Surfaces

Surfaces describe the geometry of planar or curved structural components whose surface dimensions are significantly larger than their thicknesses. The stiffness of a surface results from its material and thickness. When generating the FE mesh, 2D elements are created for surfaces. For the calculation, these elements are assumed in the surface's centroidal axis.

To enter a surface, you can use existing boundary lines. If you describe the surface graphically using the options in the Navigator context menu or the toolbar buttons (see image Buttons for planar surfaces), the definition lines are automatically created after defining the surface parameters and clicking OK in the dialog.

The Basic tab manages elementary surface parameters. By ticking check boxes, further tabs are added where you can make the specific entries.

Stiffness Type

The stiffness type controls how internal forces can be absorbed or which properties are assumed for the surface. Various types are available for selection in the list.

Standard

The surface transfers moments and membrane forces. This approach describes the general behavior of a homogeneous and isotropic surface model. The stiffness properties of the surface are direction-independent.

Without Thickness

The surface has no stiffness. This type is to be used for the boundary surfaces of a solid.

Rigid

This stiffness type allows you to model very stiff surfaces in order to model a rigid connection between objects.

Membrane

The surface has a uniform stiffness in all directions. However, only membrane forces in tension (n_x, n_y) and membrane shear forces (n_xy) are transferred. In the case of compression, transverse forces, and moments, the affected surface elements fail.

Without Membrane Tension

Only moments and membrane forces in compression are transferred. For membrane forces that cause tension, the affected surface elements fail (example: bearing).

Load Transfer

This type allows surface loads to be applied to areas that are not filled with surfaces, such as wind loads on windows or the members of a hall. The load of this surface is distributed to the edges or the integrated objects. If member loads are generated, the load is converted into the global directions relative to the true member lengths (load directions X_L, Y_L, Z_L). The surface itself has no stiffness.

You can define the criteria for the load transfer in the Load Transfer tab.

The 'Load transfer direction' describes the direction(s) in which the load should be applied to the objects. The list offers options for an isotropic distribution based on an FEM calculation as well as for an orthotropic arrangement on surface strips, which are used to determine the load width in one or both local surface axes.

With the option 'Isotropic | FEM', RFEM uses a separate partial model to determine the load distribution, in which the surface is represented by a rigid surface element. All objects integrated into the surface (members, line and nodal supports, lines connected with model elements, couplings or nodes, etc.) are replaced by rigid line supports or rigid nodal supports. The responses of this partial model are then applied as loads for RFEM's 3D calculation. If certain objects should not transfer loads, you can specify them in the 'Without effect on' section.

For load transfer via surface strips, you can define how RFEM should perform the 'load distribution'. By default, the load is distributed to the adjacent objects with a variable distribution. However, if you want to achieve a constant load distribution, select the corresponding entry in the list. The difference between the two variants is contrasted in the following image.

The input options for the 'surface strip width', the 'smoothing factor', and the 'minimum number of strips on the surface' are accessible when the Advanced distribution settings check box is activated in the 'Options' section. Adjustments are only necessary for problematic load distributions. The effect of these parameters is explained using an example in the technical article Advanced Distribution Settings for Load Transfer Surfaces.

For the load transfer surface, you can also define a 'basis weight', for example to take into account the self-weight of glazing.

In the 'Without effect on' section, you can exclude members, lines, and nodes from the load transfer (for example, bracing). Define the objects individually or select a pattern object that lies parallel to the load-free members or lines.

When the boundary lines of the surface are defined, the loaded members, lines, and nodes are specified in the 'Loaded objects' section. If you want a specific load distribution, tick the Load distribution factor check box in the 'Basic' tab. You can then individually define the factors for the load-bearing objects in the Load Distribution Factors tab.

For load transfer via surface strips, you can take into account the 'member eccentricity' or the 'section distribution' in order to correctly capture the geometric position of a member or its course (see chapter Section). The 'Neglect rotational equilibrium' check box is not activated by default. This means that the moment from the surface loads is formed about the centroid and compared with the moment from the member loads about the centroid. For nodal loads, however, this option is irrelevant. The following image shows how a free line load is distributed to the opposite members with and without taking the rotational equilibrium into account.

deactivate Deactivate for calculation]] check box is automatically ticked and cannot be deactivated.

