Difference between revisions of "AY Honors/Bridges/Answer Key"
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==7. Build a bridge using materials such as craft sticks, toothpicks, yarn, thread, and glue.== | ==7. Build a bridge using materials such as craft sticks, toothpicks, yarn, thread, and glue.== | ||
| − | Directions can be found in such magazines as Popular Mechanic and craft manuals specific to “model wooden | + | Directions can be found in such magazines as Popular Mechanic and craft manuals specific to “model wooden bridges.” There are extensive competitions that use wood craft sticks. Some bridges hold well in excess of 120 pounds! |
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==8. Recite John 3:16, then tell how this verse describes Jesus’ role as a bridge between heaven and earth.== | ==8. Recite John 3:16, then tell how this verse describes Jesus’ role as a bridge between heaven and earth.== | ||
Revision as of 23:05, 12 April 2013
1. Define the following terms:
- a. Bridge
- A structure built to span a valley, road, body of water, or other physical obstacle, for the purpose of providing passage over the obstacle.
- b. Aqueduct
- A bridge that carries water.
- c. Falsework
- The temporary support scaffolding used during the construction of some types of bridges.
- d. Compression force
- The downward pressure (usually vertical) that is applied to the supporting columns of a bridge design by the weight of the bridge materials as well as the traffic load that is on the bridge.
- e. Tension force
- The tightening or pulling together force (usually horizontal or at an angle other than vertical) that helps to stabilize a bridge design, and transfers compression force back to the supports of a bridge design.
- f. Bascule bridge
- A movable bridge with a counterweight that continually balances the span throughout the entire upward swing in providing clearance for boat traffic. They are the most common type of movable bridge in existence because they open quickly and require relatively little energy to operate.
- g. Covered bridge
- A structure with enclosed sides and a roof that often has only a single lane, and typically is made of wood. Since wooden bridges tended to deteriorate rapidly from exposure to the elements, they only had a useful lifespan of nine years. Covering them protected their structural members, thus extending their life to 80 years or more. Covering them also eliminated the need to plow them during or after snowstorms.
2. Describe these six main types of bridge design and give two examples of each:
a. Arch bridge
Arch-shaped bridges that have abutments at each end. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side. A viaduct (a long bridge) may be made from a series of arches, although other more economical structures are typically used to span a long distance today. The greatest bridge builders of antiquity were the ancient Romans. The Romans built arch bridges and aqueducts that could stand in conditions that would damage or destroy earlier designs. An example of one that still stands today is the Alcantara Bridge in Spain.
The earliest known arch bridges were built by the Greeks and include the Arkadiko Bridge.
Dubai in the United Arab Emirates is currently building the Sheikh Rashid bin Saeed Crossing which is scheduled for completion in 2012. When completed, it will be the largest arch bridge in the world.
b. Beam bridge
The beam bridge is the simplest kind of bridge today. The earliest beam bridges were simple logs that sat across streams and similar simple structures. In modern times, beam bridges are more commonly made from shallow steel “I” beams, box girders, reinforced concrete, or post-tensioned concrete.
Like most bridges that are characterized by how they are supported, beam bridges consist of one horizontal beam with 2 supports, usually on either end. Weight on top of the beam pushes straight down on the piers at either end of the bridge.
Examples include most highway overpasses, and any single log bridge.
c. Cable-stayed bridge
A bridge that consists of one or more columns (normally referred to as towers or pylons), with cables supporting the bridge deck. Like suspension bridges, cable-stayed bridges are held up by cables. However, in a cable-stayed bridge, less cable is required and the towers holding the cables are proportionately shorter. The first known cable-stayed bridge was designed in 1784 by C. T. Loescher.
The longest cable-stayed bridge is the Sutong Bridge over the Yangtze River in China.
There are two major classes of cable-stayed bridges: In a harp design, the cables are made nearly parallel by attaching cables to various points on the tower(s) so that the height of attachment of each cable on the tower is similar to the distance from the tower along the roadway to its lower attachment. In a fan design, the cables all connect to or pass over the top of the tower(s).
