External Distance of a Crest Curve
Hardvertical curvegrades
A crest vertical curve 300 m long joins a grade of +2% to a grade of −4%. The external distance from the point of vertical intersection down to the curve is most nearly:
- A1.13 m
- B2.25 m
- C4.50 m
- D6.00 m
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Answer: B. 2.25 m
The algebraic grade change is A = |g₁ − g₂| = |2 − (−4)| = 6%. The mid-curve offset from the tangent intersection is E = AL/800 = (6)(300)/800 = 2.25 m. Using the arithmetic difference |2| − |4| = −2 rather than the algebraic change is the classic sign error and halves the result.
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