Millennium Legacies - Pushing the Limits: New Adventures in Engineering - page 70

London Millennium Bridge, leading to St. Paul's
While it is still a set of drawings on paper or a computer screen, such a complex structure is
routinely checked by engineers for strength, stiffness, and stability through the use of computer
models. The natural frequencies of the structure are determined, and its response to various loading
conditions is checked to be sure that no sympathetic modes of vibration can be expected to be ex-
cited and amplified beyond safe limits. In the course of designing the London Millennium Bridge,
Arup engineers assumed that people using the bridge would exert vertical forces on it but that they
would not be in step. The possibility of pedestrians exerting significant horizontal forces in unison
was not even considered. The assumptions made by the engineers were customary and hardly worth
a second thought. In fact, the form of the bridge seemed much more innovative than its function,
and so its artistic and architectural elements may have gotten more attention than its engineering.
The materials of the Millennium Bridge are aluminum and stainless steel, and it has been fre-
quently described as looking “high-tech.” Its location and the absence of vehicle traffic were expec-
ted to draw four million people annually to use it. Because the Millennium Bridge was the first new
river crossing built in the area since Tower Bridge was completed in 1894—and the first pedestrian-
only bridge—opening day was a much-anticipated event. When the day arrived, on June 10, 2000,
almost one hundred thousand people eventually showed up to admire the bridge and walk across it.
To everyone's surprise, the lightweight bridge deck swayed noticeably side to side. The bridge was
closed within three days, after being used by as many as a quarter of a million pedestrians, and it
remained closed while engineers sought to understand the motion and work out ways to check it.
A bridge moving under the footfalls of people is not a new phenomenon. Many a hiker has ex-
perienced how easy it is to set into motion a light, narrow bridge suspended over a mountain ravine.
Nineteenth-century suspension bridges were known to have been brought down by the cadenced
march of soldiers, and to this day some spans display a sign warning soldiers to break step when
crossing. London's Albert Bridge bears the terse notice “All Ranks Break Step.” As recently as
September 1999, a new pedestrian bridge in Paris exhibited excessive sideways motion under the