Bridge Engineering
Prof. Piyali Sengupta
Department of Civil Engineering
Indian Institute of Technology (ISM), Dhanbad
Module - 01
Introduction to Bridges
Lecture - 01
Components, Materials, Forms, Evolutions and Classifications of Bridges.
Hello everyone, this is Dr. Piyali Sengupta from Department of Civil Engineering of Indian
Institute of Technology, Dhanbad. I welcome you all to the course on Bridge Engineering.
This course on bridge engineering aims at knowledge development on principles of
engineering mechanics, load transfer mechanisms, analysis methodology, design principles,
damage mechanics, failure mechanisms, repair and retrofit strategies, field-based
construction, inspection and maintenance techniques in the realm of bridge engineering.
This course is designed to be a 12 weeks course, the first module of this course is on
Introduction to Bridges. Today we are going to cover the first lecture under module 1. The
first lecture is on Components, Materials, Forms, Evolutions and Classifications of Bridges.
Let us see what we are going to learn in today’s class.
(Refer Slide Time: 01:19)
The concepts to be covered in today’s class are introduction to bridges. Here, we are going to
learn about definitions and functions of bridges. We are also going to consider a classic
example of a prominent bridge of our country.
The next topic of discussion is components of bridges. Here, we are going to learn about the
concepts of superstructure, sub structure and foundations. Subsequently, we are going to see
the schematic diagram of different components of a typical bridge.
The next topic of discussion is load transfer mechanisms. Here, first we are going to learn
about different types of loads that may act on bridges. Subsequently we are going to learn
about the concepts of load path, alternate load path and load transfer mechanisms.
The next topic of discussion is basic forms of bridges. Here, we are going to learn about 6
basic forms of bridges which are derived based on load transfer mechanisms. We are also
going to learn about features of six basic forms of the bridges.
The next topic of discussion is structural materials. Here, we are going to learn about
different types of materials that may be used for construction of bridges. Subsequently we
will learn about the advantages and disadvantages of different types of materials when used
for construction of bridges.
The next topic of discussion is historical evolution of bridges. Here, we are going to see the
earliest forms of bridges constructed by primitive men. We would also see how those earliest
forms of bridges got evolved towards the latest technology enabled bridges through different
steps using a flowchart.
The last topic of discussion is classifications of bridges. Here, we are going to learn about
different key parameters based on which classifications of bridges takes place.
(Refer Slide Time: 03:32)
The first topic of discussion is introduction to bridges.
(Refer Slide Time: 03:36)
What is a bridge? A bridge is a structure that provides passage over an obstacle without
closing the way beneath. The required passage may be for a road, railway, pedestrian
walkway, canal or a pipeline. The obstacle that is required to be crossed may be a river, road,
railway or valley.
We all know that bridges play a crucial role in infrastructural and economic development of a
country, by providing fast and smooth transportation network across different regions. There
are several prominent bridges across different countries. You can see the photo of one of the
prominent bridges of our country. Can anybody recognize which bridge is this? This photo is
of Howrah Bridge which was renamed as Rabindra Setu after Nobel Laureate Rabindranath
Tagore. This bridge was constructed to overcome Hooghly River as an obstacle and to
connect two cities Howrah and Kolkata formerly known as Calcutta. This bridge is located in
West Bengal, India.
(Refer Slide Time: 04:55)
The next topic of discussion is components of bridges. There are several components of
bridges, like decking system, which consists of deck slabs longitudinal and cross girders,
trusses, stiffeners, suspenders, connections etc.
The deck slab rests on longitudinal and cross girders. The loading from deck slab also gets
shared by the girder network. From the deck slab and the other components from the decking
system, the loads get transferred to the abutments and piers through the next component, that
is, bearing. The primary functions of the bearings are to ensure that the load transfer happens
uniformly so as to make sure that the bearing pressure remains within the allowable limit.
