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This article provides examples and problems related to geometric design, including overpass construction, raceway design, curve adequacy, highway ramp design, and pavement design.
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Problem 3.15 Due to crashes at a railroad crossing, an overpass (with a surface 24 ft above the existing road) is to be constructed on an existing level highway. The existing highway has a design speed of 50 mph. The overpass structure is to be level, centered above the railroad, and 200 ft long. What length of the existing level highway must be reconstructed to provide an appropriate vertical alignment?
Problem 3.27 A developer is having a single-lane raceway constructed with a 100 mph design speed. A curve on the raceway has a radius of 1000 ft, a central angle of 30 degrees, and PI stationing at 1125+10. If the design coefficient of side friction is 0.20, determine the superelevation required at design speed (do not ignore the normal component of the centripetal force). Also, compute the degree of curve, length of curve, and stationing of the PC and PT.
Geometric Design Example • A 2-lane (10 ft wide lanes) combined horizontal and crest vertical curve is currently posted for a 35 mph speed limit and is designed for 40 mph. Curve data are: • Horizontal: L = 580 ft, Δ = 60°, e = 6%, Ms = 30 ft • Vertical: L = 580 ft, G1 = 3%, G2 = -2.5% • Is this curve adequately designed for 40 mph? • Because this curve is often covered in packed snow and ice, the county wants to post a recommended speed limit for these conditions. What speed limit do you recommend?
Problem 3.37 • Two straight sections of freeway cross at a right angle. At the point of crossing, the east-west highway is at elevation 150 ft and is a constant +5.0% grade (upgrade in the east direction), the north-south highway is at elevation 125 ft and is a constant -3.0% grade (downgrade in the north direction). Design a 90-degree ramp that connects the northbound direction of travel to the eastbound direction of travel. • Design the ramp for the highest design speed (to the nearest 5 mph) without exceeding a minimum D of 8.0. (Assume the PC of the horizontal curve is at station 15+00, and the vertical curve PVIs are at the PC and PT of the horizontal curve). Maximum allowed superelevation by design standards is 12%. • Determine the following: • Stationing and elevations of the horizontal curve PC and PT • Horizontal curve radius and length • Stationing and elevations of the vertical curve PVCs and PVTs • Vertical curve lengths • Length of constant grade in between the 2 vertical curves (if any)
North Downhill (-3.0%) Uphill (+5.0%) EW freeway Elevation difference = 25 ft NS freeway
Pavement Design Example • The pavement on Stevens way is slowly being switched to PCC a few thousand yards at a time. Assuming all the heavy vehicle traffic on Stevens Way is due to bus traffic, estimate a rigid pavement design using the following data: • Design life = 40 years • Articulated bus average axle weights: • Front = 15,000 lb • Middle = 18,500 lb • Rear = 11,500 lb • Regular (non-articulated) bus average axle weights: • Front = 12,200 lb • Rear = 20,000 lb • Typical school day = 80 regular, 40 articulated in one direction • Bus traffic growth rate = 2% per year • Use undowel joints and crushed stone base material
Pavement Design Example • What would a flexible pavement design be for the same loading? Assume: • Two layers (HMA and a granular base course) • Subgrade MR = 10,000 psi
