Structures Summary

Structures are the bones of our world, whether in nature or human design, and understanding their behavior is the key to building things that last.

1. Structures Are Everywhere, Both Natural and Man-Made

Structures are all around us, from the bones in our bodies to the bridges we drive across. A structure is essentially a collection of materials designed to bear loads, and they exist in both the natural and human-made worlds. Biological structures, like bones and tree bark, have evolved over millions of years to be efficient and resilient. Meanwhile, human-made structures, such as buildings and airplanes, are relatively new but have benefited from scientific study.

The formal study of structures began in the seventeenth century, thanks to Galileo. After being forced to abandon astronomy, he turned his attention to understanding the strength of materials. His work laid the foundation for structural engineering. Later, Robert Hooke expanded on this by studying how materials behave under stress, introducing the idea that structures resist loads by pushing back with equal force.

This understanding has allowed humans to build everything from towering skyscrapers to intricate bridges. But the principles of structure are not just for engineers; they are universal. Even a bird’s nest or a termite mound follows these same rules, demonstrating how nature and human ingenuity often overlap.

Examples

• Galileo’s shift from astronomy to material science in the 1600s.
• Bones in animals, which are natural load-bearing structures.
• The construction of bridges that balance forces to stay intact.

2. Stress and Strain: The Forces Inside Materials

Stress and strain are forces that act inside a material, determining how it reacts to external pressure. Stress measures how much force is being applied to a material, while strain measures how much the material deforms under that force. Together, they reveal a material’s stiffness, or its ability to resist deformation.

For example, a rope under tension experiences stress as its molecules are pulled apart. If the rope is thin, the stress is higher because the force is concentrated over a smaller area. Strain, on the other hand, is the change in the rope’s length compared to its original length. These concepts are essential for understanding why some materials, like rubber, are elastic, while others, like steel, are rigid.

The relationship between stress and strain is quantified by Young’s modulus of elasticity. This measure helps engineers choose the right materials for specific purposes. For instance, rubber’s low modulus makes it ideal for tires, while diamond’s high modulus makes it incredibly hard and resistant to deformation.

Examples

• A thin rope breaking under stress faster than a thick one.
• Rubber stretching easily due to its low modulus of elasticity.
• Steel beams in buildings resisting deformation under heavy loads.

3. Tensile Forces Stretch Materials, Sometimes Permanently

Tensile forces pull materials apart, causing them to stretch. This stretching happens because the atoms inside the material are being pulled away from each other. Over time, this can lead to a phenomenon called creep, where the material deforms permanently under constant stress.

Creep is an adaptation mechanism seen in both natural and artificial structures. For instance, new shoes become more comfortable over time as the material adjusts to the stress of your weight. However, not all materials stretch indefinitely. Blood vessels, for example, maintain their shape because the tensile forces acting on them are balanced by other forces.

Tensile forces are also responsible for the behavior of pressurized structures like balloons and sails. When a balloon is inflated, its molecules are stretched apart, creating tension. Similarly, sails stretch when hit by wind, allowing them to capture energy. Understanding tensile forces helps engineers design structures that can withstand stretching without breaking.

Examples

• Shoes becoming more comfortable due to creep.
• Balloons stretching as they are inflated.
• Blood vessels maintaining their shape despite constant stress.

4. Compression: The Secret to Ancient Structures’ Longevity

Compression forces push materials together, making them incredibly strong. Ancient builders intuitively understood this, which is why many old structures, like castles and cathedrals, rely on compression. These buildings are made of masonry, where each piece pushes against the others, creating stability.

A structure is stable when compressive forces are evenly distributed. If one part of a structure experiences more force than another, it can lead to cracks or collapse. This is why ancient builders avoided designs that relied on tensile forces, which are more likely to cause failure.

Even children understand compression intuitively. When building a tower with blocks, they know it will collapse if it’s too tall or uneven. This simple principle has allowed humans to create structures that have stood for centuries, proving the power of compression.

Examples

• Masonry buildings like castles and cathedrals.
• The stability of block towers built by children.
• The use of compression in modern concrete structures.

