In this blog post, we’ll explore what engineering beauty is by examining the engineering principles and structural characteristics of truss bridges.
What is engineering beauty? When we think of beauty, we often imagine the aesthetic value found in landscapes such as natural scenery, parks, or beautiful buildings. However, beauty exists in far more diverse places around us. Have you ever visited Hangang Park? The scenery along the Han River is well-known for its beauty, and the musical fountain show at Banpo Bridge, in particular, offers a unique spectacle for many people. This kind of beauty can be described as scenic beauty.
So, what is engineering beauty? Let’s consider the Han River Steel Bridge. Unlike typical bridges, it consists of a complex steel frame structure, which might initially give the impression of being somewhat rigid and rough. However, it is precisely within that structure that engineering beauty lies hidden. Let’s now examine where that beauty comes from.
A steel frame structure consisting of a continuous series of triangles is called a truss structure, and a bridge built using this design is known as a truss bridge. While it may appear simple and rough at first glance, it is both very lightweight and exceptionally strong, making it widely used in various bridges and structures even today. In particular, because they can effectively withstand heavy loads, they are used as a key structural form not only in general automobile bridges but also in heavy-load bridges such as railway bridges. So, what principle allows truss bridges to withstand such heavy loads?
First, let’s examine the principle by which a truss structure supports loads. A truss is a structure formed by connecting multiple steel members to create triangles; each of these steel members is called a “member.” Additionally, the wide section over the columns where vehicles or trains pass is called the “deck”—it’s easy to understand if you think of it as the part where we actually walk or where vehicles travel.
The point where the ends of two members connect is called a “joint.” In a truss structure, since these joints are connected to the deck, when a load is applied to the deck, the force is transmitted to each member through the joints. In this process, each member experiences almost no bending force—the force that causes bending—and instead primarily experiences tensile force pulling from both ends or compressive force pushing from both ends toward the center.
This is because each member is connected to the others in a triangular configuration. The triangle is one of the most stable geometric shapes, structurally resistant to deformation. Therefore, even when a load is applied, each member shares the tensile and compressive forces rather than bending.
Let’s consider an everyday example. It is relatively easy to bend and break a wooden chopstick by hand, but it is much harder to break it by pulling on both ends or compressing it straight. As this example shows, even with the same material, the amount of force it can withstand varies greatly depending on the direction of the applied force. Truss structures are designed using precisely this principle to utilize the same material more efficiently while withstanding much heavier loads.
Like conventional bridges, truss bridges consist of a deck resting on piers, but they are characterized by the presence of truss structures installed on both sides or beneath the deck. So how can these truss bridges stably support extremely heavy loads, such as those from subways or trains? The secret lies in the structure of the beams installed beneath the deck and the internal structure of the trusses.
The beams referred to here are structural members installed lengthwise, much like the main beams in traditional houses, and they play a crucial role in supporting the load. If you look beneath the deck, you’ll see that between the deck and the columns—starting from the top—vertical beams, horizontal beams (or cross bracing), and, at the very bottom, the main structural elements are all interconnected in an organic manner.
Let’s take a moment to recall our experiences in gym class back in school. You’ve probably tried forming a circle with four people, resting your heads on each other’s knees and leaning against one another to stay upright. This is an activity where you maintain balance by supporting each other’s bodies without relying on any additional equipment. In this scenario, the weight isn’t borne by just one or two people; all four people share the load and support each other. Truss bridges utilize a similar principle.
When a subway or train passes over the deck, the load is first transferred to the vertical members, which then transfer the load to the horizontal members (or cross-braces). The horizontal members then transfer the load to the main structure, and the main structure transfers the load to the truss structure connected to the deck. The truss structure, in turn, distributes the force to the deck and each member, ensuring that all members bear the load collectively. In this way, truss bridges are designed to utilize the principle of repeatedly distributing the load along multiple structural members, preventing stress from concentrating on any single part. Consequently, because each member shares the load with the others, the structure can stably support very heavy loads despite being relatively lightweight.
Furthermore, a truss is a structure capable of effectively resisting forces acting from multiple directions internally. Unlike typical road bridges, railway bridges must adequately account for not only vertical loads but also horizontal loads. For example, when a train or light rail passes over only one track, or during braking and acceleration, horizontal forces act on the structure, causing the bridge to sway from side to side.
A basic rectangular structure can easily deform when subjected to horizontal forces. In contrast, a truss structure effectively suppresses such deformation through its internal diagonal members. Since a structure composed of triangles as its basic units does not easily change shape, it can stably distribute forces transmitted from various directions. For this reason, truss structures exhibit excellent resistance not only to vertical loads but also to horizontal loads.
Therefore, when constructing a railway bridge that must repeatedly bear very heavy loads, such as those from trains, it is a far more rational and economical design to use a truss structure—which is lightweight, efficiently distributes loads, and can withstand forces acting from various directions—rather than simply making the deck thicker.
So far, we have examined how truss bridges can support the weight of subways and trains, focusing on engineering principles. The essence of engineering lies in designing structures efficiently to achieve maximum performance while using minimal materials. If you look at the Han River Railway Bridge—built on this engineering philosophy—with fresh eyes, you will appreciate its value in a whole new light. This is the true beauty of engineering.
If you ever visit Hangang Park, it’s certainly nice to admire the beautiful scenery from Banpo Bridge, but why not also take a moment to look at a truss bridge like the Hangang Railway Bridge and appreciate the structural principles and engineering beauty it embodies?
