Damper Retrofit

Damper Retrofit of the London Millennium Footbridge -- A Case Study in Biodynamic Design

DAMPER RETROFIT OF THE LONDON MILLENNIUM FOOTBRIDGE – A CASE STUDY IN BIODYNAMIC DESIGN Douglas P. Taylor Taylor Devices, Inc. 90 Taylor Drive North Tonawanda, NY 14120-0748 716-694-0800 ABSTRACT The Millennium Footbridge was opened to the public on June 10, 2000 – the first new bridge across the River Thames in historic London in more than a century. Nearly 100,000 people used the new bridge in its first day of operation. On June 12, 2000, the Millennium Bridge was ordered closed, due to hazardous deck motions. Seemingly random pedestrian footfalls were causing resonance of the bridge deck, with lateral accelerations measuring up to 0.25 g. The selected method of retrofit was to add fluid damping to the bridge – and test the structure with groups of up to 2,000 people. INTRODUCTION The London Millennium footbridge is sited on the River Thames in London, United Kingdom, between St. Peter’s Hill and St. Paul’s Cathedral on the north bank of the river, and the Borough of Southwark with the nearby Globe Theater and Tate Modern Art Museum on the South.  The Millennium Bridge is the first new bridge across the Thames in London in more than a century, and was the result of an intense competitive bid process, with more than 200 competing design entries.  Each team consisted of an architect, an engineer, and an artist.  The winning team was Foster and Partners (architects), ARUP (engineers), and sculptor Sir Anthony Caro. As with any modern construction in a historic area, considerations were expected in the final bridge design to accept the latest design codes and local design ordinances, while preserving and protecting the historic context of the site.  In this case, the bridge design constraints included a maximum height limitation, so that tourists would be provided an unobstructed view of the area.  An additional constraint was the requirement for the bridge design to allow adequate clearance for marine traffic on the River Thames.  When these two constraints were applied, only a very small vertical window remained for construction of the bridge itself. The bridge design team elected to use lateral suspension cables, where the cables are located at the level of the bridge deck.  Two piers are located in the river, with a main span of 144 m between piers, and end spans of 81 m on the north and 108 m on the south.  The bridge deck is 4 m wide, and uses articulated sliding joints spaced at regular intervals along its length.  The architectural design theme for the Millennium Bridge is that of a “Blade of Light”; expressed and exemplified by the slender, ribbon-like cross section of the structure.  A photograph of the bridge is provided as Figure 1. BRIDGE OPENING – JUNE 10, 2000 The Millennium Bridge was officially opened to the public on June 10, 2000, and immediate problems were noted. Maximum pedestrian loads of 2,000 people filled the entire bridge deck to capacity, with a resulting loading density of approximately 1.5 people per square meter.  Under these conditions, the bridge exhibited severe lateral sway in a frequency band of 0.5 to 1.1 Hz, with lateral accelerations of up to 0.25 g.  As many as five separate structural modes were being excited, and pedestrians found it virtually impossible to walk on the bridge.  Many held on to deck handrails

for support.  On June 11, the number of people allowed on the bridge at one time was reduced, but the lateral shaking periodically reoccurred.  On June 12, 2000, the bridge was closed and an extensive analysis and study of the vibration phenomena began.  Dallard, Fitzpatrick et al (2001) [1] report on the bridge design and the subsequent extensive research that transpired after the bridge was closed.  The severity of the problem was exacerbated by the ever-prolific media, with press headlines such as these: “Wobbling Bridge Will Stay Shut”  – BBC NEWS “£2 Million to Fix the Wibbly Wobbly Way and it Won’t Open Until the Spring” – DAILY PRESS “The Sleek New Footbridge Across the Thames Swayed and Wobbled in the Wind so Much that Some Feared London’s Bridge was Falling Down” – WASHINGTON POST THE PROBLEM The phenomena of forced harmonic excitation of bridge structures is well understood, and has been well documented by many sources.  Most military manuals dating back well into the 1800’s have warnings about soldiers marching in step over bridges of any type.  This was true even for substantial bridges.  For example, in 1860 the famous 1854 Roebling two-deck railway suspension bridge across the Niagara River at Niagara Falls, NY USA was posted with a warning notice to pedestrians against walking in-step.  This heavily built, record-setting span is depicted in Figure 2, from an 1850's engraving.  The warning notice, reproduced from period photographs, is shown in Figure 3. FIGURE 1 THE MILLENNIUM BRIDGE

D.L. Glover FIGURE 2 JOHN ROEBLING'S RAILWAY SUSPENSION BRIDGE AT NIAGARA FALLS, NY – 1854 FIGURE 3 WARNING NOTICE RAILWAY SUSPENSION BRIDGE AT NIAGARA FALLS, NY USA , CIRCA 1860 What was unique about the Millennium Bridge was that resonance was occurring without any expected forced motion or marching.  Indeed, the pedestrian motion appeared to be purely random in nature.  After extensive review of available video footage during the period the bridge was open, a series of tests were performed to study the observed pedestrian- induced motion.  These identified a unique biodynamic feedback phenomena, later called “synchronous lateral footfall,” which  resulted  in  seemingly  random  walking  motions  becoming  synchronized  over  time  among  members  of  an unrelated group of people on the bridge. In essence, when groups of more than 200 people were on the bridge, the loadings induced by their footfalls were indeed random, up until the point when a significant number of the people would, by pure chance, step in unison.  This would produce a tiny, but still perceptible lateral motion at the first lateral mode frequency of the bridge.  Depending on group size and location on the bridge, this first lateral mode was in the range of 0.5 Hz to 1.0 Hz.  Frequencies in this bandwidth are coincident with a normal walking pace, and the bridge structure would respond at the same frequency, thus providing positive feedback to the pedestrians.  This positive feedback would cause other group members to also begin walking in phase with the motion, providing an amplified input to the bridge structure, with the resultant amplified feedback.  Since the bridge structure was essentially undamped, the amplification would continue until a large number