Have you ever wondered why some things bend under pressure while others break? How do some buildings remain standing for centuries?
In Structures, scientist and engineer James Edward Gordon reveals why some materials and structures can withstand forces like gravity and wind while maintaining their form and stability, while others collapse under pressure.
Read more in our Structures book overview.
Overview of Structures by J.E. Gordon
What is structural integrity? According to scientist and engineer James Edward Gordon, it’s how much force a structure can withstand without losing its form or collapsing. He explains that all structures, both natural and man-made, are subjected to external forces that they must withstand to maintain their form and stability. For example, trees must counter forces such as wind and heavy rain, while buildings must withstand the weight of furniture and people. In his book Structures, Gordon examines the factors that make some objects sturdy while others are vulnerable to failure. (Shortform note: Structures was originally published in 1978. This overview is based on the 2003 updated edition.)
Gordon was a pioneering materials scientist who bridged the gap between theoretical physics and practical engineering. He served as head of plastic structures research at the Royal Aircraft Establishment and professor of materials technology at the University of Reading. In addition to Structures, he authored several other books on engineering, including The Science of Structures and Materials and The New Science of Strong Materials. His research was key to British military aircraft development during World War II and continues to influence generations of scientists and engineers.
Principle 1: Material Properties
According to Gordon, a structure is only as stable as the materials it’s made of. Therefore, to understand how a structure can withstand external forces, you first need to grasp how its individual materials respond to forces. Gordon suggests that three key properties determine how a material responds to external force: elasticity, energy storage, and strength. Let’s explore each of these in detail.
Material Property #1: Elasticity
A material’s elasticity is its ability to return to its original shape after being misshapen by an external force. Gordon explains that every material has a given internal molecular structure—the bonds between its molecules are organized in a specific way. When a force acts on a material, its internal molecular structure shifts as the bonds between molecules stretch. This may or may not impact the material’s visible shape, depending on the material itself and the amount of force applied. In any case, elasticity is the material’s ability to recover its original molecular structure after being subjected to force. (To picture elasticity on a larger scale, think of how a spring compresses when you squeeze it and expands when you release it.)
According to Gordon, some materials are naturally more elastic than others because their molecular bonds can withstand more force—which means they can stretch further without breaking. If the force applied to the material isn’t too strong, the change to the material’s shape is just temporary. When you remove the force, the molecules return to their normal positions, and the material regains its original structure. But if the force is too strong for the material to withstand, the change becomes permanent. The force stretches the molecular bonds beyond their ability to recover, preventing them from returning to their original positions.
For example, think of stretching a rubber band. If you stretch it a little before letting go, the band deforms temporarily before bouncing back. However, stretching it too far permanently deforms the rubber band—it becomes saggy or snaps.
Material Property #2: Energy Storage
As we just discussed, applying force to a material causes its molecular bonds to stretch. Gordon explains that this stretching process allows the material to absorb and store the force’s energy—in other words, once the force hits the material, the material stores its energy within its internal structure. The more force applied to the material, the more energy the material has to store—and the further its molecular bonds have to stretch to accommodate the amount of energy. Put simply, the greater the force, the more energy it creates for molecular bonds to absorb. And the only way molecular bonds can store this energy is to keep stretching until they absorb it all.
But, as we explained before, molecular bonds can only stretch so far as their material’s internal structure allows. It follows that a material’s elasticity (how far bonds can stretch) determines its capacity for energy storage—how much energy its molecular bonds can safely store while maintaining their ability to return to their original positions.
Gordon explains that when the stored energy exceeds a material’s storage capacity, the material must release that energy—and this makes the material deform or break. This is because stored energy increases pressure within the material’s internal structure (think of a balloon inflating). Eventually, the material must release that energy to relieve the building internal pressure (think of an over-blown balloon about to pop). This release manifests either through permanent deformation (where bonds stretch too far to recover) or through breakage (where bonds snap completely).
How Materials Release Excess Energy
Gordon describes the energy release process: Excess energy concentrates at certain points within the material, creating areas of high internal pressure. Due to this high pressure, the molecular bonds in these areas stretch beyond their capacity and break, forming tiny cracks in the material’s structure. These cracks then serve as paths of least resistance through which the stored energy escapes.
As the energy escapes through these paths, it breaks more molecular bonds, causing them to violently release their stored energy and weaken or break neighboring bonds. The more energy stored within the material before this process begins, the faster and more catastrophic the breakage becomes. Gordon says this chain reaction explains why materials often break suddenly rather than gradually—once released, the stored energy provides the force needed to rapidly spread damage through the material.
For example, consider a glass window that’s struck by a small stone. Initially, the glass absorbs and stores the energy from the impact. However, this energy quickly concentrates at the point of impact, creating a high-pressure area that exceeds the glass’s energy storage threshold. This causes a crack to form, providing a path for the stored energy to escape. The sudden release of this energy causes the crack to spread rapidly across the entire window, resulting in a shattered pane rather than just a small chip at the point of impact.
