Surfaces

Surfaces describe the geometry of planar or curved structural components whose surface dimensions are significantly greater than the thicknesses. The stiffness of a surface results from its material and thickness. When generating the FE mesh, 2D elements are created on surfaces. They are applied in the centroidal axis of the surface for calculations.

For entering surfaces, you can use any existing Boundary Lines. If you graphically define the surface using the options in the Navigator shortcut menu or the toolbar buttons (see the image Buttons for Planar Surfaces), the definition lines are automatically generated after you set the surface parameters and click OK in the dialog box.

The Main tab manages the basic surface parameters. By activating the check boxes, further tabs are added where you can enter specific information.

Stiffness Type

The stiffness type controls the way internal forces and moments can be absorbed, or which properties are required for the surface. You can select from various types 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 do not depend on directions.

Without Thickness

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

Rigid

This type of stiffness 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 the tension state (n_x, n_y) and membrane shear forces (n_xy) are transferred. The affected surface elements fail under compression and shear forces and moments.

Without Membrane Tension

Only moments and membrane forces under compression are transferred. For membrane forces causing tension, however, a failure of the affected surface elements occurs (example: hole bearing).

Load Transfer

With this type of stiffness, any surface load can be applied to areas that are not filled by surfaces, such as wind loads on windows or hall members. The load of this surface is distributed to the edges or the integrated objects. If member loads are generated, the load is converted in the global directions in relation 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 provides options for an isotropic distribution based on FEA calculations and for an orthotropic arrangement on surface stripes that are applied to one or both local surface axes for determining the load application width.

In the case of the "Isotropic | FEA" option, RFEM uses a separate submodel to determine the load distribution where the surface is represented by a rigid surface element. All objects integrated into the surface (members, line and nodal supports, lines, couplings, or nodes connected with model elements, and so on) are replaced by rigid lines or rigid nodal supports. The reactions of this partial model are then applied as loads for the 3D calculation of RFEM. If certain objects should not transfer any loads, you can specify them in the "Remove influence from" section of the dialog box.

When transferring loads by surface stripes, you can define how the "Load distribution" is carried out by RFEM. By default, the load is distributed with a variable distribution on the adjacent objects. However, if you want to achieve a uniform load distribution, select the corresponding entry in the list. The difference between the two options is shown in the following image.

The input options for the "Stripe width", the "Smoothing factor", and the "Minimum Number of Stripes on Surface" are accessible if the Advanced distribution settings check box is selected in the "Options" section. Adjustments are only necessary for problematic load distributions. The effect of these parameters is explained in the Knowledge Base article Advanced Distribution Settings for Load Transfer Surfaces using an example.

For the load transfer surface, you can also "Set surface weight" to consider the self-weight load of a glazing, for example.

In the "Remove Influence from" section, you can exclude members, lines, and nodes from the load transfer (for example, bracings). Define the objects individually, or select a template object that is parallel to the load-free members or lines.

When the boundary lines of the surface are defined, the stressed members, lines, and nodes are displayed in the "Loaded Objects" section. If you want to set a specific load distribution, select the Load distribution factor check box in the "Main" tab. Then, you can individually define the coefficients for the load-bearing objects in the Load Distribution Factors tab.

When transferring loads via surface strips, you can take into account the "Member Eccentricity" or the "Section Distribution" in order to correctly determine the geometric position of a member or its distribution (see the chapter Section). The "Neglect equilibrium of moments" check box is deactivated by default. It is used to form the moment from the area loads to the centroid and compare it with the moment from the member loads to the centroid. However, this option is irrelevant for nodal loads. The following image shows how a free line load is distributed to the opposite members with and without considering the equilibrium of moments.

Bracing

This stiffness type allows you to easily incorporate the stabilizing effects of trapezoidal sheeting, purlins, and bracing into your model realistically.

A bracing surface is not treated as an object that contributes to stiffness. Rather, it serves as the geometric basis for defining the members and objects located within the bracing surface and for determining their stiffening effect. Therefore, for a bracing surface, the Deactivate for Calculation check box is automatically selected and cannot be deselected.

You can define the criteria for the bracing surface in the Bracing, Parameters, and Tolerances tabs.

Bracing

There are two types of bracing you can select from, which differ fundamentally.

• Surface

The stiffening effect of trapezoidal sheeting is modeled using an orthotropic surface that is connected to the adjacent members via rigid links. The stiffness of the fasteners is taken into account via line hinges in the rigid links.

