Why do most animal bones have a hollow ring-like cross-section? About 500 million years ago, the first vertebrates appeared on Earth, which developed internal skeletons for movement, support, and protection. Have you ever noticed that their bones are hollow? Most animal bones are hollow and lightweight, because the forces they typically face are bending and compression. When bending, the maximum bending stress is at the edges, and the central part is hardly subject to any force, which is why animal leg bones are hollow. Under pressure, the stress is also dispersed along the edges.
Similarly, a thin wheat straw can support a much heavier wheat head. These supporting structures in plants are called mechanical tissues. Some varieties of wheat and rice have a tendency to fall over, indicating that the mechanical tissues in their stems are not well developed enough and need to be strengthened through water control and fertilization. Many plant stems are also hollow, such as bamboo, reeds, celery, and many weeds.
Nature inspires people to create round pipes. Steel pipes can withstand almost the same amount of bending and pressure as steel bars made from the same amount of material. Round hollow pipes not only have strong resistance to bending and pressure, but also save materials and are lightweight. They can also transport gases and liquids. Stone bridges are designed in an arch shape to maximize their resistance to pressure. The arch structure can transfer pressure to the supporting bases on both sides of the bridge, and the arch is just the pressure curve of the bridge. The arched roofs of cave dwellings on the Loess Plateau in China and the arched roofs of most underground tunnels are also built using this principle. The supports on the edges of arched bridges and cave dwellings are very thick, precisely to withstand the transferred pressure.