Welcome to the Wobbly Bridge

Opened in 2000, the Millennium Bridge in London was first conceived in response to an international call to action by the Financial Times in 1996. The competition, held in cooperation with the London Borough of Southwark and the Royal Institute of British Architects, solicited designs for a footbridge that would allow pedestrians to cross the Thames River.

The endeavor was to be a monument to modern engineering techniques and technologies, while serving as a part of the surrounding environment that offered gorgeous views for all pedestrians. It was to celebrate the future while honoring the past.

With that idea in mind, the winning design, spear-headed by engineering and design firm Arup, placed support cables underneath the bridge, allowing unobstructed views of London. Transverse arms grip these cables and stainless steel balustrades were used to provide railing. The deck is four meters wide and constructed of aluminum.

Construction on the bridge began in 1998 and finished two years later. Much excitement surrounded the opening of the walkway and so it was no surprise when a large crowd of around 100,000 gathered to celebrate its first day of use. Engineers stated that the working load limit* of the bridge was 5,000 people. Given the way that number is calculated, it’s possible that the bridge could have supported 20,000 or 30,000 walkers without breaking.

So while the crowd was expected, what happened next was not. As the Londoners began crossing the bridge, it began to sway. With structures such as these, sway is normal, but not to the extent that the Millennium Bridge swayed. On June 12, 2000, two days after opening, the bridge was shut down for investigation.

So what caused such a problem? Well, it turns out that engineers had overlooked the effects of the human walking motion. When a person steps, a vertical force of approximately 55 pounds is applied to the walking surface, in this case, the bridge. There is also a smaller horizontal force of approximately 5.5 pounds applied, attributed to the spacing of our legs.

The physics of the individual walking motion were multiplied by people’s subconscious tendency to synchronize. As the small, almost undetectable sways were created by the first walkers, subsequent walkers adjusted their stepping motions to the sway, exaggerating the effect.

After careful study and consideration of solutions, Arup engineers decided to install a passive damping system. They installed viscous dampers under the bridge, which serve to correct to the horizontal forces on the bridge, and tuned mass dampers, which serve to absorb the vertical.

Almost two years after first opening and (depending on the currency conversion rate used) more than $7 million in upgrades later, the new force-absorbent Millennium Bridge was reopened for use on the February 22, 2002. No problems have been reported. The bridge is still referred to by Londoners as “The Wobbly Bridge.”

People are often cited for the uniqueness of their personalities: Everyone’s different. But what should not be overlooked is the fact as humans, we are more similar than different. Whether it’s a conscious desire or not, we have a natural tendency to be drawn to a larger group. Those Londoners crossing the Millennium Bridge all arrived that day with unique histories, personalities, and beliefs, but yet, as the natural swaying began, they all fell in line. To varying degrees and with varying consequences, the forces cast upon us by social groups, traditions, fears, and other pressures cause us to react similarly. Those forces create very homogenous lives for most of us. We drop our kids off at daycare, we go to work, we come home, and we attend school functions. We build similar houses, we like similar vehicles, and our clothes all look the same. No one mandated such behavior, it’s just the natural sway with which we synchronize our steps.

Maybe our journey through life is just that: a long walk across a wobbly bridge.

*Working load limit (WLL) is a calculation that represents a fraction of the minimum breaking limit (MBL) of a structure, tool, or product. For example, a ladder might be built with a 200 pound working load limit, but might not reach failure until 600 pounds, its minimum breaking limit. The ratio between MBL and WLL is referred to as the safety factor. The safety factor is typically higher for applications that have a higher risk of failure, such as a space shuttle or bridge. In the ladder example, the safety factor was 3.

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