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 on surfaces. These are applied in the surface centroidal axis for the calculation.

To enter 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 fundamental surface parameters. By ticking check boxes, further tabs are added where you can make the specific entries.

Stiffness Type

The stiffness type controls the way in which 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 direction-independent.

Without Thickness

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

Rigid

With this stiffness type, you can model very stiff surfaces to represent 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 compressive and shear forces as well as moments, the affected surface elements fail.

Membrane Without Tension

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

Load Transfer

With this type, you can 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 generated, the load is converted into the global directions, related to the true member lengths (load directions X_L, Y_L, Z_L). The surface itself has no stiffness.

You can specify 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 provides selection options for an isotropic distribution based on an FEM calculation, as well as for an orthotropic arrangement on surface strips, which are applied in one or both local surface axes to determine the load influence width.

For 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 lines or rigid nodal supports. The responses of this partial model are then applied as loads for the 3D calculation of RFEM. If specific objects are not to transfer loads, you can specify them in the 'Without effect on' section.

For load transfer via surface strips, you can specify how RFEM is to 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 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 if the Advanced Distribution Settings check box is ticked 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 'surface weight' to consider, for example, the self-weight of a glazing.

In the 'Without effect on' section, you can exclude members, lines, and nodes from the load transfer (e.g., 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 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 'Base' tab. You can then define the factors for the load-transferring objects individually in the Load Distribution Factors tab.

For 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 distribution (see chapter section). The 'Neglect moment equilibrium' check box is not ticked by default. Thus, the moment from the surface loads is formed about the centroid and compared with the moment from the member loads about the centroid. However, this option has no meaning for nodal loads. 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 transfer stresses and forces from other objects into surface internal forces via an integration method. This allows you, for example, to determine the membrane and bending stresses for a solid, which are to be designed with different partial safety factors.

You can specify 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 region. In the 'Include objects' section, specify the relevant surfaces and solids. Alternatively, select 'All' objects and then exclude specific elements in the 'Excluded from inclusive objects' section.

If the results of a specific region "below" and "above" the surface are to be integrated, you can define the relevant distances in the 'Parameters' section. They 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

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

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

Quadrangle

This surface type, in its basic form, 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, specify 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 freeform surfaces.

In the 'New Surface' dialog, specify the boundary lines of the NURBS surface. The respective 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 given in the 'Coordinates - Control Point' section.

Trimmed

If surfaces intersect, you can quickly create the intersection: Select the surfaces and then call 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 'Split by Intersection' options, RFEM creates partial surfaces and assigns them the 'Trimmed' type. You can then delete components if, for example, you want to remove overhanging surfaces.

Rotated

A rotated surface is created when an existing line is rotated around 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, specify the boundary line of the surface to be rotated. Enter the rotation angle α. You can determine the points of the rotation axis using the coordinates or graphically with the button.

Pipe

A pipe surface is created when the center line of the pipe is rotated in 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, specify 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 with the button.

If the pipe section is conical, tick the 'Different radius at end' check box and enter the corresponding value.

Minimum Curvature Spline

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

Specify the 'coordinate system' of the reference plane and enter the 'Sample coordinates in 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 with the button.

Thickness with Material

Select the appropriate type in the list of existing 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 or redefine a different material in the list of already created materials (see chapter Materials). This material is then assigned to the thickness type.

Hinges

With a hinge, the transfer of internal forces along a line of the surface can be controlled (see chapter Line Hinges). After ticking the check box, you can specify the hinge type in the 'Hinges' tab.

Supports

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

Release

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

Eccentricity

An eccentricity allows you to model a height offset of the entire surface (see chapter Surface Eccentricities). You can specify the offset type in the 'Eccentricity' tab.

Load Distribution Factor

For a surface of the Load Transfer type, there is the option to define distribution factors for the load-transferring 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 into 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 adjusted to the geometry of the surface (see chapter Surface Mesh Refinements). It is thus independent of the global mesh settings. In the 'Mesh Refinement' tab, you can select or redefine the surface mesh refinement.

Specific Axes

Every 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 for adjusting the axis position:

• Angle rotation: Rotation of the xy-surface axes about the z-axis by the angle α
• Axis parallel to lines: Alignment of the x- or y-axis on 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 determine the reference objects graphically with the button.

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

Result Axes

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

Grid for Results

Every 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 spacing' 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 adjusted to the new geometry when the surface is changed.

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

Integrated Objects

RFEM generally 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. Complete the missing object number or use the button to determine the object graphically.

Activate Load Transfer

This check box allows you to distribute the load on the surface – regardless of its stiffness type – using a load transfer surface. Thus, the surface acts through its stiffness in the model. 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 function is primarily relevant for surfaces of the Beam Panel thickness type.

Deactivate for Calculation

The check box offers the option 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 centroid and orientation of the surface. Openings are considered accordingly.