lec 24, ch.16, pp.720-723: superelevation runoff (objectives)

Lec 24, Ch.16, pp.720-723: Superelevation runoff (objectives)

Know the definition of superelevation runoff

Know the minimum superelevation runoff lengths for 2-lane pavements and how to adjust them to wider pavements

Become familiar with methods for attaining superelevation

What we discuss in class today…

Definition of superelevation runoff Minimum superelevation runoff lengths for

2-lane pavements How to adjust minimum superelevation

runoff lengths for wider pavements Methods to attain superelevation Relation between superelevation runoff

What is superelevation runoff?

Superelevation runoff the length of highway needed to accomplish the change in cross slope from a section with adverse crown (in a typical 2-lane pavement case) removed to a fully superelevated section, or vice versa.

Tangent runout the length of highway needed to accomplish the change in cross slope from a normal crown section to a section with the adverse crown removed, or vice

versa.

PC Superelevation runoff

Tangent runout

(Normal Crown)

(Adverse Crown)

(Full Superelevation)

Where is the superelevation runoff located in relation to PC?

Use similar grades

Adverse crown

Relative grade

Min. superelevation runoff lengths for 2-lane pavements

See the handout given out in class for new values.

A few considerations about superelevation runoff lengths…

3-lane pavements: 1.2 times the corresponding length for 2-lane highways

4-lane, undivided pavements: 1.5 times the corresponding length for 2-lane highways

6-lane, undivided pavements: 2.0 times the corresponding length for 2-lane highways

Attainment of superelevation

1. A crowned pavement is rotated about the profile of the center line. Most popular for 2-lane pavements

2. A crowned pavement is rotated about the profile of the inside edge after the superelevation with the normal crown % is reached. Because of this the center line profile elevation rises after point C.

Attainment of superelevation (cont)

3. The center line profile is lowered until adverse crown is reached then this continues till you get superelevation with the normal crown %. Then after that the pavement is revolved around the outer edge. Because of this the center line profile continues to go down.

4. A straight cross-slope pavement is rotated about the profile of the outside edge.

Removing angular breaks…

In practice, the angular breaks are appropriately rounded by using short vertical curves.

Sample problem:Given:

Design speed: 70 mph

Superelevation: 6%

2-lane 2-way highway

Lane width = 12 ft

Cross slope = 1.5%

From Exhibit 3-29, the length of superelevation runoff is 180 ft. Now what is the length of tangent runout and the station of beginning and ending of superelevation runoff and tangent runout when the station of PC is STA 100 + 50?

PC: STA 100+50

Rise at the outer edge is: 12ft x 0.06 = 0.72ft

Relative gradient is: 0.72ft / 180ft = 0.004=0.4%

Tangent runout is: (12ft x 0.015)/0.004 = 45 ft180ft45ft

45ftAbout 1/3 of superelevation runoff in in the curve = 180/3 = About 60 ft in the curve.

About 2/3 is in the tangent = 120 ft.

STA 101+10STA 99+30

STA 98+85 STA 99+75