Bridge introduction

CE 5154 Introduction to Bridge Engineering
Lecture No. 1 -- Historical Overview and Introductio

Golden Gate Bridge, USA
Firth of Forth Bridge, Scotland Sunshine skyway Bridge, USA

Topics
• Bridge Definition
• Bridge type
• Aesthetics in bridge design
• Factors considered in deciding bridge types
• Bridge components
• Bridge specification
• Role of Bridge Engineer

What is a BRIDGE?
•Bridge is a structure which covers a gap
•Generally bridges carry a road or railway across a natural or
artificial obstacle such as, a river, canal or another railway or
another road
•Bridge is a structure corresponding to the heaviest
responsibility in carrying a free flow of transport and is the most
significant component of a transportation system in case of
communication over spacings/gaps for whatever reason such as
aquatic obstacles, valleys and gorges etc.

Bridge is the KEY ELEMENT
in a Transportation System

It Controls the Capacity of the System
 If the width of a bridge is insufficient to carry the
number of lanes required to handle the traffic volume,
the bridge will be a constriction to the flow of traffic.
If the strength of a bridge is deficient and unable to
carry heavy trucks, load limits will be posted and
truck traffic will be rerouted.
 The bridge controls both the volume and weight of
the traffic carried by the transportation system.

Highest Cost per Mile of the System
Bridges are expensive. The typical cost per mile of a
bridge is many times that of the approach roads to the
bridge.`
Since, bridge is the key element in a transportation
system, balance must be achieved between handling future
traffic volume and loads and the cost of heavier and wider
bridge structure.

If the Bridge Fails, the System Fails
The importance of a Bridge can be visualized by considering the
comparison between the two main components of a highway system
i.e. a road and bridge itself.
EXAMPLE: Suppose in a road there occurs deterioration and
ultimately a crack, thus making a sort of inconvenience but it wont
result in stopping of the flow of traffic as traffic can pass or
otherwise a bypass can be provided. The traffic no doubt will pass
with a slower speed but in case of a bridge its flow is completely
stopped incase of the failure of the bridge, that is the reason its often
called “If the bridge fails the structure fails” as the function of the
structure could no longer be served at all.

Classification of Bridges
Steel Concrete Wood
Hybrid Stone/Brick
Pedestrian Highway Railroad
Short Medium Long
Slab Girder Truss Arch
Suspension Cable-Stayed
Material
Usage
Span
Structural
Form
Structural Arrangement

Discussion on Classification According To
STRUCTURAL FORM
Distinctive Features of Girder Bridge
Distinctive Features of Arch Bridge
Distinctive Features of Truss Bridge
Distinctive Features of Suspension Bridge
Distinctive Features of Cable-Stayed Bridges

Distinctive Features of Girder Bridges
•Widely constructed
•Usually used for Short and Medium spans
•Carry load in Shear and Flexural bending
•Efficient distribution of material is not possible
•Stability concerns limits the stresses and associated economy
•Economical and long lasting solution for vast majority of bridges
•Decks and girder usually act together to support the entire load in
highway bridges

•Arch action reduces bending moments ( that is Tensile Stresses )
•Economical as compared to equivalent straight simply supported
Girder or Truss bridge
•Suitable site is a Valley with arch foundations on a DRY ROCK
SLOPES
•Conventional curved arch rib has high Fabrication and Erection
costs
•Erection easiest for Cantilever Arch and most difficult for Tied
Arch
•Arch is predominantly a Compression member. Buckling must be
worked to the detail so as to avoid reductions in allowable stresses.

•Classic arch form tends to favor Concrete as a construction
material
•Conventional arch has two moment resistant components :
The deck and the Arch Rib.
•Near the crown of the arch and the region where Spandrel
Columns are short, undesirable B.M. can occur. By using Pin
ended columns it can be avoided
•Space beneath the arch is less and hence danger for collision
with the Rib, specially on a highway
•Curved shaped is always very pleasing and arch is the most
successful and beautiful structure

•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
•These advantages are achieved at the expense of increased
fabrication and maintenance costs
•Other bridge types have rendered the truss bridge types less
likely to be used due to its high maintenance and fabrication
costs.
•The truss is instead being used widely as the stiffening
structure for the suspension bridges due to its acceptable
aerodynamic behavior since the wind gusts can pass through
the truss as is not with the case in girder, arch bridges.

