In this blog post, we’ll examine the relationship between capacity and demand—the fundamental principle for assessing a building’s structural safety—as well as the significance of safety factors used in structural design.
Buildings must be sturdy. If a building isn’t strong enough, an accident can lead to catastrophic consequences. Even decades later, the Sampoong Department Store collapse is still cited as a prime example of shoddy construction and is recorded as one of the building collapse accidents with the highest number of casualties in South Korea’s modern history. Furthermore, following the September 11, 2001, terrorist attacks, various studies were conducted on the structural safety and collapse mechanisms of the Twin Towers. It is clear that as buildings become larger, taller, and capable of accommodating more people, they must be built to be stronger and safer. So, just how strong must a building be?
In architecture, the concepts of “capacity” and “demand” are used. Capacity refers to the load a structure can withstand, while demand refers to the load expected to act on the structure. A structure’s capacity must always be greater than or equal to its demand.
Capacity ≥ Demand
The passenger capacity and load limit displayed inside an elevator are classic examples that make it easy to understand the concept of demand. When you exceed the passenger capacity and the doors barely close, you might find yourself thinking, “Is this elevator going to fall?” However, because elevators are designed to move vertically while suspended by cables and carrying passengers, they are engineered with a very high safety margin. Generally, elevator wire ropes are designed to withstand loads far greater than their rated capacity, and sufficient safety factors are incorporated to meet relevant safety standards. Therefore, the structure is not designed such that the cables would snap immediately just because the passenger capacity is slightly exceeded.
So, by how much should the capacity of a typical building exceed the required load? Of course, the larger the margin, the safer the structure can be. However, designing a structure to withstand a larger load unconditionally is not necessarily the best approach. To make columns sturdier, one must increase their size or number, which reduces usable space and increases construction costs. Above all, the weight of the structural framework itself also increases. Concrete, the most widely used structural material in modern architecture, has very high compressive strength but is also quite heavy. If this dead load is taken into account, a building could end up with columns as densely packed as those of the Parthenon or thick walls like those of a fortress. However, such spaces struggle to meet the spatial efficiency and functionality required by modern architecture. Therefore, architecture must consider not only safety but also spatial efficiency and economy, and standards are needed to appropriately balance these conflicting factors.
Structural calculations generally use two safety factors and one inequality. The two safety factors are the load factor, applied to the design load, and the strength reduction factor, applied to the design capacity.
First, the load factor is used to ensure that the design load is greater than the actual expected load. After calculating the expected load based on the building’s structure and use, a load factor greater than 1 is multiplied by this value to ensure a sufficient safety margin. Loads acting on a building are classified into dead loads (D), which remain nearly constant—such as the building’s own weight—and live loads (L), which fluctuate—such as people, furniture, or cargo. In typical limit-state design, a standard load combination widely used applies a load factor of 1.2 to dead loads and 1.6 to live loads.
Demand = 1.2D + 1.6L
Strength reduction factors, on the other hand, are applied to conservatively evaluate a structure’s capacity. In typical reinforced concrete structures, different strength reduction factors are applied depending on the type of member and the mode of failure; for members dominated by bending, a factor of 0.9 is generally applied. For example, if a structure is designed to withstand a force (F) of 100, the structural calculation multiplies this by 0.9 to evaluate its capacity as 90. This method ensures a safety margin by accounting for construction errors, variations in material quality, and various unforeseen variables.
Capacity = 0.9F
Finally, a design is considered structurally safe only when the structure’s capacity is greater than or equal to the required load.
0.9F ≥ 1.2D + 1.6L
By following this design procedure, the structure generally achieves a safety margin sufficient to withstand the expected service loads. However, this cannot be simply expressed as, for example, “a 10-story building can be added on top of a 20-story building and it will still hold.” The actual safety margin of a building varies depending on the structural system, load conditions, material properties, and design codes, and is ensured through a comprehensive review of various load combinations and safety factors.
In addition, various safety techniques are being researched and applied to actual buildings in structural design. These include designs that ensure the structure exhibits warning signs—such as deformation or cracks—before collapse when a failure occurs; fire-resistant designs that maintain structural performance for a specified period during a fire; and ductile designs that prevent sudden collapse during earthquakes or under extreme loads, thereby ensuring sufficient evacuation time. However, the fundamental principle of structural design remains constant. First, the design load is conservatively overestimated using a load factor, and the structural capacity is conservatively underestimated using a strength reduction factor. Then, within the range where the inequality “capacity is greater than or equal to the design load” is satisfied, the optimal structure is designed by comprehensively considering the building’s use, economic feasibility, and constructability. Structural design that faithfully follows this process can be considered the fundamental principle for creating safe and rational buildings.