Material Property #3: Strength
A material’s strength determines how much force it can withstand before its molecular structure permanently deforms or breaks. As we discussed, applying force to a material changes its internal structure (depending on its elasticity) and creates internal pressure (depending on its capacity for energy storage). The combination of these two effects can cause the material to deform both internally and externally. Gordon writes that the extent of this deformation depends on the material’s strength.
In simple terms, materials that can respond to force while maintaining their original molecular structure have more strength than those that can’t maintain their structure.
However, Gordon clarifies that materials unable to maintain their original structure don’t always collapse. Rather, the way materials respond to excess force depends on their elasticity and capacity for energy storage. This response can play out in multiple ways:
- Some materials gradually weaken as force increases, showing signs of impending collapse through visible deformation—for example, a wooden beam might bend before breaking.
- Some materials maintain stability up to their breaking point and then suddenly collapse—for example, a ceramic cup will likely shatter when dropped.
- Some materials deform permanently without breaking, losing their original shape but retaining some structural integrity—for example, a metal paperclip can be bent out of shape but still hold together.
Principle 2: Force Distribution
We’ve just explained the properties that determine how individual materials respond to force: elasticity, energy storage, and strength. Since structures tend to be composed of different materials, each with its own molecular composition, we’ll now explore how the combination of different materials impacts a structure’s ability to withstand force and maintain structural integrity.
Structures composed of multiple materials respond collectively to force. Gordon explains that applying force to a structure doesn’t just affect each constituent material in isolation. Rather, the energy of the force moves through all the materials that make up the structure, triggering complex interactions between them. How a structure responds to this moving energy depends on two key factors:
- Each material’s capacity for energy storage
- The structure’s shape
Let’s examine how each of these factors impacts a structure’s response to force.
Factor #1: Each Material’s Capacity for Energy Storage
As we’ve established, applying force to a material causes the material’s molecular bonds to stretch and absorb the energy carried by a force. Gordon explains that in a structure made of multiple materials, the differences between each material’s capacity for energy storage determine how energy flows through that structure. Energy doesn’t flow through a multi-material structure evenly from one material to the next. Rather, it interacts with one material at a time and only moves to another when the current material reaches its capacity for energy storage.
However, Gordon states that energy doesn’t move to a neighboring material at random. Instead, it takes the quickest path through the structure. To clarify, let’s examine how different energy storage capacities impact the speed at which energy flows:
- When force meets a material with a high capacity for energy storage, the transfer of energy slows down. The material absorbs a large portion of the force, reducing the amount of energy that immediately transfers to neighboring materials.
- When force meets a material with a low capacity for energy storage, the transfer of energy speeds up. The material can’t absorb much of the force’s energy, so it must immediately transfer as much as possible to neighboring materials.
Of course, a force can’t “choose” what material it interacts with first. Think of a boulder rolling into a house—whatever material it happens to land on is the first material it interacts with. However, Gordon explains that after the initial impact, the force’s energy always moves to materials with the lowest capacity for energy storage because these materials channel energy through structures more efficiently. In simple terms, materials that can’t absorb a lot of energy help it flow faster through a structure than materials that can absorb more.
To visualize this, imagine setting a sponge and a sheet of smooth plastic next to each other and pouring water over both. In this scenario, the sponge is a high-capacity material because it can absorb water, and the plastic is a low-capacity material because it can’t. When you pour water (which represents a force) onto these materials, the water will race across the surface of the plastic, flowing outward and away. Some goes into the sponge, where it quickly slows and gets absorbed. The sponge soaks up the water, holding it in place, while the plastic does nothing to stop or contain the flow.
Uneven Energy Flow Creates Weak Points
By opting for the most efficient path, energy flows unevenly through the structure, concentrating more on materials with a low capacity for energy storage. Gordon says this concentration of energy creates areas of high internal pressure within those materials. While energy initially veers toward these materials because they’re efficient channels, this high concentration of pressure can quickly exceed their capacity for energy storage, causing them to deform or break. This uneven energy flow creates weak points in the structure, undermining its integrity.
Additionally, Gordon explains that structural weak points tend to occur where two materials join. This is due to two reasons:
- First, when energy moves from one material to another, it must adapt its speed, which momentarily disrupts it from flowing smoothly. This disruption causes it to concentrate more heavily and increase internal pressure at the location where the materials join.
- Second, joining materials must absorb energy long enough to transfer that energy between other materials. However, joining materials are often smaller than the materials being joined (for example, nails used to join planks of wood). Each time energy flows from materials being joined into smaller joining materials, it must squeeze through those joining materials. This squeezing concentrates more energy on the joining materials, increasing their risk of exceeding their capacity for energy storage.