•It’s a light weight structure it can be assembled member by
member using lifting equipment of small capacity.
•Rarely aesthetically pleasing complexity of member
intersections if viewed from oblique direction
•In large span structures poor aesthetic appearance of the truss
bridge is compensated with the large scale of the structure. For
moderate spans its best to provide a simple and regular
structure

•Major element is a flexible cable, shaped and supported in such a
way that it transfers the loads to the towers and anchorage
•This cable is commonly constructed from High Strength wires,
either spun in situ or formed from component, spirally formed wire
ropes. In either case allowable stresses are high of the order of 600
MPA
•The deck is hung from the cable by Hangers constructed of high
strength ropes in tension
•As in the long spans the Self-weight of the structures becomes
significant, so the use of high strength steel in tension, primarily in
cables and secondarily in hangers leads to an economical structure.
•The economy of the cable must be balanced against the cost of the
associated anchorage and towers. The anchorage cost may be high
where foundation material is poor

•The main cable is stiffened either by a pair of stiffening trusses or
by a system of girders at deck level.
•This stiffening system serves to (a) control aerodynamic
movements and (b) limit local angle changes in the deck. It may be
unnecessary in cases where the dead load is great.
•The complete structure can be erected without intermediate
staging from the ground
•The main structure is elegant and neatly expresses its function.
•It is the only alternative for spans over 600m, and it is generally
regarded as competitive for spans down to 300m. However, shorter
spans have also been built, including some very attractive
pedestrian bridges
•The height of the main towers can be a disadvantage in some
areas; for example, within the approach road for an AIRPORT

Distinctive Features of Cable-stayed Bridge
•The use of high strength cables in tension leads to economy in
material, weight, and cost..
•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. For economical design, the deck must participate in
carrying these forces
•All individual cables are shorter than full length of the
superstructure. They are normally constructed of individual wire
ropes, supplied complete with end fittings, prestretched and not
spun.
•There is a great freedom of choice in selecting the structural
arrangement
•Less efficient under Dead Load but more efficient in support Live
Load. It is economical over 100-350m, some designer would
extend the upper bound as high as 800m

Distinctive Features of Cable-stayed Bridge
•Aerodynamic stability has not been found to be a problem in
structures erected to date
•When the cables are arranged in the single plane, at the longitudinal
center line of the deck, the appearance of the structure is simplified
and avoids cable intersections when the bridge is viewed obliquely

SPAN
Small Span Bridges (up to 15m)
Medium Span Bridges (up to 50m)
Large Span Bridges (50-150m)
Extra Large ( Long ) Span Bridges (over 150m)

Small Span Bridges (up to 15m)
Culvert Bridge
Slab Bridges
T-Beam Bridge
Wood Beam Bridge
Pre-cast Concrete Box Beam Bridge
Pre-cast Concrete I-Beam Bridge
Rolled Steel Beam Bridge

Medium Span Bridges (up to 50m)
Pre-cast Concrete Box Beam & Pre-cast Concrete I-Beam
Composite Rolled Steel Beam Bridge
Composite Steel Plate Girder Bridge
Cast-in-place RCC Box Girder Bridge
Cast-in-place Post-Tensioned Concrete Box Girder
Composite Steel Box Girder

Large Span Bridges (50 to 150m)
Composite Steel Plate Girder Bridge
Cast-in-place Post-Tensioned concrete Box Girder
Post-Tensioned Concrete Segmental Construction
Concrete Arch and Steel Arch

Extra Large (Long) Span Bridges
(Over 150m)
Cable Stayed Bridge
Suspension Bridge

Structural Arrangement
•Main Structure Below the Deck Line
•Main Structure Above the Deck Line
•Main Structure coincides with the Deck Line
The classification of the bridge types can also be according to
the location of the main structure elements relative to the
surface on which the user travels, as follows:

Main Structure Below the Deck Line
Arch Bridge
Masonry Arch
Concrete Arch
Inclined Leg Frame Arch
Rigid Frame Arch
Truss-Arch Bridge
Steel Truss-Arch
Steel Deck Truss

