Structures

Bridges



The beam bridge...(also known as a girder 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 80 metres.
By increasing the height of the beam, the beam has more material to dissipate the tension. To create very tall beams, bridge designers add supporting latticework, or a truss, to the bridge's beam. This support truss adds rigidity to the existing beam, greatly increasing its ability to dissipate the compression and tension. Once the beam begins to compress, the force spreads through the truss.

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The truss bridge...(a beam bridge with braces)
consists of an assembly of triangles. Truss bridges are commonly made from a series of straight, steel bars. Rigid arms extend from both sides of two piers. Diagonal steel tubes, projecting from the top and bottom of each pier, hold the arms in place. The arms that project toward the middle are only supported on one side, like really strong diving boards. These "diving boards," called cantilever arms, support a third, central span.
A single beam spanning any distance undergoes compression and tension. The very top of the beam gets the most compression, and the very bottom of the beam experiences the most tension. The middle of the beam experiences very little compression or tension. This is why we have I-beams, which provide more material on the tops and bottoms of beams to better handle the forces of compression and tension.
bridge-thru.gif bridge-deck.gif
And there's another reason why a truss is more rigid than a single beam: A truss has the ability to dissipate a load through the truss work. The design of a truss, which is usually a variant of a triangle, creates both a very rigid structure and one that transfers the load from a single point to a considerably wider area.

The Firth of Forth Bridge in Scotland is a cantilever bridge, a complex version of the truss bridge

firthofforth2_bridge_1.jpg

The arch bridge...
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 250 metres.
Tensional force in arch bridges, on the other hand is virtually nil. 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.
But as with beams and trusses, even the mighty arch can't outrun physics forever. The greater the degree of curvature (the larger the semicircle of the arch), the greater the effects of tension on the underside of the bridge. Build a big enough arch, and tension will eventually overtake the support structure's natural strength.
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The suspension bridge...
can span 600m to over 2 km-- way farther than any other type of bridge! Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.
As the name implies, suspension bridges, like the Golden Gate Bridge or Brooklyn Bridge, suspend the roadway by cables, ropes or chains from two tall towers. These towers support the majority of the weight as compression pushes down on the suspension bridge's deck and then travels up the cables, ropes or chains to transfer compression to the towers. The towers then dissipate the compression directly into the earth.
The supporting cables, on the other hand, receive the bridge's tension forces. These cables run horizontally between the two far-flung anchorages. Bridge anchorages are essentially solid rock or massive concrete blocks in which the bridge is grounded. Tensional force passes to the anchorages and into the ground.
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In addition to the cables, almost all suspension bridges feature a supporting truss system beneath the bridge deck called a deck truss. This helps to stiffen the deck and reduce the tendency of the roadway to sway and ripple.

But not every suspension bridge is an engineering marvel of modern steel. In fact, the earliest ones were made of twisted grass. When Spanish conquistadors made their way into Peru in 1532, they discovered anIncan empire connected by hundreds of suspension bridges, achieving spans of more than 150 feet (46 meters) across deep mountain gorges.
goldengate_bridge_1.jpg



Cable-Stayed Bridge

The cable-stayed bridge, like the suspension bridge, supports the roadway with massive steel cables, but in a different way. The cables run directly from the roadway up to a tower, forming a unique "A" shape.

Cable-stayed bridges, like the Sunshine Skyway in Florida, require less cable and can be built much faster than suspension bridges. Cable-stayed bridges are becoming the most popular bridges for medium-length spans (between 150m and 1km).
he tower of a cable-stayed bridge is responsible for absorbing and dealing with compressional forces. The cables attach to the roadway in various ways. For example, in a radial pattern, cables extend from several points on the road to a single point at the tower, like numerous fishing lines attached to a single pole. In a parallel pattern, the cables attach to both the roadway and the tower at several separate points.
external image bridge-cable-stay.gif


sunshineskyway_bridge_1.jpg

Sunshine Skyway Bridge- Florida


Forces and Motion

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Tension and Compression: Two Forces Every Bridge Knows Well

What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to stretch over a distance seven times that of an arch bridge? The answer lies in how each bridge type deals with the important forces of compression and tension.

Tension: What happens to a rope during a game of tug-of-war? Correct, it undergoes tension from the two sweaty opposing teams pulling on it. This force also acts on bridge structures, resulting in tensional stress.

