The Indian road congress recommends the use of the spiral as a transition curve in the horizontal
alignment of highways because of the following reasons:
1) The Spiral curve satisfies the necessity of an ideal transition
2) The geometric property of spiral is such that the calculations and setting out the curve in the field is simple and easy.

Length of Transition curve (L)

a) According to the Rate Of Change Of Centrifugal Acceleration

The centrifugal acceleration is 0 on the tangent point due to its infinite radius. But on the circular curve radius has a min value of R. So the rate of extra de of centrifugal acceleration must be selected such that there may be a consolation to the drivers. If c is the rate of extra de of centrifugal acceleration then,
 L=\frac{0.0215V^2}{CR} where V is the speed of the vehicle in kmph
C= Allowable rate of change of acceleration
R= Radius of the curve in m
L = Length of transition curve ‘m’

b) According to the rate of change of Superelevation

The length of the transition curve has to be enough sufficient to extrude the street surface from its cambered form to a completely fantastic elevated surface with no distortions. This requires that the length of the transition curve be decided so that it will make sure enough area for this maneuver with the desired rate.
 L=(W+E_{W})e if the pavement is rotated about the inner side
 L=(W+E_{W})e/2 if pavement is rotated about centre line

c) According to Empirical Formula

IRC recommends the length of the transition curve is minimum for plain and rolling terrain, The layout of the length of the transition curve is primarily based totally on those 3 criteria. The maximum cost amongst those 3 standards is followed because the deigned period of the transition curve
 L=\frac{2.7V^2}{R} (For Plain and rolling terrain)
 L=\frac{V^2}{R} (For Hilly area)
v = the design speed in kmph

Objectives / Purpose Of TRANSITION CURVE

Transition curves provide Super Elevation in a slow way and reduce the radius from infinity at an instant junction to a radius R at a curved junction gradually. Transition curves additionally lesser impact of a sudden jerk on passengers because of the impact of centrifugal pressure induced on cars/Vehicles at curves.

The ideal Transition curve must fulfill the subsequent requirements,

  • The curve must be tangential at its junction factors in order that the radius of transition is infinity on the instant junction and R on the curved junction.
  • The rate of the alternate of superelevation is the same as the rate of the alternate of curvature in order that complete super-elevation may be supplied inside the duration of the transition curve.

Spirals and clothoid are basically followed to provide transition curves, as counseled via way of means of design codes. Apart from those, cubic parabolic curves and lemniscates also are used as in step with alignment requirements. Governing parameters for starting off the one’s curves are the acceleration of cars, the preferred terrific elevations, lengths available for transition, and deflection angles The empirical formula is used to layout transition curves.

Types of Transition Curves

Euler’s spiral \phi = \frac{l^2}{2RL}
Cubical spiral  y = \frac{l^2}{6RL}
Bernoulli’s lemniscateThis curve is not used on railways. It is uniform beyond a 30° deflection angle.
Cubic parabola y = \frac{x^3}{6RL}
Shift S = \frac{L^2}{24R}

What is meant by transition curve?

Transition curve because the call suggests, is mostly a horizontal curve in plan supplied to permit the transition from a direct alignment to a circular curve gradually. In different words, it’s miles a curve which connects a with countless radius and radius R. Transition curves are generally furnished among a straight and curved track/roads.

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