Lec 03 Highway Engineering - Horizontal Alignment

7/25/2019 Lec 03 Highway Engineering - Horizontal Alignment

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Lecture 03 32

Highway Eng. Highway Horizontal Alignment 14 15

Dr. Firas Asad

In this lecture;

---------------------

Highway Horizontal Alignment.

A-Tangents.

B-Circular curves (Simple,

Compound, Broken-back and

Reversed).

C-Transition spirals.

Highway Horizontal Alignment

The information listed in this lecture is mainly taken from the Policy on Geometric Design of

Highways and Streets (AASHTO, 2011), Iraqi Highway Design Manual (SORB, 2005) and

Traffic and Highway Engineering (Garber and Hoel, 2009).

Highway Horizontal Alignment.

Geometric design of a highway includes three major components; horizontal

alignment, vertical alignment as well as the cross-sectional elements. The design of

the alignment depends primarily on the design speed selected for the highway.

Firstly, horizontal alignment of a highway defines its location and orientation in plan

view. Secondly, vertical alignment of a highway deals with its shape in profile. The

third component is the cross-section

This lecture discusses

of the highway which includes elements such

as the travelled lanes, shoulders, curbs, medians, and others.

Horizontal Alignment

Horizontal alignment of roads usually comprises of

of highways only. Vertical alignment and

cross- section elements will be discussed in the next lectures.

tangent distances, that are

connected by very gradual circular curves, that may be complemented by

transitional spiral curves, to accommodate a given design speed with comfort and

safety.


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Dr. Firas Asad

Tangents are the straight segments of

horizontal roads that can be defined in termsof their directions and lengths.

Distance along a horizontal alignment is

measured in terms of stations. A full station

is defined as 100m or 1000m.

If 1 Sta. = 100m ---> Sta 03+50.1 = 350.1 mIf 1 Sta. = 1000m ---> Sta 07+050 =7050 m

There are four common types of horizontal circular curves; simple, compound,

broken-back, and reversed.

According to both AASHTO and Iraqi

Highway Design Manuals, the following

are the general design controls for

horizontal alignment:

1-Alignment is as smooth as possible;

2-

When using compound curve R2 must

not be more than 1.5R1;

3-Avoid using broken-back curves;

4-Avoid using reverse curves;

5-Curves should be 150 m long for = 5

deg. and increased 30 m for each one

degree decrease in .

A- Tangents.

B- Circular curves.


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Dr. Firas Asad

1- Simple Circular Curve


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Sta PI = Sta PC + T

Sta PT = Sta PC + L

Example

Sol.:

: Calculate the station of PI and PC, R, Lc, M and E for horizontal curve

with D=1.35 , =21 45, sta., PT=100+93?

m

D

R 424435.1

58.572958.5729===

,

mRT 8152

75.21tan4244

2tan ==

=

L = R = 4244 * 21.75 (3.14/180) = 1610m

Sta PC = Sta PT L = 100+93 16+10 = 84+83

Sta PI = Sta PC + T = 84+83 + 8+15 = 92+98

mRLC 4.16012

sin2 =

= , mRM 22.762

cos1( =

= mRE 61.77)12

(sec =

=


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Dr. Firas Asad

Reverse curves usually consist of two

simple curves with equal radii turning in

opposite directions with a common

tangent. They are generally used to

change the alignment of a highway.

For safety reasons, the use of this curve should be avoided when possible. As with

broken back curves, drivers do not expect to encounter this arrangement on typical

highway geometry.

2- Compound Curve

Compound curves are a series of two or more simple circular curves with deflections

in the same direction immediately adjacent to each other. Compound curves areused to transition into and from a simple curve and to avoid some control or

obstacle which cannot be relocated.

3- Reversed Curve


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Dr. Firas Asad

Broken back curve occurs when a short

tangent exists between two curves in

same direction. As in the reversed curve,

this curve should be avoided because

drivers do not expect to encounter this

arrangement on typical highway

geometry.

Example

Sol.:

: Two parallel tangents of two roads 36m apart are connected by a

reverse circular curve, R2=860m and the chord from PC to PT is 320m. Find the

central angle and R1?

5.1932

sin22

''555129.12459.62

459.6320

36sin

2

1

=

=

===

==

RLC

LC1=320-193.5=126.5

mR

R

RLC

2.562

459.6sin25.126

2sin21

1 =

=

=

================================

4- Broken-back Curve

R1

/2

36

R2=860m

320m


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Dr. Firas Asad

Transition curves are placed between

tangents (straight segments) and

circular curves especially when speeds

are high and curvature is sharp. Also

they are placed between two adjacent

circular curves having substantially

different radii.

C- Transition spirals.

1-Providing gradual increases or decreases the radial force.

The main advantages of transition curves are:

2-Providing a suitable location for the superelevation runoff.

3-Facilitating the transition in width when widening is needed.

4-Enhancing the appearance of the highway.

Length of Spiral Curves. If the transition curve is a spiral, the degree of curvature

between the tangent and the circular curve varies from 0 at the tangent end

(radius = at tangent) to the degree of the circular curve D at the spiral curve end

(radius = Rc).

The factor C is an empirical value

representing the comfort and safety

levels provided by the spiral curve,

(0.3 to 0.9 m/s3.

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Lec 03 Highway Engineering - Horizontal Alignment

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