16-07-2012, 05:08 PM
SUPERELEVATION
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INTRODUCTION
When a particles moves from straight to curve, then a force (known as centrifugal force) acts upon it and tends to push it away from the centre. The force acts horizontally and tends to push the particle off the road or track. This is occurs because there is no component force to counterbalance this force. So to counteract this effect, the outer edge is raised above the inner one and the particle moves smoothly. The superelevation is provided both on roadway and railway track.
SUPERELEVATION IN HIGHWAYS
Superelevation in highways is generally the inclination provided to the carriageway of the horizontal curved portion of the road. OBJECTS ::
To counteract the effect of centrifugal force. So the vehicle moves savely at high speed on a curved path.
To ensure safety the fast moving vehicle from skidding or overturning.
To prevent damaging effect of the road due to improper distribution of load.
Due to inclination there is no necessity of provide side drain
ANALYSIS OF SUPERELEVATION ::
When a vehicle is moving on a circular curve of radius ‘R’ at a speed ‘V’, then some forces acting on the vehicle.
Weight ‘W’ of vehicle acting vertically downward.
The frictional force FA and FB acting along the pavement surface to the centre of the curve.
The centrifugal force ( ) acting horizontally outwards.
MAXIMUM AND MINIMUM VALUES
:: The values of superelevation increases with increases with speed or decreases in radius of curve. It is always necessary to limit maximum values of superelevation. As per IRC maximum values of superelevation for different terrain are follows : In plain and rolling terrain – 7% In snow bound areas – 7% In hilly area area not bound by snow – 10% For expressways in urban areas 8% superelevation is used When the design values of superelevation is less than or equal to the road camber, the normal cambered section should be continued on the curved portion without providing any superelevation.
METHODS OF DESIGN ::
The IRC has recommended to provide superelevation to counteract the centrifugal force fully due to 75% design speed and limiting the maximum superelevation to 7%. The required superelevation is designed by following steps :
The superelevation for 75% of design speed is calculated by e = 2225
If the value of ‘e’ is less than 0.7, then the obtained value is provided, if exceeds then maximum value 0.07 is provide.