Main Structure Above the Deck Line
Suspension Bridges
Cable Stayed Bridges
Through-Truss Bridge

Main Structure Coincides with the
Deck Line
Girder Bridge
Slab (solid and voided)
T-Beam (cast-in-place)
I-beam (pre-cast or pre-stressed
Wide-flange beam (composite & non-
composite
Concrete Box (cast-in-place, segmental
& pre-stressed
Steel Plate Girder (straight & haunched)
Steel box (Orthotropic deck)

FACTORS CONSIDERED IN DECIDING
BRIDGE TYPE
•Geometric Conditions of the Site
•Subsurface Conditions of the Site
•Functional Requirements
•Aesthetics
•Economics and Ease of Maintenance
•Construction and Erection Consideration
•Legal Considerations
In general all the factors are related to economy, safety and
aesthetics.

Geometric Conditions of the Site
•The type of bridge selected will always depend on the horizontal
and vertical alignment of the highway route and on the clearances
above and below the roadway
•For Example: if the roadway is on a curve, continuous box girders
and slabs are a good choice because they have a pleasing
appearance, can readily be built on a curve, and have a relatively
high torsion resistance
•Relatively high bridges with larger spans over navigable
waterways will require a different bridge type than one with
medium spans crossing a flood plain
•The site geometry will also dictate how traffic can be handled
during construction, which is an important safety issue and must be
considered early in the planning stage

Subsurface conditions of the soil
•The foundation soils at a site will determine whether abutments and
piers can be founded on spread footings, driven piles, or drilled shafts
•If the subsurface investigation indicates that creep settlement is going
to be a problem, the bridge type selected must be one that can
accommodate differential settlement over time
•Drainage conditions on the surface and below ground must be
understood because they influence the magnitude of earth pressures,
movement of embankments, and stability of cuts or fills
•For Example: An inclined leg frame bridge requires strong
foundation material that can resist both horizontal and vertical thrust. If
it is not present, then another bridge type is more appropriate.

•The potential for seismic activity at a site should also be a
part of the subsurface investigation. If seismicity is high,
the substructure details will change, affecting the
superstructure loads as well
•All of these conditions influence the choice of
substructure components which in turn influence the choice
of superstructure
Subsurface conditions of the soil

Functional Requirements
•Bridge must function to carry present and future volumes of traffic.
•Decisions must be made on the number of lanes of traffic,
inclusion of sidewalks and/or bike paths, whether width of the
bridge deck should include medians, drainage of the surface waters,
snow removal, and future wearing surface.
•For Example: In the case of stream and flood plain crossings, the
bridge must continue to function during periods of high water and
not impose a severe constriction or obstruction to the flow of water
or debris.
•Satisfaction of these functional requirements will recommend some
bridge types over others.
•For Example: if future widening and replacement of bridge decks
is a concern, multiple girder bridge types are preferred over
concrete segmental box girders.

Aesthetics
•It should be the goal of every bridge designer to obtain a
positive aesthetic response to the bridge type selected
•There are no equations, no computer programs or design
specifications that can make our bridge beautiful.
•It is more an awareness of beauty on our part so that we can
sense when we are in the presence of something good.
•Aesthetics must be a part of the bridge design program from
the beginning. It can’t be added on at the end to make the
bridge look nice. At that time it is too late. From the beginning,
the engineer must consider aesthetics in the selection of spans,
depths of girders, piers, abutments, and the relationship.

Economic and ease of maintenance
•The initial cost and maintenance cost over the life of the bridge
govern when comparing the economics of different bridge types.
•A general rule is that the bridge with the minimum number of spans,
fewest deck joints, and widest spacing of girders will be the most
economical.
•For Example: (1) By reducing the number of spans in a bridge
layout by one span, the construction cost of one pier is eliminated.
(2) Deck joints are a high maintenance cost item, so minimizing their
number will reduce the life cycle cost of the bridge. (3) When using
the empirical design of bridge decks in the AASHTO (1994) LRFD
Specifications, the same reinforcement is used for deck spans up to
4.1m. Therefore, there is little cost increase in the deck for wider
spacing for girders and fewer girders means less cost although at the
“expense” of deeper sections.