Use the “Span direction” to specify the orientation of the trapezoidal sheeting. It affects the alignment of the orthotropic surface and is taken into account when defining the orthotropic 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 secondary beams and longitudinal beams of the trapezoidal sheeting are automatically entered. These are all members within the defined tolerances in the plane of the bracing surface and perpendicular or parallel to the direction of tension of the trapezoidal sheeting.

The “Remove influence from” section provides the option to exclude certain members from being automatically assigned as connected members.

• Member Support

The stiffening effect of trapezoidal sheeting, purlins, or bracing is determined by member supports connected to the members of the surface: Corresponding member shear panels and rotational restraints are generated for these connected members.

Use the “Bracing direction” to define 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 limit the exterior span of the bracing surface, are closest to a limitation of the bracing surface, or are located within an inner span of the surface are automatically entered. The tolerances apply in the plane of the bracing surface and for the orientation perpendicular to the bracing direction.

The “Remove influence from” section provides the option to exclude specific members from being automatically assigned as connected members.

Parameters

The “Parameters” tab is aligned with the bracing type: For a surface, you define the properties of the trapezoidal sheeting and the fasteners; for a member support, you specify the stabilizing structural components.

• Surface

Specify the trapezoidal sheeting in the ‘‘Panel’’ category. In the list, you can select a predefined cross-section or create a trapezoidal sheeting using the ‘‘New Cross-Section’’ option. To do this, use the library accessible via the button. The properties required for the orthotropic surface are automatically derived from the cross-section parameters. They are applied to a thickness with the Shape Orthotropy thickness type and the “Trapezoidal Sheeting” orthotropy type.

In the “Side lap joints” category, specify whether the trapezoidal sheets are arranged regularly or irregularly. A separate orthotropic surface is generated for each region between two adjacent side lap joints. These surfaces are coupled at the side lap joints via line hinges. The stiffness of the side lap joint is taken into account using the spring stiffnesses defined in the line hinges, which result from the flexibility of the fasteners selected in the “Seam fastener” category. The positions of the side lap joints are automatically derived from the arrangement of the trapezoidal sheeting.

The “Eccentricity” describes the location of the trapezoidal sheeting relative to the connected secondary beams. With the “Top” option, for example, the trapezoidal sheeting is arranged so that its bottom side lies on top of the secondary beams.

In the “Panel – cross member fastener” category, you define the parameters for connecting the orthotropic surface to the secondary beams. This is achieved using rigid links. The line hinge of the rigid links models the stiffness of the connection, including the effect of cross-section deformation. Selected fasteners are available for securing the trapezoidal sheeting to the secondary beams.

For trapezoidal sheeting in a negative location, you can specify whether the sheeting is fastened to the bottom chord or the top chord. For trapezoidal sheeting in a positive location, fastening always occurs at the bottom chord.

The connection of the orthotropic surface to the longitudinal beams is also achieved via rigid links with line hinges, which result from the stiffnesses of the fasteners. In the “Panel – longitudinal member fastener” category, several fasteners are available for selection.

In the “Seam fastener” category, specify the fasteners for the longitudinal joints. Here, too, specific types are available in the list.

• Member Support

Select which structural components contribute to stabilization in the analysis. In addition to the components “Sheeting,” “Purlins,” and “Bracing,” the “Manual Stiffness” option is also available. Sheeting and purlins are mutually exclusive. Bracing, on the other hand, can be combined with both sheeting and purlins.

Define the geometry in the “Dimensions” category. The shear panel length is derived by default from the geometry of the bracing surface but can also be adjusted manually; in both cases, it is necessary to check the “Consider as shear panel” option in the “Sheeting” category. The beam spacing is also adapted automatically as the maximum distance between the members of the inner and exterior spans. Manual definition is also possible.

The “Eccentricity” describes the location of the trapezoidal sheeting with respect to the connected members. With the “Top” option, for example, the trapezoidal sheeting is arranged so that its bottom side lies on top of the members.

In the “Sheeting” category, you can specify the parameters of the trapezoidal sheeting. If you check the “Consider as rotational restraint” option, the program generates a member support for each member in the end and inner spans with the Φ_x nonlinearity Restraint around x. If you select the “Consider as shear panel” option, the program generates a member support for each member in the end and inner span panels with the nonlinearity shear panel in the y or z direction. The direction is determined by the local member axis, which lies within the bracing surface.

For a sheeting, you can select a predefined cross-section from the “Cross-Section and Material” list or create trapezoidal sheeting using the “New Cross-Section” option. To do this, use the library accessible via the button. Then specify the additional parameters, such as fastening arrangement, determination of connection stiffness, member and sheet stiffness, and so on. If you select the “Consider as shear panel” check box, the shear panel stiffness is also taken into account in accordance with DIN 18807.

