Vertical Parameters

To enter geometry data relating to the vertical alignment, click the Vertical tab. Vertical geometry data is divided into three groups: Grades, Curves, and Straights.

Grades

Specify the following in the Grades group:

Design Limits

Design limits define the range of grades as observed from a vehicle traveling from the defined start to the defined finish. They permit the limiting gradient to depend on the direction of travel, an important consideration when planning the development of roads and railways where loaded vehicles will normally travel in only one direction.

Tip: Enter a negative grade for downhill and a positive grade for uphill.

Sustained Limits

Sustained limits constrain the average gradient over a nominated distance to fall within a smaller range than the design grades. These can be used to avoid long climbs or descents at the limiting design grades. It is calculated by ensuring that the average alignment grade over the distance defined is equal to or less than the sustained grade figures entered into the Grades fields.

Coordination (road geometries)

Horizontal and vertical curves must be coordinated so that drivers can see objects on the road in time to stop, or bends in time to turn. Select values here to help prevent the creation of horizontal curves during or beyond crests.

Compensation (Rail Geometries)

Rail alignments have their maximum allowable grade while in horizontal curves governed by a curve compensation factor. Curve compensation adjusts the limiting grades for sections of a railway during horizontal curves. Tight curves reduce the maximum grades.

Enter the percentage of compensation for the allowable grade, per degree of horizontal curvature. The value is expressed as the central angle subtended by a chord of 100 feet:

The equation used to define the maximum and minimum grades during horizontal curves is:

Where:

Examples

If the alignment is on a straight section, the horizontal radius of curvature at that location is close to infinity. 1746 divided by a very large number will be close to zero, and X multiplied by zero will be zero, so GMAX will equal GDESIGNMAX

If the alignment is in a tight horizontal bend and the radius of curvature value is R=300 m at that point on the alignment, and if x = 0.04%, then 0.0004 multiplied by 1746/300 is 0.002328 or 0.2328%.

If GDESIGNMAX = 2%, and GDESIGNMIN = –2%, then

GMAX = 0.02 – 0.002328 = 0.017672 = 1.7672%

GMIN = –0.02 + 0.002328 = –0.017672 = –1.7672%

Curves

Specify the following in the Curve group:

Vertical Curve type

The vertical curve type can be parabola or circular.

Minimum Radii or K-value

The vertical curve geometric properties can be defined either by a radius (meters or feet) or a K-value. To change the preference settings to/from radius and K-value please refer to Preference Settings. K-values define the rate of change of gradient on parabolic curves. As a guide, the local radius of curvature at the peak of the parabola is 100 times the K-value.

Straights

These settings are used for generating warnings. They have no effect on the generated alignment.

Specify the following in the Straights group:

Vertical Minimum

Straights are measured from the end of the vertical curve to the start of the next vertical curve. If the straight is less than the Vertical minimum a warning will appear.

Back to Back

Check Back to Back if you want to ignore small straights and allow back-to-back curves in the same direction or in the reverse direction.

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