If superelevation of 0.064 is adopted at a horizontal curve and width of the pavement at the curve is 7.5 m, then by how much should the outer edges of the pavement be raised above the surface if the pavement needs to be rotated about center line?
- A0.28 m
- B0.56 m
- C0.48 m
- D0.24 mCorrect
Solution
Concept
Attainment of superelevation by rotating about the center line balances earthwork and maintains vertical alignment, causing the outer edge to rise by half the total cross-fall height while the inner edge drops by the same amount.
Formula
Raise of outer edge above center line = (e × W) / 2
Step-by-step solution
1. Identify given values: Superelevation rate (e) = 0.064, Width of pavement (W) = 7.5 m. 2. Calculate the total height difference between inner and outer edges: E = e × W = 0.064 × 7.5 = 0.48 m. 3. Since rotation is about the center line, the outer edge is raised by half of the total height difference: Height = E / 2 = 0.48 / 2 = 0.24 m.
Explanation
When the pavement is rotated about its center line to attain superelevation, the inner edge is lowered by E/2 and the outer edge is raised by E/2 with respect to the center line. Here, total superelevation E = e × W = 0.064 × 7.5 = 0.48 m. Therefore, the outer edge is raised by 0.48 / 2 = 0.24 m above the center line.
Why the other options are wrong
A: Incorrect numerical calculation. B: Incorrect calculation involving twice the total rise. C: Represents the total height difference between inner and outer edges (or rotation about inner edge).
Shortcut
Raise about center line = e × W / 2 = 0.064 × 7.5 / 2 = 0.24 m.
Common mistake
Confusing rotation about the center line with rotation about the inner edge, leading to selecting 0.48 m instead of 0.24 m.
Correct answer: D) 0.24 m