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Alignment Design. I. Key points in this presentation factors influencing locating and configuration of horizontal alignment criteria for horizontal alignment design factors influencing vertical alignment design coordination of horizontal and vertical alignments
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Alignment Design I. Key points in this presentation factors influencing locating and configuration of horizontal alignment criteria for horizontal alignment design factors influencing vertical alignment design coordination of horizontal and vertical alignments 1Friday, November 7, 2014
Alignment Design II. Introduction Definition of alignment: Definitions from a dictionary In a highway design manual: a series of straight lines called tangents connected by circular curves or transition or spiral curves in modern practice Definition of alignment design: also geometric design, the configuration of horizontal, vertical and cross-sectional elements (first treated separately and finally coordinated to form a continuous whole facility) III. Horizontal alignment design Components of horizontal alignment Tangents (segments of straight lines) Circular/simple curves Spiral or transition curves 2Friday, November 7, 2014
Alignment Design Horizontal curves Simple curves Continuous arcs of constant radius which achieve the necessary highway deflection without an entering or existing transition Compound curves A series of two or more simple curves possessing a common tangent at any points of meeting which turn in the same general direction and have their centers on the same side of the line. Any two adjacent areas must, of course, have different radii if they are to be compounded pair rather than a single curve 3Friday, November 7, 2014
Alignment Design Horizontal curves Spiral curves A parabolic type of curve used as a transition from a tangent to a circular curve, or from a circular to another circular of different radii Reverse curves S–curves, are similar to compound curves except that their centers are on opposite sides of the line and their direction of turning is opposite to each other Broken-back curves Two closely spaced simple curves with deflection in the same direction and a short intervening tangent 4Friday, November 7, 2014
Alignment Design 3. Factors influencing the location and configuration of horizontal alignment Physical controls Topography, watercourses, geophysical conditions, land use, and man-made features Environment considerations Affect on adjacent land use, community impacts, ecologically sensitive areas Economics Construction costs, right-of-way costs, utility impacts, operating and maintenance costs Safety Sight distance, consistency of alignment, human factor considerations Highway classification and design policies Functional classification, level of service, design speed, design standards 5Friday, November 7, 2014
Alignment Design 4. Design criteria Design speed: principal factor Consistency: consistent and as directional as possible preferable to long, flat curves no circular curves of different radii placed end to end; no short tangents placed between two curves; Long, flat curves for small changes in direction no sudden changes from flat to sharp curves no long tangents followed by sharp curves Length of curves: Adjacent curves: 6Friday, November 7, 2014
Alignment Design 4. Design criteria High fills: Bridges: at tangents positions Miscellaneous 5. Notations in reading alignment drawings PC: point of curve BC: beginning of curve PT: point of tangent TS: tangent to spiral SC: spiral to circular curve CS: circular curve to spiral ST: spiral to tangent VPC: vertical point of curvature—the point at which a tangent grade ends and the vertical curve begins. VPI: vertical point of intersection—the point where the extension of two tangent grades intersect VPT: vertical point of tangency—the point at which the vertical curve ends and the tangent grade begins 7Friday, November 7, 2014
Alignment Design IV. Vertical alignment Terms Elements of vertical curve Grade line/vertical alignment: a profile taken along the road centerline, consisting of a series of straight lines connected by parabolic or circular vertical curves to which the straight grade are tangents. Gradient: the rate of slope between two adjacent VPIs expressed as a percent Convex curves Concave curves Plus grade/upgrade: the grade line increasing from the horizontal Minus grade/downgrade: the grade line decreasing from the horizontal 8Friday, November 7, 2014
Alignment Design IV. Grade line design 2.Principles for vertical alignment design Consistency A smooth grade line with gradual changes, consistent with the type of highway and character of terrain Environmental Impacts Long Grades On a long ascending grade, it is preferable to place the steepest grade at the bottom and flatten the grade near the top. Intersections Maintain moderate grades through intersections to facilitate turning movements Broken-Back Curvature Coordination with Natural/Man-Made Features VPI Locations 9Friday, November 7, 2014
Alignment Design V. Coordination of horizontal and vertical alignment In order to enhance highway safety and improve the facility's operation, the following coordination of horizontal and vertical alignment must be achieved: Balance Curvature and grades in proper balance for safety, capacity, ease and uniformity of operations and a pleasing appearance Coordination Vertical curvature superimposed upon horizontal curvature (i.e., vertical and horizontal P.I.'s at approximately the same stations) generally results in a more pleasing appearance and reduces the number of sight distance restrictions Crest Vertical Curves (convex curves) No sharp horizontal curvature at or near the top of pronounced crest vertical curves 10Friday, November 7, 2014
Alignment Design V. Coordination of horizontal and vertical alignment Sag Vertical Curves (concave curves) No sharp horizontal curves at or near the low point of pronounced sag vertical curves or at the bottom of steep vertical grades Passing Sight Distance Provision of long tangent sections to secure sufficient passing sight distance Intersections Horizontal and vertical alignment be as flat as practical Divided Highways Provision of independent alignments for the two 1-way roadways on divided facilities with wide medians Residential Areas Aesthetics 11Friday, November 7, 2014
Alignment Design Definitions of alignment from a dictionary: an adjustment to a line; arrangement in a straight line. the line or lines so formed. the proper adjustment of the components of an electronic circuit, machine, etc., for coordinated functioning: The front wheels of the car are out of alignment. a state of agreement or cooperation among persons, groups, nations, etc., with a common cause or viewpoint. a ground plan of a railroad or highway 12Friday, November 7, 2014
