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Superelevation

Superelevation. CTC 440. Objectives. Know how to determine superelevation transitions on simple circular curves and spirals Know how to use maximum relative gradients to determine superelevation length transitions. Superelevation.

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Superelevation

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  1. Superelevation CTC 440

  2. Objectives • Know how to determine superelevation transitions on simple circular curves and spirals • Know how to use maximum relative gradients to determine superelevation length transitions

  3. Superelevation • Used to partially overcome the centrifugal force on a vehicle as it goes around a curve • Transition lengths are needed to change the cross slope from normal crown to full bank and then back down to normal crown • In New York State the allowed maximum superelevation rates are: • Rural and interstates/freeways 8% • Suburban 6% • Urban 4%

  4. Methods • There are various methods for transitioning pavement from normal crown to a superelevated section • The most common method is to rotate the pavement around the centerline (which is also the HCL and TGL)

  5. Runout • Runout is the distance used to change the section from normal crown to where the adverse crown is removed (to level)

  6. Runoff • Runoff is the distance used to change the section from where the adverse crown is removed (to level) to the point where full superelevation is achieved • Runoff length is also the length of spiral length • Refer to Exhibit 5-15 to get the length (function of e, design speed and number of lanes rotated) but first you must determine e

  7. Reverse Crown • The point at which the whole pavement is sloped at 2% (in the direction of the superelevation)

  8. Spirals • Runout occurs before the TS (on the tangent) and after the ST • Runoff occurs on length of spiral • There is full superelevation between the SC and CS

  9. Circular Curves • Runout also occurs on the tangent • 0.7*Runoff occurs before the PC and after the PT • 0.3*Runoff occurs on the curve (right after the PC and right before the PT). • The circular arc is not fully superelevated because part of the transition falls on the curve

  10. Determining Superelevation Rate, e • Use Exhibits 2-11 through 2-14 (English) • or Exhibits MT 2-11 through MT 2-14 (Metric) • 2-11 (low-speed urban streets) • 2-12 (emax=4%) • 2-13 (emax=6%) • 2-14 (emax=8%) • Function of design speed, emax and radius

  11. Runoff • Refer to Exhibit 5-15 of HDM to get the length (function of e, design speed and number of lanes rotated) • Runoff length is also the length of spiral

  12. Determining Runout Lengths • Rout=(Roff*NC)/e • NC is normal crown (usually 2%) • e is the superelevation rate (%)

  13. Basic steps • Determine e, Roff • Calculate Rout • For circular curves calculate 30% and 70% of Roff • Draw diagram working back and forth from the PC/PT or TS/SC

  14. Example • Last existing curve of Paris Hill project • Design speed=100 km/hr • Emax=8% • Radius=590 m • PC STA 4+340.78 • PT STA 4+901.88 • Curves to the RT

  15. Step 1 (find e, runoff, runout) • e= (7%) (table M2-134)---see next slide • Roff= 57 m (Exhibit 5-15)---see following slide • Rout=(Roff)(NC)/e=(57m)*(2%)/7%=16 m

  16. Step 2 (.7 & .3 Roff) • 0.3*57m=17m • 0.7*57m=40m

  17. Step 3 – Draw Diagram

  18. Other pavement transitions • Sometimes it makes more sense to transition directly from one curve to another • Can determine minimum length of transition by using a maximum relative gradient (Exhibit 5-12 of HDM and equation on page 5.7.3.3)

  19. Equation Variables • Lr=transition length • w=pavement width • ed=% change in super rate • n=# of lanes • bw=adjustment factor • n*bw factor is combined see HDM page 5-59 • Δ=maximum relative gradient from HDM Table 5-4 (in %); it is a function of design speed

  20. Example-minimum transition lengths • A county road- reverse curves 2% rt & 3% lt • Design speed = 40 km/hr • 2 lanes-3.6 m in width • What is the minimum transition length for superelevating directly from 2% to 3%

  21. Example of determining minimum transition lengths using maximum relative gradient

  22. Minimum transition length • Lr=w*ed*(n*bw)]/Δ • w*ed= Δ y=0.18 m • n*bw=1 (since only 1 lane is superelevated) • Δ=0.7% • Lr=26 m (compare to exh. 5-15; 40km/hr; .05)

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