This article describes several peculiarities relating to heavily skewed portal structures. The ‘heavy skew’ designation is applied to portal structures with over 20 degrees of rotation from perpendicular. These peculiarities exist as a product of the simplification of the relationships between traditional 2D and modern 3D representations of overhead line equipment.
The measurement of Running Edge to Centre of Steel (RECOS) in Lineardraft, is always measured parallel to the structure axis.
The image below shows how RECOS is measured for an STC structure (shown in green) and how RECOS is measured for a portal structure (shown in red).
The perpendicular RECOS of the portal is shown in blue below to illustrate the effect of heavily skewed portal structures.
Lineardraft calculates REFOS using RECOS (shown by the red line in the image below) minus half the width of the mast.
The REFOS value is never stored and is just presented as a helpful value in the 'Structure' menu. This approximation was chosen to simplify arguments about how to measure REFOS.
The three blue lines marked '1', '2', and '3' in the image below, are all potential ‘REFOS’ measurements.
Cantilever Alignment & Reach
Cantilevers are always parallel to the structure axis. In the case of a portal structure, the structure axis is parallel to the gantry axis. Below the image shows the cantilever parallel to the gantry in blue and tries to illustrate the extreme effects introduced by a heavy skewed portal coupled with the expectation that a cantilever should be perpendicular to track, in red.
The effect on Reach measurement is it will increase proportional to the amount of rotation from perpendicular. i.e., if the portal was perpendicular to the track the blue and red lines in the image would be of identical length.
Cross-Sectional Views & Measuring Effects
Views are always parallel with the structure axis in Lineardraft. Consider the image below, where the dotted view line in blue is parallel to the structure axis. The view line in red is shown to illustrate the effect of aligning the view with a cantilever perpendicular to the track axis.
Considering both views in the image below, red and blue, it is easy to see that neither is ideal if one wants to precisely measure and dimension a cross-section drawing.
Taking the red cantilever as an example case: in the blue view, the reach value for the cantilever will be incorrect whilst the mast and gantry will be correct. In the red view, the cantilever will be measured correctly but the mast and gantry incorrectly.
Stagger is always calculated along the structure axis, parallel with the cantilver axis. The blue arrowed line in the image below shows the axis along which the wire will deviate according to stagger. The red arrowed line seeks to illustrate the difference if one expects the stagger to deviate perpendicular to track.