The Physics of Bridges: How Structures Connect Us
- Science Holic
- Aug 6
- 3 min read
Author: Eric Lin
Editors: Linzi Yeung, Jonathan Chen, Justin Tai
Artist: Alvina Zheng
Bridges, serving as a cornerstone of transportation, enable humans to traverse rivers, valleys, and highways with ease. Since the dawn of civilization, humans have used bridges. The oldest bridge in the world, the Kazarma Bridge or Arkadiko Bridge, is located in Argolida, a regional area of the Peloponnese in Greece. This enduring structure dates back to c.1300 BC during the Mycenaean period and is still used today. Behind the graceful arch and imposing span lies a complex web of physics, material science, and motion. From suspension bridges to beam bridges, each type of bridge has a unique design tailored to withstand specific weight loads, environmental conditions, and terrain challenges.
At the heart of bridge design are four main physical forces: compression, tension, shear, and torsion. Compression is the force that pushes materials inward and is mainly seen in piers or arch supports. The Rialto Bridge in Venice is a prime example when analyzing the importance of these substructures. When people move across the bridge, their weight creates a downward force that pushes inward along the arch. Reinforcements located at the ends of the bridge, known as abutments, resist compression by pushing back with an equal force, creating an equilibrium that stabilizes the bridge. Tension, on the other hand, stretches materials apart. The Golden Gate Bridge, for example, possesses suspension cables that counteract the downward force caused by people and cars with an upward tension force that pulls the bridge up. The weight of objects on the bridge creates a force that pushes downwards, but it doesn’t fall due to the tension force created by the suspension cables. The next force to consider when building a bridge is shear. Shear is the force that acts parallel to the surface of the bridge, resulting in tearing between layers of material. Shear can occur when loads push on different parts of the bridge in opposing directions, causing a split similar to tearing paper in half. Shear forces can happen anywhere on the bridge, but are particularly critical in certain areas, including the supports and piers. These locations are especially vulnerable because the deck load – the total weight a bridge is designed to support – may shift due to winds, traffic, or seismic activity. In such a case, the support can be pushed sideways at the top but remain in place at the bottom, creating a shear force that can lead to structural damage. Engineers focus on reducing failure by redesigning critical joints with shear metal plates and adding rebar to support beams that are more resistant to shear forces. Finally, the last force to consider when designing a bridge is torsion. Torsion is the twisting force that acts on a structure when separate parts rotate in different directions. This force can be demonstrated when one side of a bridge twists contrary to the other, a phenomenon caused by uneven loading, wind, or earthquakes. A simple way engineers counteract torsion is by designing bridges to be symmetrical, as asymmetry almost always leads to torsion. All four forces mentioned are variables that engineers have to take into account when building bridges.
In summary, engineers consider a multitude of factors when constructing bridges, such as the design and materials used to account for compression, tension, shear, and torsion. Shockingly, something seemingly so mundane and as common as a bridge can have various layers of math and science as its foundation.
Citations:
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“How Bridges Work.” Explain That Stuff, 29 Mar. 2024,
Lamb, Robert, et al. “How Bridges Work.” HowStuffWorks, 12 Nov. 2021,
Masters, Bridge. “How It Works: Engineering Bridges to Handle Stress.” Bridge Masters, 17
Spinning the Main Cables - the History of the Design and Construction | Golden Gate.
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