In this blog post, we’ll examine the concepts of lateral loads and moment of inertia acting on supertall buildings, and explore how Lotte World Tower resists strong winds and earthquakes through its mega-column structure.
Generally, when constructing a building, design considerations for vertical loads caused by gravity are important. However, for super-tall buildings, structural design to withstand lateral loads—such as those caused by strong winds acting on the upper floors and the building’s great height above ground—is particularly critical. If you imagine a long stick standing on the ground—no matter how well balanced it is, it can easily topple over from even a small impact—you can appreciate just how important and challenging the design for lateral loads in super-tall buildings is.
Currently, the Lotte World Tower, standing 555 meters tall with 123 above-ground floors, has been completed in Jamsil, Seoul. The Lotte World Tower had its grand opening in April 2017 and continues to hold the record as the tallest building in South Korea. One of the primary structural systems applied to the Lotte World Tower is the mega-column structure. A mega-column structure refers to a system in which massive columns are positioned at the corners when viewed from the building’s floor plan. Let’s explore the structural principles behind how these massive columns help the building resist lateral loads.
To understand a building’s behavior under lateral loads, we first need to review the concepts of cantilevers and moments of inertia. A cantilever refers to a part of a building or a beam where only one end of the member is fixed, while the other end is unsupported. A cantilever deflects under its own dead load or external loads, generating a bending moment in the member. The ability to resist this bending is related to the moment of inertia of the member’s cross-section—that is, the cross-section perpendicular to the axial direction. The moment of inertia of a cross-section can be understood as a value indicating how far the area comprising the cross-section is distributed from the reference axis.
As this concept illustrates, for members with the same cross-sectional area, the greater the distance (y) from the reference axis, the more material is concentrated in that region, resulting in a larger moment of inertia. Therefore, to ensure that a cantilever effectively resists lateral loads, more material must be placed toward the outer edges rather than near the central axis.
Now, let’s consider the design of a high-rise building with a typical square column shape. According to the concept explained earlier, a building can be understood as a cantilever supported at its base against wind loads; for this building to effectively resist lateral loads, the members comprising the cross-section must be arranged toward the outer periphery of the building. Here, the cross-section refers to a plane section perpendicular to the building’s length, and the structural members visible in this plane section would be columns. To illustrate this, let’s say 36 columns are needed in a cross-section to resist gravity loads. While the columns could be spaced at regular intervals, this is not an optimal approach for achieving a large moment of inertia. By concentrating the columns along the building’s perimeter, the moment of inertia can be effectively increased while maintaining the same number of columns. This type of structure is known as a “tube” structure. Many early supertall buildings, such as the World Trade Center and the Sears Tower, utilized tube structures to resist lateral loads.
However, a tube structure is not the most efficient method for increasing the moment of inertia of a cross-section. If we consider a lateral load acting in a single direction, we can see that, depending on that direction, some of the columns placed along the perimeter—which are farther from the reference axis—exhibit high efficiency, whereas the columns located in the middle are relatively less efficient. Similarly, when forces act in a different direction, the columns at the corners—which are farther away from that direction—exhibit high efficiency. The megacolumn structure is designed to address this issue by concentrating the corner columns at the building’s corners. This ensures that regardless of whether lateral loads act from above, below, left, or right, the building can efficiently resist them through structural members concentrated along the perimeter and at the corners. In fact, compared to a tube structure, this is a simple concept that is easy to grasp; however, in the past, it was difficult to implement a structure of this form because high-strength concrete and construction technologies for super-tall structures had not yet advanced sufficiently to install mega-columns with such large cross-sections. Lotte World Tower incorporates core walls and eight megacolumns to support vertical gravitational loads, and is designed so that outriggers and belt trusses resist lateral forces caused by wind and earthquakes.
In typical buildings, especially high-rises, corner spaces are highly valuable because they offer views in two directions. However, mega-column structures have the drawback of restricting the use of corner spaces due to the placement of massive columns there. To address this issue, Lotte World Tower divided the columns at the corners, thereby maintaining their structural function while preserving the views.
Lotte World Tower is no longer a building under construction but a completed supertall building that officially opened in April 2017. Standing 555 meters tall with 123 above-ground floors, Lotte World Tower incorporates various supertall structural technologies, including not only mega-columns but also core walls, outriggers, belt trusses, and a diagonal grid structure at the topmost level. In particular, Lotte Construction explains that it implemented wind- and earthquake-resistant designs to ensure Lotte World Tower can withstand strong winds of 80 meters per second and earthquakes of magnitude 9 at a height of 555 meters. Simply stacking a massive structure higher and higher is not enough to complete a supertall building. The key to super-tall construction lies in understanding the vertical and lateral loads acting on the building and determining where and how to arrange structural members to most effectively resist those loads. Lotte World Tower is one of the prime examples of applying these structural principles to an actual building and has established itself as one of South Korea’s premier super-tall landmarks.
