Curves and Super elevation - Introduction
Curves are introduced on a railway track to bypass obstacles, to provide longer and easily traversed gradients, and to pass a railway line through obligatory or desirable locations. Horizontal curves are provided when a change in the direction of the track is required and vertical curves are provided at points where two gradients meet or where a gradient meets level ground. To provide a comfortable ride on a horizontal curve, the level of the outer rail is raised above the level of the inner rail. This is known as superelevation.
This section describes the defining parameters, elements, and methods of setting out circular curves.
Radius or degree of a curve
A curve is defined either by its radius or by its degree. The degree of a curve (D) is the angle subtended at its centre by a 30.5-m or 100-ft chord.
The value of the degree of the curve can be determined as indicated below. Circumference of a circle = 2p R
Angle subtended at the centre by a circle with this circumference = 360 o
Angle subtended at the centre by a 30.5-m chord, or degree of curve
In cases where the radius is very large, the arc of a circle is almost equal to the chord connecting the two ends of the arc. The degree of the curve is thus given by the following formulae
D = 1750/R (when R is in metres) D = 5730/R (when R is in feet)
A 2 o curve, therefore, has a radius of 1750/2 = 875 m.
Relationship between radius and versine of a curve
The versine is the perpendicular distance of the midpoint of a chord from the arc of a circle. The relationship between the radius and versine of a curve can be established as shown in Fig. 13.1. Let R be the radius of the curve, C be the length of the chord, and V be the versine of a chord of length C.
In Eqn (13.1), V, C, and R are in the same unit, say, metres or centimetres. This general equation can be used to determined versines if the chord and the radius of a curve are known.
Case I: Values in metric units Formula (13.1) can also be written as
where R is the radius of the curve, C is the chord length in metres, and V is the versine in centimetres, or
Case II: Values in fps units When R1 is the radius in feet, C1 is the chord length in feet, and V1 is the versine in inches, Formula (13.1) can be written as
Using formulae (13.2) and (13.3), the radius of the curve can be calculated once the versine and chord length are known.
Determination of degree of a curve in field
For determining the degree of the curve in the field, a chord length of either 11.8 m or 62 ft is adopted. The relationship between the degree and versine of a curve is very simple for these chord lengths as indicated below.
Versine on a 11.8-m chord
This important relationship is helpful in determining the degree of the curve at any point by measuring the versine either in centimetres on a 11.8-m chord or in inches on a 62-ft chord. The curve can be of as many degrees as there are centimetres or inches of the versine for the chord lengths given above.
Maximum Degree of a Curve The maximum permissible degree of a curve on a track depends on various factors such as gauge, wheel base of the vehicle, maximum permissible superelevation, and other such allied factors. The maximum degree or the minimum radius of the curve permitted on Indian Railways for various gauges is given in Table 13.1.
Table 13.1 Maximum permissible degree of curves
Elements of a circular curve
In Fig. 13.2, AO and BO are two tangents of a circular curve which meet or intersect at a point O, called the point of intersection or apex. T1 and T2 are the points where the curve touches the tangents, called tangent points (TP). OT1 and OT2 are the tangent lengths of the curve and are equal in the case of a simple curve. T1T2 is the chord and EF is the versine of the same. The angle AOB formed between the tangents AO and OB is called the angle of intersection ( Ð1) and the angle BOO1 is the angle of deflection (Ðf) . The following are some of the important relations between these elements:
1.Setting Out a Circular Curve
A circular curve is generally set out by any one of the following methods.
Tangential offset method
The tangential offset method is employed for setting out a short curve of a length of about 100 m (300 ft). It is generally used for laying turnout curves.
In Fig. 13.3, let PQ be the straight alignment and T be the tangent point for a curve of a known radius. Let AA ¢ , BB¢ , CC ¢ , etc. be perpendicular offsets from the tangent. It can be proved that
Value of offset O1 = C12= 2R
where C1 is the length of the chord along the tangent. Similarly,
The various steps involved in the laying out of a curve using this method are as follows.
(a) Extend the straight alignment PT to TQ with the help of a ranging rod. TQ is now the tangential direction.
(b) Measure lengths C1, C2, C3, etc. along the tangential direction and calculate the offsets O1, O2, O3, etc. for these lengths as per the formulae given above. For simplicity, the values of C1, C2, C3, etc. may be taken in multiples of three or so.
(c) Measure the perpendicular offsets O1, O2, O3, etc. from the points A, B, C,
¢ , B¢ , C¢ , etc. on the curve.
In practice, sometimes it becomes difficult to extend the tangent length beyond a certain point due to the presence of some obstruction or because the offsets become too large to measure accurately as the length of the curve increases. In such cases, the curve is laid up to any convenient point and another tangent is drawn out at this point. For laying the curve further, offsets are measured at fixed distances from the newly drawn tangent.
