Structures summary

Stress isn’t just a psychological problem affecting humans – structures can suffer from stress as well.

In physics, stress is a force that can exist at any point inside a solid. It measures the degree to which atoms and molecules within the material are being pushed apart by external forces.

When you pull on either side of a piece of rubber, it stretches out. But what allows a solid to change its shape under pressure?

Rubber changes shape when it's pulled because the atoms inside it move around. These atoms are being acted upon by tensile forces.

Some structures, like ancient churches, castles or monuments, have managed to stay intact for centuries or even millennia. What makes it possible for them to stay standing for so long?

Structures stay in place largely thanks to the compressive forces acting upon them. Unlike tensile forces, compressive forces push on a structure rather than pulling it.

The roof of your house is obviously an important part of its design, since it protects the house from the elements. A roof might seem a simple enough component of a structure, but for engineers, a roof is much more complicated than it seems.

Roofs present a lot of challenges for engineers. A permanent and stable roof can be dangerous because it puts a lot of weight on the surrounding walls of the building. And because so many roofs are triangular, the resulting force usually doesn't push down vertically; it pushes outward against the walls. The resulting tensile forces on the walls could cause the building to collapse on itself.

Even seemingly well-designed or newly built bridges can collapse sometimes. There are a few possible reasons for this, but cracks and material problems are often the main culprits.

Structures can fall apart when they develop cracks or when new material is added onto them. C. E. Inglis wrote about this in 1913, when he published a book about the significance of irregularities within a material.

If you keep stretching out a rubber band, it eventually reaches a point where it will break; the tensile forces acting upon it cause its material to develop cracks or holes.

When tensile forces are applied to a material, the interatomic bonds within it stretch out – and they can only stretch to a certain extent. After about a 20 percent increase of tensile strain, the chemical bonds become weaker and eventually unstick from each other, producing a crack or hole.

When a person gets frustrated or stressed out, they will often dwell on their problems, which only serves to increase their stress levels. A similar phenomenon occurs in metal – and, appropriately enough, it's called fatigue.

Fatigue occurs when the heavy load on a metal fluctuates and causes it to lose strength. People first noticed this phenomenon during the industrial revolution, when machinery would sometimes break when it was moved.