Superelevation Of Road – Advantages And Design Of Super Elevation
Superelevation Of Road:
The inward transverse inclination provided to the cross-section of the carriageway at horizontally curved portion of a road is called superelevation, cant or banking.
Superelevation is provided to counteract the effect of centrifugal force and to minimize the tendency of the vehicle to overturn or skid by raising the outer edge of pavement with respect to the inner edge, providing a transverse slope throughout the length of the horizontal curve.
It is expressed as the ratio of elevation of the outer edge above the inner edge to the horizontal width of carriageway or as the tangent of the angle of slope of the road surface. It is generally denoted by ‘e’ or S.E.
Objects Of Providing Superelevation:
1. To counteract the effect of centrifugal force acting on the moving vehicle to pull out the same outward on a horizontal curve.
2. To help a fast moving vehicle to negotiate a curved path without overturning and skidding.
3. To ensure safety to the fast moving traffic.
4. To prevent damaging effect on the road surface due to improper distribution of load.
Advantages Of Superelevation:
1. It allows running of vehicle at high speed on a curved path as on a straight path without any danger of overturning and thus results into increased volume of traffic.
2. It provides more or less even distribution of load on wheels and hence uniform stress is offered on the foundation which results into less wear on wheel tyres and springs as well as economy in maintenance cost of the road.
3. It also helps to keep the vehicles to their proper side on the pavement and thus prevents collision of vehicles moving in opposite directions on a curved portion of the road.
4. It provides drainage of the whole width of road towards the inner side. Thus, there is no necessity of providing side drain on the outer side of the road.
Design Of superelevation:
To design superelevation for mixed type of traffic is a complex problem. The Indian Roads Congress has recommended to provide superelevation to counteract the centrifugal force fully due to 75% of the design speed and limiting the maximum superelevation to 1 in 15 or 7%.
Following are the steps adopted to design the superelevation of a road:
Calculate the superelevation for 75% of design speed neglecting the friction i.e f = 0
V = 75%(V) = 0.75 V
We know that, e+f = V²/127R
e + 0 = (0.75 V)²/127R
If the calculated value ‘e’ is less than 7% or 0.07, then provide the obtained value. If it exceeds 0.07 then provide the limiting value of superelevation emax = 0.07 and proceed to the next step.
Check the coefficient of friction developed for the maximum value of e = 0.07 at the full value of design speed,
If the value of ‘f’ thus calculated is less than 0.15, the superelevation of 0.07 is safe for the design speed. If not, calculate the restricted speed as given in step 4.
If the calculated value of ‘f’ exceeds 0.15, then the speed of the vehicles is restricted to the value Vr m/sec or Vr km/hr as calculated after allowing the limiting values of e and f in the following equation:
In case of an important highway, it is always desirable to design the road without speed restriction at curves. Hence, the curve should be realigned, if possible with a longer radius of curvature so that the design speed can be maintained without any restriction.