Long chord offset method
The long chord offset method is employed for laying curves of short lengths. In such cases, it is necessary that both tangent points be located in such a way that the distance between them can be measured, and the offsets taken from the long chord.
In Fig. 13.4, let T1T2 be the long chord of a curve of radius R.
Let the length of the long chord be C and let it be divided into eight equal parts T1 A, AB, BC, CD, etc., where each part has a length x = C/8. Let PW be a line parallel to the long chord and let O1, O2, and O3 be the offsets taken from points R, Q, and P.
Versine V from the long chord C is calculated by the formula
During fieldwork, first the long chord is marked on the ground and its length measured. Then points A, B, C, etc. are marked by dividing this long chord into eight equal parts. The values of the perpendicular offsets V1, V2, V3, etc. are then calculated and the points A¢ , B¢ , C¢, etc. identified on the curve.
Quartering of versine method
The quartering of versine method (Fig. 13.5) is also used for laying curves of short lengths, of about 100 m (300 ft). In this method, first the location of the two tangent points (T1 and T2) is determined and then the distance between them is measured. The versine (V) is then calculated using the formula
V is measured in the perpendicular direction at the central point O of the long chord. The tangent points T1 and T2 are joined and the distance AT1 measured. As AT1 is almost half the length of chord T1T2 and as versines are proportional to the square of the chord, the versine of chord AT1 is V/4.
For laying the curve in the field, the versine V/4 is measured at the central point B on chord AT1 and the position of point B is thus fixed. Similarly, a point is also fixed on the second half of the curve. AB is further taken as a sub-chord and the versine on this sub-chord is measured as V/16. In this way the points D and F are also fixed. The curve can thus be laid by marking half-chords and quartering the versines on these half-chords.
Chord deflection method
The chord deflection method of laying curves is one of the most popular methods with Indian Railways. The method is particularly suited to confined locations, as most of the work is done in the immediate proximity of the curves. In Fig. 13.6, let T1 be the tangent point and A, B, C, D, etc. be successive points on the curve. Let X1, X2, X3 and X4 be the length of chords T1A, AB, BC, and CD. In practice, all the chords are of equal length. Let the value of these chords be c. The last chord may be of a different length. Let its value be c1. It can be proved that
1. The position of the tangent point T1 is located by measuring a distance equal to the tangent length R tan f/2 from the apex point O. In this case, f is the deflection angle.
2. A length equal to the first chord (c) is measured along the tangent line T1O and point A¢ is marked.
3. The zero end of the tape is placed at the tangent point T1. It is then swung and the arc A1A marked. Then the first offset on the arc is measured. The value of the offset is c2/2R. The position of point A is thus fixed.
4. The chord T1A is extended to point B and AB¢ is marked as the second chord length equal to c.
5. The position of point B is then fixed on the curve since the value of the second offset is known and is equal to c2/R.
6. Similarly, the positions of other points C, D, etc. are also located.
7. The last point on the curve is located by taking the value of the offset as
c1 (c + c1 )/R , where c1 is the length of the last chord.
The various points on the curve should be set with great precision because if any point is fixed inaccurately, its error is carried forward to all subsequent points.
The theodolite method for setting out curves is also a very popular method with Indian Railways, particularly when accuracy is required. This method is also known as Rankine's method of tangential angles. In this method, the curve is set out using tangential angles with the help of a theodolite and a chain or a tape.
In Fig. 13.7, let A, B, C, D, etc. be successive points on the curve with lengths
T1A = x1, AB = x2, BC = x3, CD = x4, etc. Let d1 , d 2 , d 3 , d4 be the tangential angles OT1A, AT1B, BT1C, and CT1D made by the successive chords amongst themselves.
Let D1 , D 2 , D 3 , and D4 be the deflection angles of the chord from the deflection line.
Angle subtended at centre by a 100-ft chord = D o Tangential angle for a 100-ft chord = D/2
Tangential angle for an x-ft chord, d = (D/2) × (1/100) x degree
= (5730/2R) × (1/100) × 60 x minutes
where d is the deflection angle in minutes, x is the chord length in feet, and R is
the radius in feet. It is seen that
D1 = d1
D 2 = d 1 + d 2 = D1 + d2
D 3 = d 1 + d 2 + d 3 = D 2 + d3
The procedure followed for setting the curve is as follows.
(a) The theodolite is set on the tangent point T1 in the direction of T2O.
(b) The theodolite is rotated by an angle d1 , which is already calculated, and the line T1A1 is set.
(c) The distance x1 is measured on the line T1A1 in order to locate the point A.
(d) Now the theodolite is rotated by a deflection angle d2 to set it in the direction of T1B1 and point B is located by measuring AB as the chord length x2.
(e) Similarly, the other points C, D, E, etc. are located on the curve by rotating the theodolite to the required deflection angles till the last point on the curve is reached.
(f) If higher precision is required, the curve can also be set by using two theodolites.