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Why and how using built-in angles

The built-in angle allows to specify a given orientation at one line end. The purpose is to define a reference frame and, by convention, the local Z-built- in axis will represent the axial vector of the connected line whereas the X-built-in axis and the Y-built-in axis will be the first and second bending axis of the line (click here to see the related conventions).

Note that this may be useful only when a line is composed of beam elements since neither bars or cables may account for bending or torsion.

The built-in angle defines a direction of reference with respect to a fixed reference frame or with respect to a local frame such as an object frame (Sea&Ground, floater, buoy, equipment). Built-in angles are defined by two parameters noted ENCASD and PSID.

  • PSID is the rotation angle around the local Z-axis, and is positive from local X-axis towards the local Y-axis.

  • ENCASD denotes the rotation angle around the axis opposite to the local Y-built-in once the rotation PSID is applied (In other words, ENCASD defines a rotation around Y-built-in).

ENCASD depends on the line orientation, as shown in the sketch below :

Note that the X, Y, Z axes may be the local axes of any object as well as the global reference frame axes.

Tip

Use the pop-up menu that displays when right-clicking over the 3D View window and activate the Show local axes option to display the local axes corresponding to your built-in angles. This way, you can easily check whether you have correctly defined your ENCASD and PSID parameters.

This information is needed when at least one rotational degree of freedom is constrained (clamp, flexjoint, user defined connection). In this case, the local Z axis defined by ENCASD and PSID should correspond to the local orientation of the line at its equilibrium position, local axis 3 of the beam element. Otherwise, a nominal bending moment would be induced, as illustrated below :

Warning: When a line is constrained at both extremities, ENCASD and PSID values must be carefully defined and checked (using Show local axes). The values defined at both extremities should be consistent in order to:

  • Ensure that the Z local axis defined by ENCASD and PSID is aligned with beam local 3 axis, from one line extremity to the other one.

  • Avoid induced torsion

See figures below:

Left column: The Z axis corresponds to the beam local 3 axis from the left extremity to the right one. The Y axis remains the same at both sides. The local frame defined at left extremity becomes the local frame at right extremity sliding along the line. Right column: The Z axis corresponds to the beam local 3 axis from the left extremity to the right one. But, the Y axis direction changes which induce a torsion moment in the line.

Built-in angles definition is not required with a pin connection. Nevertheless, it may be useful to define the ENCASD and PSID values in order to output angular deflections during post-processing. Built-in angle values are the angular deflections with respect to the nominal axis defined by ENCASD and PSID: