AASHTO 01 Chapter III 2nd part

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AASHTO 01Chapter III2nd part

• Instructor Dr. Nedal Ratrout

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Sight Distance on Horizontal Curves

• Stopping Sight DistanceThe sight line is a
  chord of the curve, and the stopping sight
  distance is measured along the centerline of the
  inside lane around the curve. Exhibit 3-57 is a
  design chart showing the middle ordinates needed
  for clear sight areas that satisfy stopping sight
  distance criteria presented in Exhibit 3-1
• Equation 3-40 applies only to circular curves
  longer than the sight distance for the pertinent
  design speed.
• Where sufficient stopping sight distance is not
  available .the alternatives are
• Increase the offset to the obstruction.
• Increase the radius.
• Reduce the design speed.

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Sight Distance on Horizontal Curves

• Passing sight distance
• the minimum passing sight distance for a
  two-lane roads or street is about four times as
  great as the minimum stopping sight distance at
  the same design speed. Equation(3-40).

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General Controls for Horizontal Alignment

• Alignment should be as directional as practical.
  A flowing line that conforms to the natural
  contours.
• The minimum radius of curvature for that speed
  should be avoided wherever practical.
• Sharp curves sudden changes should be avoided.

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General Controls for Horizontal Alignment

• For small deflection angles, curves should be
  sufficiently long to avoid the appearance of a
  kink, the minimum length for horizontal curves on
  main highways, Lcmin, should be about three times
  the design speed or Lc min3v. On high speed
  controlled-access facilities for aesthetic
  reasons, Lc des6v 30v.
• Abrupt reversals in alignment should be avoided.
• The broken-back or flat-back arrangement of
  curves (with a short tangent between two curves
  in the same direction) should be avoided ..
• To avoid the appearance of inconsistent
  distortion, the horizontal alignment should be
  coordinated carefully with the profile design.

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Vertical Alignment

• Terrain
• The topography influence the alignment of roads.
• In level terrain, sight distances, are generally
  long.
• In rolling terrain, natural slopes consistently
  rise above and fall below the road offer some
  restriction.
• In mountainous terrain, longitudinal and
  transverse changes in the elevation of the ground
  with respect to the roads or street are abrupt,
  excavation are frequently needed.

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Vehicle on Grades

• Passenger cars. Can readily negotiate grades as
  steep as 4 to 5 percent without an appreciable
  loss in speed .
• In trucks the effect is more pronounced.
• The effect of rate and length of grade on the
  speed of a typical heavy truck is shown in
  Exhibits3-59 and 3-60. form Exhibits 3-59 it can
  be determined how far a truck, starting its climb
  from any speed.
• Similarly Recreational vehicles in Exhibit 3-61

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Control Grades for Design

• Maximum grades. of about 5 percent are considered
  appropriate for a design speed of 110 km/h
  70mph.for a design speed of 50km/h 30mph
  generally the range of 7 to 12 percent. Maximum
  grade controls are presented in Chapter 5 though
  8.
• Minimum grades. Flat grades can typically be used
  without problem on uncurbed highways where the
  cross slope is adequate to drain the pavement
  surface laterally. minimum grade is typically 0.5
  percent.

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Critical Lengths of Grade for Design

• The term critical length of grade is used to
  indicate he maximum length of a designated
  upgrade on which a loaded truck can operate
  without an unreasonable reduction in speed.
• In the past, the general practice has been to use
  a reduction in truck speed of 25 km/h below the
  average running speed of all traffic to identify
  the critical length of grade.
• It is recommended that a 15 km/h reduction be
  used.

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Critical Lengths of Grade for Design

• The length of any given grade that will cause the
  speed of a representative truck (120 kg/kw
  entering the grade at 110 km/h to be reduced by
  various amounts below the average running speed.
  Exhibit 3-63
• Where an upgrade is approached on a momentum
  grade, heavy trucks often increase speed on order
  to make the climb. increase of about 10 km/h can
  be considered for moderate downgrades and a speed
  increase of 15 km/h for steeper grades of
  moderate length or longer.
• The critical of grade in Exhibit 3-63 is derived
  as the length of tangent grade.
• Some downgrades are long and steep enough that
  some heavy vehicles travel at crawl speeds to
  avoid loss of control on the grade.

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Climbing Lanes

• Climbing Lanes for Two-lane HighwaysA highway
  section with a climbing lane is not considered a
  three-lane highway, but a two-lane highway with
  an added lane for vehicles moving slowly uphill.
  Exhibits 3-65A and 3-65B. Climbing lanes are
  designed for each direction independently.

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Vertical Curves

• Vertical curves The gradual changes between
  tangent (crest or sag) Ex 3-73.
• the major control is he provision of ample sight
  distances for the design speed. it is recommended
  that all vertical curves should be designed to
  provide at least the stopping sight distance
  shown in Exhibit 3-1.
• For driver comfort, the rate of change of grade
  should be kept within tolerable limits. In sag
  vertical curves where gravitational and vertical
  centripetal forces act in opposite directions
• A long curve has a more pleasing appearance than
  a short one.
• Drainage of curbed roadways on sag vertical
  curves profile design (0.35.)
• K is useful in determining the horizontal
  distance form the vertical point of curvature
  (VPC) to the high point of Type I curves or to
  the low point of Type III curves. K is also
  useful in determining minimum lengths of vertical
  curves of various design speed.

