4.4 Surfaces

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).

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].

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 4.67). 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.

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.

A B-Spline surface is similar to a quadrangle surface (see Figure 4.66). 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 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.

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.

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.

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

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.

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).

Use this stiffness model for surfaces with different stiffnesses in both surface directions (see Chapter 4.12). Use the [Edit] 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.

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.

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.

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

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

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

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

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.

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.

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 [Edit] button to define the parameters.

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, Membrane 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).

The plane in the surface center represents the reference surface for the thickness, which is assumed to be in equal proportions on both sides of the "centroidal plane". To check the center, use the Display navigator option Rendering → Model → Solid Model → Surface and Filled incl. thickness (see Figure 4.118).

By specifying an Eccentricity e_z, you can define an offset of height for the surface. In this way, you can create uniform top or bottom edges for adjoining surfaces that have different thicknesses.

The eccentricity in the form of additional moments has an influence on the surface's internal forces.

In general, RFEM automatically recognizes all objects that lie on a surface and are not used for the surface definition. In the table columns or text boxes of the dialog box, all numbers of nodes, lines, and openings are displayed.

If an object is not recognized, it is possible to integrate it manually: Double-click the surface to open the Edit Surface dialog box: Then, deactivate Automatic object detection in the Integrated tab. The text boxes of the dialog sections to the left are thus enabled for access. Use to select the objects graphically.

The area of each surface is shown in the table column. Areas of openings are not taken into account, thus the value represents the net area.

The mass of each surface is indicated in the penultimate column. It is determined from the area and the material's specific weight.

Enter a user-defined note or select an entry from the list.

Each surface has a local coordinate system. The axis system of the surface is significant for various input parameters such as orthotropic and foundation properties or directions of surface loads. The base internal forces are also related to a surface axis system.

RFEM displays the coordinate systems as soon as you move the pointer over a surface. They can also be displayed or hidden by using the shortcut menu of a surface (see Figure 4.76).

If required, you can adjust the local surface axes:

• Reverse Local Axis System shortcut menu option

• The orientation of the local z-axis is reversed and the remaining axes are aligned according to the right-hand rule. As a result, foundations are put on the other side of the surface, or the "top" and "bottom" reinforcement layers for the reinforced concrete design change surface sides.

• Edit Surface dialog box

• To open the Edit Surface dialog box, double-click the surface. In the Axes tab, you can adjust the local surface axes for Input as well as for Results.

In the two sub-tabs you, can direct the local surface axis x or y parallel to a line, to the intersection of a line and the surface (Direct to line, for radial axis system), or direct the axes of the customized coordinate system (see Chapter 11.3.4).

Each surface is covered by a grid, which is used for the results output in the tables. This grid is independent of the FE mesh.

For detailed information about surface grid and customization options of the grid points, see Chapter 8.13.

For the Standard and Without membrane tension surface types, the Modify Stiffness dialog tab is available. In it, you can influence the surface stiffnesses.

You can select the Definition Type of the stiffness adjustment in the list. If you select None (no modification of stiffness), all stiffness components with the factor 1.00 are taken into account for the calculation.

Use the Multiplier Factors option to customize the stiffness factors k for the surface's torsion, bending, shear, membrane, and eccentric stiffnesses. The surface stiffness elements are displayed in Equation 4.20.

The definition type According to ACI 318-14 Table 6.6.3.1.1(a) sets the reduction factors to be according to the American reinforced concrete standard and depending on the component type. The list provides different options to set the appropriate factors for walls or plates, for example.