Therefore, for a structure to maintain stability, Gordon argues that both the size and material properties of joining materials must be able to accommodate increased pressure, since breaking a single connection can cause a chain reaction that cascades through the structure, weakening or destroying it.
Factor #2: The Structure’s Shape
According to Gordon, a structure’s shape influences how gravitational force flows through it. Gravitational force is the continuous downward pull that acts on all matter; it can spread evenly across structures or concentrate in particular areas. Let’s explore how gravity interacts with vertical and horizontal structures.
Category #1: Vertical Structures
When a vertical structure maintains a consistent width throughout its height (like a rectangular column), it provides a uniform path for gravitational force to flow down. According to Gordon, a consistent width allows the energy of gravitational force to spread evenly down and across the structure, preventing concentrations of internal pressure in any single area. This effect is even more pronounced when a vertical structure widens as it descends (like a pyramid). Because there’s more material at the bottom of the structure, energy spreads progressively across an expanding area as it moves downward, preventing excessive pressure from building up at any single point.
When a vertical structure narrows as it descends (like the Louvre’s inverted pyramid), the energy of gravitation initially spreads across a wider area before compressing into a smaller space. Gordon explains that this compression concentrates more energy and increases internal pressure in the materials in that smaller area, potentially leading to structural weak points.
Category #2: Horizontal Structures
When a horizontal structure remains flat between support points (like a tabletop resting on its legs), it directs the energy of gravitational force toward two paths: downward across its entire length and laterally toward its support points. Gordon explains that the greater the distance between support points, the further energy has to travel laterally before it can flow down. And the further the energy has to travel laterally, the more it concentrates in the middle of the structure’s length. This increases internal pressure in that area, raising the risk of sagging or collapse.
When a horizontal structure incorporates curves between support points (like an arch), it redirects the energy of gravitational force—which would otherwise flow downward across the length—along the curve. As a result, energy spreads more evenly across the structure, minimizing the build-up of pressure at any single point along the curve’s length.
Principle 3: Time Effects
Until now, we’ve been discussing the immediate effects of force on materials and structures. Now, we’ll examine how these effects can accumulate over time, impacting a structure’s long-term stability.
Gordon explains that the effects of force eventually add up, changing a material’s molecular structure. As we’ve discussed, when force impacts a material, its molecular bonds stretch and return to their original positions. When this happens frequently, the cycle of stretching and returning can cause these molecular bonds to settle into slightly different positions, permanently altering their internal structure and their ability to withstand force. Gordon explains that three conditions impact how a material’s internal structure changes over time:
1) Force variations: Consistent force (like the same weight on a floor) creates predictable stretch-return patterns that make it easier for molecular bonds to maintain their original structure. However, Gordon writes that variations in force (like abrupt spikes in weight) cause molecular bonds to stretch in unpredictable patterns. The more these stretching distances vary, the more difficult it becomes for molecular bonds to find their way back to their original positions.
2) Material elasticity: In materials with higher elasticity, molecular bonds have freedom to stretch and settle into different positions. In materials with lower elasticity, the movement of molecular bonds is restricted, which helps them maintain their original positions.
3) Environmental exposure: Molecular bonds of materials subjected to frequent temperature changes or moisture are more likely to alter positions. Gordon writes that temperature fluctuations force bonds to adapt their spacing—heat stretches bonds and cold makes them contract, so it’s hard for them to maintain their original positions. Meanwhile, moisture enters the spaces between bonds and pushes them apart, which prevents them from returning to their original positions.
The Cumulative Impact of Force on Structures
Gordon says that when materials’ molecular bonds alter positions, they change how the energy of a force moves through those materials. This inevitably impacts how energy flows through an entire structure. For example, imagine a sponge that’s repeatedly used and dried over months or years. Each time it absorbs water, the sponge’s cellulose fibers swell as molecular bonds loosen and shift. When it dries, those bonds contract—but never quite return to their original form. Over time, this repeated cycle gradually breaks down the sponge’s molecular structure. The sponge becomes thinner, less elastic, and more brittle, reshaping how the sponge handles force—until one day, instead of absorbing water evenly, it tears or crumbles.
Furthermore, if a structure is composed of different materials, the molecular bonds in each material will change at different rates. As a result, the way energy flows between materials will change more radically in some parts of the structure than in others.
Each time energy takes a different path through a structure, it creates new areas of high internal pressure. According to Gordon, this decreases structural integrity because, as previously explained, high internal pressure causes materials to deform or break—therefore, new pressurized areas create new structural weak points. Further, every new pressurized area accelerates the formation of structural weak points—because they cause even more molecular bonds to shift positions, which then creates more flow path changes and new areas of high internal pressure.
Given these effects, Gordon emphasizes that structures maintain long-term stability only when their design accounts for how force variations, material elasticity, and environmental exposure will affect the molecular bonds of their constituent materials over time.
