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1
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By
Dr. Attaullah Shah
Swedish College of Engineering and Technology Wah Cantt.
CE-407
Lec-03
Structural EngineeringBridges
Bridges
700 A.D. Asia700 A.D. Asia
1,304 years 1,304 years agoago
100 B.C. Romans100 B.C. Romans2,104 years ago2,104 years ago
Clapper Bridge
Tree trunkStone
Arch design evenly distributesstressesNatural concrete made from mud and straw
Roman Arch Bridge
History of Bridge DevelopmentHistory of Bridge Development
Great Stone Bridge in China
Low bridgeShallow archAllows boatsand water to passthrough
History of Bridge DevelopmentHistory of Bridge Development
Truss BridgesMechanics of DesignWood
Suspension Bridges
Use of steel in suspending cables
1900 1900
1920 1920
Prestressed ConcreteSteel
2000 2000
Compression Tension
Basic Concepts Basic Concepts
Span - the distance between two bridge supports, whether they are columns, towers or the wall of a canyon.
Compression –
Tension -
Force -
Concrete has good compressive strength, but extremely weak tensile strength. What about steel cables?
Basic Concepts Basic Concepts
Beam - a rigid, usually horizontal, structural element
Pier - a vertical supporting structure, such as a pillar
Cantilever - a projecting structure supported only at one end, like a shelf bracket or a diving board
Beam
Pier
Load - weight on a structure
The type of bridge used depends on the obstacle. The main feature that controls the bridge type is the size of the obstacle.
Types of BridgesTypes of BridgesBasic Types:
•Truss Bridge•Beam Bridge•Arch Bridge•Suspension Bridge•Floating Bridge
Truss Beam ArchSuspension
Floating
Truss Bridge
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.
Types of BridgesTypes of Bridges
Beam BridgeBeam Bridge
Consists of a horizontal beam 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.
Forces
When something pushes down on the beam, the beam bends. Its top edge is pushed together, and its bottom edge is pulled apart.
Types of BridgesTypes of Bridges
Beam BridgeBeam Bridge
Arch BridgesArch Bridges
The arch has great natural strength. Thousands of years ago, Romans built arches out of stone. Today, most arch bridges are made of steel or concrete, and they can span up to 800 feet.
Types of BridgesTypes of Bridges
Forces
The arch is squeezed together, and this squeezing force is carried outward along the curve to the supports at each end. The supports, called abutments, push back on the arch and prevent the ends of the arch from spreading apart.
Types of BridgesTypes of Bridges
Arch BridgesArch Bridges
Suspension BridgesSuspension Bridges
This kind of bridges can span 2,000 to 7,000 feet -- way farther than any other type of bridge! Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.
Types of BridgesTypes of Bridges
Forces
In all suspension bridges, the roadway hangs from massive steel cables, which are draped over two towers and secured into solid concrete blocks, called anchorages, on both ends of the bridge. The cars push down on the roadway, but because the roadway is suspended, the cables transfer the load into compression in the two towers. The two towers support most of the bridge's weight.
Types of BridgesTypes of Bridges
Suspension BridgesSuspension Bridges
•Pontoon bridges are supported by floating pontoons with sufficient buoyancy to support the bridge and dynamic loads.
•While pontoon bridges are usually temporary structures, some are used for long periods of time.
•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.
Types of BridgesTypes of Bridges
Floating BridgeFloating Bridge
Floating Bridges
Retractable!
But high maintenance!
How do the following affect your structure?Ground below bridgeLoadsMaterialsShapes
Bridge Bridge Engineering Engineering
Some Uses of Bridges
− Walkways
− Highways/Roads
− Railways
− Pipelines
− Connecting lands
− Crossing rivers and canyons
Types of Bridges
− Arch
− Truss
− Cantilever
− Cable-Stayed
− Suspension
What makes a bridge stay up?
− Forces− Compression – a
pushing or squeezing force
− Tension – a pulling or stretching force
Arch Bridges
− Keystone – the wedge-shaped stone of an arch that locks its parts together
− Abutments – the structures that support the ends of the bridge
Arch Bridges
− Works by
Compression
Arch Bridges
− Where have you seen these bridges?
Cold Spring Arch Bridge, Santa Barbara, CA
Marsh Rainbow Arch, Riverton, KS
Pont du Gard, Nimes, France
Cable-Stayed Bridges
− Piers – the vertical supporting structures
− Cables – thick steel ropes from which the decking is suspended
− Decking – the supported roadway on a bridge
Cable-Stayed Bridges
− Works by Tension AND Compression
Cable-Stayed Bridges
− Where have you seen these bridges?
