Friday, 20 January 2017

Superelevation Analysis and its importance in Highway Engineering

Explanation of Superelevation

In order to counteract the effect of Centrifugal force and to reduce the tendency of vehicle to overturn or skid,the outer edge of the pavement is raised with respect to the inner edge.Thus,providing the transverse slope throughout the length of the horizontal curve.This transverse inclination to the pavement surface is known as "Superelevation" . Little bit Confusing right ? Don't worry I will give you clear idea about it and also about Superelevation Analysis

When you read on top about Superelevation you may got confusion about what is Centrifugal force ? Why must we counteract it ? 

Assume a body is revolving around a center in circle.Then there will develop a force which will attract body towards center.This Force is called as Centripetal Force.

You may have read about Newton's third law "For every action there will be an equal and opposite reaction" 

Same law applies here To counteract Centripetal force a Force which will drives body away from center acts.This Force is called as Centrifugal Force.

So you can remember now that Centrifugal Force will drive body away from Center revolving in a Circle.

How Centrifugal Force involved in Highway Engineering ?

Bro,roads will not be straight always.Sometimes we have to face curves (turnings).Those curves are nothing but circles as mentioned above.So when Vehicles take turning,there will a centrifugal force acting.We must counteract it otherwise Centrifugal force drives vehicle farther from center,which means it gets skidded or overturn leading to dangerous accidents.

To avoid this we are introducing Superelevation at turnings.

How Superelevation helps counteracting Centrifugal Force ? 

You can get Answer for that question from Superelevation Analysis

Analysis of Superelevation

Superelevation is also called as Cant or banking. Superelevation is expressed with the letter "e"

Superelevation (e) is the ratio of the height of the outer edge with respect to the horizontal width.

Time to observe the above Figure :

what you Notice from Figure ;

  • Centrifugal force as I mentioned Above acting outwards away from the center
  • Weight of the Vehicle acting vertically downwards through the center of gravity.
  • Angle of inclination between inner edge and outer edge of the road
  • "E" is the height of outer edge with respect to inner edge ( From N to L)
  • "B" is the horizontal width (From M to N)
As the angle of super elevation is small and we get super elevation as shown in figure.

 we can see Frictional forces FA and FB at A and B as shown in figure.Those are the frictional forces and the values of FA abd FB reach maxium value of fxRB and fxRA respectively where "f" is the coefficient of lateral friction and RA and RB are the normal reactions of the wheels A and B

The centrifugal force is thus opposed by corresponding value of the friction developed and by the component of the force of the gravity due to the superelevation provided.

Considering the equilibrium of the components of forces acting parallel to the plane and component of centrifugal force is opposed by the component of gravity and the frictional forces FA and FB

 Above we have considered 1-ftanθ =1,the reason is that lateral friction "f" is taken as 0.15 for design purposes.The value of tanθ due to superelevation rarely exceeds 0.07.So the value of ftanθ - 0.15x0.07=0.01 .Thus the value of 1-f tanθ = 1.0.01=0.99 which is approximately "1"
In above equation,

e= rate of superelevation =tan θ
f=design value of lateral friction coefficient=0.15
v=speed of the vehicle,m/sec
R=radius of the horizontal curve,m
g=acceleration due to gravity=9.8 m/s2

The above equation is when velocity in m/sec,For velocity in kmph we have to multiply velocity in m/sec with 5/18=0.278 

Here in above equation,
V is Speed in Kmph
R is radius of the horizontal curve

This is all about Superelevation today.I am going to discuss about Superelevation design in highway engineering in my upcoming article.Until then stay tunes to our website.

If you have any queries with respect to Superelevation Analysis please let me know in comments.

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