bridge introduction

CE 5154 Introduction to Bridge Engineering

Lecture No. 1 -- Historical Overview and Introduction

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.

Tacoman arrows

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


Stone Arch Bridge Movie


• 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

Truss Action Movie


• 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

Discussion on Classification According To 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 BridgeSlab BridgesT-Beam BridgeWood Beam BridgePre-cast Concrete Box Beam BridgePre-cast Concrete I-Beam BridgeRolled Steel Beam Bridge

Medium Span Bridges (up to 50m)

Pre-cast Concrete Box Beam & Pre-cast Concrete I-BeamComposite Rolled Steel Beam BridgeComposite Steel Plate Girder BridgeCast-in-place RCC Box Girder BridgeCast-in-place Post-Tensioned Concrete Box GirderComposite Steel Box Girder

BOX GIRDER

Large Span Bridges (50 to 150m)

Composite Steel Plate Girder BridgeCast-in-place Post-Tensioned concrete Box GirderPost-Tensioned Concrete Segmental ConstructionConcrete Arch and Steel Arch

Extra Large (Long) Span Bridges(Over 150m)

Cable Stayed BridgeSuspension Bridge

Discussion on Classification According To 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 BridgeSteel 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.

Anchor Block Movie

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

GIRDER BRIDGE

Bridge Cap and Damper

Truss bridge

Truss Bridge

ARCH BRIDGE

Suspension Bridge