Alignment Design

Alignment Design

I. Key points in this presentation factors influencing locating and con

figuration of horizontal alignment criteria for horizontal alignment de

sign factors influencing vertical alignme

nt design coordination of horizontal and vert

ical alignments 1 Thursday, April 20, 2023

Alignment Design

II. Introduction

1. 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

2. 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

1. Components of horizontal alignment Tangents (segments of straight lines) Circular/simple curves Spiral or transition curves

2 Thursday, April 20, 2023

Alignment Design

2. 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


Alignment Design

2. Horizontal curves Spiral curvesA parabolic type of curve used as a transition from a tang

ent to a circular curve, or from a circular to another circular of different radii

Reverse curvesS–curves, are similar to compound curves except that thei

r centers are on opposite sides of the line and their direction of turning is opposite to each other

Broken-back curvesTwo closely spaced simple curves with deflection in the sa

me direction and a short intervening tangent


Alignment Design3. Factors influencing the location and configuration

of horizontal alignment Physical controlsTopography, watercourses, geophysical conditions, l

and use, and man-made features Environment considerationsAffect on adjacent land use, community impacts, eco

logically sensitive areas EconomicsConstruction costs, right-of-way costs, utility impact

s, operating and maintenance costs SafetySight distance, consistency of alignment, human fact

or considerations Highway classification and design policies

Functional classification, level of service, design speed, design standards


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:


Alignment Design4. Design criteria

High fills:

Bridges: at tangents positions

Miscellaneous5. 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 tang

ent grade ends and the vertical curve begins. VPI: vertical point of intersection—the point where the ext

ension of two tangent grades intersect VPT: vertical point of tangency—the point at which the ver

tical curve ends and the tangent grade begins


Alignment Design

IV. Vertical alignment

1. 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 f

rom the horizontal8 Thursday, April 20, 2023

Alignment DesignIV. 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 stee

pest grade at the bottom and flatten the grade near the top. Intersections Maintain moderate grades through intersections to facilitat

e turning movements Broken-Back Curvature

Coordination with Natural/Man-Made Features

VPI Locations


Alignment DesignV. Coordination of horizontal and vertical alignment In order to enhance highway safety and improve the facil

ity's operation, the following coordination of horizontal and vertical alignment must be achieved: Balance Curvature and grades in proper balance for safet

y, 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


Alignment DesignV. Coordination of horizontal and vertical alignment

Sag Vertical Curves (concave curves) No sharp horizontal curves at or near the low poi

nt of pronounced sag vertical curves or at the bottom of steep vertical grades

Passing Sight Distance Provision of long tangent sections to secure suffic

ient passing sight distance Intersections Horizontal and vertical alignment be as flat as pr

actical Divided Highways Provision of independent alignments for the two

1-way roadways on divided facilities with wide medians

Residential Areas

Aesthetics11 Thursday, April 20, 2023

Alignment Design

Definitions of alignment from a dictionary: an adjustment to a line; arrangement in a stra

ight line. the line or lines so formed. the proper adjustment of the components of a

n 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


Simple curve elements

Simple curve in full superelevation

Reverse curves

Compound curve

Spiral transition curve in full superelevation

Superelevation transition for divided highways

Vertical curves

Convex and concave curves

Coordination of vertical and horizontal alignments

Coordination of vertical and horizontal alignments

Alignment Design

Documents

alignment design4

consistency of alignment

alignment designii

alignment design3

transition curves

adjacent curves

reading alignment drawingspc

design standards