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Frequently Asked Questions (FAQ)
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AnswerIt is quite likely that the high deformations are caused by the consideration of shrinkage and the horizontal storage in the model.The shrinkage is taken into account internally on the load side as elongation, in which connection a failure due to the prevention of shrinkage is also possible. If the shrinkage is prevented by a non-displaceable horizontal bearing, forces are created which can lead to failure of the concrete and thus to a significant increase in deformation or even instability of the model.In this context, it is important that when using the nonlinear deformation calculation, the boundary conditions of the model are mapped as realistically as possible.
Basically, you should pay attention to the following points:
The structure in RF‑STAGES and RFEM may differ due to the definition in RF‑STAGES. Therefore, the structure in RF-STAGES may be different than in RFEM. In order to find the instability at a certain construction stage, it is necessary to model the structure in this construction stage in RFEM and take it into account separately. In this context, it should also be noted that the entries are not synchronized between RFEM and RF-STAGES. For example, a member end release removed in RFEM is not automatically removed in the RF-STAGES model.
Method of Analysis
RF-STAGES calculates permanent load cases according to the large deformation analysis. As a result of this analysis, instabilities may occur which are not present in a load case when calculating according to the linear static analysis (critical load problems), for example.
Special Structural Elements
Some of the structural elements available in RFEM are not supported in RF‑STAGES. These structural elements can also cause the instability in certain cases. The following structural elements are not fully supported in RF‑STAGES:
- Line hinges
- Member elastic foundations
- Sets of members
- Nodal releases
- Line releases
- Surface releases
- Nodal constraints
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 behaviour 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.
The member end rests geometrically exactly on the pipe surface, but the automatic integration of structural elements is only possible for plane surfaces. Since the end point of the member was not integrated into the pipe surface, no common FE node is created. For RFEM, the member has no connection to the pipe, thus resulting in a termination of the calculation.
You can manually integrate the member end node into the pipe surface: double-click the pipe surface to open the 'Edit Surface' dialog box. In the 'Integrated' tab, you can integrate the node as long as the 'Automatic object detection' option is deactivated (Figure 01).
Due to the integration, the member is connected to the pipe surface. A common FE node is created and the calculation is performed successfully (Figure 02).
The calculation can be terminated due to an unstable structural system for various reasons. There can be a 'real' instability due to overloading the system, but the instability effects may also be caused by failing members.
In the calulcation parameters, you can deactivate the nonlinearity 'Members due to member type' (see Figure 01). If the calculation is then possible without the error message, the problem is probably caused by failing members.
The option 'Failing members to be removed individually during successive iterations' in the Global Calculation Parameters dialog box (see Figure 02) allows you to prevent the complete failure of tension members. This will help in most cases. For this, the number of possible iterations should be sufficiently large.
An alternative method is to apply a prestress to the tension members in order to prevent the failure of them.
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