Introduction

Bridges are the typical infrastructures that connects people, communities, and civilizations. They
have played a crucial role in human history, facilitating trade, travel, and the exchange of
knowledge. This comprehensive introduction will explore the rich history of bridges, delve into
their fascinating designs and various types, and shed light on modern construction techniques
that have revolutionized bridge engineering. Bridges, whether they span across rivers, valleys, or
ravines, are not just magnificent engineering marvels but also integral components of
transportation and infrastructure that connect communities and facilitate the movement of people
and goods.

History of Bridges
The history of bridges stretches back thousands of years, reflecting mankind's ingenuity and
evolutionary progress. The earliest known bridges were simple, natural structures, such as fallen
trees or stepping stones, used to cross small rivers or chasms. As civilizations evolved, so too did
the complexity and sophistication of bridge construction techniques.

Ancient civilizations made significant contributions to bridge building. The Mesopotamians, for
instance, constructed the world's oldest known bridge, the Arkadiko Bridge. This stone arch
bridge, located in Greece, was built around 1300 BCE and spanned over 22 m long, 5.60 m wide
at the base and 4 m high, spans a 1 m culvert. The width of the roadway is about 2.50 meters.
The Romans were renowned for their mastery of arch bridge construction, with iconic examples
like the Pont du Gard aqueduct in France.

Figure 1 Arkadiko Bridge, Greece Figure 2 Pont du Gard aqueduct, France

In the middle Ages, Europe saw the emergence of new bridge-building techniques, such as
timber truss bridges. These structures utilized triangulated wooden beams to distribute weight
efficiently, allowing for longer spans. The Industrial Revolution in the 18th and 19th centuries
brought significant advancements in materials, leading to the widespread use of iron and steel in
bridge construction.

Historical Evolution of Bridge Design

 Ancient Civilizations: From simple log or stone structures to impressive aqueducts, early
civilizations like the Romans and Egyptians laid the foundation for early bridge design.
 Medieval Era: The development of arches, vaults, and buttresses paved the way for the
construction of stone bridges during the medieval period.
 Industrial Revolution: Introduction of wrought iron and steel marked a significant
turning point in bridge design, enabling the construction of longer and more resilient
bridges.
 Modern Innovations: Advances in materials, including concrete and pre-stressed
concrete, along with computer-aided design (CAD) technology have revolutionized
bridge architecture in recent years.

Purpose and Impact of Bridges

 Promoting Connectivity: Bridges connect previously divided regions, enhancing

accessibility, and fostering economic growth.
 Transportation Sustainability: Bridges provide alternate routes, relieving congestion,
reducing travel time, and promoting environmentally friendly modes of transportation,
such as walking or cycling.
 Architectural and Cultural Significance: Many bridges have become iconic landmarks,
representing the identity and culture of a region or nation.
 Emergency Response and Disaster Management: Bridges often serve as vital evacuation
routes during natural disasters, allowing for the safe and efficient movement of people
and resources.

Components of Bridge
 Super-structure: Deck, Truss, Girders etc.
  Sub-structure: Piers/Abutments, Bearing and foundation.

Figure 3 Components of Bridge

Classification of Bridges
Bridges are mainly classified according to:

1. Materials used in their construction.

 Timber
 Masonry
 Concrete (R.C.C or Pre-Stressed)
 Steel
2. Various structural forms.
 Slab (0-12m)
 Beam (10-30m)
 Cantilever/Balanced Cantilever (30-500m)
 Box-Girder (18-30m; 60-70m With Pre- Stressing) (Cellular/Multi-Celled Bridges)
 Truss 35- 300m
 Arch 20-500m
 Cable Stayed 90-350m
 Suspension 300-2000m
3. Construction and function.
 According to inter-span relations
 Simple: Independent spans; large Bending Moment and Shear Force
 Continuous: Design moments are reduced; foundation supported on good
rock or differential settlement of supports is eliminated
 Cantilever/Balanced bridges: Slight differential settlements under the pier or
abutment are not detrimental
 According to position of bridge floor
 Deck
 Through
 Half/Semi Through and
 Suspended Bridge
 According to Method of connections
 Pin jointed
 Riveted or
 Welded
 According to road level
 High level or
 Submersible.
 According to the method of clearance for navigation
 High level
 Movable-bascule
 Movable swing or transporter bridge.
 According to duration of use and service
 Permanent/temporary
 Military (Bailey)
 According to function
 Aqueduct
 Viaduct
 Pedestrian
 Highway
  Railway or pipe bridges
 According to span length
 Culvert <= 6m
 Minor/Short span bridge 6m-60m
 Major/Medium span bridge >60m-150m
 Long span bridge >150m
 According to degree of redundancy
 Determinate or
 Indeterminate

Types of Bridges based on various structural forms and functions

Bridges can be classified based on various factors, including structural form, function, and
geography.

