PDF Summary:Structures, by J. E. Gordon

Have you ever wondered why some things bend under pressure while others break, or how 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.

This guide walks you through Gordon’s key ideas, explaining the principles that underpin structural integrity. You’ll learn how a material’s internal composition determines its ability to withstand force, why force always seeks the path of least resistance through a structure, and how time can gradually weaken even the sturdiest constructions. Additionally, we’ll expand upon Gordon’s ideas with up-to-date research and insights from other materials scientists and engineers.

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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:

1. Each material’s capacity for energy storage
2. 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.

(Shortform note: Understanding how energy moves through materials has enabled materials scientists to manipulate energy flow to create more stable structures. They do this by strategically placing materials with different energy storage capacities to direct the flow of energy in a way that either amplifies or dampens pressure at specific points in the structure. This ensures that energy only flows to materials that can efficiently absorb and redirect it. An example of how engineers have leveraged strategic material placement can be seen in structures that redistribute seismic forces, channeling them away from critical areas to reduce the impact of earthquakes.)

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.

(Shortform note: Even materials with a low capacity for energy storage can withstand pressure concentrations, provided the pressure remains below the material’s fatigue limit. The fatigue limit is the maximum pressure a material can endure repeatedly without its internal structure changing. When pressure concentrations stay below this limit, materials are less likely to deform or break—meaning they won’t undermine a structure’s 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.

FGMs Improve Joining Material Integrity

Gordon’s explanation of how weak points occur, specifically between materials, has led to the development of functionally graded materials (FGMs). FGMs are engineered with properties that change gradually across their structure. For example, an FGM might have a higher capacity for energy storage on one side and gradually transition to a lower capacity on the other.

Because FGMs possess a gradient of material properties, they directly address the problem of uneven energy flow between materials. This is because gradients act as a buffer between materials with different properties: As energy moves through the FGM, it encounters a series of incremental changes in material properties. Each small change allows the energy to adjust slightly, rather than having to adapt all at once. This gradual adjustment prevents the sudden concentration of energy that typically occurs between materials, allowing for a more even distribution of internal pressure throughout the structure.

As such, FGMs make efficient joining materials: When used to join materials with different energy storage capacities, FGMs avoid transmission disruptions that would otherwise cause increased internal pressure and create potential weak points. This versatility has led to their application in a variety of fields, including aerospace, where they’re used to join ceramic heat-resistant materials with metal components in jet engines to manage thermal stress. In orthopedics, they’re used to integrate dental implants with jawbones more naturally, enhancing comfort and durability.

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.

(Shortform note: While vertical structures with consistent widths or wider bases do allow energy to spread more evenly, they aren’t necessarily more stable. Engineers explain that a column’s height-to-width ratio significantly affects its stability—specifically, columns that are too tall and thin are more likely to collapse. Taller, thinner columns have a higher center of gravity and less lateral support, making them more likely to buckle or bend under their own weight or external forces. This also applies to columns with wide bases that narrow too steeply toward the top; the sharp tapering can create instability similar to that of overly tall, thin columns. An example of this is Egypt’s “Bent Pyramid,” constructed around 2500 BC)

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. (Shortform note: While narrowing structures often seem unstable, engineers have found ways to make them more structurally sound. Their techniques include adding internal supports to help distribute energy more evenly as it descends, and incorporating materials that can withstand increased pressure at the narrowest point in the structure.)

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.

(Shortform note: Engineers counterbalance this downward force by incorporating a slight upward curve into horizontal beams—a technique called precambering. When gravitational force pushes down on the beam, it first flattens out this curve. As a result, the beam remains straight under the force, rather than sagging in the middle.)

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.

(Shortform note: According to research, only a catenary curve allows for perfectly even energy distribution. A catenary curve is the natural shape a hanging chain or cable assumes under gravity when supported only at its ends. It’s possible to design arched structures to follow the design of this curve. However, if an arch doesn’t follow this exact shape, some parts of the arch will have to bear more energy than others, potentially creating weak points in the structure.)

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. (Shortform note: Research adds that no material can withstand force infinitely, since even very small, consistent forces can cause molecular bonds to shift or break over extremely long periods.)

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. (Shortform note: Studies reveal that the molecular bonds of materials with high elasticity can shift even under minimal force if that force is applied at a high frequency. Rapid, repetitive force can cause more changes to the molecular structure of highly elastic materials compared to gradually applying the same force to nonelastic ones. This means that the same amount of force can impact elastic materials more than nonelastic ones, but only if it is applied at a faster rate.)

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.

(Shortform note: Research on the effects of environmental exposure adds that these factors don’t act independently, but interact in complex ways to accelerate structural changes. For example, temperature fluctuations can make molecular bonds more susceptible to moisture damage, and vice versa.)

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.

(Shortform note: This understanding of how structural weak points accumulate—otherwise known as progressive collapse—has led engineers to design structures with distinct sections. This approach, known as compartmentalization or structural isolation, involves creating separated zones within a structure to limit the spread of energy flow variations and prevent localized weak points from compromising entire structures. Examples of this design strategy can be seen in nuclear power plants with isolated sections built to contain potential radioactive releases, and in high-rise buildings divided into upper and lower parts that limit the impact of seismic forces.)

Why Older Buildings Are Often More Structurally Sound Than Modern Ones

If molecular damage accumulates over time, why do some century-old structures outlast newer constructions? Experts suggest it’s because their engineers incorporated durable materials and overestimated safety requirements.

1) Material selection: Older buildings were often constructed with durable materials like stone or hardwood. These materials are less prone to molecular degradation over time since they’re highly resistant to environmental factors like moisture and temperature fluctuations. In contrast, modern constructions often prioritize cost-efficient materials like particleboard, plywood, or concrete reinforced with steel. While these materials are strong initially, they don’t hold up well over time because their molecular bonds shift more readily under force, creating weak points that progressively compromise structural integrity.

2) Overestimation: Because historical engineers lacked computational tools to calculate precisely how much force buildings could withstand, they overcompensated by incorporating deeper foundations, heavier materials, and thicker walls. These design choices create material redundancy, meaning that if some materials suffer molecular damage, the structure has sufficient material to maintain structural integrity. On the other hand, modern construction tends to favor a more minimal, just-enough approach—leaving fewer materials to compensate when damage occurs.