You can define the criteria of the stiffening surface in the Stiffening, Parameters, and Tolerances tabs.

Stiffening

Two stiffening types are available for selection, which differ fundamentally.

• Surface

The stiffening effect of trapezoidal sheeting is modeled by an orthotropic surface that is connected to the attached members by rigid couplings. The stiffness of the fasteners is taken into account by line hinges in the rigid couplings.

The 'stress direction' defines the orientation of the trapezoidal sheeting. It influences the orientation of the orthotropic surface and is taken into account when defining the orthotropy properties. The x and y axes refer to the local xyz axis system of the surface.

In the 'Members connected to surface' section, the members that are considered as cross beams and longitudinal beams of the trapezoidal sheeting are automatically entered. These are all members within the defined Tolerances in the plane of the stiffening surface and perpendicular or parallel to the stress direction of the trapezoidal sheeting.

The 'Without effect on' section offers the possibility to exclude certain members from the automatic assignment as connected members.

• Member support

The stiffening effect of trapezoidal sheeting, purlins, or bracing is captured by member supports on the members of the surface: For these connected members, corresponding member shear panels and rotational restraints are created.

The 'stiffening direction' defines the orientation of the trapezoidal sheeting or purlins. The x and y axes refer to the local xyz axis system of the surface.

In the 'Supported members' section, the members that bound the end panel of the stiffening surface, lie closest to a boundary of the stiffening surface, or are located in an interior panel of the surface are automatically entered. The Tolerances apply in the plane of the stiffening surface and for the orientation perpendicular to the stiffening direction.

The 'Without effect on' section offers the possibility to exclude certain members from the automatic assignment as connected members.

Parameters

The 'Parameters' tab is matched to the stiffening type: For a surface, you define the properties of the trapezoidal sheeting and fasteners; for a member support, you define the stabilizing components.

• Surface

In the 'Panel' category, define the trapezoidal sheeting. In the list, you can select an already defined profile or create a trapezoidal sheeting using the 'New cross-section' option. Use the library that is accessible with the button for this. From the parameters of the cross-section, the properties required for the orthotropic surface are automatically adopted. They are used for a thickness with the thickness type Shape Orthotropy and the orthotropy type 'trapezoidal sheeting'.

In the 'Longitudinal joints' category, define whether the trapezoidal sheets are arranged regularly or irregularly. A separate orthotropic surface is created for each area between two adjacent longitudinal joints. These surfaces are coupled at the longitudinal joints by line hinges. The stiffness of the longitudinal joint connection is taken into account by the spring stiffnesses defined in the line hinges, which result from the flexibility of the fasteners selected in the 'Longitudinal joint fastening' category. The positions of the longitudinal joints are automatically derived from the arrangement of the trapezoidal sheets.

The 'Eccentricity' describes the position of the trapezoidal sheeting relative to the connected cross beams. With the 'Top' option, for example, the trapezoidal sheeting is arranged so that its underside lies on the top side of the cross beams.

In the 'Panel - cross beam fastening' category, define the parameters for connecting the orthotropic surface to the cross beams. This is done via rigid couplings. The line hinge of the rigid couplings represents the stiffness of the connection including the influence of the profile deformation. Selected fasteners are available for fastening the trapezoidal sheeting to the cross beams.

For trapezoidal sheeting in negative position, you can define whether the sheeting is fastened at the bottom flange or the top flange. For trapezoidal sheeting in positive position, fastening always takes place at the bottom flange.

The connection of the orthotropic surface to the longitudinal beams is also made via rigid couplings with line hinges, which result from the stiffnesses of the fasteners. In the 'Panel - longitudinal beam fastening' category, some fasteners are available for selection.

In the 'Longitudinal joint fastening' category, specify the fasteners for the longitudinal joints. Here, too, certain types are available in the list.