Bearings are located beneath the decking system and above piers and abutments. Abutments
and piers are the vertical members of bridges. They are like columns of bridges. The corner
columns are named as abutments, while the central columns are named as piers. The loads
from bearings get transferred to abutments and piers and from abutments and piers the
loading gets transmitted to the ground through foundation system.
That is why it is necessary to provide foundation to each of the piers and abutments.
Foundations can be of two types: shallow foundation and deep foundation. There are other
components like river training works, like revetment for slopes for embankment at abutments
and aprons at river bed levels. The next set of components are approaches which are provided
to each side of the bridges so as to make sure that the bridge remains connected to the
surrounding transportation network.
Approaches can be in the form of approach slab or approach embankment. The next set of
components are handrails and parapets, they act as safety barrier and absorbs impact loading
or collision load, then there are components like guard stones.
(Refer Slide Time: 07:17)
The components of bridges can be broadly classified into 3 groups: superstructure, sub
structure and foundation. The components which are located above the level of the bearings
are grouped as superstructure. The components which are located below the level of the
bearings, but above the foundation are classified as substructure. The components which are
located below the bed level are named as foundations.
(Refer Slide Time: 07:48)
You can see the components of a typical bridge through the schematic diagram. You can see
that road level is marked here, then bed level is marked here. The bridge has approaches in
the form of approach slab at one end and approach embankment at the other end.
This is the decking system which consists of deck slabs as marked here. Then, the deck slab
is resting on girder network. We can see longitudinal girders, that is marked here, cross
girders, that is shown here. Then, we have expansion joints which are provided at the
locations of structural discontinuities. Then we have parapets and handrails, parapets and
handrails act as safety barriers and absorbs impact loading as discussed earlier.
So, up to bearing whatever components are present, everything is part of superstructure. Now,
we can see the decking system. Beneath the decking system, we have bearings. Bearings are
the components that transfers the loading from the decking system to the piers and abutments
uniformly. So, that is why bearings are located beneath the decking system, but above piers
and abutments. We can see piers are drawn here, that is a central column, abutment is shown
here that is a corner column.
Abutments have breast wall, back wall and wing wall as shown here. Now, the
center-to-center distance between the bearings are marked as effective span, then we can see
HFL that stands for Highest Flood Level. This is one of the preliminary data that is necessary
before construction and site selection of a bridge. We can see HFL is shown here. Now, for
every pier and abutment we need to provide foundation, for load transfer from the piers and
abutments to the ground. You can see foundations are drawn and marked here.
So, these were the preliminary components of a typical bridge. We are going to learn about
functions and design principles of every component in the subsequent lectures.
(Refer Slide Time: 10:40)
The next topic of discussion is load transfer mechanisms. There are several types of loads that
may act on bridge, like dead load, then there can be live load, wind load, seismic load, snow
load, impact load due to vehicles or floating vessels, buoyancy forces, longitudinal forces
caused by tractive effort or braking of vehicles, longitudinal forces due to frictional resistance
of expansion bearings, centrifugal forces due to curvature, horizontal forces due to water
currents, earth pressure due to live load surcharge, wave pressure, temperature effects,
erection stresses, deformation effects and there can be miscellaneous secondary stresses.
Depending on the site conditions and the type of the bridge that is going to be constructed,
reconnaissance survey and preliminary data are required to be collected. And based on which
out of these loads which are going to be the design loading that is going to be decided.
(Refer Slide Time: 11:50)
Whenever any loading acts on a particular point, then it is necessary to understand how that
load gets transferred to the ground or soil. Now, whenever any load acts on a bridge, it passes
through interconnected structural members before it gets transferred to ground.
These interconnected structural members actually define the mechanism of load transfer. The
path followed by a load to transmit itself from its point of application to the foundation
through these interconnected structural members is named as load path. This is a very
important concept in structural engineering. We can identify this load path by keeping in
mind two aspects, first we need to identify the structural members or elements that is going to
participate in load resistance. Next, we need to identify which of the structural elements or
members are going to contribute to the load transfer to the successive element. Once we
identify these element sets or member groups, then it will be easy to identify what will be the
load path.
