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Solid ground for pedestrians on footbridges

Bringing walkers into the urban transport fold, 'YEAR 2010' award winner Sifiso Nhleko has developed an innovative and inexpensive method for testing the structural integrity of footbridges before they are built.

The Millennium Footbridge
London's spectacular Millennium Footbridge
© Peter Gutierrez

When London's spectacular Millennium Footbridge was unveiled in 2000, participants in the opening event felt an unexpected and, for some, uncomfortable movement underfoot. Much to the embarrassment of designers, tests showed the motion was caused by a 'positive feedback' phenomenon: small sideways oscillations caused walkers on the bridge to sway in step, which in turn increased the oscillations. The bridge eventually had to be closed for almost two years for modifications.

"Footbridges form an important part of a transport network," says PhD student Sifiso Nhleko, "where design is governed by pedestrian traffic demand, aesthetic value and sustainable use of materials. The 'lateral walking force coefficient', first identified a decade ago in connection with the Millennium Footbridge, is now a major factor in the modern bridge construction process.

"However," he adds, "expensive field tests are still required to determine this parameter on the actual structure after it has already been built. Furthermore, the use of human subjects to induce 'deliberate' structural vibrations is extremely unsafe."

Sifiso Nhleko holds a BSc (honours) degree in civil engineering and an MSc degree in structural engineering, both received from the University of Cape Town. He is now working on his PhD in structural dynamics at the University of Oxford. His paper on 'Lateral excitation mechanism of slender structures induced by crowds' won the first place ' YEAR 2010external link ' award for 'Design and production of vehicles and infrastructure'.

An innovative approach

Sifiso Nhleko
Sifiso Nhleko
© Peter Gutierrez

Nhleko has been exploring the relationship between the lateral walking force coefficient and 'pedestrian gait parameters' – limb movement patterns used to achieve human walking locomotion. This has led to a new technique for predicting the lateral walking force coefficient of large virtual pedestrian crowds.

"We are using a novel and inexpensive method that involves a typical subject walking on an aluminium foil sheet," Nhleko says, "with a portable circuit designed to be switched on and off by the subject's walking mechanism."

The YEAR competition is funded by the European Commission and coordinated by University College Dublin's Urban Institute Ireland. Other partners included FEHRL, ZAG and Continental Automotive. Organisers say Nhleko's work promises to change the way footbridges are designed and tested.

For more on the YEAR 2010 competition and its winners, click  hereexternal link.