For purlin-member supports, select the purlin cross-section in the “Purlins” category and specify the purlin spacing. Taking additional stiffness parameters into account, member supports with the Φ_x nonlinearity restraint about x are generated for the members of the end and inner spans.

In the “Bracing” category, select the cross-sections of the diagonals and posts. Specify the spacing of the posts and the number of bracing elements. Based on these parameters, member supports with the shear panel nonlinearity in y or z are generated for the members of the end and inner spans. The direction is determined by the local member axis, which lies within the bracing surface.

Tolerances

In the “Tolerances” tab, you define the allowable deviations according to which members are automatically assigned to the longitudinal beams (Surface bracing type) or to the exterior span and inner span (Member Support bracing type).

The “Tolerances for Members in Plane” control the deviation within which members are considered to lie within the bracing surface. You can specify this value absolutely as a distance or relatively as an angle. Similarly, you can define the “Tolerances for Members Parallel/Perpendicular to the x-Axis of the Surface.”

• Show and Hide Objects Generated from Bracing Surfaces

The program uses the parameters to generate orthotropic surfaces, rigid links, and line hinges with the corresponding geometric and stiffness properties. Use the button in the toolbar to control whether these generated objects are displayed in the model. Alternatively, in the Navigator – Display, use the Model → Surfaces → Bracing check box.

When rendering the full model, the selected trapezoidal sheeting is displayed. This allows you to visually check the stress direction. The stress direction is also indicated by a red direction arrow.

• Convert Generated Objects from Bracing Surfaces to Standard Objects

The generated objects from bracing surfaces—orthotropic surfaces, rigid links, line hinges—are marked in purple text in the Navigator – Data and cannot be edited. If you want to perform editing on the objects, the program provides an option to convert them into “real” surfaces. Proceed as follows:

1. Double-click a generated surface or a generated line hinge in the Navigator – Data.
2. In the Object dialog box, click the button at the bottom.

This function converts all generated objects of the bracing surface into standard orthotropic surfaces and, if applicable, standard joints. The generated state is thereby dissolved, and the generated objects are lost as such. A corresponding note therefore appears before the function is executed.

If you have used the function by mistake, you can restore the original state using the Undo button . Or you can close the file without saving.

Result Surface

This stiffness type allows for the conversion of stresses and forces from other objects into surface internal forces using an integration method. This enables you, for example, to determine the membrane and bending stresses in a solid—which should be designed using different partial safety factors.

You can define additional criteria for integrating results in the Result Surface tab.

In the “Integrate stresses and forces” section, select whether the results should be captured purely on an object-by-object basis or also geometrically within a region. In the “Include objects” section, specify the relevant surfaces and solid bodies. As an alternative, select “All” objects and then exclude specific elements in the “Excluded from included objects” section.

If the results for a specific region “below” and “above” the surface are to be integrated, you can define the relevant distances in the “Parameters” section. These distances are related 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.

Plane

In the case of a planar surface, all boundary lines lie in one plane. Use the list button on the toolbar to access various shapes of planar surfaces.

You can define the surface graphically (after clicking OK in the dialog box) by drawing a rectangle, circle, and so on. If you select the "Select Boundary" option, RFEM will automatically recognize the surface as soon as a sufficient number of boundary lines is defined.

Quadrangle

In its basic shape, this surface type describes a general quadrilateral surface. You can use straight lines, arcs, polylines, and splines serving as boundary lines. This allows for modeling curved surfaces.

Define the boundary lines of the quadrangle surface in the "New Surface" dialog box. If a closed surface cannot be formed by four lines, more than four lines are also allowed. Then, in the "Quadrangle" tab, four corner nodes have to be specified. They control the way the curved surface is spanned.

NURBS

NURBS surfaces are created by four closed NURBS lines (see the chapter Lines). Almost any freeform surfaces can be modeled in this way.

Define the boundary lines of the NURBS surface in the "New Surface" dialog box. NURBS lines forming opposite pairs must have the same number of control points so that the order of these NURBS lines is "compatible". In the "NURBS" tab, you can then specify the shape of the surface by means of "Control Point Weights". The coordinates of the selected control point have to be entered in the "Coordinates – Control Point" section.

Trimmed

When surfaces intersect, you can create the corresponding intersection quickly: Select the surfaces, then open the shortcut menu. Different options are available for selection.

