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Line data: Pre-bend |
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Line data: Pre-bend |
Pre-bend is provided for modelling lines which are not straight in their unstressed state, e.g. spool pieces, rigid jumpers etc. Pre-bend can only be used if torsion is included and the line's length and end orientations are explicitly specified.
| Note: | When pre-bend is used, curvature results and bend radius results are reported relative to the pre-bent curvature. |
| Warning: | Pre-bend is not consistent with the assumptions made in calculating stress results or in fatigue analysis. |
The structure and segmentation of the line are fundamental when setting up pre-bend so, for convenience, the relevant structure data are repeated on the pre-bend page. This means that you can easily adjust, for example, the line segmentation as you set up the pre-bend, to ensure that the curvature remains sufficiently smooth. Note that the underlying data are the same: changes you make to the structure values here will be mirrored on the structure page, and vice versa.
Pre-bend can be specified in one of two ways, in terms of either curvature or bend angle.
To specify pre-bend by curvature, you give the curvature values, for each section, in radians per unit length, in the local $x$ and $y$ axis directions of the first node of that section. These together define the constant curvature and plane of bending for the section.
To specify bend radius by bend angle, in addition to the angle itself OrcaFlex requires either the section length or the bend radius (to define the extent of the bent section) and the bend axis direction (to define the plane of bending). The three values bend angle, bend radius and section length are not independent. To maintain their consistency, OrcaFlex treats bend radius and section length as surrogates: if you change one, the other will be updated to preserve the relationship between the three.
The angle, $\alpha$, through which the section bends.
If you enter a value for bend radius $r$ (for non-zero bend angle), the section length will be updated accordingly. Conversely, if you enter a value for the section length, the bend radius will be adjusted.
The axis about which the section bends, defined by the angle $\theta$, in the local $xy$ plane of the first node of the section.
It can be difficult to envisage the shapes generated by pre-bend data in either form, particularly for compound bends in more than one plane. To help you with this, OrcaFlex provides the view pre-bend shape button. Click this to open a new wire frame 3D view showing only the line in its unstressed shape. You may find that displaying node axes will be helpful when defining either the curvature components or the bend axis direction. You can open multiple view windows, with differing view parameters; if you edit the pre-bend data with these windows open, they will be updated automatically.
OrcaFlex reports the line section pre-bend curvature components on the details page of the line properties report. If pre-bend is specified by bend angle, the corresponding curvature components are calculated and reported using the formula \begin{equation} \left(c_x, c_y \right) = \left( \frac{\alpha\cos\theta}{l}, \frac{\alpha\sin\theta}{l} \right) \end{equation} where $l$ is the section length.