G3 Program 15: Bridges

Have you ever looked up at a skyscraper, or a bridge, or even a roller coaster…and wondered just who made them and how they were made? Well, all of those things were made by men and women who are structural engineers. Structural Engineering is a field of science that “deals with the analysis and design of structures that support or resist loads.” What does that mean? Well, as an example, if an engineer wants to build a bridge, he or she needs to make sure that it is designed well enough to support the weight of all of the people, cars, trucks, etc. that want to cross it. It also needs to be strong enough to hold steady during high wind storms and other rough weather.  Even though structural engineers build so many different kinds of things, our group focused on bridges.

Types of Bridges

There are 5 main types of bridges:

• Beam bridges
• Truss bridges (not discussed in our program)
• Arch bridges
• Suspension bridges
• Cantiliver, or Cable-Stayed bridges

Like all scientists, structural engineers draw on the experiments and knowledge of all of the people that came before them when attempting new structures. One of the most dramatic bridge failures occurred in 1940. The Tacoma Narrows Bridge (aka “Galloping Gertie”) was located in Washington state, and at the time of its completion, it was the 3rd longest suspension bridge in the world (The Golden Gate Bridge in San Francisco and the George Washington Bridge in New York held spots #1 and #2). During a severe wind storm in November of 1940, the bridge fully collapsed.

But like all failures, “Galloping Gertie” taught structural engineers a lot about bridge design and how to prevent another such disaster from occurring. But science is always evolving, and there are always new things to be learned. In 2000, The Millennium Bridge (a pedestrian bridge) was opened in London, England. On opening day, however, it quickly became apparent that something was wrong with the structure. The bridge was shaking and moving noticeably from side to side as pedestrians walked across! Londoners started to call the bridge “The Weebly-Wobbly Bridge” in honor of this unexpected motion.

Structural Engineers discovered that the masses of people that were walking across the bridge had actually been syncing up with the natural vibration of the bridge. [All bridges have a small amount of natural vibration, so small that it is practically unnoticeable.]  The combination of the people swaying to keep their balance coupled with the natural swaying of the bridge led to the dramatic sideways swaying motion of the bridge. So again, yet more things for structural engineers to learn about the design of structures that support all sorts of weight in motion.

Our mission for the day was based on the PBS FETCH project entitled “What Gives.” Our G3 scientists were asked to design a suspension bridge using nothing more than:

• 2 sheets of cardboard (each sheet was 24″ x 18″)
• Duct tape
• String
• A hole-puncher
• Scissors

Success!

They had the freedom to do any design they wanted. The only rules were that they could not attach the cardboard sheets (the bridge deck) to the chairs it was balanced on. And they could not tie the 2 black strings (main cables) to the chairs either. The success of their suspension bridge would hinge on how well they placed/designed the vertical strings (hangers) coming down from the black strings (cables) to the cardboard sheets (bridge deck). The scientists were divided into groups of at least 2 people, though many scientists preferred to be in a group of 3-4 people. When bridges were ready to be tested, I helped the groups test the stability of their bridges by putting piles of books on the bridge deck (the cardboard) to see how much weight the bridges could support.

Cool graffiti!

All of the scientists had very solid bridge designs. There was a lot of attention to where the hangers should be placed, how the duct tape should be used…and what the graffiti on the deck should look like! In fact, the scientists in Track B produced some very successful bridges that supported the weight of all of the books I had! Very impressive. Well done, scientists! See you next session 🙂