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Frequently Asked Questions (FAQ)
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The effectiveness of a plate structure subjected to compression depends on its shape. For each load distribution, there is an associated optimal shape that transfers the loads to the foundations using as little material as possible.
Figure 01 - Form-Finding of Compression-Loaded Plate Structure
Since the use of a structural system usually specifies several load states, it is recommended to align the optimal shape with the main load (permanent load) of the model for purely strategic reasons.
The RFEM program helps you to find these model shapes in conjunction with the RF-FORM-FINDING add-on module. The underlying form-finding process shifts the position of the FE mesh nodes in space based on a specified load so that the new position of the elements transfers the load mainly via the membrane's load-bearing capacity. The explicit use of the normal force resistance of members and the membrane force resistance of the surface elements results in constant stress states in the cross-sections. This stress-free state makes optimal use of the cross-sections and allows for a very effective structure with minimum material input.
This process can be applied both for the usual cable and membrane structures consisting of flexible elements under tension as well as for pressure-loaded rigid beams and shell structures.
To develop an optimal pressure plate shape in RFEM, proceed as follows:
- Activation of the RF-FORM-FINDING module in the 'General Data' of the 'Options' tab
Figure 02 - Activation of Form-Finding Options
- Specification of form-finding settings for the structural elements
Figure 03 - Form-Finding Setting for Beam
Figure 04 - Form-Finding Setting for Surface Element
- Defining the main loading for the shaping
Figure 05 - Defining External Form-Finding Loading
- Form-finding analysis
- Activation of the RF-FORM-FINDING module in the 'General Data' of the 'Options' tab
The directions of the warp and weft are linked to the axes of the surface. With the default setting, only an isotropic prestress can be applied. If the axes are aligned, orthotropic or radial prestress will be available as well.
The procedure is demonstrated in the video.
AnswerThe sag can be influenced by the prestress of the membrane. Please note that a uniform increase in both directions does not cause any changes in the shape. Thus, a modification must only be made in the desired direction. The video shows the procedure.
AnswerDeformation of members are displayed as single colored lines per default. Please let them displayed as colored cross-sections. Therefore, it is always a good idea to decrease the Display factor by the deformation to 1.0, otherwise, the results might be misinterpreted.
AnswerSuch 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.
AnswerThe 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.
Generally, the form-finding process in RFEM always considers all model data. In form-finding, the elastic supporting elements on the membrane react with the deformations in the direction of the membrane tensile stress acting. The integral process gives the prestressed model to the subsequent calculation if the forces of all deformed elements are in equilibrium with the membrane geometry subjected to the specified prestress.Figure 01 - Form-Finding with Deformed Supporting StructureHowever, the flexibility of the supporting structure can be suppressed for the pure form-finding by defining the support with the "Form-finding stage only" nonlinearity (also called form-finding support) at the membrane edges.In this case, the form-finding process results in a shape whose prestress is in equilibrium with the specified form-finding support forces and the remaining boundary conditions.For the structural analysis of all other load cases and combinations, the form-finding supports are deactivated, and the form-finding support forces are applied as external loads to keep the global equilibrium on the entire model.Due to the removal of the supports, the membrane reacts with the semi-rigid supporting structure in the subsequent analyses (LC and CO). Without additional loading, this reaction is similar to the relaxation with the associated reduction of prestress.
AnswerThe form-finding warps the surfaces into each other because the defined prestress on the boundary line between two membranes does not coincide at the same angle, and thus the affected FE nodes are not in equilibrium with the boundary conditions. In this case, the FE nodes of the membrane surfaces move in the direction of the remaining resultants until the form-finding process finds a solution within the tolerance limit. Since it is usually possible to only achieve this solution with large displacement of FE nodes in space, the surfaces assigned to the FE nodes seem to "swim" into each other.This behavior results in an unfavorable FE mesh arrangement in conjunction with a singular membrane force distribution. In this case, it is necessary to adjust the prestress in order to reach the equilibrium, or to set an element at the limit situation in order to transfer the forces due to the disequilibrium.
The perimeter of membrane cutting patterns is described by boundary lines. These boundary lines can be as follows:
Figure 01 - Boundary Lines of Cutting Patterns
- Globally defined boundary lines of the assigned membrane surfaces, or
- Cutting lines applied subsequently for the distribution of cutting patterns on the membrane surfaces
The global boundary lines are untouchable due to their fixed geometry description (arc, circle, spline, and so on), and are also implemented in the design of planar cutting patterns in this way.
On the other hand, the subsequently applied cutting lines are based on the FE mesh of the surfaces assigned in the cutting line specification and have no influence on the mesh itself.The cutting units encircled by the boundary lines and cutting lines take over the FE mesh of the assigned surfaces for flattening. Since the cutting lines run through the FE elements themselves, with no regard to the global mesh in the edge area, the edges of the original FE elements cannot be used to describe the cutting pattern limitation. In this case, the affected FE elements within the cutting line area are divided by the cutting lines.
Depending on the orientation of the cutting line, the FE elements are cut in the middle or almost on the edge. Since the FE elements on the edge can cause some problems with the geometry, a certain tolerance limit has been entered for the decision. This limit controls the critical length ratio between the FE edge length specified by the cutting line and the original FE edge length. If the ratio is smaller than the given limiting value, the cutting line refers to the original FE node.Figure 03 - SmoothingThis fact may lead to an "Irritation" of the cutting line if the cutting lines are close to the FE element edges. This situation can be optimized by reducing the given tolerance limit.
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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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