Chapter 25
Roading
The calculation process of the grade is shown by the following decision chart: Is the super NULL?
Yes
No Is this element subject to super?
No
Yes Yes
Is this the first element of the template? No
Super
Normal crossfall + super difference
Normal crossfall
Figure 26: Calculation process decision chart
Once the grade of the element is determined, the horizontal distance is calculated. The horizontal distance is the normal distance plus widening if the element is subject to widening. If the element is widened then the widening occurs by extending the element at its calculated grade. All grade calculations are carried out using grades expressed as percentages, even though these values may be entered and displayed as ratios. NOTE
25.13.4
Calculating a template using the default values of superelevation and widening will give the same results as would be given if all elements of the template were defined to have neither applied.
Pivot calculation The pivot point is the fixed point in terms of which the cross-sections are calculated when superelevation is applied. If the pivot point is defined as “center," the centerline is held. Should the pivot point be defined as “left" or “right," the pivot calculation will find the last element of the template on that side of the road which can be subject to superelevation and uses the end of that element as the pivot point. The position of the pivot point relative to the centerline is calculated without superelevation being applied and held fixed. After superelevation has been applied to the cross-section, the whole superelevated cross-section is raised or lowered until it is anchored on the pivot point.
25.13.5
Cross-section calculation The cross-section at a given station (chainage) is calculated by reference to the crosssection definition and to the templates to which it refers. Interpolation between the templates in adjacent cross-section records occurs. Interpolation is based on the end of each corresponding element of the two templates concerned. For the cross-section to be calculated, there must be a XSEC record defined both before and after the station being calculated. The cross-section can then be made up one element at a time by evaluating the same element in both templates and interpolating between them. This interpolation is based on the differences in horizontal and vertical distances of the ends of the individual elements.
25-30
SRX