For different types of loads in a particular structure there can be several load paths. In case of
local damage to a structural component, that load path gets disturbed and that is why, it is
necessary that the structure is designed in such a way that it should be smart enough to locate
an alternate load path and transmit the loading to the ground and prevent imminent damage or
collapse.
(Refer Slide Time: 13:48)
The next topic of discussion is basic forms of bridges. There can be six basic forms of
bridges, which is derived based on their load transfer mechanisms; Beam Bridges, Truss
Bridges, Arch Bridges, Cantilever Bridges, Suspension Bridges and Cable Stayed Bridges.
We are going to learn about basic features of each of the six basic forms.
(Refer Slide Time: 14:12)
The first basic form to be discussed is beam bridge. Beam bridges carry the vertical loading
by flexure. We can see the schematic diagram of a beam bridge here. We can see that the top
fiber of this beam bridge identified in green color indicates that it is a compression load
carrying member While the bottom fiber which is demarcated in red color indicates that it is
carrying tension.
(Refer Slide Time: 14:50)
The second basic form to be discussed is truss bridge. Truss bridges of simple span behave
also like a beam, because it also carries vertical loading by bending. The top chords carry
compression loading and that is why it is a compression load carrying member. Bottom
chords carry tensile forces and that is why it is a tensile force member. Vertical and diagonal
members can be either compression or tension load carrying member depending on their
orientations.
We can see the schematic diagram of a truss bridge here, where the top chord shown in green
color is a compression load carrying member. Bottom chord shown in red color is a tension
load carrying member. Verticals for this configuration are found to be compression load
carrying member. Diagonals are either compression or tension load carrying member and that
is dependent on the fact that how they are oriented. Like the diagonals which are marked in
red color are tension load carrying member, while the diagonals that are marked in green
color indicates that they are compression load carrying members.
(Refer Slide Time: 16:12)
The next basic form to be discussed is arch bridge. Arch bridges carry the loading in
compression and the primary structural component in arch bridge is the arch rib itself. We can
see the schematic diagram of arch bridge here, where the arch bridge is in compression. Here
in the schematic diagram, we can see that the deck slab is connected to the arch bridge at the
ground point and we can see that vertical suspenders are oriented in such a way, they are
connecting the deck at one end and connected at the arch bridge at the other end.
(Refer Slide Time: 16:57)
The next basic form to be discussed is cantilever bridge. Cantilever bridges consists of 3 or
more spans, of which the outer or anchor spans are the corner spans, then we have cantilever
spans and subsequently we have the central suspended span. We can see the schematic
diagram of a 5 span cantilever bridge here. We can see that this one is span 1, span 2, span 3,
4 and 5. Span 1 and span 5 are corner spans or outer spans also named as anchor spans. They
are anchored at the ground, on both side we can see the same. The other end is connected to
the tower as we can see here. So, these spans as we have mentioned are named as anchor
spans.
The next spans are named as cantilever spans. So, the span 2 and span 4 are cantilever spans,
these spans are connected to the tower at one end and the other end is cantilever end, as we
can see here. Then span 3, that is the central span is suspended span. This is resting on the
cantilever ends and it acts as if it is simply supported beam or truss. Now, if you look
carefully, you will be able to see that the top chords on the anchor span and the cantilever
span are tension load carrying members and they are marked in red colors, as we can see
here.
The top chord of the central suspended span is compression load carrying member. The
bottom chord of the anchor span and cantilever span are compression load carrying members,
as we can see here and they are shown in green colors. The bottom chord of the central
suspended span is also a tension load carrying member as we can see that the drawing is color
coded to identify tension members in red color and compression members in green color.
This form of bridge is found to be one of the latest configurations of bridges which was
evolved after the 2nd World War.
