Transition curve-Highway engineering

Jobghost.blogspot.com explains most important topics in Civil engineering for the GATE-2016 examination. Here the “TRANSPORTATION ENGINEERING-Transition curve and Set-back distance” topics are explained. Each and every word given below is important for all your civil engineering competitive examinations. And also the GATE-2016 practice sets, Example problems, previous year Question papers are available in jobghost.blogspot.com.

Transition curve:

When the Straight road meets a horizontal curve the road should be turned gradually. That gradual joining curve of straight and circular curve is called transition curve

Length of Transition curve calculated based on three criteria

1)   For gradual Centrifugal transition:

The centrifugal force should not give a jerk to the vehicle due to sudden turning. So it should be counteracted to provide a smooth turning

v---> in m/sec  

If v given in KMPH convert to m/sec by multiplying  (5/18)

2)   For Gradual increase of rate of change of super elevation:      

The transition curve should gradually increase the super elevation up to its maximum percentage.  So that the curve will not offer any sudden jerk to the road users. And also the centrifugal force will be counteracted at its main curve. So the length of transition curve based on rate of change of super elevation is calculated as

L_s = B.e.N                            

N ----> Slope

R ----> Radius of Curve

B ----> Width of road (including widening)

3) Empirical Formulae:

Based on the terrain condition the length of transition curve can be calculated by using the following empirical formulas.

a.For plain and rolling terrain:

b.    For Steep Terrain:

The “highest value” of the above three calculated values are taken as the design value of length of curve.

Set-Back Distance :(m)

It is the visible distance required by a driver to see the vehicle coming from the opposite side of circular curve. So that the driver can safely proceed without colliding with upcoming vehicle due to insufficient visibility.

There are two Cases involved in designing set back distance.

Case-1: If L > SSD      

When the Length of curve is greater than the required sight distance then the set-back distance required is calculated as below..

Let us take “m” as required set back distance shown in figure.

R is radius of curve.

Considering OAB as a triangle the distance X can be calculated as below

The radius of curve is at the center of road. So the distance from outer edge to center of road to be deduced to form a perfect triangle.

d- Distance b/w center of road to center line of inner lane (d = 0 for single lane)

Now we have Found X in the triangle, Hence the set-back distance

Case-2: If L < SSD               

When the length of curve is lesser then required sight distance then the curve should be designed as below.

Here the distance “C” is calculated as same as above , and the distance G is need to be added to find a total distance.

          m= C+G

To find G Triangle OPQ formed.

          OQ= S-L/2

Adding C and G we get

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