• Member support

Select which components contribute to the stabilization in the calculation. In addition to the components 'profile sheeting', 'purlins', and 'bracing', the 'Manual stiffness' option is also available. Profile sheeting and purlins are mutually exclusive. Bracing, on the other hand, can be combined with both profile sheeting and purlins.

In the 'Dimensions' category, define the geometry. The shear panel length is derived by default from the geometry of the stiffening surface, but can also be adjusted manually; in both cases, the option 'Consider as shear panel' must be ticked in the 'Profile sheeting' category. The beam spacing is also automatically preset as the maximum distance between the members of the interior and end panels. Manual definition is also possible.

The 'Eccentricity' describes the position of the trapezoidal sheeting relative to the connected members. With the 'Top' option, for example, the trapezoidal sheeting is arranged so that its underside lies on the top side of the members.

In the 'Profile sheeting' category, you can define the parameters of the trapezoidal sheeting. If you tick the 'Consider as rotational restraint' option, the program creates a member support with the Φ_x nonlinearity rotational restraint about x for the members of the end and interior panels. With the 'Consider as shear panel' option, a member support is generated for the members of the end and interior panels with the nonlinearity shear panel in y or z. The direction results from the local member axis, which lies in the stiffening surface.

For a profile sheeting, you can select an already defined profile in the list for 'cross-section and material' or create a trapezoidal sheeting with the 'New cross-section' option. Use the library that is accessible with the button for this. Then define the further parameters such as fastening type, determination of the connection stiffness, member and sheeting stiffness, etc. If the 'Consider as shear panel' check box is activated, the shear panel stiffness according to DIN 18807 is also taken into account.

For purlin member supports, select the purlin profile in the 'Purlins' category and specify the purlin spacing. Taking into account further stiffness parameters, member supports with the Φ_x nonlinearity rotational restraint about x are created for the members of the end and interior panels.

In the 'Bracing' category, select the profile of the diagonals and posts. Specify the spacing of the posts and the number of braces. From these parameters, member supports with the nonlinearity shear panel in y or z are created for the members of the end and interior panels. The direction results from the local member axis, which lies in the stiffening surface.

Tolerances

In the 'Tolerances' tab, you define the permissible deviations according to which members are automatically assigned to the longitudinal and cross beams (stiffening type Surface) or to the end panel and interior panel (stiffening type Member support).

The 'Tolerances for members in plane' control within which deviation members are considered to lie in the stiffening surface. You can define this value absolutely as a distance or relatively by an angle. Similarly, you can define the 'Tolerances for members parallel/perpendicular to the x-axis of the surface'.

Result Surface

This stiffness type makes it possible to transfer stresses and forces of other objects into surface internal forces using an integration method. This allows you, for example, to determine the membrane and bending stresses of a solid, which are to be designed with different partial safety factors.

You can define further criteria for integrating results in the Result surface tab.

In the 'Integrate stresses and forces' section, select whether the results should be captured purely object-related or also geometrically within a range. In the 'Include objects' section, define the relevant surfaces and solids. Alternatively, select 'All' objects and then exclude certain elements in the 'Excluded from inclusive objects' section.

If the results of a certain range "below" and "above" the surface are to be integrated, you can define the relevant distances in the 'Parameters' section. They refer to the local z-axis perpendicular to the surface plane.

Geometry Type

The geometry type describes the formal concept of a surface. Various types are available for selection in the list.

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Quadrangle

This surface type describes a general four-sided surface in its basic form. Straight lines, arcs, polylines, and splines are possible as boundary lines. This allows curved surfaces to be modeled.

In the 'New Surface' dialog, define the boundary lines of the quadrangle surface. If the closed surface cannot be formed by four lines, more than four lines are also permitted. In the 'Quadrangle' tab, the four corner nodes are then specified. They control how the curved surface is spanned.

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Trimmed

When surfaces intersect, you can quickly create the intersection: Select the surfaces and open the context menu. Various options are available for selection.

With the option 'Create intersection', only the intersection line is generated. If you select one of the options 'Split by intersection', RFEM creates partial surfaces and assigns them the type 'Trimmed'. You can then delete components, for example if you want to remove protruding surfaces.

