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Bridge Problems
Many problems must be looked at before a good bridge can be designed.

Bridges are built to last, even more than 100 years at a time. Bridge designs, like that of the designs of buildings, dams, and tunnels have to, not only consider the impact of their own weight, temperature change and so on, but catastrophes like hurricanes, floods earthquakes, industrial accidents, or terrorist attacks as well.
Designers must look at all the known and proven data that will insure a safe and long lasting structure, and even must try to consider the unknown at times. History helps us learn from our mistakes, whether it the mistakes were from miss calculations, arrogance, ignorance or simply a freak of nature.

Even professional bridge builders make mistakes. In 1940, a brand new suspension bridge that had been built across the Tacoma Narrows in Washington State, collapsed because the engineers did not account for the constant 40 mile per hour wind that would work on the bridge. A resonance, a natural vibrating frequency, developed causing the bridge to to twist and sway so violently that it eventually collapsed. The bridge had been called “Galloping Gertie” because it moved and twisted so much in the wind.
Click here to see it collapse: http://members.tripod.com and here for the play-by-play:

http://www.howstuffworks.comits/Tacoma_Narrows/DSmith/photos.html

3d animated yellow on red rolling zooming rectangular click here sign Infamous Bridge Disasters

Those mistakes can cost not just millions of dollars but human lives as well. On the 29th of August 1907,"the Quebec Bridge", soon to become the largest cantilever bridge in the world, while under construction, collapsed because the the chief engineer under estimated the weight of the bridge by over 8 million pounds. The bridge structure plunged over 150 feet taking with it the lives of 75 workers. Reconstruction completion was in August 1917, but not before a second disaster. In 1916,one of the four rocker arms failed while the bridge's 5000 pound center was being lifted into place. This caused the span fall into the river below, killing 11 more workmen. http://www.civeng.carleton.ca/Exhibits/Quebec_Bridge/intro.html

In December, 2002, we had our own little bridge construction disaster next door to our school. A crane working on the new bridge construction, fell into the Margaree River just before the bus traffic to school was about to start crossing the old bridge. The supports under the crane collapsed when a girder support failed. The crane flipped over board, causing it's boom to crash across the old bridge adjacent to the new one under construction. (Shown below) Fortunately there were no serious injuries, nor any casualties. The only casualty was the yellow crane seen in the first thumbnail.

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The bridge below are of the iron truss bridge in West Mabou. On the evening of February 20th, 2004, a snow plow struck the left corner of the bridge circled in red in the first picture. The corner strut had a slight wrinkle which quickly caused the deck to drop slightly on that end. The stress was no longer properly dispersed through out the bridge. The deck gradually began to sag near that end of the bridge as the structure twisted and slowly crumbled. By late morning, with the wrinkles very distinct, and brackets beginning to let go; the sag was 3 to 4 feet. By late evening of the 21st, A structure that, the day before could carry loaded pulp trucks, collapsed under the stress of it's own weight.


The Tay Bridge collapse in 1879 resulted as a result of poor bridge design. However, many future bridges have benefited from this disaster because of what was learned from this tough lesson. http://www.tts1.demon.co.uk/tay.html

Many lives were lost in Kansas City when an indoor walkway collapsed and fell on people below.

All of these disasters happened because of human error and or neglect; designers either used the wrong materials and/or poor architectural design decisions to deal with all the stresses working on the bridge. Terrorism, especially since "September 11th, has put even more emphasis on the importance of design to with stand a variety of forces. In order to maximize our safety, engineers not only have to design structures to handle forces as a result of the conditions surrounding the project requirements (the environmental, structural and esthetic concerns), what mother nature has to through ( temperature change, earth quakes and so-on), but also destructive human intervention (accidental or otherwise). It is interesting to note that the "Twin Towers" were designed to handle airplane collision, perhaps not of that magnitude.