(Refer Slide Time: 20:08)
The next basic form is suspension bridge. Suspension bridges carry the loads through these
flexible cables and these flexible curved cables offer little resistance to the lateral loading.
These cables carry the loading in tension and from the cables, the loads get transferred to the
ground through the towers and the anchorages. As we can see this is the anchorage and these
are the towers.
Now, due to the flexibility as we have said that the cables offer little resistance to lateral
loading, that is why they tend to get deflected. That is why these vertical suspenders are
provided so as to connect the cable to the deck and to help the cables retain its shape to some
extent under lateral loading.
If we draw the top of the tower and connect the top of the tower to the central lowest
elevation point for the cable, then this distance is named as sag. This is actually cable sag.
This cable sag is a very important parameter and it helps in designing the suspension bridge.
So, we can see that schematic diagram of the suspension bridge containing the curved cables,
sag is shown here and the suspenders towers and anchorages.
(Refer Slide Time: 22:00)
The next basic form is cable stayed bridge. Cable stayed bridges carry the vertical loads on
the decks through tensions and this tension load is taken care of by nearly straight inclined
cables in tension. These cables as we can see in the schematic diagram of the cable stayed
bridge are not flexible like the cables in suspension bridge. They are hold tight and inclined at
different angles and the inclination range from 25 to 45 degree.
These cables carry the loading in tension and from the cable tension the loads get transferred
through the tower to the foundation and the tower experiences vertical compression force as
we can see that the tower is in compression, while the cables are in tension. Also, the cable
stays as connected to the deck induces horizontal compression in the deck itself.
(Refer Slide Time: 23:22)
The next topic of discussion is structural materials. There can be several materials that can be
used for construction of bridges, like timber, stone, brick, steel, aluminium, reinforced
concrete, prestressed concrete or composites. When timber is used as construction material, it
can be only applicable to short span temporary bridges that is carrying very light amount of
loading.
If we use masonry as construction material then it can be only applicable to short span
permanent bridges. Economy can be achieved in masonry bridges depending on availability
of good quality building materials and skilled workers. Generally, masonry bridges are found
to be in the form of arch bridges across the world.
After industrial revolution, iron started to be used as construction material for bridges, which
got advanced to structural steel. Steel bridges are found to be applicable for long span
bridges. Reinforced concrete is found to be suitable for different spans and different site
conditions. It has an added advantage that it can be designed to have good durability
performance against aggressive exposure conditions.
Prestressed concrete can also be used for bridge construction due to its inherent benefits for
prestressed concrete, it helps in offering high resilience in bridges at slender cross-sectional
dimensions. That is why prestressed concrete is found to be suitable for long span slender
bridges at it can be constructed at comparable cost. It is also found to be suitable for fast
construction.
Composite materials are also being used for construction of bridges these days. In this way,
benefits of multiple materials can be retained so as to offer greater resilience at comparable
cost.
(Refer Slide Time: 25:34)
The next topic of discussion is historical evolution of bridges. Primitive man was a wanderer.
He used to roam around different places in search of food and shelter and that is why it was
observed that bridges were the earliest civil infrastructure that was built by primitive man.
A tree accidentally fallen across the stream became the earliest form of beam bridges. A
natural rock arch formed by erosion of the loose soil below the arch became the earliest form
of the arch bridges. Creepers hanging from tree to tree were used by animals to cross one
bank to another and that gave the idea to form rope bridge and subsequently suspension
bridges.
Around 4000 BC, it was believed that men were tending to settle down to lead a community
life and then only the requirement of permanent bridge was felt. And from there only, the
journey of evolution of bridges started. After industrial revolution, it was further progressed
so as to get it towards the latest technology-enabled bridges. We can see the steps of
evolution of bridges in the form of a flowchart.
(Refer Slide Time: 26:54)
Like we can see natural rock arches got advanced to corbelled arches. Subsequently, it got
advanced towards true arches. Then, it evolved into masonry arch bridges. Subsequently, RC
arch bridges and steel arch bridges came into existence. Natural stepping stones got advanced
towards artificial stepping stones. Then, stone slabs on stepping stones or rough piers were
formed. From there, stone beam bridges were constructed which got advanced towards stone
cantilever bridges.