Zakim Bridge, Boston, MA
Sunshine Skyway Bridge, Tampa, FL
Sundial Bridge, Redding, CA
Suspension Bridges
− Similar to Cable-Stayed
− Different construction method
Suspension Bridges
− Works by Tension and Compression
Suspension Bridges
− Where have you seen these bridges?
Golden Gate Bridge, San Francisco, CA
Brooklyn Bridge, Brooklyn, NY
Verrazano-Narrows Bridge, New York, NY
Other Types
Truss
Southern Pacific Railroad Bridge, Tempe, AZ
Cantilever
Firth of Forth-Forth Rail Bridge, Edinburgh, Scotland
FUNCTION OF A BRIDGEFUNCTION OF A BRIDGE
To connect two communities which are To connect two communities which are separated by streams, valley, railroads, separated by streams, valley, railroads, etc.etc.
• Replaces a slow ferry Replaces a slow ferry boat tripboat trip• Connects two continentConnects two continent• Built in 1973Built in 1973• Total length is 5000 ftTotal length is 5000 ft
Bosporus Straits Bridge Bosporus Straits Bridge at Istanbul, Turkey –at Istanbul, Turkey –
COMPONENTS OF A BRIDGECOMPONENTS OF A BRIDGE
• Deck or Slab:Deck or Slab: supported roadway on abridge supported roadway on abridge
• Beam or Girder:Beam or Girder: A rigid, usually horizontal, A rigid, usually horizontal, structural elementstructural element
• Abutment:Abutment: The outermost end supports on a The outermost end supports on a bridge, which carry the load from bridge, which carry the load from the deckthe deck
• Pier:Pier: A vertical supporting structure, such as a A vertical supporting structure, such as a pillarpillar
• FoundationFoundation
DeckDeck
GirderGirder
AbutmentAbutment
PierPier
COMPONENTS OF A BRIDGECOMPONENTS OF A BRIDGE
TYPES OF BRIDGESTYPES OF BRIDGES
•Beam or Girder BridgeBeam or Girder Bridge•Truss BridgeTruss Bridge•Rigid Frame BridgeRigid Frame Bridge•Arch BridgeArch Bridge•Cable Stayed BridgeCable Stayed Bridge•Suspension BridgeSuspension Bridge
GIRDER BRIDGEGIRDER BRIDGE
Chesapeake Bay Chesapeake Bay Bridge, VirginiaBridge, Virginia
• Typical span length 30 to 650 ft
• World’s longest: Ponte Costa e Silva, Brazil with a center span of 1000 ft
TRUSS BRIDGETRUSS BRIDGE
Firth of Forth Bridge, ScotlandFirth of Forth Bridge, Scotland
• Typical span length 150 to 1500 ft
• World’s longest: Pont de Quebec, Canada with a center span of 1800 ft
RIGID FRAME BRIDGERIGID FRAME BRIDGE
•Girders and piers act togetherGirders and piers act together
•Cross-sections are usually I-shaped or box-shaped.Cross-sections are usually I-shaped or box-shaped.
•Design calculations for rigid Design calculations for rigid frame bridges are more frame bridges are more difficult than those of simple difficult than those of simple girder bridges.girder bridges.
ARCH BRIDGEARCH BRIDGE
• After girders, arches are the second oldest bridge type.After girders, arches are the second oldest bridge type.
• Arches are good choices for crossing valleys and rivers Arches are good choices for crossing valleys and rivers
• Arches can be one of Arches can be one of the more beautiful the more beautiful bridge types.bridge types.
• Typical span lengthTypical span length 130 ft – 500 ft.130 ft – 500 ft.
• World’s longest:World’s longest: New River Gorge Bridge, U.S.A. with a center span of New River Gorge Bridge, U.S.A. with a center span of 1700 ft.1700 ft.
Larimer Avenue Bridge, Pittsburgh Larimer Avenue Bridge, Pittsburgh
CABLE STAYED BRIDGECABLE STAYED BRIDGE
Normandie BridgeNormandie Bridge
• Continuous girder with Continuous girder with one or more towers one or more towers erected above in the erected above in the middle of the span.middle of the span.• From these towers From these towers cables stretch down cables stretch down diagonally and support diagonally and support the girder.the girder.• Typical span lengthTypical span length 350 to 1600 ft.350 to 1600 ft.• World’s largest bridge:World’s largest bridge: Tatara Bridge, JapanTatara Bridge, Japan center span: 2900 ft.center span: 2900 ft.
