geometric design. homework ch 3 # 1,3,7,8,9 geometric design roadway is designed using v, r, e –...
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![Page 1: Geometric Design. Homework Ch 3 # 1,3,7,8,9 Geometric Design Roadway is designed using v, R, e – Need to know the required Stopping Sight Distance for](https://reader036.vdocuments.site/reader036/viewer/2022062407/56649c755503460f94928b38/html5/thumbnails/1.jpg)
Geometric Design
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Homework
• Ch 3 # 1,3,7,8,9
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Geometric Design• Roadway is designed using v, R, e– Need to know the required Stopping Sight
Distance for the roadway– D = 1.47vt +v2/(30(f+g)
• v is in mph (1.47, 30 are conversion factors)• SSD – distance required to make a full stop• Decision Sight Distance – adds in a maneuver time (3.5 –
4.5s) to the reaction time based on AASHTO guidelines– D = 1.47v(tr+tm) +v2/(30(f+g)
• Gives Decision Sight Distance
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Alignment
• 2 types of alignment– Vertical– Horizontal
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Vertical Curves
• 2 types of curves– Crest– Sag
• Different criteria for SSD– crest - 6” object over hill– sag headlights
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Vertical Curves
• Grade controls – Trucks and f control– Max grade Interstate = 9%– Max grade local street = 15%– min grade = 0.5% - drainage– Fig 3.15– length of grade is important as well– Also look at length of curve and change in grade• Don’t want cars grabbing air
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Vertical Curves
• Truck performance• Note crawl speed• Fig 3.16, 3.17, 3.18
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Vertical Curves
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Vertical Curve Design
• Vertical Curves are Parabolas• A = G2 - G1 –algebraic diff in grades (in %)
• E = AL/8– L is in stations (ft/100)– gives E in ft
• y = [(G2-G1)/2L]*x^2 + G1x+ Y0– Gives y in ft– x is in stations, G1, G2 in %
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Vertical Curve Design
• Hi/Lo point occurs at dy/dx = 0– x = -LG1/(G2-G1)
– Not in middle of L– All calcs in stations
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Vertical Curve Design• SSD for Crest curves (dead cat distance)– 2 equations - L>=SSD,
– L=(!G2-G1!*(SSD^2))/(200*(h1^0.5+h2^0.5)^2– L<=SSD
– L=2*SSD - (200*(h1^0.5+h2^0.5)^2/(!G2-G1!)– These provide minimum values– only one is true– eye height h1 = 3.5 ft– object height h2 = 0.5 ft– 200*(h1^0.5+h2^0.5)^2 = 1329 (not what is in the
book)
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• Given a design speed of 45 mph, approach grade of +3% and a departure grade of -4% meeting at VPI Station = 100+50, VPI Elevation = 347.85. – What is the PVC & PVT stations and elevations– Where is the high point located and what is
elevation
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Vertical Curve Design
• L for Sag curves– L>SSD• L=(!G2-G1!*(SSD^2))/(200*(h+SSD*tan)
– L<SSD• L=2*SSD - (200 *(h+SSD*tan) /(!G2-G1!)
– 200 *(h+SSD*tan) = 400+3.5SSD– only one is true– headlight height h = 2 ft– beam angle = 1 degree (dispersion)
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• Given a design speed of 45 mph, approach grade of -4% and a departure grade of -1% meeting at VPI Station = 100+50, VPI Elevation = 347.85. – What is the PVC & PVT stations and elevations– Where is the high point located and what is
elevation
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Horizontal Alignment
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Horizontal Alignment
• 3 parts to alignment– tangents - straight sections– curves– transitions between curves and tangents
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Curves
• L=2R/360 • D - degree of curve• D different for RR • and highways
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Curves
• External Distance PI -> LE = R(sec(I/2)-1)• Middle Ordinate L->LC M = R(1-cos(I/2)• Tangent PC->PI T = Rtan(I/2)• Curve Length PC->PT L = 100(I/D)• Chord Length PC-> PT LC = 2Rsin(I/2)• Degree of curve D = 5729.58/R– How many degrees to create an arc of 100’ for a
given radius
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Horizontal Distance
• Need to maintain SSD around curve• Look at M and L to determine safe speed– L = SSD– I changes to I=L*D/100– M = R(1- cos (I/2))– If M > available M then NO GOOD
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Example
• Given the following info for a horizontal curve, determine if a speed limit of 60 mph is safe.– I = 45 degrees, R =1500 ft, 2-12 foot lanes, 6 foot
shoulder, e = 0.04, f=0.12, barn located 14 feet off centerline of inside lane.
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Design Vehicles
• Fig 2.4 • Gives tightest turn radius for a low speed turn
by a vehicle• 10 specific types of vehicles used– determine speeds, curb radius, pavement width
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Delineation of Vehicle Paths
• Flow Control• pavement markings, medians, islands• Channelization provide improved flow
through an intersection
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Channelization
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Channelization
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Lab
• Develop an Excel Spreadsheet which will solve horizontal and vertical curve problems