4.2 Lines

The geometry of the model is defined by lines. They are essential for creating members, surfaces, and solids. Every line is defined by a start and end node. To define complex types of lines, additional intermediate nodes are required.

The line number is automatically assigned in the New Line dialog box and can be changed in it. The order of the line numbers is not important.

To adjust the order of line numbers retroactively, select Tools → Renumber in the menu (see Chapter 11.4.18).

In RFEM, the following line types are available for selection in the menu and in the table list:

• Single line
• Polyline
• Arc
• Circle
• Ellipse
• Elliptical arc
• Parabola
• Hyperbola
• Spline
• NURBS
• Trajectory
• Line on surface

The different types of lines are described on the following pages.

Every line is geometrically defined by a start and end node. They define the line orientation that also affects the position of the line coordinate system. The nodes can be entered manually, selected graphically, or redefined (see Chapter 4.1). If a line requires control points or intermediate nodes, they are also included in the nodes list.

The display of line orientations can be activated in the Display navigator.

The coordinate systems of lines can be activated in the Display navigator: Go to Model → Lines and select Line Axis Systems x, y, z including Indexes (see Figure 4.102).

This table column shows the total length of the line.

Table column D provides information about the global axis the line is parallel to, or in which plane spanned by global axes the line lies. If there is no entry, the line is in an arbitrary spatial position.

This field allows you to enter user-defined notes. Use the [Apply Comment] button to import saved comments (see Chapter 11.1.4). A Generated comment appears for lines created by RFEM (e.g. a pipe).

Selecting the line entry in the pull-down menu opens the New Line dialog box shown in Figure 4.24. The figure above shows the general dialog box for entering lines graphically. Use the list button in the toolbar to open it.

A "real" line is defined by only one start node and one end node. Such a line represents a direct connection between both nodes.

A polyline is a polygonal chain consisting of several straight line sections. The intermediate nodes are therefore listed in addition to the numbers of the start and end nodes in the dialog box (see Figure 4.24). To simplify line management, "real" lines are handled as polylines.

When entering polylines graphically, existing nodes, grid points, or snap objects can be selected as definition nodes. It is also possible to set nodes freely into the work plane.

In the second tab of the dialog box, you can define a Rotation of the line. Specify either an Angle β or a Help node that the line axis y or z is to be aligned with. The help node can be selected in the list, defined graphically, or newly created.

A rotation of the line may facilitate the input of line loads acting in local line direction. Line rotation does not affect surfaces or members because they have their own coordinate system.

The local line axis systems are shown in Figure 4.102.

An arc can be defined by the following parameters:

• Three nodes
• Center node, edge node, and opening angle
• Edge nodes and radius, opening angle, or rise
• Tangents and radius

In the Nodes at Arc dialog section, you can define the start, intermediate, and end nodes directly, select them graphically, or newly create them. The node order is shown in the small dialog graphic.

Based on these three nodes, RFEM determines the Arc Parameters specified in the dialog section below. It is possible to change the radius r, the rise h, and the opening angle α. The node coordinates are adjusted accordingly.

The coordinates of the arc center that result from the arc nodes or parameters are displayed in the Center of Arc dialog section. When you change data manually or select nodes graphically with the button, the coordinates of nodes are adjusted as well.

In the Subsequent adjustment by displacing node at list, you can specify the node you want to change the coordinates of.

When you define the arc graphically using a toolbar button, the via Three Nodes option allows you to select or create the nodes directly in the graphic.

If you select one of the other input options, you have to select two nodes first, as shown on the left in Figure 4.30 and Figure 4.31. Then, another dialog box (shown on the right) appears where you define the arc parameters.

In the Options dialog section (in Figure 4.30 and Figure 4.31 on the right), you can select the arc's plane from the list. The Included Angle can be defined directly in the graphic or entered manually and applied with [Apply].

To adjust an already defined arc, double-click its arc line. The Edit Line dialog box opens where you can modify the data in the Arc - Three Nodes dialog tab (see Figure 4.29).

In the Definition Type dialog section (right box), select the appropriate arc parameter from the list. Then, set the arc directly in the graphic or enter it manually and click the [Apply] button.

In the Step text box, you can enter the spacings after which the pointer snaps when dragging the arc radius, angle, or rise.

The orientation of the circular arc can be adjusted by selecting the Reverse orientation check box, which determines whether the arc is placed to the "right" or the "left" of both nodes.

A circle can be defined by the following parameters:

• Three nodes
• Center point and radius

You can enter the Circle Radius and the coordinates for the Center of Circle manually or graphically by using the button. The Point of Normal to Circle Plane determines the plane in which the circle is generated. Use the three buttons in the dialog section to select one of the global axes.

When you define the circle graphically by using one of the toolbar buttons, you can select or create the three nodes or the center and radius directly in the graphic.

To define an ellipse, three nodes are required.

The ellipse is generated by the Nodes on Ellipse: The largest distance between the three entered nodes is assumed to be the principal axis of the ellipse.

When you use the toolbar button to define the ellipse graphically, you can set it directly by selecting three nodes in the work plane.

The following curves of conic sections can be defined:

• Elliptical arc
• Parabola

• Hyperbola

Enter the curve parameters (focus, angle, axis rotation, etc.) manually in the corresponding tab of the New Line dialog box, or define them graphically.

When you enter line data graphically by using one of the toolbar buttons, you can define the curve parameters directly in the graphic.

Splines are used to model any kind of curves. Enter a spline line graphically by selecting determinant nodes of the curved line one after the other, or create nodes by mouse-click.

NURBS (Non-Uniform Rational B-Splines) are required for modeling free form surfaces. NURBS are splines whose control points are not placed on the curve itself. Usually, these lines are entered graphically by selecting control points one after the other or by generating the points per mouse-click.

Use trajectory curves to create helical lines. Usually, they are entered graphically by using the toolbar button shown on the left. The following dialog box appears:

First, define the Coordinates of the line start. This enables the Parameters dialog section where you can specify the total Rotation of the helix.

Then you can define the Coordinates of the line end graphically or enter them manually and click the [Apply] button. Alternatively, you can use the Length text box. The coordinates of the line end are subsequently determined in consideration of the specified rotation in the work plane.

To adjust an already defined trajectory curve, double-click its trajectory line. The Edit Line dialog box opens where you can modify the data in the Trajectory dialog tab.

In general, lines in plane surfaces are automatically recognized as integrated objects, so the line type Polyline is usually sufficient. However, to set a line on a curved surface, use the line type Line On Surface. You can also use this option to insert lines in plane surfaces that are not parallel to the global axes, without having to create a new user-defined coordinate system.

The input dialog box is similar to that of a polyline (see Figure 4.24).

Enter the start and end nodes of the line in the dialog box or select them graphically. You can use the floating New Line dialog box of the type 'On Surface' to set nodes directly on a curved surface, if the surface was selected before and the model display option Solid or Solid Transparent is active. RFEM generates nodes of the type On Surface.

The surface where the line lies is defined in the On Surface tab. You can also check the parameters δ₁ and δ₂ of the start and end nodes (see Figure 4.14), but it is not possible to change them.

If you choose the graphical input by using the [Line on Surface] button, you can place the nodes directly on the curved surfaces. Please note that the Wireframe Display Model is not suitable for this.

The line types Cut via Two Lines and Cut via Section represent special line functions that are relevant for the RF-CUTTING-PATTERN add-on module. They allow you to define cutting patterns via partial strips if the division of the membrane surface is too large for the cutting. These features are described in the RF-FORM-FINDING / RF-CUTTING-PATTERN Manual, Chapter 5.2.