SUSPENSION BRIDGESUSPENSION BRIDGE
• Continuous girder with one or Continuous girder with one or more towers erected above in the more towers erected above in the middle of the span.middle of the span.
• At both ends of the bridge, large At both ends of the bridge, large anchors or counter weights are anchors or counter weights are placed to hold the ends of the placed to hold the ends of the cables.cables.
• Typical span lengthTypical span length 250 to 3000 ft.250 to 3000 ft.
Golden Gate Bridge, CaliforniaGolden Gate Bridge, California
Factors Describe a Bridge Factors Describe a Bridge
Four main factors are used in describing a bridge:
• Span (simple, continuous, cantilever)
• Material (stone, concrete, metal, etc.)
• Placement of the travel surface in relation to the structure (deck, through)
• Form (beam, arch, truss, etc.).
Basic Span TypesBasic Span Types
Simple Span
Continuous Span
Cantilever Span
LOADS ON BRIDGESLOADS ON BRIDGES
• Permanent Loads: remain on the bridge for an extended period of time (self weight of the bridge)
• Transient Loads: loads which are not permanent - gravity loads due to vehicular, railway and pedestrian traffic - lateral loads due to water and wind, ice floes, ship collision, earthquake, etc.
VEHICULAR DESIGN LOADS (HL 93)VEHICULAR DESIGN LOADS (HL 93)
• AASHTO – American Association of State Highway and Transportation Officials
This model consists of:
• Design Truck• Design Tandem• Design Lane
145 kN 145 kN 35 kN
4.3 to 9.0 m 4.3 m
9.3 N/m
DESIGN TRUCK
DESIGN TRUCKDESIGN TRUCK
110 kN 110 kN
9.3 N/m
1.2 m
DESIGN TANDEM
DESIGN TANDEMDESIGN TANDEM
DESIGN PRINCIPLESDESIGN PRINCIPLES
Resistance ≥ effect of the applied loads
Strength of the Member ≥ Factor of Safety x Applied Load
Allowable Stress Design (ASD):
Load and Resistance Factor Design (LRFD):η ∑γiQi ≤ φi Rn
Where, Qi = Effect of loads Rn = Nominal resistance γi = Statistically based resistance factor applied to the force effects
φi = Statistically based resistance factor applied to the nominal resistance η = Load modification factor
MATERIALS FOR BRIDGESMATERIALS FOR BRIDGES
• Concrete
• Steel
• Wood
CONCRETE BRIDGESCONCRETE BRIDGES
• Raw materials of concrete: cement, fine aggregate coarse aggregate, water• Easily available• can be designed to satisfy almost any geometric alignment, straight to curved• can be cast-in-place or precast• Compressive strength of concrete range from 5000 psi to 8500 psi• Reinforced concrete and prestressed concrete
STEEL BRIDGESSTEEL BRIDGES
• Minimum construction depth
• Rapid construction
• Steel can be formed into any shape or form • Predictable life
• Ease of repair and demolition
WOOD BRIDGESWOOD BRIDGES
• Convenient shipping to the job site
• Relatively light, lowering transportation and initial construction cost
• Light, can be handled with smaller construction equipment
• Approx. 12% of the bridges in US are wood bridges
• Commonly used for 20-80 ft span
Wood Bridge on Concrete Wood Bridge on Concrete AbutmentsAbutments
Three Span Wood Bridge Three Span Wood Bridge
GIRDER CROSS-SECTIONS GIRDER CROSS-SECTIONS COMMONLY USED IN BRIDGESCOMMONLY USED IN BRIDGES
COLLAPSE OF BRIDGESCOLLAPSE OF BRIDGES
• Poor design
• Inadequate stability of the foundation
• Fatigue cracking
• Wind forces
• Scour of footing
•Earthquake
BeforeBeforeCollapseCollapse
After CollapseAfter Collapse
AKASHI KAIKYO BRIDGE, JAPAN
Completion Date: 1998 Cost: $4.3 billion
Length: 12,828 feet Type: Suspension
Materials: Steel Span: 6,527 feet
SUNSHINE SKYWAY BRIDGE, USA
Completion Date: 1987 Cost: $244 Million
Length: 29,040 feet Type: Cable Stayed
Materials: Steel, Concrete Span: 1200 feet
NEW RIVER GORGE BRIDGE, USA
Completion Date: 1978 Cost: $37 Million
Length: 4,224 feet Type: Arch
Materials: Steel Span: 1700 feet
