Curve setting

 

Curve setting

Degree of a curve:

Degree of a curve is the angle (in degree) subtended at the of a circular curve by a chord or arc of length 30 metres.

Simple curve: 

When a curve consists of a single arc with a constant radius connecting the two tangents, it is known as simple curve.

Reverse curve: 

A reverse curve consists of two arc bending in opposite Their centres lie on opposite sides of the curve. Their radii may be either equal or different and they have one common tangent.

Compound curve: 

When a curve consists of two or more arcs with different radii is called a compound curve. Such a curve lies on the same side of a common tangent and the centre of the different arcs lie on the same side of their respective tangents.

 Combined curve: 

When transition curves are introduced at both ends of a circular curve, the resulting curve is known as a combined curve.

Transition curve: 

A curve of varying radius is known as a transition curve. The radius of such a curve varies from infinity to a certain fixed value. A transition curve is provided on both ends of a circular curve. The curvature varies from zero at the tangent point to a definite value just at the junction with the circular curve.

  Different types of transition curve:

(i) Enter's spiral.

 (ii) Cubical spiral. 

(iii) Cubic parabola.

 (iv) Lemniscate curve

The types of Horizontal curves are:

Simple circular, compound, transition, reverse and combined.

5º curve: 

The unit chord which subtends an angle of 5° at the centre.

 Radius of a lºcurve is 1719 m. So, Radius = 1719/D

Rear tangent: 

The first tangent along the direction of progress of a survey. 

Forward tangent: 

The second tangent along the direction of progress of a survey. 

Point of curve: 

The first tangent point along the direction of progress of a survey.

Point of tangency: 

The second tangent point along the direction of progress of a survey.

 Tangent length: 

Tangent length = R tan φ/2

• Length of curve, L = πRφ/180

• Length of long chord = 2R sinφ/2

Where, R = Radius of circular curve in φ

φ= Deflection angle.

• Chainage of 1s' tangent point = Chaihage of point of intersection - Tangent length 

• Chainage of 2nd tangent point = Chainage of 1s tangent point + Length of curve

• Deflection angle, S = 1719 x C/R

Centrifugal ratio: 

The ratio between the centrifugal force and the weight of the vehicle is known as centrifugal ratio.

Centrifugal ratio = P/W =WV²/gRW=V²/ gR

Allowable value for centrifugal ratio in roads = 0.25 

Allowable value for centrifugal ratio in railways = 0.125

Filed procedure of setting out compound curve:

AB, BC and DE are rear, forward and common tangent .Tangent lengths, BT1, BT3 are calculated and so are the curve lengths between T, and T2 and T2, and T3, The chainage T1, T2, T3 are set out on the ground Deflection angles are calculated and setting out table is prepared. The theodolite is centred over T, and levelled. Deflection angles for the for the chords of short curve are set out serially and points P1, P2,........ are marked until the tangent point T2 is reached . Similarly after shifting the theodolite at T2 P4, P5,........ are marked until T3 is reached All the points are joined to get the shope of the curve . [T1, T2,T3 is measured =(180°–P1+P2/2)