If you select the "Create Intersection" option, only the intersection line is generated. If you select one of the "Divide by Intersection" options, RFEM creates partial surfaces and assigns the "Trimmed" type to them. Next, you can delete components if you want to remove, for example, overhanging surfaces.

Rotated

A rotated surface is generated when an existing line is rotated about an axis. RFEM creates the surface from the start and end nodes as well as the rotated definition points of the line. New lines are generated in this process.

In the "Rotated" tab, define the boundary line of the surface to be rotated. Enter the rotation angle α. You can define the points of the rotation axis using the coordinates or graphically using the button.

Pipe

A pipe surface is generated when the center line of the pipe is rotated by a certain radius about the axis. New lines are generated in this process: two circles and a polyline that is parallel to the pipe axis.

Define the radius of the pipe in the "Pipe" tab. This value describes the distance from the pipe axis to the center of the surface. Enter the number of the center line or select the pipe axis graphically using the button.

If the pipe section is conical, activate the "Different radius at end" check box and enter a corresponding value.

Spline with Minimum Curvature

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

Define the “Coordinate system” of the reference plane and enter the “Sample coordinates in the coordinate system”. These points represent the control nodes of the spline surface. Then specify the “Boundary lines of the reference plane” or select the lines graphically using the button.

Thickness with Material

Select the appropriate type in the list of available thicknesses, or define a new thickness (see the chapter Thicknesses).

Material of Thickness

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

Hinges

A hinge can be used to control the transfer of internal forces and moments along a line of the surface (see the chapter Line Hinges). After selecting the check box, you can define the hinge type in the "Hinges" tab.

Support

If the surface has elastic foundations, you can select a surface support in the "Support" tab, or define a new one (see the chapter Surface Supports).

Release

To decouple the model on the surface, you can select a surface release in the "Releases" tab or define a new one (see the chapter Surface Releases).

Eccentricity

An eccentricity can be used to model an offset of height for the entire surface (see the chapter Surface Eccentricities). You can define the offset type in the "Eccentricity" tab.

Load Distribution Factor

For a surface of the Load Transfer type, it is possible to define distribution factors for the load-transferring objects. If you select the check box, you can assign the factors individually in a new tab.

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

Mesh Refinement

The mesh size of the FE mesh can be adjusted to the geometry of the surface (see the chapter Surface Mesh Refinements). Thus, it is independent of the general mesh settings. In the "Mesh Refinement" tab, you can select a surface mesh refinement or define a new one.

Specific Axes

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

Input Axes

The orientation of the input axes is important, for example, for orthotropic and foundation properties or the effect of a surface load.

The list in the "Category" section provides various options for adjusting the axis position:

• Angular rotation: Rotation of xy-surface axes about the z-axis with an angle α
• Axis parallel to lines: Alignment of the x- or y-axis with a line
• Axis directed to point: Alignment of the x- or y-axis with the intersection point of a line and a surface
• Axis parallel to coordinate system: Alignment of axes with a user-defined coordinate system

You can define the reference objects graphically using the button.

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

Result Axes

Currently, the orientation of the result axes is only possible for a setting that is "identical to input axes".

Grid for Results

Each surface is covered by a grid, which is used for the results shown in tables. It allows for a results output in equidistant, adjustable result points, which is independent of the FE mesh.

The default setting is a Cartesian surface grid with regular distances of 0.5 m between the grid points in both directions. If necessary, you can adjust the "grid distances" in the x-direction (b) and y-direction (h), apply a "grid rotation", or change the "grid origin". For circular surfaces, the "Polar" grid type provides an alternative for the numerical results output.

If activating the "Adapt automatically" check box in the "Options" section, the grid points will be adapted to the new geometry when changing the surface.

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

Integrated Objects

Generally, RFEM automatically detects all objects lying in a surface that are not used for the surface definition.

The numbers of the surface-related nodes, lines, and openings 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. Now, the input text boxes in the "Integrated Objects in Surface" section become accessible. Add the missing object number, or use the , button to determine the object graphically.

Activate Load Transfer

The check box makes it possible to distribute the load of the surface – regardless of its stiffness type – using a load transfer surface. Thus, the surface acts in the model due to its stiffness. The distribution of the load to the neighboring objects, however, is controlled via the parameters that you can define in the Load Transfer tab. This feature is primarily relevant for surfaces of the Beam Panel thickness type.

Deactivate for Calculation

This check box provides you with the possibility not to consider the surface for the calculation; for example, in order to simulate construction stages or to analyze varying modeling versions. In this case, the stiffness, boundary conditions, and loads of the surface are not applied.

Information | Analytical

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