When the Millennium Bridge opened this past June it was expected to be a source of pride for the city of London, offering a pedestrian link across the Thames River and spectacular views of the city. After all, the picturesque single-arch footbridge was the first new river crossing in London in over a century. Instead, the structure had to be closed within days of opening due to excessive swaying, behavior its engineers curiously attributed to synchronized walking.
While its European designers claim that this pedestrian effect was unpredictable, several U.S. engineers have indicated that the problem is well documented. In an interview with Failure, Mark Ketchum, of San Francisco-based design firm OPAC, who engineered the similarly-designed (but non-pedestrian) Arroyo Cangrejillo bridge in Argentina, noted, the problems exhibited on the Millennium Bridge are source-similar to the problems that plagued a footbridge connecting the BART [Bay Area Rapid Transit] station to the football stadium in Oakland, California about 25 years ago.
In a nutshell, the Millennium bridge has the same vibration frequency laterally as it does vertically, so when the structure moves up and down that energy bleeds over, leading to a side-to-side motion. The Millennium has got problems, says Ketchum. They are going to have a hard time retrofitting their way out of that one.