In this blog post, we’ll explore why buildings can be safely constructed to great heights, as well as the principles of compressive and tensile forces and reinforced concrete.
Why must buildings withstand tensile forces?
Buildings are iconic symbols that showcase the level of human civilization. Among them, the massive structures of antiquity continue to inspire awe in many people even today.
The Lighthouse of Pharos was one of the most iconic structures of the ancient world. It is said that a massive statue of Poseidon, the god of the sea, stood at the top of the lighthouse, while the first floor featured 30 statues of half-human, half-fish sea gods, including a remarkable statue that moved its fingers in sync with the sun’s movement. Although it has since disappeared due to wars and earthquakes, in ancient times it was such an outstanding structure that it was counted among the “Seven Wonders of the Ancient World.”
The main reason the Lighthouse of Pharos was called a wonder was that it reached a height of over 100 meters—a truly astonishing feat for its time. Today, thanks to advances in construction technology, skyscrapers over 800 meters tall have emerged, and projects for structures exceeding 1 kilometer in height are underway. So, if ancient people had had the technology, could they have built the Lighthouse of Pharos much taller than it is today? The answer is closer to “no.” This is because the building materials available at the time could not sufficiently withstand “tensile forces.”
Hearing this explanation may raise a question. Buildings bear the weight of people and furniture from top to bottom, and when the wind blows, they are subjected to lateral forces pushing against them from the sides. Since it seems that buildings are primarily subjected to compressive forces, why is it a problem that they are weak against tensile forces?
What are compressive and tensile forces?
The forces acting on a building can be broadly categorized into compressive force (Compression) and tensile force (Tension).
First, compression refers to the force that presses down on or pushes against an object. For example, the force acting on food when you chew it is compression. The maximum compression a material can withstand is called compressive strength; if this limit is exceeded, the material will break or fail.
Conversely, tension is the force generated when an object is pulled. A classic example is the force applied when you tear apart chewed food with your hands. The maximum tensile force an object can withstand is called tensile strength; if this limit is exceeded, the object will tear or break.
Now that we understand the concepts of compressive and tensile forces, let’s examine how these forces occur in actual buildings.
Generally, compressive forces act on a building from top to bottom due to the weight of people, furniture, equipment, and the building itself. These forces are transmitted downward through horizontal members, such as floors and beams, and vertical members, such as walls and columns.
During this process, the horizontal members bend very slightly. Although the actual amount of deformation is too small to be seen with the naked eye, structural bending clearly occurs. When a horizontal member bends, its lower surface stretches and its upper surface shortens.
Due to this deformation, tensile forces pulling in the left-right direction are generated on the lower surface, while compressive forces pushing in the left-right direction are generated on the upper surface. If the horizontal member is made of stone, like the Lighthouse of Pharos, its compressive strength is very high, so no major problems occur in the upper part. However, because its tensile strength is relatively low, cracks form first on the lower surface, eventually leading to failure.
How Do Building Materials Break?
Let’s examine the process by which a horizontal member fails in a bit more detail. In structural engineering, this is explained using the stress-strain curve.
In the early stages, when the force applied to an object is small, it remains in an elastic state. An elastic state is one in which the degree to which an object’s length increases or decreases is proportional to the magnitude of the compressive or tensile force. This is the same principle as how more force is required the more you stretch a rubber band.
However, if the force applied to the object exceeds its compressive or tensile strength, this proportional relationship is no longer maintained. This state is called “yield.” An object does not break immediately upon yielding. It first enters the “plastic deformation” stage.
During plastic deformation, the object’s length easily increases or decreases even without a significant increase in the applied force. This phenomenon can be easily observed when eating pizza. After taking a bite of pizza and pulling on the cheese, the cheese initially stretches in proportion to the pulling force. However, at a certain point, the cheese begins to stretch downward on its own and elongate without the need for additional force. This state is plastic deformation.
If plastic deformation continues, the material will eventually fail, just as the cheese snaps. The process by which a rubber band snaps or a horizontal beam breaks can also be explained by the same principle.
Let’s apply this principle to the Lighthouse of Pharos. The Lighthouse of Pharos was constructed primarily from stone, including marble. Stone has excellent compressive strength but relatively low tensile strength. Therefore, when tensile force is applied, it yields relatively easily; this leads to plastic deformation and can eventually result in cracking and failure.
This principle is easy to understand if you imagine a human tower. In a three-story human tower, the person on the second story supports the weight of the person on the third story, but the person on the first story must bear the weight of both the second and third stories.
The same applies to buildings. As a building gets taller, the structural members on the lowest floor must bear the load of all the structures above them. As the number of floors increases, the tensile forces acting on the horizontal members also increase; therefore, there are limitations to safely constructing a building beyond a certain height using only stone, which has low tensile strength.
So how have today’s supertall buildings become possible? The reason modern high-rise buildings can be constructed safely—even though they also use concrete as their primary material—lies in the advancement of reinforced concrete technology.
How has reinforced concrete made supertall buildings possible?
Reinforced concrete is a representative composite structural material that combines concrete and steel reinforcement.
Concrete is relatively inexpensive, easy to work with, and has very high compressive strength. Therefore, it performs exceptionally well under compressive forces. However, because its tensile strength is very low compared to its compressive strength, there are limitations to safely constructing tall buildings using concrete alone.
In contrast, rebar is a material with excellent strength under both compressive and tensile forces. In particular, its tensile strength is very high, allowing it to effectively withstand tensile forces. However, constructing a building using only rebar significantly increases material costs, making it less economical.
Reinforced concrete is a structural system that combines the advantages of these two materials. Most of the building is constructed using cost-effective concrete, while rebar is placed in areas where tensile forces are concentrated to compensate for the concrete’s lack of tensile strength. In a typical reinforced concrete beam, rebar is concentrated in the lower section—where tensile forces are greatest—to increase resistance to bending.
Thanks to this structure, concrete effectively handles compressive forces and rebar effectively handles tensile forces, working together as a single structural unit. This principle is at the heart of modern reinforced concrete structures.
Today, countless high-rise buildings around the world actively utilize this reinforced concrete technology. However, in the case of super-tall buildings, it is common to use not only reinforced concrete but also steel structures, steel-reinforced concrete (SRC), and composite structures. Selecting the most suitable structural system based on a building’s height and purpose is the standard practice in modern architecture.
Technology that overcame tensile forces changed the history of architecture
The Lighthouse of Pharos was once a marvelous structure that transcended the limits of science and technology at the time. However, from a modern perspective, its height may no longer seem particularly remarkable. This is not because the Lighthouse of Pharos has lost its value, but because architectural technology has advanced so dramatically.
Ancient architecture achieved remarkable feats by utilizing stone with excellent compressive strength, but it was inevitably limited in height because it could not overcome the limitations of tensile strength. Later, with the advent of reinforced concrete and various composite structural technologies, these limitations were overcome, and today, super-tall buildings exceeding hundreds of meters and approaching 1 km in height have become a reality.
Ultimately, the advancement of architectural technology did not stop at simply constructing taller buildings. Understanding and overcoming the invisible “tensile force” is arguably one of the most important changes that made modern architecture possible.
