7.0 super elevation design

8/13/2019 7.0 Super Elevation Design

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Lecture 7: Super-elevation

Design

TR 320 Highway Geometric Design


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Objective of the Lecture

Lecturer:

To introduce super-elevation design

The learner should be able to:

To drawn super-elevation diagram

and provide details for the criticalpoints

TR 320 Lecture 6: Horizontal AlignmentDesign

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Key points

Design super-elevation

Super-elevation Transition

Criteria for superelevation runoff

length

Methods for attainment of super

elevation

3TR 320 Lecture 6: Horizontal AlignmentDesign


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Super elevation (re-cap)

Refers to banking of curves to counteract the

centripetal force experienced by a vehicle

negotiating a curve

Super-elevation is expressed as a slope (% or

in the minimum radius equation as a decimal)

Design values: 8% (sometimes up to 12%) is

adopted as a maximum value for rural roads

and 4% or 6% for urban roads


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Design super-elevation

Max. super-elevation rates are limited by:

The need to prevent slow moving vehicles

from sliding to the inside of the curve

Keeping parking lanes relatively level in

urban areas

The need to keep differences in slopebetween road and streets intersecting with

it within reasonable bounds


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Super-elevation transition

Involves the modification of roadway cross

section from normal crown to full super-

elevation

E.g. for a normal crown of 2% to full super-

elevation of 6% (Please make a sketch!!)

Terms: Tangent run-out, super-elevation

runoff; normal crown, adverse camber

removed


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Interpretation of Super-elevation diagram


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Computation of levels

Reference: the centre line

Levels of other parts are computed in

reference to the centre line

To get final elevation add to the centre line

elevation

See figure below


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Super-elevation diagram for twolane

road with 3.6 m wide lanes


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Super elevation transition is normally linear,

i.e. the rate of rotation of the pavement is

constant with respect to distance through the

transition

Spiral transitions are used in conjuction with

superelevation transitions

Normally coincide with super elevation runoff


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Methods for attainment of

superelevation

For highways superelevation is attained most

commonly by rotating the cross section about

the profile grade line

This means the centre line for two lane

roadways and undivided multilane highways

For divided highways with wide medians the

rotation is about the inside travelled way edge

(For railways it is the top of lower rail)


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Criteria for Length of

superelevation runoff

Vehicle dynamics, or

Appearance criteria

Vehicle dynamics criterion is associated with

the use of transition curves and is based on

the need to limit the rate of increase of

centripetal force as one traverses the

transition curve (comfort)


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The minimum length is determined

from the formula:

L = 0.0702 V3/RCL = minimum length of spiral

V = design speed, km/hr

R = curve radius, m

C rate of increase of centripetalacceleration, m/s3

C value from 1.0 to 3 are usedTR 320 Lecture 6: Horizontal Alignment

Design13


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AASHTO Recommends:

L = 0.0214 V3/RC For minimum length of spiral for highways

(comfort) Provide minimum shift

Maximum radius for use of a spiral for safety

reasons is also recommended, see Exhibit 3-36


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Appearance criteria

Based on rate of rotation of pavement during

the development of superelevation so that the

relative slope of the outside edge is 1:200 or

more for V greater than 80 km/hr otherwise1:100


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7.0 super elevation design

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