Wednesday, March 19, 2014

Grade Compensation for the Horizontal Curves on Highways


Grade or Gradient is  the ratio of vertical rise or fall of the road center line with respect to its length. It has to be provided for the vertical alignment of the road. 

When we provide gradient to the road, vehicle has to apply more power to overcome the component of the gravity force. Higher the grade/slope,  larger is the effort required.

In a horizontal curve the effective tractive effort of the vehicle is reduced because of the turning angle. 

Suppose 'T' is the tractive effort of the vehicle, and the turning angle of the front wheel is 'theta' then the tractive effort in that direction will be less.

There will be a loss of tractive effort equal to (T-T.cos.theta). This loss in the tractive effort has to be compensated at the horizontal curves which also have the vertical grade. It is done by reducing the grade, this is known as the grade compensation.

According to IRC, a grade compensation  of,  (30+R/R), maximum 75/R  has to be provided for the grades more than 4%.
No grade compensation is required for the lower values of the the grade.
Here, R is the radius of the horizontal curve in meters.

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