I had the good fortune of spending a week in Pittsburgh, Pennsylvania, not too long ago. Unlike many other large U.S. cities, the Steel City has undergone and continues to undergo a very significant downtown revitalization. The heart of the city lies at the confluence of the Monongahela and Allegheny Rivers. These good sized rivers converge to form the mighty Ohio.
Pittsburgh has more bridges than any city in the world. According to a Pittsburgher I met on the street, Venice, Italy used to hold the record before the criteria were refined to exclude foot bridges. Many of the 446 bridges are of course steel. Standing downtown on the Roberto Clemente Bridge, from which you can see both PNC Park (home of the Pirates) and Heinz Field (home of the Steelers), I looked and could see multiple major bridges spanning the Allegheny River in both directions.
Engineers use a variety of materials for building bridges and other structures including concrete, steel, wood, stone, and even plastic! Steel is a preferred choice because of its ability to withstand compressive forces (pushing down which includes the force of gravity) and tensile forces (tension). Steel is also very ductile, that is it is easily drawn into wire and hammered thin. I saw this at the large city convention center overlooking the Allegheny. Inside the building were several massive bundles of steel cable anchoring various key points between the roof and the 3rd level.
Bridges excite everyone. They are amazing structures, that, when well-engineered, not only get us from a to b but also impart to us a sense of awe and appreciation for the creativity and ability of the bridge designer and builders. How many bridges are there in your town? What are they made of? Do a bridge exploration in your. You are sure to gain a new appreciation for bridges and for their builders!
For more on bridges and engineering principles take a look at this teaching resource.
force, compressive, tensile, ductile