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Frequently Asked Questions (FAQ)
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AnswerThe shrinkage and creeping parameters can be found in Table 1.3 in the last column of the table Creep Coefficient / Shrinkage Strain.
No, the "Isotropic Nonlinear Elastic 1D" material model is not suitable for a bending beam because the nonlinear stress distribution over the height of the cross-section cannot be modeled here. The reason for this is that there are no stress points/FE mesh points over the height of the cross-section. Thus, it is not possible to simulate the cross-section cracking.
On the other hand, the "Isotropic Nonlinear Elastic 1D" material model would be suitable for a cracking of the entire cross-section subjected to a pure axial force loading, but not for bending and compression.
For the simulation of a cross-section subjected to bending in the cracked state, it is recommended to perform a nonlinear analysis with RF‑CONCRETE Members and the RF‑CONCRETE NL module extension. Creep and shrinkage can be considered by using the module extension in RF‑CONCRETE Members.
After the calculation, the nonlinear stiffness of the cross-section can be imported back into RFEM (see Image 01) and the internal forces can be determined again, taking into account the cracked concrete cross-section.
You can find more details about this procedure under the following links:
Warning No. 1136 ("During the calculation of material nonlinearity, the material with a decreasing branch of the diagram can be calculated with one load increment only.") refers to the entries in the global calculation parameters.
In our example file, the number of load increments has been set globally to 10 for load cases and load combinations:
For this material, one load increment can only be expected. If you adjust the number of load steps for load cases and load combinations globally to 1, you can define your material:
While Member 1 is an upstand beam, Member 2 is a downstand beam. This results in a compression axial force for Member 1, and a tensile force for Member 2.
For the concrete design, a compressed cross-section is more favorable than a tensioned cross-section. For comparison purposes, here are the axial forces of the members:
Now, if you deactivate the axial forces for the design in RF‑CONCRETE Members, the result is a required reinforcement that is affine to the moment distribution:
With this setting, you are on the safe side for Member 1, but on the unsafe side for Member 2.
The deformation uz,local under the "Serviceability Limit State Design" category only refers to the simplified analytical method. However, in the case of a nonlinear calculation, you can display the deformations in the "Nonlinear Analysis" category for the serviceability limit state by using the "Nodal Displacement ug" entry.
Since the boundary surfaces of contact solids can only be of the Null Surface type, it is necessary to model additional surfaces with a section. The following is an example for this.
There is a reinforcement of a plate with an additional plate. Both plates are connected by means of a contact solid:
In order to simulate a weld, borders are required on both plates:
You can simply create the bottom border by inserting a new rectangular surface. In this case, the program asks whether the surface should be integrated into the base surface, which must be confirmed by clicking Yes. There are then two openings in the bottom surface; one in the outermost surface where lies the surface for the border, and another in the surface for the border where lies the bottom surface of the contact solid:
For the upper border, it is also necessary to create a new surface, and the opening to the upper surface of the contact solid must be inserted manually:
After creating the borders, you can insert the actual surfaces for the weld:
If the reinforcing plate is subjected to tension now, the modeled weld transfers the tension stresses:
The best way is to create the bolt as a single solid and not from several partial solids (within the hole). Then, you can arrange the surface release between the contact surfaces of the solids. In the case of a bolt connection, the setting for the surface release type would have to be defined with a nonlinearity in the uz direction.
As an alternative, you can also enter releases and spring stiffnesses for the other directions.
The corresponding example can be downloaded here.
Basically, there are six methods available for solving the nonlinear algebraic system of equations. If your model does not include any nonlinearities (that is, you have created a purely linear system), the access of this selection field is blocked.
As soon as you define a nonlinearity in the model, the selection field for the solving methods is activated, and you can select the desired solving method.
In the calculation parameters of RFEM or RSTAB, there are the "Number of load increments for load cases/load combinations" text boxes under the "Global Calculation Parameters" tab. These two entries control the numerical incremental application of the defined load boundary conditions in the respective load cases and load combinations. The reciprocal value of the entry describes a fraction of the load. The solving process then applies the defined load fractions successively to the model in so-called load increments until the complete load is reached. In the respective load increments, the equation solver tries to find an equilibrium within the maximum allowed iterations, and thus to specify suitable start values for the next load increment.
It is possible to imagine that the solving process collects the complete load of a load case or a load combination in a "watering can" and pours it onto the load-collecting model in portions. In this case, the number of load increments correlates with the speed of the load application. The speed is not to be understood as a real time parameter, but purely numerically.
The incremental load application has only an effect in the case of nonlinear structural systems. It usually provides a correspondingly higher result quality with increasing number of load increments. The basic aim of this method is to find a micro convergence in the respective load increments to specify new high-quality start values for the next load increment, and thus finally to achieve a macro convergence for the entire load case.
Materials are required to define surfaces, cross-sections, and solids. The material properties affect the stiffnesses of these objects.
There are 13 material models available if you have a license for the RF‑MAT NL add-on module.
In the case of the abundance of material models, it is necessary to make sure that you assign the corresponding material model to the members and their surfaces/solids.
In the example shown here, surfaces have been generated from a member for a detailed analysis. There is still an unused cross-section defined (marked in blue) and the material is entered for the member cross-section as well as for the surfaces. When editing an existing material to Isotropic Nonlinear Elastic 2D/3D , the 2D/3D material model is also defined for the created member cross-section, which leads to the error message.
When working with members and surfaces / solids, it is recommended to create more than one material.
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Wind Simulation & Wind Load Generation
With the stand -alone program RWIND Simulation, you can simulate wind flows around simple or complex structures 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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