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Frequently Asked Questions (FAQ)
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AnswerThe deformations of load cases and load combinations always refer to the deformed form-finding and thus to the shifted FE mesh. In this case, the result is stored in the "DL_LK_DEFORMATION u" layers. The "DL_FE_MESH" layer shows the result of the form-finding. The initial structure is located in the "DL_TOPOLOGY" layer.If the DXF file was generated when the "Form-finding" load case is active (see Figure 02), that is, not via a load case or a load combination, the deformation for the initial structure can be found in the "DL_FE_MESH" layer. The deformations in the "DL_LK_DEFORMATION u" layers are of no interest in this case and should be hidden.Basically, the display factor should be set to "1" when exporting the deformed structure (see Figure 03). Defining a larger or smaller factor also affects the result in the DXF file.
AnswerWith the FE mesh refinement, it is also possible to create an aligned FE mesh in the program. Thus, the automatic FE mesh generator can be controlled to a certain extent. However, it is not possible to set a specified mesh geometry with it.
AnswerThe load from the form-finding load case (FF) is impressed 1.0-fold into the system. If you want to factor in the self-load in combination with combinatorics, you can additionally create a permanent load case for the self-load with a load of 0.35 times the load.
AnswerThe cause of such an effect could be free edges, which can deform almost infinitely during Form-Finding. A state of equilibrium is difficult to achieve.As a basis for Form-Finding, there should be no free edges on the membrane surface. Ropes must be arranged on the free edges of the membrane whose stiffness can be based, for example, on the folded or reinforced edge strips of the membrane.
For form-finding, it is primarily important to apply a prestress for the membrane and any provided edge cables. The restraints do not have to be prestressed.
The bracing can be installed as tension members with their initial length (see the figure). As a result of the prestressed membrane and edge cables, the load for bracing is obtained during the calculation.
The result of the Form-Finding can be exported to a DXF file.There are several options for this.For example, you can use the "Direct Export" in a DXF file.Please note that AutoCAD must be started as an application in the background.Alternatively, you can use the export function by using the File menu.It is important that the 3D face elements are exported.Unfortunately, there is currently no direct interface with Autodesk Revit.However, a webinar about membrane structures showed a possible workflow for integrating the export into a Revit model. See the following link.
AnswerThe geometrically nonlinear flattening process takes over the real mesh geometry of the planar, buckling, single curved or double curved surface components from the selected cutting patterns and planarizes these planar components by minimizing the distortion energy assuming a defined material behavior.The iterative calculation used for this is controlled by the parameters in the menu "Calculation Parameters"/"Cutting Patterns" tab.The "Max Number of Iterations" parameter is limiting the scope of the calculation and stops the process when the set maximum iteration is reached. Provided the convergence criterion does not depend on the "Tolerance for Convergence Criterion" parameter in the Convergence range when the maximum iteration has been reached, the program displays error message 10154.If no error message is output by the program, it is reasonable to assume a proper convergence.The error can usually be solved 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 the form-finding, the elastic supporting elements on the membrane react with deformations in the direction of the acting membrane tension stress. The integral process passes the prestressed model for the subsequent calculation if the forces of all deformed elements are in equilibrium with the membrane geometry below the specified prestress.Figure 01 - Form-finding with deformed supporting structureHowever, the support structure's non-linearity "Only form-finding -Condition" (also called form-finding support) at the membrane edges can be suppressed for pure form-finding.In this case, the form-finding process results in a shape whose prestress is in equilibrium with the set form-finding support forces and remaining boundary reactions.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 maintain the global equilibrium on the entire model.Due to the removal of supports, the membrane responds in the subsequent analyzes (LC and CO) with the semi-rigid supporting structure. Without additional loading, this reaction is similar to relaxation with a concomitant reduction in prestress
AnswerThe form-finding warps the surfaces into each other because the defined prestress on the boundary line between the two membranes does not coincide at the same angle and consequently the affected FE nodes are not in equilibrium with the boundary reactions. In this case, the FE nodes of the membrane surfaces move in the direction of the remaining resultant until the form-finding process finds a solution within the tolerance limit value. Since this solution is usually only achievable with a large displacement of the 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, the prestress must be adjusted to reach a balanced position or an element must be arranged at the limit situation to transfer forces from the imbalance.
The perimeter of membrane cuttings is described by boundary lines. These boundary lines can be as follows:
Figure 01 - Border Lines of Cuttings
- globally defined boundary lines of the assigned membrane surfaces or
- retroactively introduced cutting lines for dividing the cuttings on the membrane surfaces
The global boundary lines are untouchable due to their fixed geometry description (arc, circle, spline, etc.) and are thus also implemented in the design of the plane cuttings.On the other hand, subsequently introduced 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 boundary lines and cutting lines take over the FE Mesh of the assigned surfaces for flattening. Since the cutting lines run over the FE elements themselves, independently of the global mesh in the boundary zone, the edges of the original FE elements can not be used to describe the cutting edge definition. 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 split in the middle or almost on the edge. Since the FE elements on the edge can cause some problems with geometry, a certain tolerance limit has been entered for the decision. This checkbox controls the critical length ratio between the FE edge length specified by the cutting line and the original edge length of the FE. If the ratio is smaller than the given restrictive value, the cutting line refers to the original FE node.Figure 03 - SmoothingThis fact may lead to an "Irritation" of the cutting line when the cutting lines are passing close to the FE element edges. This situation can be optimized by reducing the given limit of tolerance.
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