Question

a) State the 3 parameters needed for design a circular curve. b) Describe the possible traditional methods for setting out circular curves. c) Describe the two types of transition curves and comment on the use of them. d) Describe the two main requirements in the design and construction of vertical curves.

Answer 1

A. Three parameters needed to design a circular curve are- 1. Radius, R 2. The deflection angle between the tangents, α 3. Tangent Distance, T.

B. There are many traditional methods of setting out a circular curve. Some Linear methods include 1. Ordinates from the long chord method, 2. By successive bisection of arc method 3. Offsets from the tangent method, 4. By offsets from chords produced. Similarly, some Angular methods are- 1. Rankine method of tangential angle 2. Two Theodolite method 3. Tacheometric method

C. A transition curve is used to join a straight line to a circular curve to smoothen the feeling of transition by the road user. Two types of transition curve are- 1. Spiral - Driver of the vehicle will feel a strong centrifugal force if it goes to a circular curve directly from the straight path. So the use of a spiral transition curve helps to avoid the sudden impact of this centrifugal force as this transition curve follows the actual path of the vehicle more closely and improves the visibility of the highway.  and 2. Cubical parabola- Used mostly in railways.

D. Two main requirements in the design and construction of the vertical curves are- 1. Stopping sight distance- It is the total distances from when the driver decides to apply the brake until the vehicle stop. and 2. Passing sight distance- it is defined as the distance that allows a driver to complete a normal pass while that driver can observe that there is no potential threat ahead before making the pass.