AASHTO 01 Chapter III Instructor: Dr. Nedal Ratrout

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AASHTO 01Chapter IIIInstructor Dr. Nedal
Ratrout
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Design Considerations

• The maximum superelevation may be different of
  different highway conditions. At the other
  extreme, no e is needed for tangent highways or
  highways with extremely long-radius curves.

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Design Considerations

• Maximum Superelevation Rates
• The maximum rates of e used on highways are
  controlled by four factors
• Climate conditions.
• Terrain conditions.
• Type of area.
• Frequency of very slow-moving.
• Using only one maximum e rate within region of
  similar climate and land use is desirable, as
  such a practice promotes design consistence.
• The highest e rate for highways in common use is
  10 percent, although 12 percent is used in some
  cases.

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Design Considerations

• 8 percent is recognized as reasonable maximum
  value for e rate.
• In summary, it is recommended that
• several rates, rather than a single rate, of
  maximum e should be recognized.
• A rate of 12 percent should not be exceeded.
• A rate of 4 or 6 percent is applicable for urban
  design.
• Superelevation may be omitted on low-speed urban
  streets.

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Design Considerations

• Minimum Radius
• The minimum radius of curvature is also an
  important control value for determination of
  superelevation rates for flatter curves.
• Exhibit 3-13, exhibit 3-14 gives the minimum
  radius for each of the five maximum e rates for
  design speeds form 20 to 130km/h 15 to 80 mph.

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Design Considerations

• Design Superelevation Tables
• Exhibits 3-21 to 3-25 show values of R and the
  resulting e for different design speed.

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Sharpest Curve Without Superelevation

• The minimum rate of cross slope is determined by
  drainage needs 1.5 percent for high-type, 2.0
  percent for low-type.
• very flat horizontal curves need no e. traffic
  entering e by normal cross slope
• Traffic entering to the left has an adverse e.
  But with flat curves the side friction needed is
  small.

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Sharpest Curve Without Superelevation

• On successively sharper curves a positive slope
  across the entire roadway is desirable
• Thus an important part of e design policy is a
  criterion for the min curvature for which e is
  needed.
• Many agencies express this criterion as a single
  R of curvature applicable to all design speeds.
  Some agencies use a different criterion for max e
  without curvature for each design speed.

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Sharpest Curve Without Superelevation

• The max curvature for sections without e is
  determined by setting low values of side friction
  factor. The result is an increasing radius for
  successively higher design speed.
• For an Av rate of cross slope of 1.5 the crossp
  min R for each design speed is shown in ex 3-26

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Effects Of Grades

• In case of divided hwy values from ex 3-21 to
  3-25 can be used a slightly higher speed for
  downgrades and lower for upgrades.

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Transition Design Controls

• The e transition sections consists of the e
  runoff tangent runout sections.
• The e runoff section consists of the length of
  roadway needed to accomplish a change in outside
  lane cross slope from zero to full e or vice
  versa.
• The tangent runout section consists of the length
  of roadway needed to accomplish change in
  outside lane cross slope from normal cross slope
  to flat or vice versa.

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Transition Design Controls

• In the alignment transition section, a spiral or
  compound transition curve may be used to
  introduce the main circular curve.
• Such transition curvature consists or one or more
  curves aligned and located to provide a gradual
  change in alignment radius.
• Tangent-to-curve The roadway tangent directly
  adjoins the main circular curve.

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Tangent-to-curve

• Tangent-to-curve The roadway tangent directly
  adjoins the main circular curve.
• Min length of e runoff The length of e runoff
  should be based on a max acceptable diff between
  the longitudinal grades of the axis of rotation
  and the edge of pavement.
• Current practice is to limit the grade diff,
  referred to as relative gradient to a Max value
  of 0.5 or a long slope of 1200 _at_ 80 km/h

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Tangent-to-curve

• Previous editions of AASHTO suggested that runoff
  length should be at least equaled to the
  distance traveled in 2 sec at the design speed.
• To clac the min length of e runoff ex 3-25 can be
  used for undivided streets and hwy, adjustment
  factors in ex 3-28. Typ min values in ex 3-29

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Ex 29
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runoff

• The location of the runoff is critical. Most
  agencies place 60 to 80 prior to the curve. Ex
  3-30 gives some values

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• Thank You