Economic and ease of maintenance
•Generally, concrete structures require less maintenance than steel
structure. The cost and hazard of maintenance painting of steel
structures should be considered in type selection studies.
•One effective way to reduce the overall project cost is to allow
contractors to propose an alternative design or designs.

Construction and Erection Considerations
•The length of the time required to construct a bridge is
important and will vary with the bridge type.
•Generally, larger the prefabricated or pre-cast members shorter
the construction time. However, the larger the members, the
more difficult they are to transport and lift into place.
•The availability of skilled labor and specified materials will
also influence the choice of a particular bridge type.
•For Example: if there are no pre-cast plants for pre-stressed
girders within easy transport but there is a steel fabrication plant
nearby that could make the steel structure more economical.
•The only way to determine which bridge type is more
economical is to bid alternative designs.

Legal Considerations
•Regulations are beyond the control of an engineer, but they are
real and must be considered.
Examples of certain regulations are as follows:
•Permits Over Navigable Waterways
•National Environmental policy Act
•Department of Transportation Act
•National historic preservation Act
•Clean Air Act
•Noise Control Act

Legal Considerations
•Fish and Wildlife Coordination Act
•The Endangered Species Act
•Water Bank Act
•Wild and Scenic Rivers Act
•In addition to the environmental laws and acts defining
national policies, local and regional politics are also of
concern

Discussion on Bridge Components
•Common bridge components
•Components of a Girder bridge (Beam Bridge)
•Components of a Suspension Bridge

General Bridge Components
Bridge Bearings: These are supports on a bridge pier, which carry the
weight of the bridge and control the movements at the bridge supports,
including the temperature expansion and contraction. They may be metal
rockers, rollers or slides or merely rubber or laminated rubber ( Rubber with
steel plates glued into it).
Bridge Dampers & Isolators: Bridge dampers are devices that absorb energy
generated by earthquake waves and lateral load
Bridge Pier: A wide column or short wall of masonry or plain or reinforced
concrete for carrying loads as a support for a bridge, but in any case it is
founded on firm ground below the river mud

General Bridge Components
Bridge Cap: The highest part of a bridge pier on which the
bridge bearings or rollers are seated. It may be of stone, brick
or plain or reinforced concrete.
Bridge Deck: The load bearing floor of a bridge which
carries and spreads the loads to the main beams. It is either of
reinforced concrete., pre-stressed concrete, welded steel etc.
Abutment: A support of an arch or bridge etc which may
carry a horizontal force as well as weight.
Expansion Joints : These are provided to accommodate the
translations due to possible shrinkage and expansions due to
temperature changes.

Components of a Girder bridge (Beam Bridge)

Components of a Suspension Bridge
• Anchor Block: Just looking at the figure we can compare it as a dead man
having no function of its own other than its weight.
• Suspension girder: It is a girder built into a suspension bridge to distribute the
loads uniformly among the suspenders and thus to reduce the local deflections
under concentrated loads.
• Suspenders: a vertical hanger in a suspension bridge by which the road is
carried on the cables
• Tower: Towers transfers compression forces to the foundation through piers.
• Saddles: A steel block over the towers of a suspension bridge which acts as a
bearing surface for the cable passing over it.
• Cables: Members that take tensile forces and transmit it through saddles to
towers and rest of the forces to anchorage block.

BRIDGE SPECIFICATIONS
• Meaning of bridge specifications.
• Need of bridge specifications.
 History
 Development
 Lack of specification and usage of proper codes and safety
factors -------reason of failure of a structure (bridge)
 Use and check of safety factors case study of wasserwork bridge
for the check of present working capacity.
 Assignment: Main reason of failure for some bridge/bridges

BRIDGE SPECIFICATION
• Basically the word specification stands in general for a
collection of work description upon which there is a
mutual agreement of the most experienced group of
people based upon their practical and theoretical
knowledge
• Bridge specification:
Applying the above mentioned definition, context to
bridge makes it self explanatory.

