"London Bridge is falling down," repeats the well-known nursery rhyme. Indeed, that famous bridge over England's Thames River has been rebuilt several times over the centuries to correct engineering deficiencies and to support increased traffic. What's the newest bridge over the river, and why has it recently been declared unstable?

The Millennium Bridge across London's Thames River is the first new river crossing constructed since the Tower Bridge was opened in 1894. The new bridge is a pedestrian bridge, on which no vehicles are allowed. In order not to block the view of nearby St. Paul's Cathedral, this suspension bridge was given a unique design.

The suspension cables fan out sideways from the concrete and steel piers, instead of upward as is common with suspension bridges like the Golden Gate Bridge in San Francisco, California. The bridge opened in June 2000. It was closed just days later due to severe swaying. The combination of wind and heavy foot traffic made the bridge sway side-to-side by as much as 20 cm (almost 8 inches), sending pedestrians grabbing for the handrails.

Although there is no danger of the bridge collapsing, it remains closed. Engineers are currently preparing a report that will help determine the future of the Millennium Bridge.

• Students can read about the closing of the bridge at ABCnews.com's "London Closes New $30 Million Bridge Because It Sways." This article includes a video news report that shows the bridge swaying. (Requires RealPlayer. Download now.)
  
  

• Students can get general information about the bridge, including engineering drawings and preliminary findings from the engineering investigations into the extreme swaying of the bridge.

Teaching the Problem

Bridges have a "natural frequency." When the wind blows or people cross the bridge at a rhythm that matches this frequency, the force can cause the bridge to vibrate. This phenomenon is called resonance, and the frequency is called resonant frequency. Soldiers are taught to march across a bridge out-of-step, so they won't create vibrations that tap into the bridge's resonant frequency. In extreme cases, the vibrations can cause a bridge to collapse, as happened when the driving force of the wind caused the collapse of the Tacoma Narrows Bridge in Washington State in 1940.

Students can learn more about resonant frequency using the following sequence of Physics Explorer activities:

Analyzing the Problem

When an oscillating system is driven at one of its resonant frequencies, energy is transferred into the system, causing increased amplitude of the vibrations. An object will not resonate if its system dissipates (releases) energy faster than energy is added.

• Have students define "damping" and "frictional force."
  
  

• Ask them to use these concepts to suggest engineering solutions to stop the swaying of the Millennium Bridge.

1. Students can read about various cases in which resonant frequency is an important factor:
   
   
   

   • Baseball--The Game and Beyond: Stinging and Broken Bats: A look at how resonant frequency causes bats to "sting" a batter's hands and how it can snap the bat altogether.
     
     

   • Shattering a Glass with Sound: A college student's demonstration of how to use resonant frequency to break a glass. (Requires QuickTime.)
     

   
   
   

2. Students can increase their understanding of resonant frequency at the following sites:
   
   

   • Exploratorium's Resonator: A hands-on experiment using wooden dowels to build a resonator and experience resonant frequency. Instructions are provided.
     
     

   • School Science's The Science of Audio Systems: Speaker Systems: Part of a larger educational unit in which students combine actual demonstrations with online information and questions.

   
   
   

3. Engineering bridges is no simple project. Students can explore some of the special problems facing civil engineers at the following sites:
   
   

   • NOVA Online's Super Bridge: An interactive multimedia site (companion to a television broadcast) presenting four major types of bridges and giving students the chance to match bridge types to typical locations. The site includes a teacher's guide.
     
     

   • How Bridges Work: An educational look at beam bridges, arch bridges, and suspension bridges.
     
     

   • Newton's Apple: Bridges: A lesson plan for having students design and build a model bridge.
     
     

   • Bridge Research: Leading the Way to the Future: A government report about the state of the nation's bridges, and the role and value of research as part of the nation's investment in highway bridges.