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• ### In a model with surface supports, the calculation does not converge or the result has very large deformations and implausible stresses. What are the reasons for this?

FAQ 004252 EN-US

The definition of surface supports should be as realistic as possible. Experience shows that the equation solver works most effectively with this. To simplify matters, degrees of freedom are often defined as 'fixed'. However, this can have a big impact on the overall stiffness matrix and cause numerical problems (see Figure 01).

It is better to work with springs in order to avoid the numerical problems. It is often sufficient to define very stiff springs (see Figure 02). The same applies to the foundation perpendicular to the surface. You can find more information in  and in the links below this FAQ.

• ### In a nonlinear system, some load cases do not converge. The load combinations, however, can easily be calculated. What is the cause of it?

FAQ 004114 EN-US

When defining non-linearities, for example, failure of support under tension, it may happen that some load cases cannot be calculated. If these are loads that cannot exist without other stabilizing loads, solving the problem is simple: You can set the load cases to 'Not To Be Calculated'. As a result, only the load combinations are considered in the option 'Calculate All' of the calculation process. This is possible because, for example, some loads can never appear without having a self-weight.

In the attached example, it is clearly evident that the system would buckle in the load case wind, and thus no convergence is found. In contrast to this, it is possible to calculate the load combination in which self-weight and wind are combined without a problem because the self-weight stabilizes the system.
• ### Why is the membrane constricted so much during the form-finding? I obtain very large deformation values.

Such an effect could be caused by free edges, which can be deformed almost infinitely during the form-finding process. In this case, it is difficult to achieve the state of equilibrium.

As a basis for the form-finding, there should be no free edges on the membrane surface. There must be cables arranged on the free edges of the membrane whose stiffness can be based, for example, on the folded or strengthened membrane edge strips.
• ### How do I recognize that the flattening process in RF‑CUTTING‑PATTERN has found the convergence?

The geometrically nonlinear flattening process transfers the real mesh geometry of the planar, buckled, curved or double-curved surface components from the selected set of cutting patterns, and flattens these planar components by minimizing the distortion energy, assuming the defined material behavior.

The iterative calculation used for this is controlled by the parameters in the "Calculation Parameters" menu, "Cutting Patterns" tab.

The "Maximum number of iterations" parameter limits the scope of the calculation, and stops the process when the set maximum iteration is reached. If the convergence criterion does not depend on the "Tolerance for convergence criteria" parameter in the convergence range when the maximum iteration has been reached, the program displays Error 10154.

If there is no error message displayed by the program, it is reasonable to assume the proper convergence.

You can usually resolve the error by adjusting the flattening geometry or increasing the maximum number of iterations.

• ### Is there a way how to easily prevent RF‑SOILIN from recalculating the foundation every time the load in the governing load case is changed?

No, that is not possible. The calculation of the foundation parameters in RF‑SOILIN is performed iteratively. The first iteration step requires that the start values for the foundation parameters are selected internally by the program. These start values can be used to perform a finite element analysis of the FE model in RFEM.

The result is the soil contact stress distribution. The soil contact stress of the first iteration step is included in the RF‑SOILIN calculation as the initial value. Together with the stiffness modulus of the entered soil layers, it is possible to calculate the settlement for each finite element. The settlement and the soil contact stress are then used to calculate the foundation parameters.

In the next iteration step, the new foundation parameters replace the old ones, and a new finite element analysis is started, which results in a new distribution of soil contact stress. As a convergence criterion, the new distribution of soil contact stress is compared with the old one.

As long as the deviation exceeds a certain convergence limit, the new distribution of soil contact stress in RF‑SOILIN is considered in the calculation of the new foundation parameters. In the case the the deviation of the soil contact stress distribution of two consecutive iteration steps is not reached for the first time, the iteration is ended and the foundation parameters of the last iteration step are given as a result in RF‑SOILIN.

• ### Currently, I have the problem that the calculation in RF-SOILIN takes extremely long time and does not converge. What can I do?

FAQ 003051 EN-US

These problems can often be solved easily and efficiently.

Especially in the edge areas of plates, it is common that the results oscillate. In this case, the result can be improved by modeling a foundation overlap with negligible thickness around the surface. The same foundation is assigned to this as the main plate. The foundation overlap shows the edge area of the plate more accurately and the convergence behavior is often better.

It is important to extend the foundation overlap at least so far that the basin is completely subsided. Thus, it is also possible to graphically display the influence of the settlement on the surrounding buildings.

• ### Why is the sum of the charges different from the sum of the support forces? What causes could it be?

FAQ 002779 EN-US

The following causes can be responsible for this:
• In most cases, these differences can be attributed to a lack of convergence. Increasing the iterations and increments in the calculation parameters and FE mesh settings should help.
• High stiffness jumps result in numerical problems, which leads to errors in the result evaluation. In RSTAB, this is not a major problem with a full and analytical approach. In RFEM, on the other hand, approximation approaches are used, so higher stiffness jumps should rather be avoided.
• Bedded bars may well be subject to deviations as well. If the bars are not or only roughly divided, there are convergence problems. A practical solution here is to select a "finer" bar pitch in the FE mesh settings.
• ### During the calculation, I get a warning that the calculation of a load case does not converge. What should I do?

FAQ 002516 EN-US

When you see this message, check the nonlinearities available in the model as well as the plausibility of the results. The sum of loads and the sum of support forces could also help with the check. If you find significant deviations there, the results should not be used for further analysis. It is often helpful to increase the number of load increments and the number of iterations in Calculation Parameters (see Figure 02).

• ### What does the calculation parameter setting "Failing members to be removed individually during successive iterations" mean?

FAQ 000769 EN-US

In the first iteration step, all members are considered. Before the next step, the program determines which members cannot resist the determined compressive forces due to their definition, for example tension members with negative axial forces. Then, the tension member with the greatest compressive force is removed from the stiffness matrix. Thus, the next iteration step follows.

Next, the member definitions are compared to the determined axial forces. For the next iteration step, the tension member subjected to the highest actions is removed from the stiffness matrix. This procedure is continued until no member is subject to the internal forces that it cannot resist.

In this way, you can often achieve a better convergence behavior for the system because of redistributing effects. This calculation option requires more time because the program must run through a larger number of iterations. Furthermore, you have to make sure that a sufficient number of possible iterations is set (see the "Settings" dialog box section in Figure).

For this method, it might also happen that the initially failed member is reinserted, because it is subjected to tension forces due to possible redistribution effects.

• ### In a load case, compressive forces in a cable arise. What is the explanation for this?

FAQ 000217 EN-US

Compressive forces in cables or tension members may arise if the number of iterations is not sufficient for this analysis so that the system did not converge. The number of iterations can be specified in the Global Calculation Parameters tab of Calculation Parameters (see figure).

For the maximum number of iterations, the value 100 is preset. However, this does not mean that all iterations will be run. Depending on the structural system, the calculation often converges much earlier.

Check also the settings of Reactivation of Failing Members. If the option "Assign reduced stiffness to failing members" is selected, small compressive forces may arise.

If this is not justifiable, select the option "Failing members to be removed individually during successive iterations." However, you should pay attention to the sufficient maximum number of iterations (see above). If not, contact us via our free e-mail, chat, or forum support, or send us your question via the online form.

#### First Steps We provide hints and tips to help you get started with the main programs RFEM and RSTAB.

#### Wind Simulation & Wind Load Generation With the stand-alone program RWIND Simulation, wind flows around simple or complex structures can be simulated by means of a digital wind tunnel.

The generated wind loads acting on these objects can be imported to RFEM or RSTAB.

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