Civil Engineering Reference
In-Depth Information
on the bridge, thus amplifying the latent effect. The resultant wobble of the walkway caused hun-
dreds of people on the bridge to grasp its handrails to steady themselves.
What happened on opening day could naturally happen again, and so the bridge structure or the
forces to which it could be subjected had to be altered. Limiting the number of people on the bridge
at any one time was ruled out, as was altering their walking patterns by installing “street furniture”
and other obstructions along the walkway. These solutions were considered not in keeping with the
intended free use of the bridge.
If the load exerted on the bridge was not to be controlled, then the bridge structure itself had to
be altered. One way of doing this was to stiffen the bridge structurally. This would involve major
construction work, would be costly and time-consuming, and would alter the appearance of what
was, after all, a piece of architectural sculpture as well as a utilitarian structure.
The third way to prevent unacceptable motion of the bridge was to install mechanical dampers,
which would dissipate energy and limit movement. Two kinds were considered. The first were vis-
cous dampers, which act as shock absorbers, like those installed on the Pont de Normandie. The
second alternative was to use tuned mass dampers, in which a large mass is connected to the struc-
ture by stiff springs. Tuned to the desired frequency, the mass and spring can absorb much of the
energy that would otherwise go into exciting the bridge.
In the end, it was decided that both kinds of dampers would be installed on the Millennium
Bridge in the most unobtrusive way possible, and the deck structure was also to be stiffened against
lateral deflection. Each of these solutions would necessarily alter the appearance of the bridge, but
the alterations would be largely invisible to those walking across it. Everything, however, would be
visible from underneath by the people on boats that cruise up and down the Thames. The strong
rectangular pattern of the underside of the bridge deck would be interrupted by the cross-bracing
used to stiffen the deck, tuned mass dampers would interrupt the slenderness of the cross members,
and the struts of the viscous dampers at the piers would also be visible. The final design for the fix
employed a total of ninety-one dampers of various kinds, but the bridge was not stiffened structur-
ally, so as not to alter its appearance any more than necessary. The work was completed about two
years after the bridge was initially opened, and though a small amount of motion could still be de-
tected by those walking across it, the damping system has kept that to a minimum.
The problems experienced by London millennium projects were embarrassing to British planners
and engineers. However, each of them was clearly pushing the envelope of experience and testing
the limits of human ingenuity in both engineering art and engineering science. It is the intent of
visionary designers to work on the edge of technology, and it is the nature of their creations to awe
others, engineers and nonengineers alike. Not every concept proves to be as exciting when realized;
not every design works as smoothly as conceived. Not every wheel turns as freely, nor does every
bridge stand as steady as it should. But if there never were an embarrassment, could any engineers
honestly say that they had been trying as hard as they might to celebrate the turning of a millennium
or to lift the spirits of the world?
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