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
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,
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