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Bridge Type Strengths

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Joshua Hill

on 16 December 2013

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Transcript of Bridge Type Strengths

Strength of Design
What Type of Bridge is the Strongest?

A bridge is a structure designed to span a distance for the use of travelling from one point to another. Since the beginning of time, man has found a way to cross over obstacles using a simple bridge. A bridge's structure can determine its efficiency and strength. The four most common types of bridge structures include the arch, flat beam, suspension and cantilever, or truss. Each design serves a specific purpose, but which design is the strongest?
An arch bridge is a simple curved shape with a flat deck and heavy side pillars. The arch bridge dates back to 1300 BC in Greece, and was commonly used in early Roman architecture. This type of bridge was originally used to cross small rivers. The arches hold compression forces which push the load outward to the sides. Arch bridges can cross a very short span, but can be linked with other arches to increase the span. Early arch bridges were made of stone; modern arch bridges are made of reinforced concrete or steel.
A flat beam bridge is a simple deck on top of heavy beams or piers. Beam bridges date far back in history, when man laid a felled tree or large stone across a stream. Bridges like these are used for people, animals, cars and trains. These bridges span short lengths between few piers, but can be longer if the deck is laid across multiple piers. Beam bridges used natural materials like stone or wood; modern bridge materials include steel and concrete.
Suspension bridges consist of a deck suspended by cables between towers which act as vertical beams. The cables and towers are anchored at each end for extra stability. Early suspension bridges date back to Asia in 1400s,and were improved in Europe in 1800s. These bridges are commonly used in mountain areas to cross canyons, or cross large bodies of water, and are mainly used for people, animals, and cars. In these bridges, tension forces build up in the cables, while compression forces are concentrated in the vertical pillars. Suspension bridges are usually medium to long spans. These bridges use cables and steel for suspension, and concrete for towers and decks.
Cantilever bridges use trusses to help decks hanging over from two or more anchored pillars meet in the middle to span up to 1500 feet. These bridges were designed in Germany and in America in the 1800s. The design of the trusses can be varied depending on the style or strength needed. Cantilever bridges are used to span great lengths over difficult crossings. Usually, forces of tension stay in the upper trusses and forces of compression are in the lower pillars. These bridges are commonly made of iron, steel and concrete to reinforce the strength.
Over the years, the design of bridge structure has improved the efficiency of a bridge’s strength. The span, use and cost of materials are considered when deciding on a bridge structure. Arch bridges are good for small rivers, but are not suitable for long distances. Beam bridges are simple and low cost, but they can’t carry heavy loads. Suspension bridges are good because of their long spans and flexibility in destructive conditions (such as earthquakes), but they are bad because they vibrate and cannot carry heavy loads. Cantilever and truss bridges can span great lengths, and can be easily constructed in sections, but compress a lot of heat buildup and might damage some of the metal. By making models of each design and testing the strength and flexibility, engineers can find the best bridge to build for their projects.

This project is a test of the most efficient bridge design for a heavy load. To test the different structural designs, models can be built and increasing weights can be placed on the deck until the structure fails. My hypothesis is that the truss bridge will be the strongest and have the most efficiency. Truss bridges are found around the world and the truss design is also used in other types of construction where strength is necessary.
For each bridge model, the dependent variable is the amount of weight held before the bridge collapsed. The independent variable for each bridge is the structural design of the bridge. The controlled variables are the mass of the bridge, materials and distance of span.
For this project, I used the following materials:
• Graph paper (for bridge designs)
• Wooden Popsicle sticks (enough for 4 bridge models)
• Wood glue (Elmer’s)
• Clothes pins or binder clips, medium and small sizes, used for clamps
• Weights (cans of soup – 1 ½ pounds each, and bricks – 7 pounds each)
• 4” x 5”x 2” wood block, large hook and 5 gallon plastic bucket
• Digital scale
• Digital camera

1. Conduct background research on the 4 main bridge structure designs. I also read about other experiments and professional tests on bridge design and strength to weight efficiency to choose a good plan for my tests.
2. On graph paper, sketch out designs for the models including the truss, suspension, and the arch bridge design. This helps to determine the length and width for each bridge and how many popsicle sticks are needed.
3. Gather popsicle sticks, wood glue, clips and other materials to build 4 bridge models that would each span 16 inches.
4. Build each bridge model by laying out the sticks and using wood glue and clothes pins to hold the sticks in place until the glue dried. This may take several days!
Hill 3
5. When the bridges are complete, weigh each bridge to find its mass. For a more accurate test, make sure all the bridges have the same mass by adding popsicle sticks until they were all equal.
6. To test each bridge, place the model on 2 bookshelves, spaced apart to make a span of 16 inches.
7. Place a wood block with a hook in the middle of the deck of the bridge with a bucket hanging below.
8. Gradually increase the weights by 1.5 pounds until each bridge cracks or breaks. For weights, I used 3 cans of soup that had a weight of 1.5 pounds each and bricks that weighed 7 pounds each.
9. Record the data for each and calculate the efficiency (weight to strength ratio). Take photographs and record other observations as well.
10. After all bridges are tested, use the data to create a graph.

The tests showed that the truss bridge was the strongest design. The flat beam bridge held 16,071 grams, the arch bridge held 22,498 grams, the truss bridge held 35,380 grams, and the suspension bridge held17,237 grams. Each bridge weighed 170 grams and spanned the same length of 16 inches. The flat beam bridge broke in half, the suspension bridge deck broke, but the fishing line (cables) held up, and the arch bridge deck broke, but the arches did not. I think the truss bridge held the most weight because it was supported on each side and the top with a simple truss design that gave it extra support. By using the formula of mass of load divided by mass of the bridge, the weight to strength ratio was calculated for each bridge.
The tests on the bridge models proved my hypothesis was correct. The truss bridge held the most weight, and was the strongest bridge. The hook actually bent before the bridge model cracked. My research also showed that the truss bridge is the most commonly used design and there are many truss bridges all over the world. Knowing the most efficient bridge structural design can help engineers choose the appropriate bridge for their spans. Engineers use many tests to improve their designs and use of materials. Different tests can help find which design can handle other forces like wind and earthquakes.

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