Designing to prepare for the unknown is the task of an engineer as well. The engineer will try to use the tell tails of the past to best address the potential of the future. Take earthquakes for example:

http://iti.acns.nwu.edu/links/bridges/disasters.html

http://filebox.vt.edu/users/aschaeff/titlepage.html

http://bridges.lib.lehigh.edu/cgi-bin/library?c=bridges&a=d&cl=CL1.26

Bridge Design
most-beautiful-bridges-in-the-world/

/top-ten-longest-bridge-world.htm

 


Shape

Torsion, resonance and aerodynamics (after several spectacular collapses) have been incorporated in better design shapes in today's structures. If you looked closely at the bridges, towers and buildings in our area, you would see that the components of these typical structures are indeed rigid structures. In the frames (skeletons) of most of these structures you will see triangular shapes. This because the triangular frame maintains it’s shape even when it is pushed or pulled. A rectangular shape is not as rigid as a triangle. This is because it changes shape when it is pushed or pulled. It may lean to one side or crush due to it’s inability to dissipate the forces applied to it. As a result, rectangular frames are often re-enforced or braced with diagonal cables or rods, forming triangles. Arches are reinforced with external supports called buttresses.
Activity:Use the following link to see how shape makes a difference, as well as how to maximize it's strength by first clicking on a shape, then on "push it", and finally on the "strengthen it" button at the bottom: http://www.pbs.org/wgbh/buildingbig/lab/shapes.html

As we said earlier; bridges range in length from a few feet or meters to several miles or kilometers. The type of bridge used depends on various features of the obstacle. The main feature that controls the bridge type is the size of the obstacle. How far is it from one side to the other? This is a major factor in determining what type of bridge to use. A beam bridge can span up to 200 feet. What allows an arch bridge to span greater distances than a beam bridge up to 1000 feet), or a suspension bridge to span a distance nearly 7 times that of a an arch bridge? The answer lies in how each bridge type deals with important forces called compression and tension. Torsion and shear are two other forces they have to contend with, as well. As you examine the material that follows you will see what causes these forces, as well as how bridges handle the forces acting on them.

All bridges must deal with two types of loads; static loads (loads which are unchanging or changing slowly) and dynamic loads (loads which are always changing). It is the loads that apply the forces. A bridge must be strong enough, first of all, to support its own weight as well as the weight of the people and vehicles that use it. These are some what obvious constant loads that the bridge must with stand. The structure also must resist the various dynamic changing forces that mother nature may through at it, including earthquakes, strong winds, and changes in temperature. Nature, no doubt, posses the strongest challenge for bridge designers to over come.

Activity: Click on the following to see how some of these loads that act on bridges, the problems they cause, and don't miss clicking on the ("Choose one") menu to the left , and click on each "try it" to get a good visuals. After you try each one click on "strengthen it" to see how to compensate for these loads: http://www.pbs.org/wgbh/buildingbig/lab/loads.html

Four common forces acting on bridges:

Compression
From the above you have probably already understand that a force is simply a push or pull. A pushing force is called Compression. Piers are always in compression as they deal with the weight of the bridge and the traffic. Compression is a force which acts to compress or shorten the object it is acting on. These objects tend to bulge due to the pressure.

Tension
A pulling force is called Tension, which acts to expand or lengthen the object it is acting on, such as in the case of the cables on a suspension bridge. These members may become thinner at the weak spot before they give way.

A simple, everyday example of compression and tension is a cylindrical spring. When we press down, or push the two ends of the spring together, we compress it. The force of compression shortens the spring. When we pull up, or pull apart the two ends, we create tension in the spring. The force of tension lengthens the spring.
if we looked at a leaf spring for a more accurate example, we would see that both forces at on the spring. As you apply a load to cause the spring to bend, the top of the spring is in compression because the molecules are being crunched together , while the underside is in tension as the molecules are being stretched apart.

Compression and tension are present in all bridges, and it's the job of the bridge engineer to design a bridge to handle these forces without buckling or snapping. Buckling is what happens when the force of compression overcomes an object's ability to handle compression. You may have witnessed this happening if ever you have stood on a long board supported on either end. The weight of the board alone can cause it sag in the middle, you standing on it may cause it to buckle in two. Snapping is what happens when tension overcomes an object's ability to handle tension. Snapping occurs when you break a piece of thread or string apart by pulling on the two ends.
The best way to deal with these forces is to either dissipate, (spread them out over a greater area, so that no one spot has to bear the brunt of the concentrated force), or transfer them, (move the forces from an area of weakness to an area of strength, and eventually to the abutments and/or piers). An arch bridge is a good example of dissipation, while a suspension bridge is a good example of transference.
For more clarification: http://www.howstuffworks.com/bridge2.htm

Shear
The weight and load of a bridge can cause materials to slide by each other, or to shear off. Shear forces that cause materials to tear and slide by each other have been proven to be the root cause of many bridge failures. Bolts and rivets that sheared off due to sudden over load or deterioration has claimed many lives.