Slab Bridges
 Usually used for Short spans
 Carry loads in Shear and Flexural bending
 Have sufficient torsional stiffness
 Bearings are not required
 Simple Shattering/formwork is required
 Becomes heavy (increase in Dead Load) for large spans. Hollow slabs are sometimes
provided for medium spans.
 Figure 4 slab bridge

Beam/Girder bridges
 Oldest and most common bridge type known
 A beam or ‘girder’ bridge is the simplest and most inexpensive kind of bridge.
 Usually used for Short and Medium spans
 Carry loads in Shear and Flexural bending
 In modern girder bridges, steel I-Beams replace Concrete Beams
 Low torsional stiffness
 Consists of a horizontal beam supported at each end by piers. The weight of the beam
pushes straight down on the piers.
 In its most basic form, a beam bridge consists of a horizontal beam that is supported at
each end by piers. The weight of the beam pushes straight down on the piers.
 The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely
span more than 250 feet.
 The beam itself must be strong so that it doesn’t bend under its own weight and the added
weight of crossing traffic.
 Figure 5 Beam Bridge

Box Girder Bridges

 In addition to flexural stiffness and shear resistance, these bridges have sufficient torsional
stiffness
 Most suitable for curved plan and longer span bridges
 https://www.youtube.com/watch?v=jd88VShhY9M

Figure 6 box girder bridge

Truss Bridge
 The primary member forces are axial loads
 The open web system permits the use of a greater overall depth than for an equivalent
solid web girder, hence reduced deflections and rigid structure
 Both these factors lead to economy in material and a reduced dead weight. The increased
depth also leads to reduced deflections, that is, a more rigid structure.
 High maintenance and fabrication costs.
 Aesthetic appearance is debatable mainly because of complexity of elevation.
  Used economically in the span range of up to 300m.
 All beams in a truss bridge are straight. Trusses are comprised of many small beams that
together can support a large amount of weight and span great distances.
 Load applied to the truss is transmitted to joint so that each individual members are in
either pure tension or compression.
 https://www.youtube.com/watch?v=fcZaLN4osIs

Figure 7 Truss Bridge

Cantilever Bridges
 A cantilever bridge is a bridge built using cantilevers structures that project horizontally
into space, supported on only one end.
 A simple cantilever span is formed by two cantilever arms extending from opposite sides
of an obstacle to be crossed, meeting at the center.
 The cantilevers may be simple beams however, large cantilever bridges designed to
handle road or rail traffic use trusses built from structural steel, or box girders built from
pre-stressed concrete.
 A structure at least one portion of which acts as an anchorage for sustaining another
portion which extends beyond the supporting pier.
 Cantilever bridges can be of steel or concrete
 In a cantilever bridge, the roadway is constructed out from the pier in two directions at
the same time so that the weight on both sides counterbalance each other
 The larger section at the support to resist negative moments
 https://www.youtube.com/watch?v=U8jGCbVIQbw

Figure 8 Cantilever Bridge: Howrah Bridge, Kolkata

Arch bridges
These bridges employ arches, either curved or segmental, to support the weight of the structure.
From ancient Bridges in Ethiopia the oldest bridge on Nile or Abay River is arch bridge, arch
bridges are known for their elegance and structural efficiency.

 The arch has great natural strength. Thousands of years ago, Roman built arches out of
stone. Today, most arch bridges are made of steel or concrete, and they can span up to
800 feet.
 Instead of pushing straight down, the weight of an arch bridge is carried outward along
the curve of the arch to the supports at each end.
 These supports, called the abutments, carry the load and keep the ends of the bridge from
spreading out.
 Arch action reduces bending moments
 Economical as compared to equivalent straight simply supported Girder or Truss bridge
 Suitable; when site is a deep gorge with steep rocky banks.
 Conventional curved arch rib has high Fabrication and Erection costs.
 Unlike girders, can be built from stones
 Considered the most beautiful of bridge types
 Used in the span range of up to 250m.
 https://www.youtube.com/watch?v=qfPt8J95R-A

Figure 9 Arch bridge: oldest bridge on Abay river

Suspension bridges
Suspension bridges utilize a series of suspended cables anchored by towers. The weight is
transferred to these towers, allowing for longer spans. Iconic examples include the Golden Gate
Bridge in San Francisco and the Akashi Kaikyō Bridge in Japan, the longest suspension bridge in
the world.

