Highway Alignment And Geometric

Design

Prepared by Wrya H.Nadir

ContentIntroductionHorizontal alignmentType of horizontal alignmentParts of circular curveParts of transition curveSuperelevationMethod of building superelevation

Introductionhighway alignment is a three-dimensional problem represented in the X, Y, and Z coordinates

Finding the dimensions of the geometric design elements of highway alignments is called geometric design

Horizontal and vertical alignments are two major components of

highway geometric design.

.

General point for design

1-Estimated future traffic volume(ADT.AADT).

2-Must be safe for driving.

3-Avoid surprise change in direction, grade site distance.

4-Design must be completed.

5-Must be economical as possible.

1-highway alignment: horizontal and vertical

alignment2-sight distance: SSD PSD DSD 3-widening on curve4-superelevation5-transition curves6-intersection7-highway cross section

Elements of geometric design

Horizontal Alignment The horizontal alignment of a highway is the plan view of the route. It consists of straight sections (tangents) of the roadway connected by curves.

Necessary for gradual change in direction when a direct point of intersection is not feasible

Tangent line

H. Curves

Type Of Horizontal Alignment1-Circular Curves:

simple

Parts Of Circular Curves

)]2/cos(1[ RM

=External distance

1146 ( )

1747.5 ( )C

C

R I raqi MD

R U S MD

2tanRT =tangent

]1))2/cos(1[(

RE

=Middle ordinate

RL 180

=Length of curve

2 sin 2L C R =long

chord

PI: point of intersection :intersection angleR: Radius of curvePC: Point of curvaturePT: point of tangency

Continue…..2-Transition Curves: a-spiral or clothoid b-cubic spiral c-cubic parabola d- lemniscatesFunction:1-to introduce gradually centrifugal force between the tangent point and the beginning of the circular curve avoiding a sudden jerk on the vehicle2-to enable the driver turn steering gradually for his own comfort and security.3-to enable gradual introduction of the designed super elevation and extra widening,

Elements Of Spiral Curves

Continue….

Super ElevationSuperelevation is tilting the roadway to help centrifugal force developed as the vehicle goes around a curve.

Along with friction. it is what keeps a vehicle from going off the road.

Must be done gradually over a distance without noticeable reduction in speed or safety

Continue….2

2

(0.2778 ( /sec ))9.832 ( )sec

127

s

s

V meter ondf e meter R meterond

Vf eR

V:Design Speed Km/HR:Radius Of Curvee:Seper Elevationfs: Coefficient Of Friction Between The Tire And Road Surface

Min and Max (e)with (fs)Max(e)=0.07=7%: plain and rolling terrain and mountains and steep terrain bounded by snow.

Max(e)=0.1=1%:for mountains and steep terrain not bounded by snow.

1/8 for non snow condition

AASHTO Max(e) = 1/1.6 for snow condition

Min(e)=2%(cross slope of the road(camber)) 0.2 for muddy roads .Fs= 0.15 for newly constructed asphalt roads.

Depend on the :1-presence of water(moisture),mud, snow2-condition of tread design and air pressure of tyres

Superelevation TransitionsConsist Of Tangent Runout And Superelevation Runoff Section.

Runout: length of roadway needed to accomplished a change in outside lane cross slope from normal rate to zero

Runoff: length of roadway needed to accomplished a change in outside lane cross slope from zero to full

Methods Of Building Seperelevation

1-center Line

Revolving Pavement About 2- Inner Edge

3-outer Edge

Thank For All