Torsion
Forces that occur at a point on a structure caused by a twist in that object or member is called torsion. Unbalanced loads such as traffic on one side and not the other may cause a bridge to twist. Wind may also tend to twist a bridge.

Activity: Check out these labs regarding forces. Again start by clicking on the ("Choose one") menu to the left , and click on each sample. Slide the bar at the top for the visual effect, don't forget to click the "the see it in real life" button at the bottom: http://www.pbs.org/wgbh/buildingbig/lab/forces.html

Applied Forces: http://www.rpi.edu/~brawi/frame_ft_jbt/bforces7.htm

When a structure is stationary, the forces acting on it are balanced. Every force applied to it is resisted by opposing force of the same size. For example, in a tug of war the rope does not move if the pulling forces are balanced. Unbalanced forces on a structure cause it to move or to deform. This is because one or more of the applied forces are greater than opposing forces. In a rigid structure, the forces are balanced. The structure resists all applied forces. Unbalanced forces cause a non-rigid structure to move or collapse.

The loads on bridges, or the forces that are incurred as a result of these loads, must be absorbed and redirected within the bridge design. Ever since humans began building their own shelters, the concept of rigidity has been important. A ridged structure of any kind, does not collapse under it's own weight or forces (push or pulls) are applied to it. For example, rigid building supports not only it's own weight, but the weight of objects placed on the floors. In addition, the building must be able to resist the force of the wind on it's sides and roof. The same can be said for bridges. As a result the shape of the design becomes an intricate part of insuring the safety and longevity of the bridge. The solid or ridge members that make up the bridge which are are in compression are called struts. Those in tension are called ties.

A very important point that was emphasized in the bridge video we watched is how bridges (as well as other structures) are designed to be flexible to some degree. Towers and skyscrapers, such as the CN Tower, for example is designed to sway as much as 3ft 4 inches at the Sky deck in order to absorb winds up to 190 km/h and gusts up to 320km/h. "The hart beat or pulse of the bridge"( movement you can actually feel) tells you that the bridge is "alive and working well". A large bridge with no pulse is probably dead or dieing and it's time to bail. Their individual components or even an arrangement of components can be very rigid , but in many cases their shape can be designed to change deliberately as you saw when rollers were added to one end of a bridge in order for the bridge not to break from being too rigid when trying to expand as a result of the heat it had to absorb.

http://www.iaw.com/~falls/bridges.html

http://www.icomos.org/studies/bridges.htm#1

Questions

1.) This is a link to a simple word quiz on some of the terms you need to know. Simply slide each term on the left to the appropriate definitions, click "check" for your score: http://eduspace.free.fr/bridging_europe/wordquiz.htm

2.) Review quiz:
Click on the following link to do the mini quiz;
http://eduspace.free.fr/bridging_europe/lesson1quiz.htm

3.) Bridge Problem Puzzle

Unscramble each of the clue words. Copy the letters in the numbered cells to other cells with the same number. For tips to unscramble the clue words check below the scrambled message.

Visit Puzzlemaker at DiscoverySchool.com

 

Discovery Channel School


TASTIC - Loads that are unchanging or changing slowly. 

CEPRISNOSMO- A pushing force.

REAHS - A tearing and sliding force.

DINCYMA- Always changing loads.

NEITONS - A pulling force.

SORTONI - A twisting force.

DIIRG - The structure resists all applied forces.

TSTUR - Bridge members in compression.

TENGIARL - Most rigid shape.

NALDAEBC - Stationary Structure.

LIBNGCKU - Overcome by compression.

NPPISGNA - Separating due to tension.

DIPTIESAS - Spread over greater area.

MIAMEXZI - To get the most of.

NARFTRES - To move .