Comparative Assessment of Wind and Seismic Actions on Steel Girder Bridges in Low-Seismicity Regions: A Parametric Study Applied to Senegal
This parametric study of steel girder bridges in Dakar, Senegal, reveals that while seismic actions govern force-based demands and longitudinal displacements, wind loads control transverse pier displacements for taller piers, demonstrating that both actions are critical and their dominance depends on the specific engineering demand parameter and pier height.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Bridges are the silent arteries of modern life, carrying people and goods across rivers and valleys, yet they exist in a constant, invisible tug-of-war with the natural world. Two of the most powerful forces engineers must account for are the wind and the ground shaking during an earthquake. In many parts of the world, one of these forces is so dominant that it dictates how a bridge is built, while the other is a minor concern. However, in regions where the ground is relatively stable but the winds are fierce, a difficult question arises: which force actually wins? This is not just a matter of academic curiosity; it is a practical dilemma for engineers designing infrastructure in places like West Africa, where the risk of a major earthquake is low but the coastal winds are strong. If an engineer designs a bridge to withstand the wind but ignores the earthquake, the structure could fail during a rare tremor. Conversely, if they over-design for an earthquake that rarely happens, they might waste resources or, more dangerously, miss the fact that the wind is actually the bigger threat to the bridge's stability.
In a recent study focused on Senegal, researchers set out to solve this specific puzzle for steel girder bridges, a common type of structure used for roads in the region. They asked a straightforward question: for a bridge in Dakar, does the shaking of the earth or the push of the wind determine the final design? To find the answer, they built a series of digital models representing bridges with different span lengths and pier heights, simulating how each would react to the specific wind speeds and earthquake risks found in Senegal. The study revealed a surprising and nuanced truth: the answer depends entirely on what part of the bridge you are looking at and how you measure the stress.
The researchers examined twelve different bridge configurations, varying the length of the bridge spans from 25 to 45 meters and the height of the supporting piers from 5 to 16 meters. They calculated the forces and movements each bridge would experience under the worst-case wind and the worst-case earthquake for the region. The results showed that for the strength of the bridge—specifically the forces trying to snap or bend the base of the piers—the earthquake is always the boss. Even though the ground shaking in Senegal is mild, the seismic forces were consistently stronger than the wind forces in every single model tested. This means that if an engineer were to design the bridge solely based on wind calculations, the piers might be too weak to survive an earthquake, even a moderate one.
However, the story changes completely when looking at how much the top of the bridge moves. In ten out of the twelve bridge models, the wind was the dominant force, pushing the top of the piers further than the earthquake ever could. This is a critical distinction because a bridge can be strong enough not to break but still fail if it sways too much, potentially damaging the road surface or the joints that allow the bridge to expand and contract. The study found that for taller piers, the wind's ability to push the structure sideways grows much faster than the earthquake's ability to shake it. In fact, for bridges with spans of 45 meters, the wind controlled the movement regardless of how tall the piers were. For shorter bridges, the wind only took over once the piers reached a certain height, roughly between 5 and 8 meters.
This finding challenges a common assumption in engineering that a low-risk earthquake zone means earthquakes can be ignored. In the United States, for example, building codes often allow engineers to skip detailed earthquake calculations for areas with very low seismic risk, assuming the wind is the only real threat. The Senegal study suggests that for bridges, this shortcut is dangerous. Even in a region with a low probability of earthquakes, the shaking forces are still strong enough to dictate the strength of the bridge's foundation. At the same time, the wind is strong enough to dictate how much the bridge is allowed to sway.
The researchers concluded that engineers in Senegal, and in similar regions around the world, cannot simply choose one hazard and ignore the other. They must design the bridge to be strong enough to resist the earthquake, while simultaneously ensuring it is stiff enough to resist the wind. The two forces govern different aspects of the design, and neither can be inferred from the other. This means that a bridge that looks perfect against the wind might collapse in an earthquake, and a bridge built to survive an earthquake might sway so violently in the wind that it becomes unusable. The study provides a clear map for designers, showing exactly where the balance shifts based on the height of the piers and the length of the spans, ensuring that the next generation of bridges in West Africa is safe from both the ground beneath and the air above.
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