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Crest Vertical Curves

• Exhibit 3-74 illustrates the parameters used in
  determining the length of parabolic crest.
• Design controls-stopping sight distance. The
  minimum lengths of vertical curves for different
  values of A to provide the minimum stopping sight
  distance for each design speed are shown in
  Exhibit 3-75.
• Exhibit 3-76 shows the computed K values for
  lengths of vertical curves corresponding to the
  stopping sight distances shown in Exhibit 3-1 for
  each design speed.
• Most States use a minimum length of vertical
  curve in use range form about 30 to 100 m.
  Minimum length of vertical curves are expressed
  as about 0.6 times the design speed in km/h,
  Lmin0.6v. These terminal adjustment show as the
  vertical lines at the lower left of Exhibit 3-75.

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Crest Vertical Curves

• Design Controls-Passing Sight DistanceFor the
  minimum passing sight distances shown in Exhibit
  3-7, the minimum lengths of crest vertical curves
  are substantially longer than those for stopping
  sight distances. Passing sight distances shown in
  Exhibit 3-77 are 7 to 10 times the corresponding
  lengths for stopping sight distance.Generally,
  it is impractical to design crest vertical curves
  to provide for passing sight distance because of
  high cost. Passing sight distance on crest
  vertical curves may be practical on roads with
  unusual combinations of low design speeds and
  gently grades or higher design speeds with very
  small algebraic differences in grades.
  Ordinarily, passing sight distance is provided
  only at locations where combinations of alignment
  and profile do not need the use of crest vertical
  curves.

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Sag Vertical Curves

• at least four different criteria
• Headlight sight distance
• Passenger comfort
• Drainage control
• General appearance
• Headlight sight distance has been used directly
  for determining the length of sag vertical
  curves. A headlight height of 600 mm and a
  1-degree upward is mainly used.
• For overall safety on highways, a sag vertical
  curve should be long enough that the light beam
  distance is nearly the same as the stopping sight
  distance. Accordingly, it is appropriate to use
  stopping sight distances as the value of S in
  Exhibit 3-78.

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Sag Vertical Curves

• The riding comfort on sag vertical curves is
  achieved when the centripetal acceleration does
  not exceed 0.3m/s. The length of vertical curve
  needed to satisfy this comfort factor at the
  various design speeds is only about 50 percent of
  that needed to satisfy the headlight sight
  distance criterion for the normal range of design
  conditions.
• Drainage affects design of vertical curves . This
  criterion corresponds to K of 51 m which is
  potted in Exhibit 3-78. the drainage criterion
  differs from other criteria in that the length of
  sag vertical curve determined for it is maximum
  whereas. The length for any other criterion is
  minimum.

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Sag Vertical Curves

• For general appearance of sag vertical curves,
  rule-of-thumb for minimum curve length of 30A or
  , in Exhibit 3-78, K30. Longer curves are
  appropriate to improve appearance.
• From the preceding discussion, the headlight
  sight distance appears to be the most logical
  criterion for general use.
• Exhibit 3-79 shows the range of computed values
  and the rounded Values of K selected as design
  controls. The length of sag vertical curves on
  the basis of the design speed values of K are
  shown by the solid lines in Exhibit 3-78.

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Sight Distance at Under-crossings

• Sight distance on the highway through a grade
  separation should be at least as long as the
  minimum stopping sight distance and preferably
  longer,
• the structure fascia may cut the line of sight
  and limit the sight distance to provide the
  minimum length of sag vertical curve discussed
  above at grade separation structures.
• The sight distance should not need to be reduced
  below the minimum recommended values for stopping
  sight distance.
• To check the available sight distance at an
  under-crossing use ex 3-54 to 3-57 using an eye
  height of 2.4 m and object height of 0.6 m

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General Controls for Vertical Alignment

• A smooth grade-line with gradual changes.
• rollercoaster or hidden-dip type of profile
  should be avoided.
• A broken-back grade-line (two vertical curves
  in the same direction separated by a short
  section of tangent grade) generally should be
  avoided.
• Where at-grade intersections occur on roadway
  sections with moderate to steep grade, it is
  desirable to reduce the grade through the
  intersection.
• Sag vertical curves should be avoided in cuts
  unless adequate drainage can be provided.

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Combinations of Horizontal and Vertical Alignment

• General Design Controls
• Curvature and grades should be in proper balance.
  Tangent alignment or flat curvature or long
  grades and excessive curvature with flat grades.
  A logical design is between those two.
• Vertical curvature superimposed on horizontal
  curvature, or vice versa, generally results in
  amore pleasing facility.
• Sharp horizontal curvature should not be
  introduced at or near the top of a crest vertical
  curve, this condition is undesirable because the
  driver may not perceive the horizontal change in
  alignment. Suitable designs can also be developed
  by using design values will above the appropriate
  minimum values for the design speed.
• Sharp horizontal curvature should not be
  introduced near the bottom of a steep grade
  approaching or near the low point of sag vertical
  curve. Because the view of the road ahead is
  foreshortened..

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Combinations of Horizontal and Vertical Alignment

• On two-lane roads and streets. It is appropriate
  to work toward long tangent sections to assure
  sufficient passing sight distance in design.
• Both horizontal curvature and profile should be
  made as flat as practical at intersections where
  sight distance along both roads or streets is
  important .
• On divided highways and streets, variation on
  width of median and the use of independent
  profiles and horizontal alignments for the
  separate one-way roadways are some times
  desirable.
• In residential areas, the alignment should be
  designed to minimize nuisance to the
  neighborhood, a depressed facility makes a
  highway less visible . Minor horizontal
  adjustments increase the buffer zone .
• The alignment should be designed to enhance
  attractive scenic views of the natural and
  manmade environment, such as rivers, rock
  formations

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• Thank You