5. Beams: A Simple Solution to Complex Problems

Beams are a critical component of many structures, helping to distribute weight and reduce stress on walls. Without beams, roofs would exert outward forces on walls, potentially causing them to collapse. Beams channel these forces downward, making structures more stable.

The concept of the beam is not limited to human-made structures. In nature, animals like horses have backbones that function like beams, allowing them to support heavy loads despite having thin legs. This natural design has inspired engineers to create beams that mimic these properties.

Beams have revolutionized architecture, enabling the construction of buildings with large windows and open spaces. By reducing the horizontal forces on walls, beams make it possible to design structures that are both functional and aesthetically pleasing.

Examples

• Roof beams preventing walls from collapsing.
• A horse’s backbone acting as a natural beam.
• Modern buildings with large windows supported by beams.

6. Cracks: Small Flaws with Big Consequences

Cracks are one of the most common reasons structures fail. They create localized stress, which can weaken a material over time. The length of a crack determines its danger; once it reaches the critical Griffith crack length, it can grow rapidly, leading to collapse.

Cracks can form for various reasons, including material defects and external forces. For example, adding a patch to a torn piece of fabric can create new stress points, causing it to tear further. Engineers must carefully monitor cracks to ensure they don’t reach a critical length.

Understanding cracks has led to safer designs. By identifying weak points and reinforcing them, engineers can prevent catastrophic failures. This knowledge is especially important in structures like bridges and airplanes, where even small cracks can have serious consequences.

Examples

• The critical Griffith crack length determining a crack’s danger.
• Patches on fabric causing additional stress.
• Engineers reinforcing weak points in bridges and airplanes.

7. Fatigue: The Hidden Danger in Metals

Metal fatigue occurs when a material weakens over time due to fluctuating stress. This phenomenon was first observed during the industrial revolution, when machinery would break unexpectedly. Fatigue is difficult to detect because it happens at a microscopic level, altering the material’s crystalline structure.

Engineers have developed methods to calculate and test for fatigue, ensuring the safety of structures. For example, airplanes undergo rigorous testing to identify weak points and improve their design. These tests often involve pushing materials to their breaking point to understand their limits.

Fatigue is not limited to metals; it can affect any material subjected to repeated stress. By studying fatigue, engineers can design structures that last longer and perform better under varying conditions.

Examples

• Machinery breaking during the industrial revolution due to fatigue.
• Airplanes undergoing destructive testing to identify weak points.
• Fatigue affecting materials like rubber and plastic.

8. Experimental Testing: Learning from Failure

Experimental testing is essential for improving the safety and efficiency of structures. By testing materials and designs to destruction, engineers can identify weaknesses and make improvements. This process has been especially important in the development of airplanes, where safety is paramount.

Testing also helps reduce waste by identifying areas where materials can be minimized without compromising strength. For example, engineers can design lighter structures by reinforcing weak points and reducing material in less critical areas.

The lessons learned from testing have led to significant advancements in engineering. By understanding how and why structures fail, engineers can create designs that are safer, stronger, and more efficient.

Examples

• Airplanes tested to destruction between 1935 and 1955.
• Reducing material in non-critical areas to save weight.
• Identifying weak points in bridges and buildings through testing.

9. The Role of Calculations in Structural Safety

Calculations are the foundation of structural engineering, helping to predict how materials and designs will behave under stress. These calculations are based on probability, meaning there’s always a small chance of failure. However, they provide a reliable framework for designing safe structures.

In the 1970s, independent groups of experts evaluated airplane designs to ensure their safety. This collaborative approach has become standard practice in engineering, combining calculations with experimental testing to minimize risk.

By using calculations, engineers can design structures that balance strength, weight, and cost. This approach has led to innovations in everything from skyscrapers to spacecraft, demonstrating the power of mathematics in solving real-world problems.

Examples

• Probability-based calculations predicting structural behavior.
• Independent evaluations of airplane designs in the 1970s.
• Balancing strength and weight in modern engineering projects.

Takeaways

1. Pay attention to the forces acting on everyday objects to better understand their design and durability.
2. When building or repairing something, consider how stress and strain might affect its materials over time.
3. Learn from failures, whether in engineering or life, to create stronger and more resilient solutions.