HISTORY AND NEED OF
BRIDGE SPECIFICATIONS
• Early bridge were design built type contract.
• No proper specifications so contract went to lowest bidder
• Statistics of built bridges in 1870’s show 40 bridges failed per year.
• Engineers thought about a mutual ground of practice that is both economical
and general along with restricting the bidding companies to follow a course of
work there by improving the quality of structures and forcing them to
compromise on quality which was a very common practice in case of absence
of any code or specification.

Development
• First practical step was taken after the collapse of a locomotive bridge on 29th
September 1876 across Ashtabula Creek at Ashtabula.
• 1914 American Association of State Highway Officials (AASHO) was formed
• 1921 committee on Bridges and Allied Structures was organized..
• The first edition of standard specifications for Highway Bridges and Incidental
Structures was published in 1931 by AASHO.
• In 1963 AASHO became AASHTO (American Association for State Highway and
Transportation Officials)
• In the beginning the design philosophy utilized in the standard specification was
working stress design (allowable stress design). In the 1970s variation in the
uncertainties of loads were considered and load factor design was introduced as an
alternative method.
• In 1986 the subcommittee on Bridges and structures initiated study of the load and
resistance factor design (LRFD) .
• The subcommittee authorized a comprehensive rewrite of the entire standard
specification to accompany the conversion to LRFD. The result is the first edition of
the AASHTO (1994) LRFD Bridge Design Specification.

CASE STUDY TO VISULAIZE THE
IMPORTANCE OF BRIDGE
SPECIFICATIONS
Location:
Waserwork strasse, Zurich Switzerland, slab bridge modeled in CUBUS
software then later on modeled in SAP 2000.
Problem:
A 70 year old slab bridge (sort of cause way) was asked to be checked for the
current code of practice in turn checking the safety factors.
Solution:
The bridge was analyzed for the current loading situations according to the
current codes of practice and the results were compared with the results of the
older bridge analysis.
Result:
The safety factors were found in accordance with the older analysis and design
of bridge on which it was being built.

ROLE OF A BRIDGE ENGINEER
The role of an engineer can be broadly classified
in two major working environments.
• Consultancy Environment
• Contractor Environment

Consultancy Environment
• Meeting the demand of clients
• Not compromising on quality control at the same time
while remaining economical.
• Must work properly on factors such as environment of
the location, traffic growth rate, population growth rate
etc before designing.
• Design should be flexible to the practical considerations.
• Following the proper design specifications.
• Proper Management both off site and on site.

Contractor Environment
• On site decision making keeping in mind factors such as cultural
& environmental factors etc
• Quality assurance to the consultants there by working up to the
needs of clients
• Be economical to the contracting firm along with not making a
compromise on quality.
• Proper time management and scheduling of works without undue
delays.
• Beneficial use of labors at various important locations of bridge.

CASE STUDY
• LOCATION:
• Arachtos, Greece.
• Arachtos bridge pier design for construction phase modeled in SAP 2000.
• Problem------Counter acting the forces just introduced for construction phase
due to heavy machinery to be used.
• Solution------Attaching with a cable or some other appropriate element with the
girder end so as to take part of loads.
• Result------calculation of the percentage of loads taken by the cable element.
• Acrachtos bridge pier design for construction phase modeled in SAP 2000 after
the introduction of cable attached to the box girder.

Aesthetics in Bridge Design
•The conventional order of priorities in bridge design is safety,
economy, serviceability, constructability, and so on. Somewhere down
this list is aesthetics. There should be no doubt in an engineer’s mind
that aesthetics needs a priority boost, and that it can be done without
infringing upon the other factors.
•The belief that improved appearance increases the cost of bridges is
unfounded and oftentimes the most aesthetically pleasing bridge is
also the least expensive.
•The additional cost is about 2% for short spans and only about 5% for
long spans
•It is not necessary that everyone agrees as to what makes a bridge
beautiful, but it is important that designers are aware of the qualities of
a bridge that influence the perception of beauty

Definition Aesthetics and Beauty
•Aesthetics is the study of qualities of beauty of an object and
of their perception through our senses.
•Even if this particular aesthetic air be the last quality we seen
in a bridge, its influence nonetheless exists and has an
influence on our thoughts and actions. ( Santayana )

