introduction to civil engineering lecture 9 · bridges. basic forms there are six basic bridge...

Introduction to Civil

Engineering

Lecture 9

1

Out lines

Road and Highway Engineering.

Road design.

Bridges.

Basic forms of bridges.

2

3Ideally, the development of a major road system is an orderly, continuous process. The process follows

several steps:

- Assessing road needs and transport options.

- Planning a system to meet those needs.

- Designing an economically, socially, and environmentally acceptable set of roads.

- Obtaining the required approval and financing.

- Building, operating, and maintaining the system.

- Providing for future extensions and reconstruction.

Estimating traffic on a route requires a prediction of future population growth and economic activity.

The key variables defining road needs are the traffic volumes, tonnages, and speeds to be expected

throughout the road's life.


4Road design

1- Alignment and profile

After a route has been selected, a three-dimensional road alignment and its associated cross-sectional

profiles are produced.

A traffic lane is the portion of pavement allocated to a single line of vehicles;

The shoulder is a strip of pavement outside an outer lane; it is provided for emergency use by traffic and to

protect the pavement edges from traffic damage.

A set of adjoining lanes and shoulders is called a roadway or carriageway, while the pavement, shoulders,

and bordering roadside up to adjacent property lines are known as the right-of-way.

In order to maintain quality and uniformity, design standards are established for each functional road type.


5Road design

1- Alignment and profile.

Standards determine:

-The number of traffic lanes is directly determined by the combination of

traffic volume and speed,

-The width of lanes and shoulders,

-roadside barriers or give the clear transverse distances needed on either side

of the carriageway in order to provide safety,

-actual alignment of the road by specifying, for each design speed, the

minimum radius of horizontal curves,

-the maximum vertical gradient, the clearance under bridges, and

-the distance a driver must be able to see the pavement ahead in order to stop

or turn aside.


6Road and Highway Engineering.

7Road design

2- Pavement

Road traffic is carried by the pavement, which in engineering terms is a horizontal

structure supported by in situ natural material.

Pavements are called either flexible or rigid, according to their relative flexural stiffness.

Flexible pavements (see figure, left) have base courses of broken stone pieces either

compacted into place or glued together with bitumen to form asphalt.

Rigid pavements (see figure, right) are made of Portland cement concrete. The concrete

slab ranges in thickness from 150 to 350 mm. It is laid by a paving machine, often on a

supporting layer.


8Road design

2- Pavement.


9Road design

2- Pavement.


10Road design

3- Drainage.

Adequate drainage is the single most important element in pavement performance

The drainage system must be able to carry the storm water produced by this

design storm without flooding the roadway or adjacent property.


11Road design

3- Drainage.


12Road design

4- Traffic management.

Road users are subject to traffic control via instructions and information provided by roadway

markings, signs, and signals, and they are subject to legal control via the rules of the road.

The marking of roadway surfaces with painted lines and raised permanent markers is

commonplace and effective.

Signs advise the driver of special regulations and provide information about

hazards and navigation

Traffic signals are primarily used to control traffic in urban street systems


13Road design


A- Traffic Marking.


14Road design


B- Traffic Signs.


15Road design


C- Traffic Signals.


16Bridges.

A bridge is a structure spanning between two elevations above a lower elevation. It is a structure that

spans horizontally between supports, whose function is to carry vertical loads.

Historical Review

The earliest bridge on record was that built on the Nile by Menes, the first king of the Egyptians about

2650 B.C. A remarkable bridge with timber deck on stone piers was built over the Euphrates in Babylon

4000 years ago.

The 19th century saw the rise of iron and steel bridges. The 177m span suspension bridge over Menai

Straits in Britain was built in 1826. The advance of railways had given bridge building a major impetus.

Cantilever trusses replaced suspension bridges in the medium to long-span range.

At the end of the 19th century, reinforced concrete was significantly applied to bridge construction. An

early example is a reinforced concrete beam 15-m span bridge in England in 1870.

17Bridges.