Compression: What happens when you push down on a spring and collapse it? That's right, you compress it, and by squishing it, you shorten its length. Compressional stress, therefore, is the opposite of tensional stress.
Compression and tension are present in all bridges, and as illustrated, they are both capable of damaging part of the bridge as varying load weights and other forces act on the structure. It's the job of the bridge design to handle these forces without buckling or snapping.

Buckling occurs when compression overcomes an object's ability to endure that force. Snapping is what happens when tension surpasses an object's ability to handle the lengthening force.

The best way to deal with these powerful forces is to either dissipate them or transfer them. With dissipation, the design allows the force to be spread out evenly over a greater area, so that no one spot bears the concentrated brunt of it. It's the difference in, say, eating one chocolate cupcake every day for a week and eating seven cupcakes in a single afternoon.
In transferring force, a design moves stress from an area of weakness to an area of strength.

Torsion, for instance, is a particular concern for engineers designing suspension bridges. It occurs when high wind causes the suspended roadway to rotate and twist like a rolling wave. As we'll explore on the next page, Washington's Tacoma Narrows Bridge sustained damage from torsion, which was, in turn, caused by another powerful physical force
The natural shape of arch bridges and the truss structure on beam bridges protects them from this force. Suspension bridge engineers, on the other hand, have turned to deck-stiffening trusses that, as in the case of beam bridges, effectively eliminate the effects of torsion.
In suspension bridges of extreme length, however, the deck truss alone isn't enough protection. Engineers conduct wind tunnel tests on models to determine the bridge's resistance to torsional movements. Armed with this data, they employ aerodynamic truss structures and diagonal suspender cables to mitigate the effects of torsion.

Shear: Shear stress occurs when two fastened structures (or two parts of a single structure) are forced in opposite directions. If left unchecked, the shear force can literally rip bridge materials in half. A simple example of shear force would be to drive a long stake halfway into the ground and then apply lateral force against the side of the upper portion of the stake. With sufficient pressure, you'd be able to snap the stake in half. This is shear force in action.


Resonance: is like the vibrational equivalence of a snowball rolling down a hill and becoming anavalanche. It begins as a relatively small, periodic stimulus of a mechanical system, such as wind buffeting a bridge. These vibrations, however, are more or less in harmony with the bridge's natural vibrations. If unchecked, the vibration can increase drastically, sending destructive, resonant vibrations traveling through a bridge in the form of torsional waves.

Check out these World Famous Bridges

Bridge of Sighs- Venice
The view from the Bridge of Sighs was the last view of Venice that convicts saw before their imprisonment. The bridge name, given by Lord Byron in the 19th century, comes from the suggestion that prisoners would sigh at their final view of beautiful Venice through the window before being taken down to their cells.
external image bridge-of-sighs-venice.jpg

Lovers bridge- Paris
Some years ago, a new fad started when love-struck sweethearts began locking padlocks onto the chain link fence of the Pont des Arts, which crosses from the left bank to the Louvre museum. The love padlocks, called cadenas d’amour, multiplied until there were thousands of love tokens on the bridge, each engraved with a message of love. After locking the love padlock onto the fence, lovers toss the keys into the Seine river – a sign of their eternal devotion.
external image lovers-bridge-pont-des-arts-paris-france.jpg


TASK:

Using a limited range of materials your job is to create a bridge that will span a 25cm gap and will hold weight without breaking.
You will need to:

  • Write a hypothesis on how your bridge will hold weight.
  • Design your bridge using what you have learned to create a bridge that you think will best hold weight.
  • Research your chosen bridge type and find existing examples of these types of bridges.
  • Photograph your build
  • Test your bridge. Did it hold up under pressure? Explain what worked well and what didn't and what you would do differently next time.
Present your research and information in a keynote and iMovie.






Roller Coasters



Roller Coaster History

Roller coasters have a long, fascinating history. The direct ancestors of roller coast­ers were monumental ice slides -- long, steep wooden­ slides covered in ice, some as high as 70 feet -- that were popular in Russia in the 16th and 17th centuries. Riders shot down the slope in sleds made out of wood or blocks of ice, crash-landing in a sand pile.

Roller Coaster Components

At first glance, a roller coaster is something like a passenger train. It consists of a series of connected cars that move on tracks. But unlike a passenger train, a roller coaster has no engine or power source of its own. For most of the ride, the train is moved by gravityand momentum. To build up this momentum, you need to get the train to the top of the first hill (the lift hill) or give it a powerful launch.
Chain Lift
The traditional lifting mechanism is a long length of chain (or chains) running up the hill under the track. The chain is fastened in a loop, which is wound around a ­gear at the top of the hill and another one at the bottom of the hill. The gear at the bottom of the hill is turned by a simplemotor.
This turns the chain loop so that it continually moves up the hill like a long conveyer belt. The coaster cars grip onto the chain with several chain dogs, sturdy hinged hooks. When the train rolls to the bottom of the hill, the dogs catches onto the chain links. Once the chain dog is hooked, the chain simply pulls the train to the top of the hill. At the summit, the chain dog is released and the train starts its descent down the hill.

