An investigation in bridges
Suspension bridges are huge bridges that cross large bodies of water. They can safely cross between 2000-7000 feet. This is much farther than any other bridge design today. Suspension bridges consist of two large towers with rope or cable connecting them in a u shape from the top of the two towers. These towers support a majority of the weight while compression pushes down on the deck which then travels up the cables to the two towers. The towers then dissipate it in to the earth. The tension goes to the supporting cables. These cables run horizontal from side to side all the way across the bridge, and connect to the anchorages on either side. The tensional forces pass from the anchorages and then into the ground. For extra support, most suspension bridges also feature a deck truss under the bridge for extra supports. This helps to make sure the bridge is more sturdy and doesn't sway. Suspension bridges are also the most costly because they are very hard to build and the materials are hard to acquire. They are made of steel to ensure that they are as sturdy as possible. However in 1532 when they were first discovered by Spanish conquistadores, they were made of twisted grass. The Incan empire was the first group of people to create them. They had hundreds of bridges that connected their empire in the mountains. However only one stands in the Andes Mountains.
Another type of bridge is the cable stayed bridge. Although it looks a lot like the suspension bridge it is different. One big difference is that they don't require anchorages or two towers. Instead they use one tower that holds all the weight. It alone is also responsible for absorbing and dealing with the compression forces. The cable attach to the bridge in a diagonal pattern in many different spots on the bridge. This bridge was invented by the Europeans shortly after the Second World War, but the basic designs date back to the 16th century by a Croatian inventor. Cable-stayed bridges are popular these days because they are much less costly, they require less steel cable, they are faster to build, and incorporate more precast concrete sections. Although they can only stretch up to 2800 feet compared to a suspension bridge which stretches 7000 feet, they are still a very popular choice today.
The basics of bridge designs/ components of a bridge is BATS. Or beams, arches, trusses, and suspension. These are the 4 basic types and components of bridges and various combinations can be used to create different bridge designs. The four bridges are classified by how far they safely stretch. The supports of the bridges can take the designs of columns, towers, and even canyon walls. Bridges can go many different lengths from beam bridges which go about 800-1000 feet all the way to 7000 feet long suspension bridges.
Each bridge stretches longer than the other due to two forces called tension and compression. Tension is like in tug of war. When the two opposing teams are both pulling this creates tension. The same force is applied to a bridge. Compression is the opposite of tension which means it is the force of pushing something together rather than stretching it. Both of these forces are present in every bridge and are capable of badly damaging it. The bridge design is what is supposed to deal with these forces because they can cause buckling and snapping. Buckling is when the compression overcomes an objects ability to endure the force. While snapping is when the tension is too much for the object to handle causing it to snap. Distributing these forces is the best was to deal with them. Transferring them throughout the design evenly will handle the buckling and snapping. It transfers the tension from a weaker spot to a stronger spot.
Another type of bridge is the beam bridge; this is the simplest for of a bridge. It consists of a horizontal road or platform and two supports on either side. This bridge is commonly seen over roads and highways. This bridge balances the forces of tension and compression very well. The next bridge is the arch bridge. This bridge has been used for thousands of years for architectural use. The semicircular structure distributes compression very evenly as it also diverts the weight to the two abutments. Tensional forces vary based on how big the arch is. The curve of the bridge dissipates the force outward which reduces the tension drastically. But the arch also has some flaws to its design. The bigger the semicircle the more tension there will be and the tension will eventually overtake the bridges natural strength. However arches are hard to build. Two more forces that affect bridges are Torino and shear. Torsion is when high wind causes the bridge to rotate and twist. While shear is when two fixed parts of an object are forced in different directions.
Underwater building: Many bridges are built to cross large bodies of water. This means that some bridges will have to be constructed underwater. When the Golden Gate Bridge was built, the water currents were very rough. This was due to California's fresh water currents colliding with the San Francisco Bay. To build the bridge they had to erect a pier 1100 feet out in the middle of the gate. Divers were very important because they had to guide the beams, panels, blasting tubes, and 40 ton steel forms into position. Most of the time they were almost blind due to the murky water.
The 4 most commonly seen bridges are the suspension, beam, truss, and arch. The truss bridge consists of an assembly made of triangles. They are made of steel bars. Rigid arms extend from each side of the pier. Steel tubes that run diagonal are what hold these arms in place. The beam bridge is very simple and rarely stretches more than 250 feet. Suspension bridges are complete giants that stretch up to 7000 feet! Arch bridges are very balanced compression wise while they only stretch up to about 100 feet.
Parts of the Experiment
Dependent- Amount of weight it can hold
Control group – Bridge with no weight
Experimental group- Beam bridge, arch bridge, and suspension bridge.
Factors held constant – Materials the bridge is made of, length of the bridge, bucket, type of weights.
- Garrett’s bridges combo kit
- Popsicle sticks
- Exacto knife/scissors
- Para chord
1) Gather materials
2) Construct a 17 in beam, arch, and suspension bridge using the instructions in the Garrett’s bridges set.
3) Gather weights, a bucket, and two chairs.
4) Carefully, but the handle of the bucket on the bridge and put the bridge on top of the two chairs so the bridge is holding the bucket.
5) Slowly add weight to the bridge until it breaks.
6) After the bridge is broken, weight the bucket to see how much weight was required to break the bridge.
7) Record all data and observations in the notebook.
8) Repeat steps 4-7 for the remaining two bridges.
Observations and Data
I used standard deviation in Microsoft Excel for my statistical analysis. Standard deviation is a value that tells how far each value was from the mean. Approximately 68% of the data will fall within 1 standard deviation of the data and 95% of the data will fall within 2 standard deviations.
1) Calculate Mean: 22
2) Subtract values from the mean: 16-22= -6 19-22= -3 22-22=0 31-22=23
3) Square all the values: -6=36 -3=9 0=0 9=81
4) Find the average: 36+9+81= 126 126/3= 42
5) Find the square root: root42= 6.480741
In the end the data did support the hypothesis which said that if weight is added to a beam, arch, and suspension bridge, then the beam bridge will hold the most weight because it has the most support and least tension.
Sources of Error
Lamb, Robert, and Michael Morrissey. HowStuffWorks. HowStuffWorks.com, n.d. Web. 29 Sept. 2014.
American Experience: TV's Most-watched History Series." PBS. PBS, n.d. Web. 29 Sept. 2014.
"Bridges and Tunnels." - How Are They Built and How Do They Work? N.p., n.d. Web. 28 Sept. 2014
"Types of Bridges." PBS. PBS, n.d. Web. 27 Sept. 2014.
Dupré, Judith. Bridges. New York: Black Dog & Leventhal, 1997. Print.
"What Is Standard Deviation?" WiseGEEK. N.p., n.d. Web. 07 Jan. 2015.