7/31/2019 06CE15

1/16

Introduction to Horizontal AlignmentNo.1

7/31/2019 06CE15

2/16

Lying out center line on ground.

Consist of tangents, curves, transition curve.

Design should be insured that safety, comfort.

Horizontal curve should be provided to

accommodate for direction change.

7/31/2019 06CE15

3/16

Basic terminology related to transition curve.

The need of providing transition curve andbasis of using spiral transition curve.

Spiral curve is also used in horizontalalignment

7/31/2019 06CE15

4/16

To understand the AASHTO approach for

transition design control considering both.

1. Tangent to curve transition.

2. Spiral curve transition.

7/31/2019 06CE15

5/16

1. Tangent-to-curve transition.

In this curve no transition curve is used we

have tangent and circular curve directly.2. Spiral Curve transition.

A transition curve is basically spiral curve thatis used in b/w tangent and circular curve

7/31/2019 06CE15

6/16

Tangent Run out :

Length of road way needed to

accomplish the change in outside lane cross slope fromnormal cross slope rate to zero or vice versa.

Super Elevation Run Off:

The length of road wayneeded to accomplish the change in outside lane crossslope from zero to full super elevation or vice versa

7/31/2019 06CE15

7/16

Now we have to discussed tangent-to-curve transition and spiral curvetransition.

Tangent-to-curve Transition:

In which totwo elements are important.

1. Length of super elevation run-off.

2. Length of tangent run-off.

7/31/2019 06CE15

8/16

Consider appearance and comfort there shouldbe maximum acceptable difference b/wlongitudent grade and edge of payment.

Relative gradient varies with design speed it isnot fix speed there is different value suggestedby AASHTO guide line.

7/31/2019 06CE15

9/16

Lr = (Wn1) ed * (bw)

n1= no of lanes rotated

W= width of lane traffic (m)ed= Design super elevation rate (%)

= Maximum relative gradient

Bw = Adjustment no. of lanes

7/31/2019 06CE15

10/16

Relative gradient same as used in super

elevation it is calculated by,Lt = eNc Lr

ed

ed= Design super elevation rate (%)

eNc= normal cross slope rate (%)Lr= Length of Run-off

7/31/2019 06CE15

11/16

In this curve there are two aspect to beconsider wherever length to be calculated.

Following are two main criteria.

1. Based on driver comfort.

2. Based on lateral shift.

7/31/2019 06CE15

12/16

1. Based on Driver Comfort

A spiral length that allows a comfortableincrease in lateral acceralation as a vehicle enters

in curve

L = 0.0214 v3

RC

L = length of transition curve

V = design speed

R = radius of circular curve

C = max change of lateral acceralation (1.2 m/sec)

7/31/2019 06CE15

13/16

2. Based on Lateral Shift:

Spiral curve issufficiently long to provide a shift in

vehicles lateral position with its lane that isconsist with the produced by vehiclesnatural spiral path.

Ls(min) =24 P(min) * R

P(min) = Lateral offset b/w tangent and circularcurve (0.2 m)

7/31/2019 06CE15

14/16

Super Elevation based on relative gradient whilelength of spiral is driver comfort and lateral shift.

Super elevation run-off accomplished over thelength of the transition curve while length of run-off is applicable for all super elevation curves.

In general length of spiral and length of run-off

are nearly same.

7/31/2019 06CE15

15/16

Q:- What are the advantages toproviding the transition curve in

horizontal alignment ?

1. To introduce gradually

centrifugal force b/w tangentpoint and circular curve.

2. To improve aesthetic

appearance of the road

7/31/2019 06CE15

16/16

Q: Why? Spiral is used as ideal shapeof transition curve in the horizontal

alignment?

IDEAL SHAPE

1. Rate of change of centrifugal acceralation

should be consistent.2. The length should inversly proportional to the

radius.

3. Spiral fulfill the condition of ideal transition

curve4. Spiral transition curve simulates the natural

turning path of vehicle.