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CE 436/536 – ROADWAY DESIGN January 29, 2008

CE 436/536 – ROADWAY DESIGN January 29, 2008. Review Homework Assignment Go over solution to problem #1 Discuss problem #2. Topics to cover this week: Sight Distance Refresher Roadway Classifications Design Standards Horizontal Alignments (Chapter 3 + AASHTO). CE 436/536 – ROADWAY DESIGN

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CE 436/536 – ROADWAY DESIGN January 29, 2008

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  1. CE 436/536 – ROADWAY DESIGN January 29, 2008 • Review Homework Assignment • Go over solution to problem #1 • Discuss problem #2 • Topics to cover this week: • Sight Distance Refresher • Roadway Classifications • Design Standards • Horizontal Alignments (Chapter 3 + AASHTO)

  2. CE 436/536 – ROADWAY DESIGN January 29, 2008

  3. Criteria for Measuring Sight Distance Last week we talked about stopping sight distance (SSD), mostly from a horizontal perspective (we did account for how grades affect SSD). AASHTO 2001 – sight distance is the distance along a roadway throughout which an object of specified height is continuously visible to the driver. This distance is dependent on the height of the driver’s eye above the road surface, the specified object height above the road surface, and the height and lateral position of any sight obstructions within the driver’s line of sight. Height of Driver’s Eye AASHTO says the average height of a driver’s eye is 3.5 feet. Height of Object AASHTO says the object height should be 2.0 feet for SSD and 3.5 feet for PSD. Height of object used to be 0.5 feet in previous versions of AAHSTO. Why did AASHTO change? Oregon, for example, chose to retain 0.5 feet as their design value.

  4. Criteria for Measuring Sight Distance Vertical curve example using driver’s eye and object height.

  5. Roadway Classifications Why does there need to be a system for classifying roadways? AASHTO 2001 – “The classification of highways (roadways) into different operational systems, functional classes, or geometric types is necessary for communication among engineers, administrators, and the general public.” Classification Systems By Design Type By Route Numbering By Administrative Needs By Functional Class Design Type = Freeways, Highways, Local Streets, etc. Route Numbering = Federal, State, County, etc. (Even vs. Odd Route #) Administrative Need = NHS, non-NHS, who is responsible, etc. Functional Class = Grouping based on the character of service they provide

  6. Roadway Classifications Functional Class System What is the intended purpose of the roadway? Mobility versus access (see figure next page) Hierarchy of the roadway system. Federal – State – Local Freeways - Arterial – Collector – Local Roads Major – Minor Characteristics and local examples of each functional class Freeway Arterial Collector Local Road

  7. Roadway Classifications

  8. Roadway Classifications Why is the functional class an important part of roadway design? Design speed Access management/characteristics Horizontal and vertical geometry Cross-sectional geometry Superelevation Roadside design Structural clearance and loading requirements Pavement design Others As a roadway design engineer, what are the two pieces of information you would need before starting a design, based on what we have covered so far? Design Speed and Functional Classification

  9. Design Standards What is a design standard? Why do we use them? Are there different design standards? Which one do we use? Typically most government agencies responsible for the design of roadways default to AASHTO as their design standard. States and larger cities often supplement AASHTO with region specific design standards intended to help AASHTO, which is very broad, focus on regionally significant issues. Standard VS Guideline - many times you will hear engineers refer to AASHTO or any design standard as a “guideline” which still requires interpretation and application of engineering judgment towards the final design product. I have found this to be both true and false depending on the jurisdiction. Legal ramifications of following, or not following, design standards and guidelines. Proof of design negligence versus driver error.

  10. Design Standards Examples of what design standards can help us with: See following slides for examples.

  11. Design Standards

  12. Design Standards

  13. Design Standards

  14. Design Standards

  15. Discussion of Roadway Design Plans • Common components of a set of roadway plans: • Title sheet • Typical sections • Details • Traffic control plans • Plan and profile • Structural • Striping • Signing • Signal • Standard drawings • Let’s talk about plan and profile drawings. • Look at sample plans.

  16. Discussion of Roadway Design Plans What is the purpose of the plan view? To show horizontal features What is the purpose of the profile view? To show vertical features What is engineering stationing and why is it used? Stationing examples

  17. Horizontal Alignments/Geometry The horizontal alignment/geometry is the most fundamental component of roadway design and is the beginning point for most all roadway design projects. Vertical alignments (profiles) are subordinate to horizontal alignments. Components of horizontal geometry: Alignment – series of tangents and curves Superelevation and transitions Cross-sectional elements Why aren’t all roads straight, why are there curves? Different types of curves: Circular Spiral Reversing Compound (multi radii)

  18. Horizontal Alignments/Geometry Examples of each type of curve.

  19. Horizontal Alignments/Geometry Elements of a circular curve: Tangent Point of intersection (PI) Point of curvature (PC) Point of tangency (PT) Point of reversing curve (PRC) Radius Degree of curve The radius and degree of curve are mathematically related by the following: D = 5729.58/R So a curve with a radius of 2000 feet is also a 2.86 degree curve.

  20. Horizontal Alignments/Geometry

  21. Horizontal Alignments/Geometry How do we know what type of curve to use and what radius or length of curve to use? The type of curve is dependant on what design standard the jurisdiction uses (i.e. some agencies use spiral curves, others do not) and what you are trying to accomplish with the curve. The radius or length of curve (spiral) is dependant on the design speed and maximum superelevation rate that is allowable for the specific conditions. Let’s look at an example: Problem 3.13 on page 77 of text

  22. Horizontal Alignments/Geometry

  23. Horizontal Alignments/Geometry Using equation 3.34 on page 77 we can calculate the minimum radius that is allowable for a given design speed and superelevation rate. Using the tables in AASHTO or Table 3.5 in our text, we can see the minimum radius for all practical design speeds and super rates are already calculated for us.

  24. Horizontal Alignments/Geometry What things would a roadway design engineer take into account in selecting the size and location of horizontal curves? Let’s work some sample problems. Terrain Cost of construction Avoid something sensitive or important Match some existing conditions or force road to go a certain place Others

  25. Homework Assignment #3 January 29, 2008 Homework, due February 5th: PROBLEM #1 Given: A 1.0 degree horizontal curve Tangent length = 600 feet PC station is 105+45 Find: What is the station of the PT?

  26. Homework Assignment #2 January 29, 2008 Homework, due February 5th: PROBLEM #2 Using the information from problem #1 plus the following: 1) Design speed of 60 mph 2) Super rate of 6% maximum 3) Coefficient of side friction of 0.120 Find: Is the curve as designed acceptable in terms of minimum radius?

  27. Homework Assignment #2 January 29, 2008 Homework, due February 5th: PROBLEM #3 Given: A roadway has a posted speed of 55 mph The PC station is 1+00 The PT station is 11+00 The curve is superelevated at a rate of 0.04 ft/ft Table 3.5 in text book Central angle of curve is 60 degrees Find: Is the design of the curve appropriate in terms of radius and super rate? If the radius is not adequate, what the super rate need to be to make it adequate? Verify the new super rate will make R >= Rv

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