In this blog post, we’ll explore the role and principles of bridge bearings—the key components responsible for bridge safety—and examine their importance through real-life accident cases.
The Banghwa Bridge Accident and the Importance of Bridge Bearings
On July 30, 2013, in South Korea, the collapse of the deck on the Banghwa Bridge resulted in the deaths of two workers and injuries to one other while the bridge was under construction. At the time, the media summarized the cause of the accident as errors in design and construction, but there was a significant issue related to the bridge bearings. So, what was the specific cause of this collapse? The answer to this question is directly related to the bridge bearings, which I will now explain.
First, what are bridge bearings? Bridge bearings are devices that connect the bridge deck to the supporting piers. To make this easier to understand, think of LEGO blocks. Each LEGO block has a round protrusion designed for assembly. Thanks to this feature, the blocks can be firmly connected to one another. If this protrusion were missing, the blocks would easily collapse under external impact. In real bridges as well, a device is needed to connect the deck to the piers to ensure safety, and that is precisely the role of the bridge bearing.
What role do bridge bearings play?
The functions of bridge bearings can be broadly divided into load transfer and absorption. The load transfer function transmits the load applied to the bridge deck to the piers below, while the absorption function accommodates changes in the bridge’s length caused by temperature fluctuations between day and night, as well as rotations caused by the bridge’s deflection. Just as a balloon expands in volume when placed in hot water, a bridge also expands in length—albeit very slightly—when the temperature rises. Among the various functions of the bridge bearings, the absorption function plays a particularly important role in maintaining the bridge’s safety. So, what principle do bridge bearings use to ensure the bridge’s safety? To understand this, let’s first examine the cause of the Banghwa Bridge accident mentioned earlier.
When we lie down on a bed, the mattress sags under the load of our weight. Similarly, bridges also experience deflection caused by loads. While a person’s weight acts as the load on a mattress, various factors combine to create loads on a bridge. The bridge’s own weight—its dead load—can act as a load, as can the weight of vehicles passing over the deck; wind, snow, and earthquakes also act as loads. When a load acts between bridge piers, the bridge deck bends downward, causing the deck near the piers to lift upward. In the Banghwa Bridge accident, the load from the crane caused the deck near the piers to lift, ultimately leading to the deck’s collapse. The bridge bearings were supposed to absorb the rotational forces caused by the bending deck, but due to a design error, an insufficient number of bearings were installed, preventing them from stopping the deck’s collapse. Consequently, the shortage of bridge bearings was one of the primary causes of the accident.
How do bridge bearings work?
Let’s now examine the principle behind bridge bearings. The core principle of bridge bearings is to absorb changes in the bridge’s length and rotation. First, let’s look at the principle of absorbing length changes. Bridge bearings are structurally connected so that they can be coupled to the deck, and rollers are installed at the points where they connect to the lower piers. Therefore, when the deck’s length changes, the rollers move along with it, causing the bearing assembly to shift as well; as a result, changes in the deck’s length can be naturally absorbed. If there were no bearing assembly, the deck would attempt to change its length but would be unable to absorb this change, leading to the generation of significant internal stress and, ultimately, the formation of cracks.
Next, let’s examine the principle of rotation absorption. When a load acts on the bridge and causes deflection, the deck near the piers lifts upward, as explained earlier. At this point, frictional force is generated within the bearing assembly connected to the deck. Frictional force can be understood as a force that resists the movement of an object; in this case, it suppresses the rotational movement of the deck, thereby absorbing rotation. In the case of the Banghwa Bridge accident, if the bearings had been properly installed, they would have been able to effectively absorb the changes in the deck’s length and rotation, and the accident that led to the deck’s collapse could have been prevented.
Bearings are the joints of a bridge
Bearings can be likened to human joints in that they connect the deck to the substructure and absorb various changes caused by external factors. If humans did not have joints, they would be unable to bend their bodies or absorb the impact generated while walking or running. The same is true for bridges. Without bearings, changes in the deck’s length or rotation cannot be absorbed, which could lead to cracks or even the collapse of the deck. Just as joints are essential for human safety, bearings are also critical components that are indispensable for the safety of bridges.
