Surfaces

Surfaces are used to describe the geometry of planar or curved components whose surface dimensions are much larger than their thicknesses. The stiffness of a surface is derived from its material and thickness. When generating the FE mesh, 2D elements are created on surfaces. These elements are applied in the surface center plane for the calculation.

To define a surface, you can use existing 'Boundary Lines'. You can also use the direct input where the program automatically creates the definition lines.

The Base tab manages the elementary surface parameters. By selecting the check boxes, additional tabs are added where you can enter the specific data.

Stiffness Type

The stiffness type controls how internal forces can be absorbed or which properties are assumed for the surface.

Various stiffness 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 independent of direction.

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 and shear forces as well as moments, the affected surface elements fail.

Membrane Without Tension

Only moments and membrane forces in compression are transferred. In the case of membrane forces causing tension, the affected surface elements fail (example: hole bearing).

Load Transfer

This type allows you to apply surface loads 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 created, the load is converted into the global directions with respect 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 in which direction(s) the load is to 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 that are applied in one or both local surface axes to determine the load distribution width.

For the option 'Isotropic | FEM', RFEM uses a separate sub-model for determining 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 to model elements, couplings or nodes, etc.) are replaced by rigid lines or rigid nodal supports. The reactions of this sub-model are then applied as loads for the 3D calculation of RFEM. If you want certain objects not to transfer any loads, you can specify them in the 'No effect on' section.

For the load transfer via surface strips, you can specify how RFEM should perform the 'Load Distribution'. By default, the load is distributed to the adjacent objects with a variable distribution. If you want to achieve a constant load distribution, however, select the corresponding entry from the list. The difference between the two variants is compared in the following image.

The input options for the 'Surface strip width', the 'Smoothing factor', and the 'Minimum number of strips on 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 in the technical article Advanced Distribution Settings for Load Transfer Surfaces using an example.

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

In the 'No effect on' 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 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, select the Load distribution factor check box in the 'Base' tab. You can then define the coefficients for the load-transferring objects individually in the Load Distribution Factors tab.

For the load transfer via surface strips, you can consider the 'Member eccentricity' or the 'Section distribution' to correctly capture the geometric position of a member or its course (see Chapter Cross-Section). By default, the 'Neglect moment equilibrium' check box is not activated. Thus, the moment is formed from the surface loads to the center of gravity and balanced with the moment from the member loads to the center of gravity. 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 considering the moment equilibrium.

Result Surface

This stiffness type allows you to convert stresses and forces from 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 that 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 are to be recorded 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 specific 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.

Planar

For a planar surface, all boundary lines lie in one plane. Various shapes of planar surfaces are accessible via the list button.

You can define the surface graphically (after clicking OK in the dialog) by drawing a rectangle, circle, etc. If you 'Select boundary', RFEM automatically recognizes the surface as soon as a sufficient number of boundary lines is defined.

Quadrangle

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

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.

NURBS

NURBS surfaces are formed from four closed NURBS lines (see Chapter Lines). This allows you to model almost any free-form surfaces.

In the 'New Surface' dialog, define the boundary lines of the NURBS surface. The respectively opposite pairs of NURBS lines 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 influence the shape of the surface via the 'Control point weights'. The coordinates of the selected control point are specified in the 'Coordinates - Control Point' section.

Trimmed

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

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

Rotation

A rotated surface is created 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 the process.

In the 'Rotation' 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 via the coordinates or graphically using the button.

Pipe

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

In the 'Pipe' tab, define the radius of the pipe. 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 cross-section is conical, activate the 'Different radius at end' check box and enter the corresponding value.

Spline with Minimum Curvature

With this geometry type, you can create a curved surface via control nodes that lie on the surface or also outside it. This allows you, for example, to model terrain surfaces.

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

Thickness with Material

Select the suitable type from the list of available thicknesses 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 from the list of already created materials or define a new one (see Chapter Materials). This material is then assigned to the thickness type.

Hinges

A hinge allows you to control the transfer of internal forces along a line of the surface (see Chapter Line Hinges). After selecting the check box, you can define the hinge type in the 'Hinges' tab.

Supports

If the surface is elastically founded, you can select or define the surface support in the 'Supports' tab (see Chapter Surface Supports).

Release

To decouple the model at the surface, you can select or define a surface release in the 'Release' tab (see Chapter Surface Releases).

Eccentricity

An eccentricity allows you to model an offset of the entire surface (see 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. When you select 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. The factor 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 suitable 'Distribution factor'.

Mesh Refinement

The mesh size of the FE mesh can be adjusted to the geometry of the surface (see Chapter Surface Mesh Refinements). It is therefore independent of the general mesh settings. In the 'Mesh Refinement' tab, you can select or 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 user-defined – separately for input and output.

Input Axes

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

The list in the 'Category' section offers various options to adjust the axis position:

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

Result Axes

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

As a standard, a Cartesian surface grid with a uniform distance of the grid points of 0.5 m in both directions is preset. If necessary, you can adjust the 'Grid distances' 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 grid type 'Polar' offers an alternative for the numerical result output.

If the 'Adjust Automatically' check box is selected in the 'Options' section, the grid points are 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

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

The numbers of the nodes, lines, and openings belonging 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 define the object graphically.

Activate Load Transfer

The check box allows you to distribute the load of the surface – regardless of its stiffness type – using a load transfer surface. Thus, the surface acts in the model through its stiffness. The distribution of the load to the adjacent objects, however, is controlled by the parameters that you can define in the Load Transfer tab. This function is primarily relevant for surfaces of the Beam Panel thickness type.

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 investigate a modeling variant. In this case, the stiffness, boundary conditions, and loads of the surface are not applied.

Information | Analytical

This section is displayed as soon as you have defined the boundary lines of the surface. It provides an overview of important surface properties such as area, volume, and mass as well as the location of the surface center of gravity and the orientation of the surface. Openings are considered accordingly.