Working With Spiral Elements
In the video below we show how to work with Spiral In, Spiral Out and Combining Spirals in an alignment. In addition we go through how to enter and work with Station Equations. We also show a data tabulation method to rationalize the data from the plans into a form that can be used to quickly enter and check the alignment as it is being created.
In the example, a railway track alignment, there are two sections where we have to use a combining spiral, in the first section a single combining spiral is used in a sequence that looks as follows
Straight
Spiral In
Arc
Combining Spiral
Arc
Spiral Out
Straight
In the second section there are two combining spirals in a sequence that looks as follows
Straight
Spiral In
Arc
Combining Spiral
Arc
Combining Spiral
Arc
Spiral Out
Straight
In both cases you can see that the Spiral In is used to enter the curve sequence and it always goes from a Tangent / Straight section to an Arc of a given radius and direction.
In both cases you can see that the Spiral Out is used to exit the curve sequence and it always goes from an Arc of a given radius and direction to a Tangent / straight section.
In the case of Spiral In and Spiral Out the Tangent or straight at one end of the spiral is an “Arc of Infinity radius”, the Arc at the other end is an Arc of specified radius and direction.
In both cases you can also see that the Combining Spiral is used to connect two arcs of different radius together - this is a different computation to that of a Spiral In or Spiral Out element.
The method of data entry also differs
- A Spiral In element has a known start point coordinate. station, and bearing (based on the prior tangent / straight line end point, and you will enter the end radius of the arc to which it will connect at its end and the length of the spiral and the direction (Left or Right). The next element will be an Arc of the radius that you defined for the Spiral In element. The direction of the Arc will match that of the spiral.
- A Spiral Out element has a known start point coordinate, station, bearing, direction and radius (based on the prior arc element end point so all you need to key in is the length of the spiral.
- A Combining spiral is entered as a length only - the radius at the start is defined by the prior element, the radius at the end is defined by the subsequent element. The start coordinate, station, bearing and direction are also known, the end point is determined once the subsequent arc is defined.
The video shows how this works in practice.
Working With Station Equations
In the video we also show how to read and enter Station Equations. There are two types of Station Equation a Gap Equation and an Overlap Equation.
- In a Gap Equation the Ahead Station is a larger number than the Back Station. This creates a Gap in stationing along the alignment i.e. if my Back Station was 100 and my Ahead Station was 150 then I would have no point on the alignment that had a station value in the range from 100 to 150, the stationing up to 100 would be written as e.g. 50.00:1 where :1 states it is in Station Equation 1 zone, the stationing after the station equation point would jump to 150.00:2 and increase from there, thereby leaving the gap in stationing. the :2 in this case states that the point is in Station Equation Zone 2
- In an Overlap Equation the Ahead Station is a smaller number than the Back Station. This creates an overlap in stationing along the alignment i.e. if my Back Station was 150 and my Ahead Station was 100 then I now have two points on the alignment that have the same exact stationing i.e. the stationing would start at e.g. 0.00:1 and increase to 150.00:1, and then would step back to 100.00:2 and start to increase again. This means that a Station 125.00 to determine where on the alignment that is requires the station equation detail i.e. 125.00:1 or 125.00:2 are two different locations on the same alignment. The :1 or :2 tells the software and the user the detail they need to work with the alignment stationing.
Notes:
- When you key in a Horizontal alignment using the geometry method, the station values are derived from the entered data, so you do not need to be concerned about Station Equations when keying in the horizontal alignment, you can get the alignment entered and then enter the station equations second before checking that your alignment end point station matches that written in the plans.
- When you key in a Vertical alignment, you will be entering the station of each grade break and the VPI of all vertical curves, so at this point you will need to enter the station equation with the station value i.e. if Station 175.00 is in Station Equation Zone 2 then you will need to enter 175.00:2 for the station of the VPI point if there are any overlap station equations that could cause ambiguity as to where Station 175.00 is on the alignment.
The Video Shows you how
The data files used in this example are provided below
Alignment PDF file
TRACK 1 DESIGN.pdf (1.5 MB)
Spreadsheet of Alignment Data
Track 1 Alignment.xlsx (17.1 KB)
CSV File Derived from Spreadsheet for Coordinate Points
Points for Track 1.csv (2.3 KB)
TBC Project File
Track 1.vce (194.0 KB)
Project created and saved in TBC v5.70 - if you need to open in an older version of TBC use CTRL + SHIFT key and then drag and drop the VCE file over the grey tab that says Start Page or Plan View in the TBC Graphics area (depends on whether you have a project open or not). Keep the CTRL + SHIFT key held down until after you release the file. This will downgrade the TBC project to the TBC version that you are running.