Click here CURVE SETTING
1. What do you mean by degree of a curve.
Answer:
Degree of a curve is the angle (in degree) subtended at the centre of a circular curve by a chord or arc of length 30 metres.
2. Define (i) simple curve, (ii) reverse curve, (ii) compound curve, (iv) combined curve.
Answer:
(i) Simple curve:
When a curve consists of a single arc with a constant radius connecting the two tangents, it is known as simple curve.
(ii) Reverse curve:
A reverse curve consists of two arc bending in opposite directions. Their centres lie on opposite sides of the curve. Their radii may be either equal or different and they have one common tangent
(iii) Compound curve:
When a curve consists of two or more arcs with different , 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.
(iv) Combined curve:
When transition curves are introduced at both ends of a circular curve, the resulting curve is known as a combined curve.
3. What is transition curve? What are the objects of providing it ? Name different types of transition curve in common use. What are the requirements of ideal transition curve?
Answer:
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. Transition curves are provided in railway tracks to ensure safe running trains.
Transition curves are provided for the following reasons:
(i) To provide the super elevation gradually from zero at the tangent point to the specified amount on the circular curve.
(ii) To maintain a constant proportion between super elevation and rate of change curvature.
(iii) To avoid overturning of train.
(iv) To minimize wear and tear of rail section due to unusual friction at point of turning.
Different types of transition curve:
(i) Enter's spiral:
(ii) Cubical spiral.
(iii) Cubic parabola.
(iv) Lemniscate curve.
Requirements of ideal transition curve:
The following conditions should be attained exactly at the junction with the circular curve.
(i) The rate of change of super elevation should be the same as that of the curvature.
(ii) The radius of the transition curve should be equal to that of the circular curve exactly at the junction.
(iii) It should meet the tangent point tangentially:
(iv) It should meet the circular curve tangentially.
(v) The specified amount of superelevation should be attained exactly at the junction with the circular curve.
Q4. Write short notes on -
(a) Tangent length
(b) super elevation,
(c) centrifugal ratio ,
Answer:-
(a) Tangent length:-
Tangent length = R tan φ/2
Where, R = Radius of circular curve in φ
φ = Deflection angle.
(b) Super elevation:
When a particle moves in a circular path, then a force known as centrifugal force acts upon it and tends to push it away from the centre. Similarly, when a vehicle suddenly moves from a straight to a curved path, the centrifugal force tends to push the vehicle away from the road or track. This is because there is no component force to counterbalance this centrifugal force. To counterbalance the centrifugal force, the outer edge of the road or rail is raised to some height (with respect to the inner edge), so that the sine component of the weight of the vehicle (W sin theta ) may counterbalance the overturning force. The height through which the outer edge of the road or rail is raised is known as super elevation.
(C) 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
5. a) What is a vertical curve?
b) Why is it provided?
c) State an expression for calculating the length of a vertical curve.
d) Explain the different types of vertical curves possible.
Answer:
(a) Vertical curves are used to attach a straight line, or in the real world a road, that goes either up or down at a continuous grade.
(b) Vertical curves are provided to change the slope in the road and may or may not be symmetrical. They are parabolic and not circular like horizontal curves. Identifying the proper grade and the safe passing sight distance is the main design criterion of the vertical curve, summit vertical curve the length should be enough to provide safe stopping sight distance and in sag vertical curve the length is important as it influences the factors such as headlight sight distance, rider comfort and drainage requirements.
(c) Expression for calculating the length of a vertical curve:
Length of vertical curve =
change of grade/rate of change of grade =( g1 – g2 )/ r
(d) Summit Vertical Curves:
Vertical curves at a crest or at the top of a hill are called also called crest curves. Summit vertical curves are used to connect two separate inclined sections. In calculating crest curves, you only need to find a correct length for the curve that will match the correct sight distance. The sight distance as well as the distance of the curve can be compared to each other in two different ways. The first is that the sight distance is less than the length of the curve and the second is that the length of the curve could be less than the sight distance.
Sag Vertical Curves:
Vertical curves at the bottom of a hill are called sag curves. Sag vertical curves are used to connect two descending grades which form an upside down parabola, or sag. Similar to crest vertical curves, the sight distance is the primary parameter needed to find the length of the curve. When designing sag curves however, you must take into account the positive change in grade which accounts for increased acceleration, or inertia. Using a dip in the road for example, sag vertical curves offer drivers a view of the roadway during daylight hours, but shorten the headlight views at night.
Comments
Post a Comment