Design of Superelevation⇒
Design of superelevation is a complex problem for mixed traffic conditions because different vehicles are moving with different speed hence the required superelevation are also different.
Note: This topic is part of the second chapter of Highway Engineering. I suggest reading this topic in the context of the complete chapter: Geometric Design of Highway. If you want to read the entire Highway Engineering, click here: Highway Engineering.
In this case IRC suggest to design the superelevation for 75% of the design speed considering coefficient of friction is 0.
Step.1: The super elevation is calculated for 75% of design speed, neglecting the lateral friction.
e+f=\(\frac{V^{2}}{127R}\), V in km/h
for design, f=0, V=0.75V.
e =\(\frac{(0.75V)^{2}}{127R}\).
e =\(\frac{V^{2}}{225R}\).
Step.2: If the calculated value of ‘e’ is less than 7%, so obtained ‘e’ is provided. If the value of ‘e’ is more than 7%, then provided e= emax (7%) check for lateral friction also
Step.3: Check for friction developed for maximum value of e=7% at full value of design speed.
e+f=\(\frac{V^{2}}{127R}\).
f=\(\frac{V^{2}}{127R}\)-e.
f=\(\frac{V^{2}}{127R}\)-0.07.
If the value of ‘f’ calculated is less than 0.15, the superelevation of 7% is safe for the design but if it comes out to be more than 0.15, then if speed of vehicle has to the restricted to safe value. which is less than design speed.
Step.4: The allowed level speed at the car peach computed as follows
e+f= \(\frac{V^{2}}{127R}\).
0.07+0.15= \(\frac{Va^{2}}{127R}\).
VA =5.25\(\sqrt{R}\)
- If the allowable speed is higher than design speed then the design speed is adequate but it is less than design speed, the speed is limited to the allowable speed.
- Speed can be reduced by warning signs of yi speed limit regulations sign.
- On important highways as for as possible speed is not reduced hence the curve should be realigned with a larger radius of curvature.
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