Surfaces

Surfaces are used to 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 for the calculation in the surface's centroidal axis.

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 elementary surface parameters. By ticking check boxes, further tabs are added in which 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, very stiff surfaces can be modeled 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. In the case of compression and shear forces as well as moments, the affected surface elements fail.

Without membrane tension

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

Load transfer

With this type, surface loads can 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 with reference to the true member lengths in the global directions (load directions X_L, Y_L, Z_L). The surface itself has no stiffness.

The criteria for the load transfer can be defined in the Load Transfer tab.

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

With the 'Isotropic | FEM' option, 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 to 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 certain objects are not to transfer loads, you can specify them in the 'Without effect on' section.

For the 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 if the Advanced distribution settings check box is activated in the 'Options' section. Adjustments are only required for problematic load distributions. The effect of these parameters is explained by means of 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' in order to take into account, 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 (for example bracings). Define the objects individually or select a pattern object that lies parallel to the load-free members or lines.

Once 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 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 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 center of gravity and compared with the moment from the member loads about 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 taking the rotational equilibrium into account.

Stiffening

With this stiffness type, the stabilizing effects of trapezoidal sheeting, purlins, and bracings can be taken into account easily and realistically in the model. The properties are described in more detail in the product feature Surface stiffness type "Stiffening".

Result surface

This stiffness type makes it possible to convert stresses and forces of other objects into surface internal forces by means of an integration procedure. 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.

Further criteria for integrating results can be defined in the Result Surface tab.

In the 'Integrate stresses and forces' section, select whether the results are to be captured purely object-related or also geometrically within an area. 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 area "below" and "above" the surface are to be integrated, you can define the relevant distances in the 'Parameters' section. They are referenced 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

In 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 dragging a rectangle, circle, etc. If you 'select the boundary', RFEM recognizes the surface automatically as soon as a sufficient number of boundary lines are fixed.

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 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. The four corner nodes are then specified in the 'Quadrangle' tab. They control how the curved surface is spanned.

NURBS

NURBS surfaces are formed from four closed NURBS lines (see chapter Lines). This allows almost any free-form surfaces to be modeled.

In the 'New Surface' dialog, define 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 using 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 'split by 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 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 about this axis at a radius. 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 with 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 using control nodes that lie on the surface or outside it. This allows, for example, terrain surfaces to be modeled.

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 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 a different 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.

Hinges

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

Support

If the surface is elastically founded, you can select or redefine the surface support in the 'Support' 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 can be used to model a height 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-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 in the next free row and select the line or member. Then assign the suitable '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 redefine 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 user-defined – separately for input and output.

Input axes

The orientation of the input axes is relevant, 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:

• Angular 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 to a line
• Axis directed to point: orientation of the x- or y-axis to 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 determine the reference objects graphically using the button.

The 'Reverse local axis z' check box makes it possible 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 by 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 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), 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 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 determine the object graphically.

Activate load transfer

The check box makes it possible to distribute the load on the surface – regardless of its stiffness type – by means of a load transfer surface. In 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 Beam Panel thickness type.

Deactivate for calculation

The check box offers the possibility not to take the surface into account in the calculation, for example to simulate construction stages or to examine 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 properties of the surface such as surface area, solid, and mass, as well as the position of the surface's center of gravity and the orientation of the surface. Openings are taken into account accordingly.