Bridges Appearance Preservation - Essay Example

Millennium Bridge was designed to carry a large load and a degree of structural movement was expected and taken into account. However, after about 80 000 people crossed it on the opening day, some vibration was detected. The bridge began to sway sideways noticeably, and the movement became so strong that people could not walk steadily anymore; many had to cling to the sides of the bridge to maintain balance.

The phenomenon of bridge sideways movement is not unique to the Millennium Bridge only. There were other structures, completely different from the given bridge that, to various degrees, suffered the same effect. However, those cases have not been widely publicized, thus the phenomenon, known as Synchronous Lateral Excitation, was not anticipated and has not been given enough attention by bridge engineers.

When people walk they have a natural sway motion. This very motion causes small regular vibrations, which, as the result of chance correlation, generated slight lateral movement of the bridge. When that happened, pedestrians instinctively adjusted and synchronized their motion with the bridge’s movement to counteract the effect and to walk more comfortably. In addition, people locked their motions together, walking in step in the same rhythm, thus creating an even greater magnitude of force, which only reinforced the oscillation of the bridge. The central span of the bridge had the most movement, with deck vibration amplitude reaching as high as 70 mm.

As an immediate solution, the number of people crossing the bridge simultaneously was limited, which helped the problem somewhat. However, to prevent any possible physical injuries of the pedestrians as well as for case investigation purposes the Millennium Bridge was closed on June 12, 2000. Thorough research included laboratory investigations with the following series of crowd tests on the site to confirm the findings of the construction engineers. On-site, tests were needed to recreate exact walking conditions as well, and later tests were carried out to confirm the viability of the solution found. The bridge was equipped with measuring instruments to document the intensity of movement. The exact timing of steps was measured by the sensors fitted in the heels of the experimental crowd.

 Two possible solutions for limiting excitation were considered: either the structure has to be stiffened in order for the bridge and footsteps’ frequency not totally, or a damping system has to be installed to absorb the vibration created.

The professionals came to the conclusion that the option of stiffening the aluminum structure of the bridge would not be possible without dramatic changes in the sleek appearance of the bridge. So the option of dampers installation was further explored.  Jones (2005) explains that an active damping system, which is comprised of powered mechanisms, is commonly used in various engineering fields and also in buildings, especially in seismic active zones of the world. However, it is not sufficiently expanded for complex systems such as bridges. Exploring the possibility of active damping installation, engineers came to the conclusion that because of the production cost and timing as well as maintenance requirements, this system will not provide a viable solution to the problem. Instead, a passive damping system, with Viscous damping mechanisms installed beneath the deck, around the piers, and the south landing will provide needed control of the sideways motions. The passive damp system functions similarly to shock absorbers, which purpose is to limit the structure’s response to the external force. In addition to viscous mechanisms, Tuned Mass dampers were attached to the discreet points under the structure and tuned to a specific frequency as a precaution for the possible vertical movement of the bridge. One major benefit of this system is that most of the damping mechanisms are installed under the structure or mounted into it, so they generally are hidden from view and the appearance of the bridge is unchanged.

The research and test results provided engineers with valuable information, on the base of which a solution suggested successfully combined the bridge’s appearance preservation and improved walking conditions. It also gave much new information for practicing bridge engineers, which was previously not explored.