Compared to other bridge types, the cable-stayed is optimal for spans longer than typically seen in cantilever bridges and shorter than those typically requiring a suspension bridge. This is the range in which cantilever spans would rapidly grow heavier if they were lengthened, and in which suspension cabling does not get more economical were the span to be shortened.
d. Cantilever bridge
A bridge built using cantilevers, structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers my be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from pre-stressed concrete. The steel truss cantilever bridge was a major breakthrough when first put into practice, as it can span distances of over 1,500 feet, and can be more easily constructed at difficult crossings by virtue of using little of no falsework.
The most famous early cantilever bridge is the Forth Rail Bridge. This bridge held the record for longest span in the world for 17 years.
e. Suspension bridge
A type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders. While modern bridges of this type date from the early 19th century, earlier bridges without vertical suspenders (simple suspension bridges) date from the 7th century in Central America.
This type of bridge has cables suspended between towers, plus vertical suspender cables that carry the weight of the deck below, upon which traffic crosses. This arrangement allows the deck to be level or to arc upward for additional clearance. Like other suspension bridge types, this type often is constructed without falsework.
The suspension cables must be anchored at each end of the bridge, since any load applied to the bridge is transformed into a tension in these main cables. The main cables continue beyond the pillars to deck-level supports, and further continue to connections with anchors in the ground. The roadway is supported by vertical suspender cables or rods, called hangers. In some circumstances the towers may sit on a bluff or canyon edge where the road may proceed directly to the main span, otherwise the bridge will usually have two smaller spans, running between either pair of pillars and the highway, which may be supported by suspender cables or may use a truss bridge to make this connection. In the latter case there will be very little arc in the out-board main cables.
The first permanent wire cable suspension bridge was Guillaume Henri Dufour’s Saint Antoine Bridge in Geneva in 1823, with two 40 meter spans. The first with cables assembled in mid-air in the modern method was Joseph Chaley’s Grand Point Suspendu in Fribourg, in 1834. The Otto Beit Bridge was the first modern suspension bridge outside the United States with parallel wire cables. Currently the longest suspended-deck suspension bridge in the world is the Akashi-Kaikyo Bridge in Japan, with a main span of 1991 meters.
f. Truss bridge
A bridge composed of connected elements (typically straight) which may be stressed from tension, compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges. The basic types of truss bridges have simple designs which could be easily analyzed by 19th and early 20th century engineers. A truss bridge is economical to construct owing to its efficient use of materials.
The ability to distribute the forces of compression, tension, shear and bending in various ways has led to a large variety of truss bridges. Modern materials such as pre-stressed concrete and fabrication methods, such as automated welding, and the changing price of steel relative to that of labor have significantly influenced the design of modern bridges.
Because wood was so abundant in the United States, early truss bridges would typically use carefully fitted timbers for members taking compression and iron rods for tension members, usually constructed as a covered bridge to protect the structure. Truss bridges became a common type of bridge built from the 1870’s through the 1930’s. As metal slowly started to replace timber, wrought iron bridges in the U.S. started being built on a large scale in the 1870’s. Bowstring truss bridges were a common truss design seen during this time, with their arched top chords. As the 1880’s and 1890’s progressed, steel began to replace wrought iron as the preferred material.
3. Getting requirements here
Hmmm... need to talk to Mark.
4. Name the bridge design that can span the longest distance.
Suspension bridge
5. Do the following activities:
a. Watch a video about bridges.
b. Make a list of at least eight notable bridges OR name two bridges that you have crossed and tell what design they are.
6. Make a video/multimedia presentation or scrapbook presentation about bridge(s) you have seen.
7. Build a bridge using materials such as craft sticks, toothpicks, yarn, thread, and glue.
Directions can be found in such magazines as Popular Mechanic and craft manuals specific to “model wooden bridges.” There are extensive competitions that use wood craft sticks. Some bridges hold well in excess of 120 pounds!
8. Recite John 3:16, then tell how this verse describes Jesus’ role as a bridge between heaven and earth.
“For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.”
Sin had placed an un-crossable gap between God and man. When God sent His Son to give His life for sinful man, a bridge was created by Jesus whereby man could once again be restored to his connection with God and heaven. By believing in God’s Son, each person has the opportunity to be re-connected to the sin-free life that God intended for humanity.