Qualities of Aesthetic Design
“ There are not HARD & FAST rules or formulas for aesthetics of bridge
design. It finally gets down to the responsibility of each designer on each
project to make personal choices that will lead to a more beautiful
structure “
•Function
•Proportion
•Harmony
•Order & Rhythm
•Contrast & Texture
•Light and shadow

Function
•For a bridge design to be successful, it must always safely perform its
function.
•For example, a bridge is designed that fulfills every requirements of
aesthetic consideration and other requirements such as economy,
constructability etc. but is somehow unable to perform the function for
which it was designed, then however beautiful it is, it won’t be
appealing.
•The very first notion of beauty in a bridge is that it performs its
function efficiently and people using it are satisfied.
•Moreover, the IMPORTANCE of function also enhances the
BEAUTY or AESTHETICS of the BRIDGE.
•For Example: A bridge across straits of Bosporus at Istanbul. This
bridge replaces a slow ferry boat trip, but it also serves the function of
connecting two continents (Asia and Europe).

Proportion
•Good proportions are fundamental to achieving an aesthetically
pleasing bridge structure
•It is generally agreed that when a bridge is placed across a relatively
shallow valley, the most pleasing appearance occurs when there are
an odd number of spans with span lengths that decrease going up the
side of the valley.
•The bridge over a deep valley again should have an odd number of
spans, but should be of equal length. And slender girders and the tall,
tapered piers can add to the aesthetic pleasure

•Another consideration is the proportion between piers and girders.
From strength viewpoint, the piers can be relatively thin compared
to the girders. However, when a bridge has a low profile, the visual
impression can be improved by having strong piers supporting
slender girders.
•Slender girders can be achieved if the superstructure is made
continuous. Infact, the superstructure continuity is the most
important aesthetic consideration
•The proportions of a bridge change when viewed from an oblique
angle.
Proportion

Harmony
•Harmony means getting along well with others. The parts of the
structure must be in agreement with each other and the whole structure
must be in agreement with its surroundings.
Harmony between the elements of a bridge:
•It depends on the proportions between the span lengths and depth of
girders, height and size of piers, and negative spaces and solid masses.
Harmony between the whole structure and its surroundings
•The scale and size of a bridge structure should be relative to its
environment.
•For Example, a long bridge crossing a wide valley can be large
because the landscape is large. But when a bridge is placed in an urban
setting, the size must be reduced.

Order and Rhythm
•Repeating similar spans too many times can become boring and
monotonous
•It can also become aggravating to be driving down the interstate
and seeing the same standard over crossing mile after mile. The
first one or two look just fine, but after a while a feeling of
frustration takes over the pleasing affect of however the beautiful
the construction.

•There is a place for contrast, as well as harmony in bridge
aesthetics.
•All bridges do not have to blend in with their surroundings. “
when a bridge is built in the middle of the country, it should
blend in with the country side, but very often, because of its
proportions and dynamism, the bridge stands out and dominates
the landscape”
•The dominance seems to be specially true in case of Cable-
stayed and suspension bridges.
•There can also be contrast between the elements of a bridge to
emphasize the slenderness of the girders and the strength of the
piers and abutments.
Contrast and Texture

•Texture can also be used to soften the hard appearance of
concrete and make certain elements less dominant.
•Large bridges seen from a distance must develop contrast
through their form and mass, but bridges with smaller spans
seen up close can effectively use texture.
Contrast and Texture

Light and Shadow
•Designer must be aware of how the shadows occur on the
structure throughout the day
•If the bridge is running north and south the shadows will be quite
different than if it is running east to west.
•For Example: When sunlight is parallel to the face of a girder or
wall, small imperfections in workmanship can cast deep shadows.
Construction joints in concrete may appear to be discontinuous
and hidden welded stiffeners may no longer be hidden.
•One of the most effective ways to make a bridge girder appear
slender is to put it partially or completely in shadow.

•Creating shadow becomes especially important with the use
of solid concrete safety barriers that make the girders look
deeper than they actually are.
•Shadows can be accomplished by cantilevering the deck
beyond the exterior girder.
•The effect of shadow on a box girder is further improved by
sloping the side of the girder inward.
Light and Shadow

End of show
• Construction & history of Brooklyn Bridge
• Construction & history of Golden Gate Bridge

Bridge introduction

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