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6.3 Line Loads
Line loads are forces and moments that act on lines (see Chapter 4.2).
To apply a line load, a line must already be defined.
Line loads are very similar to member loads. For line loads, however, it is not possible to allocate material properties (e.g. temperature loads, axial strains).
Line loads can act on members because a member is the property of a line. However, to apply a line load to a member, the line must belong to a surface. Consequently, line loads cannot be applied to members in models that consist only of members.
The number of the line load is automatically assigned in the New Line Load dialog box but can also be changed there. The numerical order is not important.
Define the objects to which you want to apply the line load. The following options can be selected:
The load acts on one line or on each among several lines.
The load acts on all lines, which have to be specified in a list. Thus, when trapezoidal line loads are used, load parameters are not applied individually to each line but as a total load to all lines of the line list (see Figure 6.17).
In the text box, enter the numbers of the lines on which the load acts. You can also select them graphically by using .
If you have selected the graphical input by clicking the toolbar button, you have to enter the load data first. After clicking [OK], you can select the relevant lines one by one in the work window.
Define the type of load in this dialog section. Depending on your selection, certain parts of the dialog box and columns of the table are disabled. The following load types can be selected:
|Load Type||Short Description|
Concentrated, distributed, trapezoidal, or variable load
Concentrated, distributed, or trapezoidal moment
The Load Distribution dialog section provides different options for displaying the effect of the load. The graphic in the right corner of the dialog box may be useful as an illustration. Use the button in the graphic below to display the load in the rendering.
|Load Distribution||Short Description|
Concentrated load, concentrated moment, or multiple concentrated loads/moments
Uniformly distributed load, uniformly distributed moment
Trapezoidal load, trapezoidal moment
Triangular-trapezoidal load, triangular-trapezoidal moment
Parabolic load, parabolic moment
Polygonally distributed load
The load distributions of line loads largely correspond to the ones of member loads. The diagrams are described in detail in Table 6.2.
The load can be effective in the direction of the global axes X, Y, Z or the local line axes x, y, z. For the calculation according to the linear static analysis, it does not matter whether a load is defined as local or equivalent global. For geometrically nonlinear calculations, however, differences between locally and globally defined loads are possible: If the load is defined with a global direction of action, it keeps this direction when finite elements start to twist. In case of a local direction of action, however, the load twists on the line according to the distortion of elements.
The orientation of line axes is illustrated in Figure 4.102. The local axis x represents the longitudinal axis of the line. The axis z is usually aligned parallel to the global axis Z.
The position of the local line axes is irrelevant for the load input if the load acts in direction of an axis of the global coordinate system XYZ.
The load impact can be related to different reference lengths:
- Related to true line length
- The load is applied to the entire, true line length.
- Related to projected line length in X / Y / Z
- The application length of the load is converted to the projection of the line in one of the directions of the global coordinate system. The projection lengths are illustrated in the dialog graphic to the right.
In this dialog section / these table columns, the load values and, possibly, additional parameters are managed. The text boxes are labeled and accessible depending on the previously activated selection fields.
Enter the load values into these fields. Adjust the signs to the global or local orientations of axes. When a trapezoidal load is selected, specify two load values. The dialog graphic in the upper right corner illustrates the load parameters.
For concentrated and trapezoidal loads, enter the distances from the line start into these fields. You can also define them relative to the line length by selecting the Distance in % check box (see below).
The dialog graphic in the upper right corner and the button in the graphic below are useful when entering parameters.
Select this check box to define the distances for concentrated and trapezoidal loads relative to the line length. Otherwise, the entries in the above-described Distance text boxes represent absolute ranges.
The check box can only be activated for trapezoidal loads. Select this option to arrange the linearly variable load from the line start to the line end. The Load Parameters A / B text boxes are no longer relevant and therefore disabled.
Often, line loads result in singularities because the load is concentrated when introduced in a single line. To reduce this effect, select the Convert Nodal/Line Load to Surface Load option in the Tools menu. The option can be used for straight lines. You can also use the shortcut menu of a line load shown on the left to access the dialog box for converting line loads. As usual, right-click the object to open the shortcut menu.
A dialog box opens where you can define the parameters for distributing the load.
The corresponding free rectangular or polygonal load is created after clicking [OK].