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Transcript of Bridges
Techniques and tools are selected to appropriately
manipulate characteristics of resources to meet
Beam & Truss Bridges
Post and Beam Bridges: Beam bridges are the most common types of bridges. To understand how a beam works, imagine a board placed over two supports, creating a bridge. If you stand on the middle of the board, it will bend. The top surface of the board grows slightly shorter because it is being compressed; the bottom surface is being stretched because it is in tension. If the board cannot withstand one or both of these forces, it will break. In addition to bending, beams must also be able to handle twisting.
A cable-stayed bridge has one or more towers from which cables support the bridge deck.
There are two major types of cable-stayed bridges: harp and fan.
In the harp design, the cables are nearly parallel so that the height of their attachment to the tower is similar to the distance from the tower to their mounting on the deck.
In the fan design, the cables all connect to or pass over the top of the towers. The fan design is structurally superior
The cable-stayed bridge is best for spans longer than cantilever bridges, and shorter than suspension bridges. This is the range where cantilever bridges would rapidly grow heavier if the span was lengthened, and suspension bridge cabling would not be more economical if the span was shortened.
Information, materials and systems (resources)
An arch bridge is a bridge with buttress at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the buttresses at either side. A viaduct (a long bridge) may be made from a series of arches, although other more economical structures are typically used today. wiki
Strengths are they are extremely strong And sturdy
Weaknesses are they are expensive and use lots if materials even for short distances
Strength & Weaknesses
Short videos on bridge building.
Construct Strengths & Weaknesses
A nice video showing how the Arch is constructed.
Construct Strengths & Weaknesses
Advantages: stable, cheap, and
Disadvantages: Short, not good
in earthquakes, and they
need multiple supports.
Bridge Fail - The Tacoma Narrows.
“Bridges” E. R. Hardesty, et al., "Bridge", in AccessScience@McGraw-Hill, 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
“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
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
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.
Forth Railway Bridge
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.
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.
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.
They are ideal for covering busy waterways.
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.
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.
The greater the degree of curvature (the larger the semicircle of the arch), the greater the effects of tension on the underside.
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.
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.
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.
How Truss Bridges Work
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.
Other types of Girder Bridges
More difficult to fabricate than I-beams
Cannot be used on bridges with curves (subject to torque)
Cannot span long distances
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.
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
What is a Girder Bridge?
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
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
Bascule Bridge or Drawbridge
They span waterways
Closed bridge to carry traffic
Open to allow marine traffic to travel under
Usually powered by electric motors
In the past they were powered by steam engines
There are three main types:
Akashi Kaikyo Bridge, Japan
Total Length : 3,911m
Center Span : 1,991m
World's Longest Suspension Bridge
Arch bridges are continuously under compression. The force of compression is pushed along the curve of the arch toward abutments.
Arch bridges are one of the oldest types of bridges and have great natural strength.
Arch bridges consist of compression and tension.
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.
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.
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.
This is the ASB Bridge located in Jackson County, Missouri
This is the Beaver Railroad Bridge located in Carroll County, Arkansas
Examples or Truss Bridges
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.
Examples of Girder Bridges
Used for longer bridges with curves
Works well with most applications
Great Seto Bridge in Japan
Clark Bridge in IL
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
Radial Attachment Design
Parallel Attachment Design
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.
Ideas for lesson.
1. Show a few pictures of Stone arch bridges and modern arch bridges for a small discussion on why this design was used and why might it still be used.
2. Provide students (working in pairs) with a strip of card around 15cm X 20cm - (to be used as an arch), a ruler (to be used as the span) and either MAB 1000's blocks or dictionaries (to be used as buttresses). Then ask students to design a bridge using the materials provided which have to support a small weight.
3.This first part of the activity is to allow students to explore the materials to see if they can successfully manipulate and use the materials to build a bridge.
4. Watch Arch video and see if students can go back to their bridge and build an arch bridge.
5. Construct a graphic organiser with the strengths weaknesses of an arch bridge - need to be guided.
Get students to use their rulers and some books to construct a bridge. Ask students to make a span of around 1 m long. Place some weight on it but not too much. Once they have created their bridge. View the video's of Beam Bridges and others and see if they can see the differences between them.
Beam & Truss Bridges.
Suspension bridges are suspended from cables. The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo. In modern bridges, the cables hang from towers that are attached to caissons or cofferdams. The caissons or cofferdams are implanted deep into the floor of a lake or river.
Using the information about Cable-Stayed bridges you can model a note taking lesson selecting the key points. From this get students to do a summary of the text in 25 words or less.
Try the activity described in the middle of the text.
Reading Comprehension & Activity