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# Bridges and Their Design - PowerPoint PPT Presentation

Bridges and Their Design. History. Bridges have been used since the dawn of humankind Stone bridges became popular in the Roman era In the 19 th century, mathematics and physics became applied to design. What is a Girder Bridge?.

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### Bridges and Their Design

• Bridges have been used since the dawn of humankind

• Stone bridges became popular in the Roman era

• In the 19th century, mathematics and physics became applied to design

• Girder bridges are among the most common and most basic bridges (for example, a tree spanning a creek).

• Girders are heavily relied upon to provide strength and stability to these bridges.

• There are two common types of girders:I-Beam Girders Box Girders

• I-beam girders get their name simply by their design.

• Consists of one vertical plate (“the web”) and two horizontal plates (“flanges”).

• I-beam girders are used for typically most small bridges without any curves in them.

• Box girders are similar to I-beam girders except for the fact that they take on the shape of a box.

• They consist of two “webs” and two “flanges” to make the shape of a box.

• Unlike I-beams, box girders are used to span longer distances and in instances where the bridge must curve for better stability.

Simple design

Works well with most applications

Easy fabrication

Box Girders

Increased stability

Increased resistance

Used for longer bridges with curves

Cannot be used on bridges with curves (subject to torque)

Cannot span long distances

Box Girders

More difficult to fabricate than I-beams

More costly

• Rolled Steel Girder Bridge: made of I-beams that are rolled into that shape at a steel mill. These are useful for spans between 10 meters and 30 meters.

• Plate Girder Bridge: Made out of flat steel sections that are later welded or fabricated into an I-beam shape. Useful for spans between 10 meters to over 100 meters.

• Concrete Girder Bridge: Made of concrete girders in an I-beam shape.

• To the left: The top bridge is a plate girder bridge while the bottom is a concrete girder bridge.

• To the right: A box girder bridge.

• Opened August 3, 1969 to connect San Diego with the island of Coronado.

• 2.12 miles (11,179-feet) long

• Approximately 200 feet tall

• Traffic ascends at a 4.67 percent grade and curves 80 degrees.

• Supported by a box girder giving it support.

• Truss bridges are a type of beam bridge made up of many small beams attached together in triangular configuration to support a large amount of weight and span great distances.

• They function by compression and tension forces and not by bending forces.

• They are identified according to the way the chords are arranged.

A truss is an interconnected framework of beams designed to hold something up.

Trusses don’t bend, they get pulled apart and pushed together.

However once the weight is increased the bridge will stag in the middle. This is due to the individual beams expanding and contracting due to the weight.

Trusses must be stable, and not able freely in any direction in order for them to work.

The beams have to be placed carefully in the right angles and in equal distances away from each other so they can distribute the weight equally.

They are usually supported at the ends by abutments and sometimes in the middle by piers.

How Truss Bridges Work

Examples or Truss Bridges hold something up.

This is the Beaver Railroad Bridge located in Carroll County, Arkansas

This is the ASB Bridge located in Jackson County, Missouri

Arch bridge hold something up.

• Arch bridges are one of the oldest types of bridges and have great natural strength.

• Arch bridges consist of compression and tension.

Compression hold something up.

• Arch bridges are continuously under compression. The force of compression is pushed along the curve of the arch toward abutments.

The natural curve of the arch and its ability to dissipate the force outward greatly reduces the effects of tension on the underside of the arch.

The greater the degree of curvature (the larger the semicircle of the arch), the greater the effects of tension on the underside.

Tension

Suspension Bridges the force outward greatly reduces the effects of tension on the underside of the arch.

• Suspension bridges in their simplest form were originally made from rope and wood.

• Modern suspension bridges use a box section roadway supported by high tensile strength cables. The development of metals brought the use of linked iron bars and chains.

• Light, and strong, suspension bridges can span distances from 2,000 to 7,000 feet far longer than any other kind of bridge.

• They are ideal for covering busy waterways. the force outward greatly reduces the effects of tension on the underside of the arch.

• This type of bridge is the only practical type suitable for very long spans or when it would be hazardous to maritime traffic to add central supports.

• A typical suspension bridge is a continuous girder with one or more towers erected above piers in the middle of the span.

• The girder itself it usually a truss or box girder though in shorter spans, plate girders are not uncommon.

• At both ends of the bridge large anchors or counter weights are placed to hold the ends of the cables.

• The main cables are stretched from one anchor over the tops of the towers and attached to the opposite anchor.

• The cables pass over a special structure known as a saddle.