Catapult-launch Lift

In some newer coaster designs, a catapult launch sets the train in motion. There are several sorts of catapult launches, but they all basically do the same thing. Instead of dragging the train up a hill to build up potential energy, these systems start the train off by building up a good amount of kinetic energy in a short amount of time.
One popular catapult system is the linear-induction motor. A linear-induction motor uses electromagnets to build two magnetic fields -- one on the track and one on the bottom of the train -- that are attracted to each other. The motor moves the magnetic field on the track, pulling the train along behind it at a high rate of speed. The main advantages of this system are its speed, efficiency, durability, precision and controllability.
Another popular system uses dozens of rotating wheels to launch the train up the lift hill. The wheels are arranged in two adjacent rows along the track. The wheels grip the bottom (or top) of the train between them, pushing the train forward.

The Brakes

Like any train, a roller coaster needs a brake system so it can stop precisely at the end of the ride or in an emergency. In roller coasters, the brakes aren't built into the train itself; they're built into the track.
This system is very simple. A series of clamps is positioned at the end of the track and at a few other braking points. A central computer operates a hydraulic system that closes these clamps when the train needs to stop. The clamps close in on vertical metal fins running under the train, and this friction gradually slows the train down.

Roller Coaster Physics

The purpose of the coaster's initial ascent is to build up a sort of reservoir of potential energy. The concept of potential energy, often referred to as energy of position, is very simple: As the coaster gets higher in the air, gravity can pull it down a greater distance. You experience this phenomenon all the time -- think about driving your car, riding your bike or pulling your sled to the top of a big hill. The potential energy you build going up the hill can be released as kinetic energy -- the energy of motion that takes you down the hill.
Once you start cruising down that first hill, gravity takes over and all the built-up potential e­nergy changes to kinetic energy. Gravity applies a constant downward force on the cars.
Capture.PNG

Click play to start the animation, which demonstrates how a roller coaster's energy is constantly changing between potential and kinetic energy. At the top of the first lift hill (a), there is maximum potential energy because the train is as high as it gets. As the train starts down the hill, this potential energy is converted into kinetic energy -- the train speeds up. At the bottom of the hill (b), there is maximum kinetic energy and little potential energy. The kinetic energy propels the train up the second hill (c), building up the potential-energy level. As the train enters the loop-the-loop (d), it has a lot of kinetic energy and not much potential energy. The potential-energy level builds as the train speeds to the top of the loop (e), but it is soon converted back to kinetic energy as the train leaves the loop.
The coaster tracks serve to channel this force -- they control the way the coaster cars fall. If the tracks slope down, gravity pulls the front of the car toward the ground, so it accelerates. If the tracks tilt up, gravity applies a downward force on the back of the coaster, so it decelerates.
Since an object in motion tends to stay in motion (Newton's first law of motion), the coaster car will maintain a forward velocity even when it is moving up the track, opposite the force of gravity. When the coaster ascends one of the smaller hills that follows the initial lift hill, its kinetic energy changes back to potential energy. In this way, the course of the track is constantly converting energy from kinetic to potential and back again.
This fluctuation in acceleration is what makes roller coasters so much fun. In most roller coasters, the hills decrease in height as you move along the track. This is necessary because the total energy reservoir built up in the lift hill is gradually lost to friction between the train and the track, as well as between the train and the air. When the train coasts to the end of the track, the energy reservoir is almost completely empty. At this point, the train either comes to a stop or is sent up the lift hill for another ride.
At its most basic level, this is all a roller coaster is -- a machine that uses gravity and inertia to send a train along a winding track.


Newton's first law of motion states that an object in motion tends to stay in motion. That is, your body will keep going at the same speed in the same direction unless some other force acts on you to change that speed or direction. When the coaster speeds up, the seat in the cart pushes you forward, accelerating your motion. When the cart slows down, your body naturally wants to keep going at its original speed. The harness in front of you accelerates your body backward, slowing you down.


Loop-the-Loops


As you go around a loop-the-loop, your inertia not only produces an exciting acce­leration force, but it also keeps you in the seat when you're upside down.

