Attainment of Superelevation in the Field
Introduction of superelevation on horizontal curve in the field is very typical as on straight portions of road cross section normal camber is provided with crown at the center, sloping downward on both sides.
But the portion of cross section on circular curve is superelevated with uniformly sloping down from outer edge up to the inner edge of pavement.
The attainment of superelevation is done in two stages
Elimination of crown of cambered section.
Rotation of pavement to attain full superelevation.
Elimination of crown of cambered section it can also be achieved by two methods:-
In the first method outer half of the cross maps is sorted about the crown at desired rate to attain the superelevation of inner half & center line is not allowed.
In the 2nd method (Diagonal crown method) the crown is shifted progressively outwards thus increasing the width of the inner half of the cross section.
Additional width of carriage ways that is required on horizontal curves is referred to as extra widening. The rear wheels follow the inner path on the curve as compared with the front wheels. This phenomenon is called tracking.
Reasons to provide extra widening:-
To avoid off tacking due to rigidity of wheels.
To counteract transverse skidding.
To increase visibility of curves
To encounter psychological tendency while overtaking operation.
The total extra widening (We) required on a horizontal curve is given by the following equation:
We (in m) = Mechanical widening(Wm) + Psychological widening(Wps) = nl²/2R + V/9.5√R
This widening done due to two reasons:-
Mechanical Widening (Wm)
The reasons for the mechanical widening are:
When a vehicle negotiates a horizontal curve, the rear wheels follow a path of shorter radius than the front wheels.
Wm = nl²/2R
Psychological Widening (Wps)
Widening of pavements has to be done for some psychological reasons also. There is a tendency for the drivers to drive close to the edges of the pavement on curves.
Some extra space is to be provided for more clearance for the crossing and overtaking operations on curves. IRC proposed an empirical relation for the psychological.
Wps = V/9.5√R
IRC RECOMMENDATION FOR EXTRA WIDENING FOR SINGLE AND TWO LANE PAVEMENT ARE GIVEN BELOW:-
If R> 300 m, then extrawidening is not provided.
If R< 50 m , then extra widening is provided in the inner edge.
If 50<R<300 , then extrawidneing is provided at both the edges.
Curve Resistance for Turning Vehicles
When a vehicle turns on a horizontal curve by tuning the front wheels, but the rear wheels do not turn as shown
The direction of rotation of the rear and front wheels are different. So there are some losses in tractive force.
Thus the loss of tractive force due to turning of a vehicle on a horizontal curve is termed as curve resistance.
curve resistance=T-T cos∅