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Spline with minimum curvature

With this geometry type, you can create a curved surface using control nodes that lie on or outside the surface. This allows, for example, terrain surfaces to be modeled.

Define the 'coordinate system' of the reference plane and specify the 'sample coordinates in the coordinate system'. These points represent the control nodes of the spline surface. Then define the 'boundary lines of the reference plane' or select the lines graphically using the button .

Thickness with Material

In the list of existing thicknesses, select the suitable type or define a new thickness (see chapter Thicknesses).

Material of Thickness

The material of the thickness defined in the section above is preset. If necessary, you can select another material in the list of already created materials or define a new one (see chapter Materials). This material is then assigned to the thickness type.

loadTransferTab Load Transfer]], it is possible to define distribution factors for the load-bearing objects. If you tick the check box, you can assign these factors individually in a new tab.

The loaded objects of the load transfer surface are preset in one row. Each object is assigned the factor 1.00, so that all objects contribute equally to the load transfer. If you want a specific distribution, click on the next free row and select the line or member. Then assign the appropriate 'distribution factor'.

Mesh Refinement

The mesh size of the FE mesh can be adapted to the geometry of the surface (see chapter Surface Mesh Refinements). It is thus independent of the general mesh settings. In the 'Mesh refinement' tab, you can select or newly define the surface mesh refinement.

Specific Axes

Each surface has a local coordinate system. As a rule, it is aligned parallel to the global axes. However, the coordinate system can also be defined by the user - separately for input and output.

Input Axes

The orientation of the input axes is relevant, for example, for orthotropy and support properties or for the effect of a surface load.

The list in the 'Category' section offers various options for adjusting the axes orientation:

• Angle rotation: rotation of the xy surface axes about the z-axis by the angle α
• Axis parallel to lines: orientation of the x- or y-axis along a line
• Axis directed to point: orientation of the x- or y-axis towards the intersection of a line with the surface
• Axis parallel to coordinate system: orientation of the axes to a user-defined coordinate system

You can define the reference objects graphically via the button .

The 'Reverse local z-axis' check box allows you to align the z and y axes in opposite directions.

Result Axes

Currently, the orientation of the result axes is only possible 'Identical to the input axes'.

Grid for Results

Each surface is covered with a grid that is used for the result output in the tables. It enables an output independent of the FE mesh in regular, adjustable result points.

By default, a Cartesian surface grid with a uniform grid point spacing of 0.5 m in both directions is preset. If necessary, you can adjust the 'grid spacings' in the x-direction (b) and in the y-direction (h) here, perform a 'grid rotation', or change the 'grid origin'. For circular surfaces, the 'Polar' grid type offers an alternative for the numerical result output.

If the 'Adjust automatically' check box is ticked in the 'Options' section, the grid points are adapted to the new geometry when the surface changes.

In the 'Points' section, you can check the coordinates of the generated grid points. Changes in the table are not possible.

Integrated Objects

RFEM usually recognizes automatically all objects that lie in the surface but were not used for the surface definition.

The numbers of the nodes, lines, and openings assigned to the surface are specified in the 'Integrated objects in surface' section.

If an object is not recognized, you should integrate it manually: Deactivate the Automatic object detection. The input fields in the 'Integrated objects in surface' section are now accessible. Add the missing object number or use the button to determine the object graphically.

Activate Load Transfer

The check box allows the loading of the surface - regardless of its stiffness type - to be distributed by means of a load transfer surface. This way, the surface acts in the model through its stiffness. The distribution of the load to the adjacent objects, on the other hand, is controlled by the parameters that you can define in the Load Transfer tab. This function is primarily relevant for surfaces of the thickness type beam panel.

Deactivate for Calculation

The check box offers the possibility not to consider the surface in the calculation, for example to simulate construction stages or to examine a modeling variant. The stiffness, boundary conditions, and loads of the surface are not applied in this case.

Information | Analytical

This section is displayed as soon as you have defined the boundary lines of the surface. It provides an overview of important properties of the surface such as surface area, volume, and mass as well as the position of the surface centroid and the orientation of the surface. Openings are taken into account accordingly.

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