 The deck is hung from the cable by Hangers constructed of high strength ropes in tension
 Cables are anchored at the abutment, hence abutment has to be massive
 The main cable is stiffened either by a pair of stiffening trusses or by a system of girders
at the deck level.
 This stiffening system serves to control the aerodynamic movements.
 Suspension bridge needs to have very strong main cables
 The complete structure can be erected without intermediate staging from the ground
 It is the only alternative for spans over 600m, and it is generally regarded as competitive
for spans down to 3000m.
 The height of the main towers can be a disadvantage in some areas; for example, within
the approach road for an airport
 They also tend to be the most expensive to build.
 These cables rest on top of high towers and are secured at each one end by anchorages.
 The towers enable the main cables to be draped over long distances.
 Inside the anchorages, the cables are spread over a large area to evenly distribute the load
and to prevent the cables from breaking free.
 https://www.youtube.com/watch?v=_anVJoFUCtk
 Figure 10 Suspension bridge: Golden Gate Bridge, San Francisco

Figure 11 Suspension bridge: Akashi Kaikyo Bridge, Japan

Cable-stayed bridges
Similar to suspension bridges, cable-stayed bridges support the deck using cables. However, in
this case, the cables are connected directly to the towers, creating a visually striking appearance.

 The cable stayed bridge is newer than the other type of bridge. Large upright steel
supports are used to transmit the load into the ground.
 Two bridges support the load of the roadway in very different ways.
 The difference lies in how the cables are connected to the towers. In suspension bridges,
the cables ride freely across the towers, transmitting the load to the anchorages at either
end.
 In cable-stayed bridges, the cables are attached to the towers, which alone bear the load.
  Cable-stayed bridge uses the pre-stressing principles but the pre-stressing tendons are
exposed/outside of the beam
 All the forces are transferred from the deck through the cables to the tower/pylon
 Roadway deck can be: Pre-stressed Concrete Box Deck, Steel Box Deck or Steel Truss
Deck
 As compared with the stiffened suspension bridge, the cables are straight rather than
curved. As a result, the stiffness is greater
 The cables are anchored to the deck and cause compressive forces in the deck.
 All individual cables are shorter than full length of the superstructure. They are normally
constructed of individual wire ropes, supplied complete with end fittings, pre-stretched
and not spun.
 Aerodynamic stability has not been found to be a problem in structures to date.
 It is economical over 200-500m.
 https://www.youtube.com/watch?v=caTaBeKUh-U

Figure 12 Abay river bridge, Bahirdar

Floating Bridge
 Permanent floating bridges are useful for traversing features lacking strong bedrock for
traditional piers.
 Such bridges can require a section that is elevated, or can be raised or removed, to allow
ships to pass.
 Pontoon bridges are usually temporary structures, some are used for long periods of time.
  Pontoon bridges are supported by floating pontoons with sufficient buoyancy to support
the bridge and dynamic loads.
 https://www.youtube.com/watch?v=JBcqs3Nsqd4

Figure 13 Floating Bridge

Modern Construction Techniques

Advancements in engineering and construction have revolutionized the way bridges are designed
and built. Modern construction techniques have made it possible to construct bridges with longer
spans, increased durability, and enhanced aesthetic appeal.

One significant development is the use of computer-aided design (CAD) and three-dimensional
modeling. These tools allow engineers to visualize and analyze the complex structural behavior
of bridges before construction begins, resulting in more efficient designs and cost-effective
solutions.

Innovative materials have also played a crucial role in modern bridge construction. High-
performance concrete, for example, offers increased strength and durability, allowing for longer
lifespan and reduced maintenance. Additionally, the use of advanced steel alloys has resulted in
lighter and more efficient bridge structures.

Another notable advancement is the implementation of prefabrication and modular construction

techniques. Components of the bridge, such as segments or entire spans, can be manufactured
off-site and assembled on-site, reducing construction time and minimizing disruption to the
surrounding environment.

Generally, Bridges have evolved hand in hand with human civilization, from humble fallen tree
logs to massive steel structures spanning vast distances. The history, design principles, types, and
modern construction techniques discussed in this comprehensive introduction highlight the
remarkable progress made in bridge engineering. As technology continues to advance, we can
expect further innovation and even more ambitious bridge projects that will shape the future of
transportation and connectivity.

Design Principles of Bridges

The design of bridges has evolved over time to meet the demands of increasing traffic,
challenging geography, and aesthetic preferences. Several important design principles guide
bridge engineers, ensuring safety, durability, and efficiency.

One crucial aspect of bridge design is the selection of the appropriate structural form. The three
primary types of bridges are beam bridges, arch bridges, and suspension bridges. Beam bridges,
the simplest form, consist of a horizontal beam supported on two piers. Arch bridges feature a
curved structure that supports the weight of the bridge through compression forces. Suspension
bridges, on the other hand, employ suspended cables and towers to carry the load.

Another crucial consideration is the choice of materials. Timber, stone, iron, steel, and concrete
have all been used throughout history. Each material possesses unique characteristics,
influencing the overall design, strength, and lifespan of the bridge. Today, most modern bridges
are constructed using reinforced concrete or steel, offering a balance of strength and cost-
effectiveness.