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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
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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
TR 320 Lecture 6: Horizontal AlignmentDesign
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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
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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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