In this blog post, we’ll explore how modern buildings ensure safety and usability by accounting for various loads and structural changes.
Buildings are subjected to various types of forces from factors such as the building’s own weight, the weight of its occupants, and the external environment; these are referred to as loads. Every object on Earth is subjected to a vertical force called gravity. This is because the Earth and the objects on it attract each other. This is Newton’s law of universal gravitation, and no object on Earth can escape this gravitational force. Buildings are similarly subject to universal gravitation; the force resulting from this gravity is called a vertical load.
Vertical loads can be broadly divided into two categories: dead load, which results from the weight of the building itself, and live load, which results from people living in or moving around the building, as well as furniture and other items. These two types of loads are the most fundamental forces acting on a building and are the first to be considered during structural design. Accurately calculating these two loads is essential in architectural design, and appropriate countermeasures must be developed through precise analysis and simulation from the earliest stages of design. In addition, buildings are subject to a complex interplay of various other loads, such as wind loads, seismic loads, rain loads, and snow loads. These loads vary depending on regional characteristics, climate, and the building’s intended use, and designers must comprehensively consider them to ensure a safe structure.
Structural engineers combine these various loads appropriately and incorporate them into their designs. When loads act on a building, forces known as shear forces and moments are generated in each major structural member (columns, beams, load-bearing walls, etc.). Shear force is a force acting on an arbitrary plane within a member; it causes two opposing faces of the member’s cross-section to slide past each other. For example, imagine asking a friend to lend you their arm and then applying force with both hands—one pushing up and the other down—as hard as you can. The force your friend’s arm experiences in this situation is called shear force. If that force becomes too great, your friend’s arm will break. This is called shear failure. In buildings, to resist shear forces, the size of structural members is increased, or they are reinforced with stirrups at intervals along their length.
Bending moment is defined as the product of the moment arm—the shortest distance from a reference point to the point where the external force acts—and the magnitude of the external force. It serves as a measure of the degree of bending. Let’s borrow our friend’s arm again for this example. Have your friend stretch out their arm and rest it on the edge of a desk so that their hand hangs off the edge. Now, press firmly on the palm of their hand. At this point, a bending moment is applied to the end of your friend’s arm. If you extend the arm a little further outward to increase the moment arm length, your friend will feel more pain even though the same force is applied. This is because the arm length has increased, thereby increasing the magnitude of the bending moment. Using this principle, structural engineers design buildings so that all structural members can effectively resist the two effects caused by loads: shear force and moment.
However, a building is not perfectly designed simply because it effectively resists shear force and moment. Unlike public art installations on the street, buildings are designed for human use; therefore, usability reviews that consider vibration and deflection are necessary to ensure user comfort. For example, in high-rise buildings, wind-induced vibrations can be a problem. If these vibrations are excessive, people inside the building may experience dizziness or anxiety.
Therefore, structural designs must incorporate measures to minimize such vibrations, and recently, vibration control systems utilizing cutting-edge technology have been developed and implemented.
A long time ago, there was a commotion when a tip was received that a building was about to collapse. It was an incident in which people inside felt strong vibrations, as if an earthquake had struck, and citizens evacuated the building. An investigation revealed that a group of middle-aged women were taking an aerobics class in the building’s gym and were all jumping in unison at a regular rhythm. The frequency of their jumping happened to match the building’s natural frequency, causing the structure to shake due to resonance. Although the incident was ultimately dismissed as a one-off episode, it serves as an excellent example of just how important it is to evaluate a building’s usability. This is because, no matter how sturdily a building is constructed, if even a slight pounding while running causes severe vibrations or the floor sags excessively, the occupants inside will be filled with anxiety and fear. To prevent such problems, usability reviews of buildings must be established as an essential procedure from the design phase onward.
So far, we have briefly examined the factors considered to ensure a building stands securely. Before proceeding with structural design, structural engineers first review the various loads that could affect the building. They then design the structure so that the members’ resistance capacity exceeds the shear forces and moments caused by these loads. Finally, in addition to addressing structural issues, a usability review must be completed to ensure that building occupants can live safely and comfortably inside.
Building collapses result in numerous casualties. Recent building collapses around the world have once again highlighted the severity of this issue. The resulting loss of life and economic damage are immense. Therefore, structural engineers must strive to design buildings that will not collapse by comprehensively considering a wide range of factors. Furthermore, these efforts must extend beyond simply ensuring the safety of buildings to encompass a comprehensive concept that includes the daily convenience and comfort of the people living within them. Only such a thorough approach will serve as the foundation for the design of safe and sustainable buildings.
