of iterations. This can be due to modeling issues, or due to the settings of the solution control parameters. 

Troubleshooting guide #

There is no accurate method to pinpoint the exact reason for the instability of nonlinear models, but assuming that the model contains no modeling errors (free hinges, inaccurate modeling, inappropriate release conditions etc), the following recommendations can be applied on the settings of the nonlinear analysis, and solution control parameters:

  • Check of applied reinforcements. The nonlinear calculation engine is prepared for SLS load levels where members are between uncracked-cracked states, and it is sensitive to the amount of reinforcement in concrete slabs. The most common reason for nonlinear convergence error in case of concrete structures is that there is not enough reinforcement in the structure, therefore it is advised to check on the applied level:
    • Check if the applied level of reinforcement reaches the minimum required amount of reinforcement. Use the “Take into account the minimum required reinforcement” option while defining reinforcement parameters to surfaces.
    • Check if the applied reinforcement is adequate to satisfy the ULS utilization limit. To do this in the case of domains, check if there are negative values at the “Reinforcement difference values” while standing on the dominant ULS load combination. A negative value means that the domain requires more reinforcement than the applied value.
  • Peaks in bending moments above columns can cause convergence issues, therefore it is advised to use the “Adjust mesh to column heads (to enable cutting of moment peaks
  • Taking into account the concrete tensile strength on the “Nonlinear analysis” part of the “Surface reinforcement parameters” dialogue will lead to better convergence results. Turning the checkbox off means that the nonlinear analysis will start from a more unfavourable state, where the concrete is already cracked.
  • On the “Surface reinforcement parameters” dialogue at the “Nonlinear analysis” part, choose the option “ε-N; κ -M”. In order to avoid convergence problems, the concrete material model has been slightly changed to provide a smoother transition between cracked and uncracked state.
  • During the nonlinear calculation of reinforced concrete structures, the method to find the modified shape through iterations is sensitive to local optimums. The incrementation between iterations can be tuned using the “number of increments” field. When changing the number of increments, it is advised to choose a value which is not the integer multiple used for the previous analysis.
  • The loadlevel at which the convergence error occurs is important information:
    • If the convergence error occurs near the full loadlevel (near 1), than tracking of the plastification part may not be accurate enough. In this case, using a custom-defined “Increment function” which has a granulated incrementation between iterations near the loadlevel of 1 can be a solution. Using an increment function it is possible to reduce the number of increments where the behaviour of the structure is linear and increase the number of increments where the behaviour is nonlinear.
    • If the convergence error occurs on low loadlevels, then the consideration of the tensile strength of the concrete, and the “ε-N; κ -M” should be checked and turned on (see above)
  • Model behaviour under the load level of the last successful incrementation can help point out the problematic parts of the structure. To be able to check the behaviour, based on the problematic load combination a new manual combination should be created, where combination factors are proportionally reduced to reflect the loadlevel of the last successful iteration. Performing the analysis and evaluating the results (unexpected deformations, irregular internal forces/stresses) of this combination can help to understand which are the weak parts of the model.  

Related articles #

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The model contains singularities, unstable mode

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