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Welcome . Highway Engineering Code No. (CM 304). Lec . 7& 8. Dr.Khaled ali. Horizontal Alignment. Horizontal Alignment. Objective: Geometry of directional transition to ensure: Safety Comfort Primary challenge Transition between two directions Horizontal curves Fundamentals

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  1. Welcome Highway Engineering Code No. (CM 304) Lec. 7& 8 Dr.Khaledali

  2. Horizontal Alignment

  3. Horizontal Alignment • Objective: • Geometry of directional transition to ensure: • Safety • Comfort • Primary challenge • Transition between two directions • Horizontal curves • Fundamentals • Circular curves • Superelevation Δ

  4. Factor Affecting the Design of Horizontal Curves • The Design Speed of the road • The Safe allowable Coefficient of Friction in the lateral direction • Maximum Allowable Rate of Superelevation, and • Permissible Centrifugal Ratio

  5. Horizontal alignments with and without transition curves.

  6. ELEMENTS OF A HORIZONTAL CURVE

  7. Transition Curves • Transition curves are used to connect tangents to circular curves.

  8. tangent to spiral point (TS), • spiral to curve point (SC), • curve to spiral point (CS), • spiral to tangent point (ST).

  9. Length of Spiral Curve 1. According to rate of acceleration 2. According to spiral geometry (A = spiral curve parameter) 3. Minimum Length

  10. SUPERELEVATION Rv ≈ Fc Fcn α Fcp α e W 1 ft Wn Ff Wp Ff α Forces acting on a vehicle running on a horizontal Curve

  11. Case 1: Side friction Only where f = coefficient of side friction V= design speed (km/h) R= radius of horizontal curve (m) Case 2: Superelevation Only where e = superelevation Speed is reduced to avoid sliding inward during wet or icy seasons. Case 3: Superelevation and Side friction For rural roads, emax = 0.10 - 0.12 For urban roads, emax = 0.08 - 0.10

  12. Side Friction Factor from AASHTO’s A Policy on Geometric Design of Highways and Streets 2004

  13. Minimum Radius Tables

  14. WSDOT Design Side Friction Factors For Open Highways and Ramps from the 2005 WSDOT Design Manual, M 22-01

  15. Design Superelevation Rates - AASHTO from AASHTO’s A Policy on Geometric Design of Highways and Streets 2004

  16. Design Superelevation Rates - WSDOT emax = 8% from the 2005 WSDOT Design Manual, M 22-01

  17. Superelevation Transition from the 2001 Caltrans Highway Design Manual

  18. Superelevation Transition from AASHTO’s A Policy on Geometric Design of Highways and Streets 2001

  19. Spiral Curves • WSDOT no longer uses spiral curves • Involve complex geometry • Require more surveying • Are somewhat empirical • If used, superelevation transition should occur entirely within spiral

  20. Spiral Curves No Spiral Spiral from AASHTO’s A Policy on Geometric Design of Highways and Streets 2001

  21. No Spiral

  22. Stopping Sight Distance SSD Ms Obstruction Rv Δs

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