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Example #6: Designing a Crest Vertical Curve

Example #6: Designing a Crest Vertical Curve. Design a length of vertical curve for a crest curve based on SSD and the following information: Design Speed = 40 mph g1 = 1.25% g2 = -2.75%. Solution: Determine the change in grade elevation A = |g2-g1| = |-2.75-1.25| A = 4.0%

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Example #6: Designing a Crest Vertical Curve

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  1. Example #6: Designing a Crest Vertical Curve Design a length of vertical curve for a crest curve based on SSD and the following information: Design Speed = 40 mph g1 = 1.25% g2 = -2.75% Solution: Determine the change in grade elevation A = |g2-g1| = |-2.75-1.25| A = 4.0% AASHTO Design Controls for SSD (Exhibit 3-76) - Min. SSD = 305 ft Assuming S > L L = 2S -2158/A L = 2*305 -2158/4 L = 610-539.50 L = 70.5 ft Assumption is correct S > L, We have found the minimum length of curve! Remember MoDOT recommends a length of curve of at least 300 ft when possible. What if we had started with this assumption ? S < L L = AS2/2158 L = 4*3052 /2158 L = 172.43ft S < L, Assumption is wrong, would have had to use the other formula to obtain the correct answer

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