- 117 Views
- Uploaded on
- Presentation posted in: General

Geometric Design (II)

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Geometric Design (II)

- To calculate minimum radius of horizontal curve
- To understand design concepts for transition curves and compute min length
- To understand the role of SSD in horizontal and vertical design
- To define and apply grade considerations
- To develop vertical curves
(Chapter 6.1 ~ 6.4)

- Minimum Curve Radius
- Curve requiring the most centripetal force for the given speed
- Given emax, umax, Vdesign

R

Point of

Tangency

Point of

Curvature

Based on circular curve

- R: radius of curve
- D: degree of curve
- : central angle
- T: length of tangent
- L: length of curve
- LC: long chord
- M: middle ordinate dist
- E: external dist

- Design baseline
- Curve radius above the minimum
- Superelevation and side-friction factor not exceeding the maximum values

- Design is revised to consider:
cost, environmental impacts, sight distances, aesthetic consequences, etc.

R

- Sight line is a chord of the circular curve
- Sight Distance is curve length measured along centerline of inside lane

Figure 6-10

- Gradually changing the curvature from tangents to circular curves

Without Transition Curves

With Transition Curves

- Gradually changing the curvature from tangents to circular curves
- Use a spiral curve
L: min length of spiral (ft)

V: speed (mph)

R: curve radius (ft)

C: rate of increase of centrifugal accel (ft/sec3), 1~3

- Use a spiral curve

- Gradually changing the cross-section of the roadway from normal to superelevated (Figure 6-9)

Keep water drainage in mind while considering all of the available cross-section options

Reduced Speed

Increased Speed

- Grade
- measure of inclination or slope, rise over the run
- Cars: negotiate 4-5% grades without significant speed reduction
- Trucks: significant speed changes
- 5% increase on short descending grades
- 7% decrease on short ascending grades

- Maximum grade – depends on terrain type, road functional class, and design speed

Rural Arterials

- Critical length of grade
- Maximum length which a loaded truck can travel without unreasonable speed reduction
- Based on accident involvement rates with 10mph speed reduction as threshold

General Design Speed Reduction

- To provide transition between two grades
- Consider
- Drainage (rainfall)
- Driver safety (SSD)
- Driver comfort

- Use parabolic curves
- Crest vs Sag curves

- Given
- G1, G2: initial & final grades in percent
- L: curve length (horizontal distance)

- Develop the actual shape of the vertical curve

point of vertical intersection

PVI

point of verticaltangency

point of vertical curvature

G1%

G2%

- Define curve so that PVI is at a horizontal distance of L/2 from PVC and PVT
- Provides constant rate of change of grade:

A

G1%

G2%

- G1 = 2%
- G2 = -4%
- Design speed = 70 mph
- Is this a crest or sag curve?
- What is A?

- Major control for safe operation is sight distance
- MSSD should be provided in all cases (use larger sight distance where economically and physically feasible)
- For sag curves, also concerned with driver comfort (large accelerations due to both gravitational and centrifugal forces)

- Critical length of curve, L, is where sight line is tangent to the crest
- Assume driver eye height (H1) = 3.5 ft and object height (H2) = 2.0 ft and S=MSSD

- Headlight sight distance
- Rider comfort
- Drainage control
- Appearance