• The saddle allows the cables to slide as loads pull from one side or the other and to smoothly transfer the load from the cables to the tower.

World's Longest Suspension Bridge are placed to hold the ends of the cables.

• Akashi Kaikyo Bridge, Japan

• Total Length : 3,911m

• Center Span : 1,991m

Cantilever Bridges are placed to hold the ends of the cables.

• To solve the problem of increasing the span distance, other alternatives to beam and arch bridges included suspension and cantilever bridges.

• Cantilever bridges are a modified form of beam bridge, with the support being placed not at the end, but in the middle of the span.

• A cantilever is a structure or beam that is unsupported at one end but supported at the other, like diving boards.

• This configuration made longer spans possible and wider clearance beneath.

• A simple cantilever span is formed by two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the center.

• In a common variant, the suspended span, the cantilever arms do not meet in the center; instead, they support a central truss bridge which rests on the ends of the cantilever arms.

• The suspended span may be built off-site and lifted into place, or constructed in place using special traveling supports.

Forth Railway Bridge extending from opposite sides of the obstacle to be crossed, meeting at the center.

• The Forth Railway Bridge (or Firth of Forth Bridge) is made of a pair of cantilever arms that extend out from two main towers.

• The beams are supported by diagonal steel tubes projecting from the top and bottom of the towers.

• These spans support a central suspended span. Some structural members of the bridge are as large as 12 feet in diameter.

### Movable Bridges extending from opposite sides of the obstacle to be crossed, meeting at the center.

• They span waterways

• Closed bridge to carry traffic

• Open to allow marine traffic to travel under

• There are three main types:

• 1.Bascule

• 2.Vertical lift

• 3. Swing

Bascule Bridge or Drawbridge extending from opposite sides of the obstacle to be crossed, meeting at the center.

• Used for short distances

• Have two movable spans the rise upward, opening in the middle

• When open the weight is supported by the stationary section of the bridge

Vertical-lift Bridge extending from opposite sides of the obstacle to be crossed, meeting at the center.

• Used for longer distances

• Straight bridge, held between two towers

• Lifted by steel ropes, attached to counterweights

-as the counterweights go down the bridge goes up and vise-versa.

• Operate in an elevator like fashion

Swing Bridges extending from opposite sides of the obstacle to be crossed, meeting at the center.

• Mounted on a central pier

• The central pier allows the bridge to rotate to the side

• Uncommonly used because the central pier is located in the area where boats like to travel

Cable-Stayed Bridge extending from opposite sides of the obstacle to be crossed, meeting at the center.

• A cable stayed bridge is a bridge with one or more pillars. They are similar to suspension bridges but defer in the way the cables are connected to the towers. The two types of cable-stayed bridges are parallel attachment design and radial attachment design. In a parallel attachment design the cables are attached at different heights along the tower and are parallel to one another and in a radial attachment design the cables are attached at a single point at the top of the tower and on several places on the road.

Parallel Attachment Design

Examples extending from opposite sides of the obstacle to be crossed, meeting at the center.

Great Seto Bridge in Japan

Clark Bridge in IL

Works cited extending from opposite sides of the obstacle to be crossed, meeting at the center.

• “Bridges” E. R. Hardesty, et al., "Bridge", in [email protected], http://www.accessscience.com, DOI 10.1036/1097-8542.095500, last modified: April 5, 2001. http://www.accessscience.com/server-java/Arknoid/science/AS/Encyclopedia/0/09/Est_095500_frameset.html

Movable bridges

• “Bridge (structure)” http://encarta.msn.com/encyclopedia_761561057/Bridge_(structure).html

• “Lift Bridge” http://en.wikipedia.org/wiki/Lift_bridge

• “Bascule Bridge” http://en.wikipedia.org/wiki/Bascule_bridge

• “Swing Bridge” http://en.wikipedia.org/wiki/Swing_bridge

Truss bridges

• http://bridges.midwestplaces.com/mo/butler/black-river-rr/

• http://en.wikipedia.org/wiki/Truss_bridge

• http://www.ce.ufl.edu/activities/trusslab/trussndx.html

• http://bridges.midwestplaces.com/browse/type/truss/through/baltimore/

Cable-stayed bridges

• http://en.wikipedia.org/wiki/Cable-stayed_bridge

• http://www.matsuo-bridge.co.jp/english/bridges/basics/cablestay.shtm

• eng.midasuser.com/.../pro_app/pro_app_civil.asp

Works Cited (continued) extending from opposite sides of the obstacle to be crossed, meeting at the center.

“Box Girder Bridge.” 18 September 2006.