Accidentally fallen trees became the first form of beam bridges. Subsequently, purposely fell
trees became the beam bridges to cross streams. Felled trees and transverse platforms helped
in the evolution of bridges. Then, timber bridges on stepping stones were used as bridges.
Timber bridges on timber piles or crib piers were started to be used which got advanced
towards timber cantilever bridges, which subsequently progressed towards timber truss
bridges.
After industrial revolution, iron was started to be used and cast-iron beam bridges came into
existence. After structural steel became prominent, then steel truss bridges were the next
upgradation from the timber truss bridges. Cast iron beam bridges got advanced towards plate
girder bridges. Then, reinforced concrete girder bridges and prestressed concrete girder
bridges were constructed.
Monkey trains or creepers branches were used for animals to cross one bank to another, that
evolved towards rope bridge. Then, suspension bridge came into existence which further
progressed towards cable stayed bridges. Earliest forms of construction was a simply
supported construction, which got advanced towards continuous construction. Then,
cantilever construction and rigid frame construction came into existence. A combination of
all these types of constructions were then evolved.
The next and last topic of discussion of today’s lecture is classifications of bridges.
(Refer Slide Time: 29:21)
We can learn about different key parameters based on which classifications of bridges take
place. According to function as the first key parameter, we can classify bridges as Aqueduct,
that is a canal over a river, Viaduct that is road or railway bridge over valley, Pedestrian
bridge, Highway bridge, Railway bridge, Road cum Rail bridge or Pipeline bridges.
The next key parameter is material of construction. According to material of construction of
superstructure, it can be timber bridge, masonry bridge, iron bridge, steel bridge, reinforced
concrete bridge, prestressed concrete bridge, composite bridge or aluminium bridge.
According to form or type of superstructure, that we have discussed earlier, we can classify
bridges into six forms, beam bridge, truss bridge, arch bridge, cantilever bridge, cable stayed
bridge or suspension bridge. According to inter-span relation, it can be simply supported
bridge, continuous bridge or cantilever bridge.
(Refer Slide Time: 30:27)
According to position of the floor with respect to the superstructure, it can be deck through,
half-through or suspended bridge. According to method of connections of different parts of
the superstructure, which is applicable to steel bridges or metallic bridges, it can be pin
connected riveted or welded bridge.
According to road level related to highest flood level of the river below, it can be high level
bridge or submersible bridge. According to method of clearance for navigation, it can be high
level movable bascule, movable swing, movable lift or transporter bridge.
(Refer Slide Time: 31:15)
According to length between the inner faces of the dirt walls, that is length of the bridge, it
can be Culverts (when the span length is less than 6 meters), Minor bridge (when the span
between 6 to 60 meters). When the span length is in between 60 to 120 meters, it can be
Major bridge. It can be a long span bridge when the main span of the major bridge is above
120 meters.
According to degree of redundancy, it can be determinate or indeterminate bridge. According
to anticipated type of service, it can be temporary bridge, permanent bridge, military bridge,
Pontoon or Bailey bridge under military bridges.
(Refer Slide Time: 32:08)
That was all for today’s discussion. If you have any queries, please feel free to post your
queries in the discussion forum. We are going to revert back to you quickly.
(Refer Slide Time: 32:18)
These are the references: Design of Bridges by N. Krishna Raju, Essentials of Bridge
Engineering by D. J. Victor, Bridge Engineering by S. Ponnuswamy, Design of Bridge
Structures by T. R. Jagadeesh and M. A. Jayaram, Bridge Engineering Handbook by W. F.
Chen and L. Duan, ICE manual of Bridge Engineering by G. Parke and N. Hewson. You can
refer to any of the books or handbooks for studying.
Thank you so much! We are ending our discussion here.