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Much like high-rise buildings and skyscrapers, bridges can also be susceptible to wind-induced vibrations, which poses a significant risk to the safety and overall structural integrity of the bridge. While damping solutions are typically considered as part of the final bridge design, adding a temporary damping solution to control bridge motion during construction should also be considered.
Bridges can be particularly vulnerable to wind-induced motion during this phase. Bridges with arches are particularly susceptible to wind-induced motion during construction when the arch is partially built, but not closed.
It is vital to understand why bridge arches are more prone to wind-induced motion during their construction and what mitigation techniques can be used to decrease this movement.
Why are bridge arches susceptible to vibration during construction?
During construction, bridge arches have low frequencies and inherent damping, in part because the arches are slim and lightweight. Arches are constructed from the ground up from both ends simultaneously. Until the arch sides are joined at the top, both ends can move, making them flexible and therefore more susceptible to vibration. Bridge arches are also increasing in flexibility and are becoming much lighter due to modern construction methods and materials.
These factors contribute to an increased susceptibility to wind-induced vibrations, which can be too much for the bridge in certain instances, especially during construction when the arches are not yet complete. If the arches move in the wind, especially on days with high winds, a damping system will often be necessary to help reduce the motion and ensure the bridge and bridge workers remain safe throughout the entire construction process.
Motioneering has put this into practice many times, such as with the I-74 Bridge that spans over the Mississippi River.
What are some damping solutions for bridge arch vibration?
Damping is perhaps the most common option for mitigating bridge arch vibrations, including during the construction phase. There are a few different types of dampers available:
Standard tuned mass dampers (TMD): Among the most popular choices, TMDs employ a mass that is situated on springs and will oscillate out of phase with the bridge’s motion and oppose the wind loads applied to the bridge.
Viscous damping devices: These dampers stroke and dissipate energy as the bridge arches move and do not require a tuning range. However, it is necessary to connect the device between two points that would move relative to each other, such as the bridge and the ground, or two parts on the bridge that always experience relative motion. This often requires long distances are necessary to connect the point of attachment on the bridge to the ground.
In-line cable dampers: Vortex-induced oscillations can benefit from an in-line cable damper attached to the arch by a cable and anchored to a firm foundation or TMD. This allows large distances to be spanned but requires the use of a very stiff cable.
The precise dampers used to mitigate vibrations are influenced by the bridge and arch design as well as the local wind conditions.
Damping needs can shift throughout construction. For instance, bridge arches can be at an increased risk of vibrations just before or after they are completed, but before the bridge deck is in place. It is important to adjust and fine-tune damping solutions as the bridge moves through construction, allowing the dampers to achieve optimized vibration mitigation.
Non-damping solutions for bridge arch vibration
While damping solutions are the most common mitigation measure structural engineers take, other methods do exist. For instance, changing the shape of the arches, such as modifying a cross-section shape, can be beneficial to mitigating vibration. These shape changes can also be temporary if the bridge arches in question are only at risk of experiencing increased vibrations during the construction process.
It may also be possible to alter the construction schedule or sequence to minimize the risk of wind-induced motion. For example, if an arch is at risk before the deck is attached, it may be possible to fast-track the attachment of the deck to minimize the exposure time, or ensure the arch is vulnerable during the least windy time of year.
Lastly, temporary supporting structures may be considered to stiffen up the vulnerable structural elements to reduce the risk of wind-induced motion. While these options are conceptually simple, they are often not enough on their own due to the scale of the structures involved. Thus, damping remains a popular mitigation strategy.
How to determine the best bridge damping solution
Damage to bridges caused by vibrations can be expensive and time consuming to fix. Vibration issues that occur even before potential damage can cause negative publicity. Thus, it is extremely important to be proactive about vibration issues, including during the construction phase of a bridge project. Being proactive means planning various damping solutions in advance, which can allow for the significant mitigation of bridge vibrations.
Vibration mitigation, including for arch vibrations throughout construction, requires the consideration of wind, especially early in the design stage. Such considerations mean a trusted engineering consultant, such as one from Motioneering, can conduct wind studies of the bridge at various stages, including during arch construction and after the bridge is complete.
As part of these studies, the consultant will conduct a wind tunnel test, which helps identify potential vibration related issues. Recommendations for damping solutions, including one to mitigate arch vibrations during construction, follow the wind tunnel test. This will help ensure that the bridge remains secure during construction and is ready for optimal performance once commissioned.
Dampers can be designed, produced, and supplied before construction starts, and the consultant can also bring a specialist on-site to monitor, set up, and launch the damper. Finetuning the damper throughout construction is another vital component of the consultative process.
Ultimately, mitigating bridge arch vibrations ensures the security of the arches during construction as well as the safety of the construction workers on the bridge. For more information on bridge vibrations and how damping can help, visit Damping for Bridges, part of our Damping Explained series.
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