Different Configurations and Structural Systems

A bridge is subdivided into:

a. The superstructure. (deck structural systems)

b. The substructure. (piers, columns, abutments)

c. The foundations. (supports the substructure)

18Bridges.

Basic forms

There are six basic bridge forms:

1. Beam Bridge:

A beam bridge, with forces of tension represented by red lines and forces of compression by green lines.

The beam bridge is the most common bridge form. A beam carries vertical loads by bending. As the

beam bridge bends, it undergoes horizontal compression on the top. At the same time, the bottom of the

beam is subjected to horizontal tension. The supports carry the loads from the beam by compression

vertically to the foundations.

When a bridge is made up of beams spanning between only two supports, it is called a simply supported

beam bridge. If two or more beams are joined rigidly together over supports, the bridge becomes

continuous.

20Bridges.

2. Truss Bridge:

A single-span truss bridge, with forces of tension represented by red lines and forces of compression by

green lines.

A single-span truss bridge is like a simply supported beam because it carries vertical loads by bending.

Bending leads to compression in the top chords (or horizontal members), tension in the bottom chords,

and either tension or compression in the vertical and diagonal members, depending on their

orientation.

Trusses are popular because they use a relatively small amount of material to carry relatively large loads.

22Bridges.

3. Arch Bridge:

An arch bridge, with forces of compression represented by the green line.

The arch bridge carries loads primarily by compression, which exerts on the foundation both vertical and

horizontal forces. Arch foundations must therefore prevent both vertical settling and horizontal sliding.

In spite of the more complicated foundation design, the structure itself normally requires less material

than a beam bridge of the same span.

24Bridges.

4. Suspension Bridge:

A suspension bridge, with forces of tension represented by red lines and forces of compression by green

lines.

A suspension bridge carries vertical loads through curved cables in tension. These loads are transferred

both to the towers, which carry them by vertical compression to the ground, and to the anchorages,

which must resist the inward and sometimes vertical pull of the cables. The suspension bridge can be

viewed as an upside-down arch in tension with only the towers in compression. Because the deck is

hung in the air, care must be taken to ensure that it does not move excessively under loading. The deck

therefore must be either heavy or stiff or both.

26Bridges.

5. Cantilever Bridge:

A cantilever bridge, with forces of tension represented by red lines and forces of compression by green

lines.

A beam is said to be cantilevered when it projects outward, supported only at one end. A cantilever

bridge is generally made with three spans, of which the outer spans are both anchored down at the shore

and cantilever out over the channel to be crossed. The central span rests on the cantilevered arms

extending from the outer spans; it carries vertical loads like a simply supported beam or a truss—that is,

by tension forces in the lower chords and compression in the upper chords. The cantilevers carry their

loads by tension in the upper chords and compression in the lower ones. Inner towers carry those forces

by compression to the foundation, and outer towers carry the forces by tension to the far foundations.

28Bridges.

6. Cable-stay Bridge

A cable-stayed bridge, with forces of tension represented by red lines and forces of compression by

green lines.

Cable-stayed bridges carry the vertical main-span loads by nearly straight diagonal cables in tension.

The towers transfer the cable forces to the foundations through vertical compression. The tensile forces

in the cables also put the deck into horizontal compression.

30Bridges.

Actions and effects on bridgesA

An action is an assembly of concentrated or distributed forces (direct action) or Imposed or constrained

deformation (indirect actions) applied to a structure.

Generally actions are classified into:

Permanent and long-term actions

1. Dead loads.

2. Superimposed dead loads.

3. Earth pressure due to retained fill.

4. Water pressure of retained water.

31Bridges.

Actions and effects on bridgesA

Transient and variable actions (live loads)

1. Vehicle traffic loading.

2. Railway loading.

3. Footway loading.

4. Cycle loading.

Actions due to natural causes

1. Wind action

2. Earthquake action

3. Flood action

32End of Lecture 9

Next Lecture :Water Supply and Sewage

Systems

introduction to civil engineering lecture 9 · bridges. basic forms there are six basic bridge...

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