# 4.4 Surfaces

General description

In addition to structure geometry, surfaces describe the stiffness that results from material and thickness properties. When generating the FE mesh, 2D elements are created on surfaces. For detailed information about the used elements, see Chapter 7.2.1.

The stiffness type Null must be used for geometry descriptions of solids.

Different geometry and stiffness properties are available for structure modeling. It is possible to combine entries of both Surface Type lists or table columns − within type-specific limits and conditions.

Color symbols help you to assign various types for Geometry and Stiffness. You can use these colors in the model to display individual surface types. Colors are controlled in the Display navigator with the Colors in Rendering According to option (see Chapter 11.1.9).

Surface Type - Geometry
Plane surface

Plane surfaces can be defined graphically by drawing a rectangle, parallelogram, circle, annulus, polygon, etc. Use the menu or the list button shown on the left to access different shapes of plane surfaces.

The following dialog box appears when you graphically enter data by using one of the toolbar buttons:

First, enter the parameters for Material, Thickness, and Stiffness in addition to the surface number. Click [OK] and define the boundary lines of the surface in the work window by selecting relevant corner points.

The [Select Boundary Lines] function allows you to select existing lines graphically. The lines must be arranged in a polygonal chain that lies in one plane. The line types are described in Chapter 4.2.

RFEM automatically recognizes the surfaces as soon as a sufficient number of boundary lines are defined.

This type of surface represents a general quadrilateral surface. In addition to straight lines, you can use arcs, polylines, and splines as boundary lines. Use this surface type to model shells, since boundary lines do not have to be arranged in one plane.

You can select the boundary lines graphically after clicking [OK].

Rotated surface

A rotated surface is created by rotating a line about a fixed axis. The surface results from the start and end position of the line, as well as the line's rotated definition points.

The New Rotated Surface dialog box consists of two tabs. In the General dialog tab, you can define the Material, Thickness, and Stiffness of the surface (see Figure 4.66). A variable surface thickness is not allowed.

In the Rotated tab, you can specify the Angle of rotation. Both points of the Rotation Axis can be defined either by entering their coordinates or graphically by using . Click [OK] and define the boundary line for rotation in the work window.

Rotated surfaces can also be created from generated lines.

Pipe

A pipe surface is created by rotating the center line of the pipe about the center axis at the distance of a specific radius.

The New Pipe dialog box has two tabs. In the General tab, you can enter the parameters for Material, Thickness, and Stiffness of the surface. In the Pipe tab, specify the Center line and Radius r. You can also define the center line graphically.

Use the pipe surface to create two circles and a polyline that is parallel to the pipe axis.

B-Spline surface

A B-Spline surface is similar to a quadrangle surface (see Figure 4.65). In addition, help nodes are created on the surface. The surface shape can be influenced by adjusting the coordinates of help nodes retroactively.

The input dialog box has two tabs. In the General tab, you can define the parameters for Material, Thickness, and Stiffness of the surface. A variable surface thickness is not allowed.

In the B-Spline tab, you can enter the number of help nodes into the Order of matrix text box: For example, if you enter "3", you create a grid of 3 x 3 help nodes across the surface. The Order of spline selection field specifies whether a polynomial of the third or fourth degree is used for the calculation of the surface.

NURBS surface

NURBS surfaces are defined by four connected NURBS lines (see Chapter 4.2). By using NURBS surfaces, you can model almost any free form surface.

When entering boundary lines, make sure that opposite pairs of NURBS lines are "compatible" with each other: Only if there is an equal number of control points are opposing NURBS lines arranged in the same order.

Trajectory surface

Use this type of surface to create a spatially curved surface from a certain start profile in relation to any trajectory.

The New Trajectory Surface dialog box has two tabs. In the General tab, you can define the parameters for Material, Thickness, and Stiffness of the surface.

In the Trajectory tab, you can enter the number of the Guide Line that represents the reference line of the surface or select it graphically. Then you can determine the Start Profile in the graphic. If necessary, define a second line as the End Profile. The Angle β describes the rotation of the generated, parallel boundary line in relation to the trajectory.

Component

This entry appears in the table column and navigator if an intersection of surfaces has been created (see Chapter 4.22). The editing functions for components of intersection surfaces provided by RFEM are the same as for "normal" surfaces. Thus, it is possible to quickly modify properties of surface components without creating an intersection again.

The original surface of a component is indicated in the Component tab of the Edit Surface dialog box.

Use the [Go to Parent Surface] button to access the edit dialog box of the original surface.

Surface Type - Stiffness

The list available in the dialog box and table provides several stiffness models, which you can select to model the structure realistically.

Standard

The surface transfers moments and membrane forces. The approach describes the general behavior of a homogeneous and isotropic material. The stiffness properties of the surface do not depend on directions.

Without membrane tension

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

Orthotropic

Use this stiffness model for surfaces with different stiffnesses in both surface directions (see Chapter 4.12). Use the  button to define the parameters.

Alternatively, you can assign an orthotropic property to the material (see Chapter 4.3). In this way, you can avoid defining properties for each individual surface.

Glass

This type of stiffness is required for the add-on module RF-GLASS. Moments and membrane forces are transferred, but stresses are not determined in RFEM. The actual stress calculation is carried out later in the add-on module RF-GLASS.

Laminate

This stiffness type transfers moments and membrane forces. The add-on module RF-LAMINATE is required to calculate the laminate model. The actual stress calculation is also carried out there. No stresses are included in the results output of RFEM.

Rigid

Use this type of stiffness to generate very stiff surfaces, which create a rigid connection between adjacent objects.

Membrane

The surface has a uniform stiffness in all directions. Only membrane forces are transferred.

Membrane Orthotropic

Only membrane forces are transferred. Stiffnesses are different in both surface directions (Chapter 4.12) and can be defined with the  button.

Null

Null surfaces are required for the definition of solids (see Chapter 4.5).

Boundary Lines No.

The boundary lines of a surface are listed in the corresponding text box or table column. The lines must form a closed continuous line.

When rotated surfaces were generated, generation parameters are displayed in the table column.

Material No.

You can select an entry from the list of materials that have already been created. Material colors make the assignment easier.

In the New Surface dialog box, there are three buttons below the list. Use the buttons to access the material library or to create and edit materials.

For more detailed information on materials, see Chapter 4.3.

Thickness
Type

You can select between two types of surface thickness.

• Constant
• The surface has the same thickness everywhere.
• Variable
• The thickness of the surface is linearly variable (see Chapter 4.11). Use the  button to define the parameters.
Thickness d

Specify the surface thickness d in this text box, unless a variable thickness or a Null surface has been defined. The thickness is used to determine the self-weight and stiffness for the stiffness types Standard, Without tension, Glass, and Membrane. For Orthotropic and Membrane-Orthotropic stiffnesses, this value is only used to calculate the self-weight (stiffnesses must be defined separately for orthotropic surfaces).

Surface thicknesses can be visualized with different colors in the model: In the Display navigator, select Model → Surfaces, and then select the Color Scale of Thicknesses in Panel check box (see figure below).