A roller coaster loop-the-loop is a sort ofcentrifuge, just like a merry-go-round. In a merry-go-round, the spinning platform pushes you out in a straight line away from the platform. The constraining bar at the edge of the merry-go-round stops you from following this path -- it is constantly accelerating you toward the center of the platform.

The loop-the-loop in a roller coaster acts exactly the same way as a merry-go-round. As you approach the loop, your inertial velocity is straight ahead of you. But the track keeps the coaster car, and therefore your body, from traveling along this straight path. The force of your acceleration pushes you from the coaster-car floor, and your inertia pushes you into the car floor. Your own outward inertia creates a sort of false gravity that stays fixed at the bottom of the car even when you're upside down. You need a safety harness for security, but in most loop-the-loops, you would stay in the car whether you had a harness or not.


external image roller-coaster-force.gif

Originally, roller-coaster designers made circle-shaped loops. In this design, the angle of the turn is constant all the way around. In order to build an acceleration force strong enough to push the train into the track at the top of the loop, they had to send the train into the loop at a fairly high rate of speed (so it would still be going pretty fast at the top of the loop). Greater speed meant a much greater force on the rider as he entered the loop, which could be fairly uncomfortable.
The teardrop design makes it much easier to balance these forces. The turn is much sharper at the very top of the loop than it is along the sides. This way, you can send the train through the loop fast enough that it has an adequate acceleration force at the top of the loop, while the teardrop shape creates a reduced acceleration force along the sides. This gives you the force you need to keep everything running, without applying too much force where it might be dangerous.

Types of Roller Coasters

There are two major types of roller coasters, distinguished mainly by their track structure.
The tracks of wooden roller coasters are something like traditional railroad tracks. In most coasters, the car wheels have the same flanged design as the wheels of a train -- the inner part of the wheel has a wide lip that keeps the car from rolling off the side of the track. The car also has another set of wheels (or sometimes just a safety bar) that runs underneath the track. This keeps the cars from flying up into the air.
Wooden coaster tracks are braced by wooden cross ties and diagonal support beams. The entire track structure rests on an intricate lattice of wooden or steel beams, just like the beam framework that supports a house or skyscraper.

he range of motion is greatly expanded in steel roller coasters. The world of roller coasters changed radically with the introduction of tubular steel tracks in the 1950s. As the name suggests, these tracks consist of a pair of long steel tubes. These tubes are supported by a sturdy, lightweight superstructure made out of slightly larger steel tubes or beams.
Tubular steel coaster wheels are typically made from polyurethane or nylon. In addition to the traditional wheels that sit right on top of the steel track, the cars have wheels that run along the bottom of the tube and wheels that run along the sides. This design keeps the car securely anchored to the track, which is absolutely essential when the train runs through the coaster's twists and turns.
The train cars in tubular steel coasters may rest on top of the track, like the wheels in a traditional wooden coaster, or they may attach to the track at the top of the car, like in a ski lift. In suspended coasters, the hanging trains swing from a pivoted joint, adding an additional side-to-side motion. In an inverted coaster, the hanging train is rigidly attached to the track, which gives the designer more precise control of how the cars move.

According to the Roller Coaster DataBase, there were 2,088 coasters in operation around the world in 2007 -- 1,921 of them steel, 167 wooden. The RCDB identifies eight main coaster types:
  • Sit-down
  • Stand-up
  • Inverted
  • Suspended
  • Pipeline: The track is attached to the middle of the train, instead of above or below it.
  • Bobsled: Wheeled trains slide down a U-shaped tube instead of being fixed to a track.
  • Flying: Riders start out in a seated position but are rotated to face the ground as the ride starts, giving the feeling of flying.
  • Fourth Dimension: Two seats from each car are positioned on either side of the track. The seats spin or rotate on their own axis - either freely or in a controlled motion. In 2007, there were only four Fourth Dimension coasters in operation.





TASK:

Using a limited range of materials your job is to create a roller coaster that will allow a marble to travel along it without falling off.
You will need to:

  • Write a hypothesis on how your roller coaster will make use of different forces to move your marble along its track.
  • Design your roller coaster using what you have learned to create a coaster that you think will make the best use of forces to create a fun ride.
  • Research roller coasters and find existing examples.
  • Photograph your build.
  • Test your roller coaster. Did it hold up under pressure? Did your loops and twists keep your marble moving? Explain what worked well and what didn't and what you would do differently next time.
Present your research and information in a keynote and iMovie.

Materials:
cardboard
paper plates
tiolet rolls
sellotape