Damper Retrofit Program The London Millennium Pedestrian Bridge. A Lesson in Biodynamics February 2002

Transcription

1 Damper Retrofit Program The London Millennium Pedestrian Bridge A Lesson in Biodynamics February 2002

2 LONDON, ENGLAND

3 LONDON NIGHTSCAPE Millennium Bridge Over the Thames

4 Millennium Bridge Facts: Pedestrian bridge, 1090 ft. length, crosses the Thames River in downtown London, England. 12 ft wide suspension deck with no support towers, uses lateral cables for support. 700-ton steel/aluminum deck, two concrete piers. Unique architecture Award-winning Blade of Light theme Lack of towers provides unrestricted view for residents and tourists.

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6 Millennium Bridge Facts: (con t) World-class design team. Architects: Sir Norman Foster and Partners, with sculptor Sir Anthony Caro. Engineers: Ove Arup and Partners Cost to build: $36 Million. Opened: June 10, 2000 Closed: June 12, 2000 Visitors in 48 hours: 150,000

7 THE PROBLEM Excessive bridge sway with more than1,000 people on the bridge. Problem traced to synchronized footfall of pedestrians stepping or shifting body weights in unison. Felt motion described as similar to a ship rolling in a storm. The induced motion would amplify once started by more and more people shifting their weight as they got their sea legs, producing feedback into the structure. Testing revealed that as few as 500 people stepping in unison were the critical mass to provide an uncontrollable deck response with accelerations of up to.25g, and displacements of up to plus or minus 3 in. at Hz. Up to five separate structural modes were excited.

8 POTENTIAL SOLUTIONS ADDED BRACING: Would destroy architectural assets. ADDING EXTRA PIERS: Would hinder navigation. LIMITING CROWDS: Unacceptable to the City of London. ACTIVE CONTROL WITH HYDRAULIC ACTUATORS: Could only control 1-2 modes. PASSIVE DAMPING Reliability of continuous control power. Cost.

9 1. (37) Viscous dampers were integrated into the bridge deck architecture or hidden under the deck to reduce away. 2. (50) Tuned mass oscillators were added to limit vertical deck deflection under footfalls. PASSIVE DAMPING: Passive damping was selected to fix the bridge.

10 Frequency response 0-5 Hz, amplitudes of inch. Maximum output force 10,000 lbs. Allowable friction = near zero, since any stick-slip of sliding seals would be perceived as an instability by pedestrians. Design life is 50 years at 0.8 Hz = 1.27 billion cycles. DAMPERS REQUIREMENTS

11 FRICTIONLESS HERMETIC DAMPERS Taylor Devices patented Frictionless Hermetic Dampers were selected for the retrofit of the Millennium Bridge: Fluid sealing by flexible welded metal bellows Tested frequency response HZ. All stainless-steel construction. Roller-bearing rod support to prevent starting from metal-metal bushing contact.

12 HERMETIC DAMPER CARTRIDGE

13 HERMETIC DAMPER CARTRIDGE 13

14 MILLENNIUM BRIDGE Frictionless Hermetic Fluid Damper 14

15 MILLENNIUM BRIDGE DAMPERS 15

16 THE MILLENNIUM BRIDGE 16

17 THE MILLENIUM BRIDGE

18 THE MILLENNIUM BRIDGE

19 THE MILLENNIUM BRIDGE

20 Tests With Over TwoThousand PeopleTook Place: Jan 2002: GROUP 1 TESTS: 700 people, average loading density of 1 person/m 2 at various speeds. GROUP 2 TESTS: 2,000 people, average loading density of 1.5 persons/m 2 spanning the entire bridge at various speeds. AN UNSCHEDULED TEST: From January 27 through January 29, 2002, the UK experienced a 10-year return period wind storm. Peak transverse wind gusts at the bridge exceeded 70 mph. January 2002: THE TESTING BEGINS

21 THE MILLENNIUM BRIDGE 21

22 TESTING PROCESS

23 Tests With Over Two Thousand People Took Place: Jan 2002: Tests started at 6 PM, using office workers from buildings near the Bridge. Crowd-holding areas were set up near each end of the Bridge. Food and beverages were available in holding area to retain the crowds along with free drinks and sandwiches at the conclusion of all tests. The entire group was sent across the Bridge four times. The first three crossings were at different walking speeds, continuously monitored by pacers. The 4 th crossing was purely random, with no pacing, to the free refreshments. January 30, 2002: GROUP 2TESTING

24 January 2002: Crowds were led across the Bridge by the Mayor of Southwick and the City of London s Town Crier, both in Victorian garb, representing the communities at either end of the Bridge. Extensive TV and media coverage. Floodlights on dampers and instrumented sections of the Bridge. Numerous British Coat Guard and rescue vessels were on station downstream of the Bridge Just in case! A Festive Atmosphere: GROUP 2TESTING

25 Hilary Wines, Mayor and The Town Crier toll the bell 25

26 The Town Crier and the Chairman of Ove Arup 26

27 Taylor Devices Engineer was confident 27

28 The Project Engineer paces nervously 28

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31 Test Results: Bridge behavior was described by observers as rock solid during all tests. No resonance or amplified responses noted. Compared to the original, undamped bridge: Dynamic response amplitudes reduced by at least 40 to 1 for all modes. Peak measured accelerations reduced from.25 g undamped to.006 g damped. An improvement of 42 to 1! A Festive Atmosphere: TEST RESULTS

32 Excerpts from the London media on January 31, 2002: The bridge was incredibly steady, I couldn t detect any signs of movement at all. BBC News. Whither the Wobble? The London Times. Repair work has been a success. The London Metro. Really solid. The Guardian. Bridge wobble free after ultimate test. The London Evening Standard. It didn t move a quiver. The London Evening Standard. CONCLUSIONS

33 THE NEW, WELL-DAMPED MILLENNIUM BRIDGE 33

34 FROM THE ARCHITECT 34

35 NEARBY, BIG BEN TOLLS THE PROJECT S COMPLETION 35

36 Taylor Devices, Inc. 90 Taylor Drive PO Box 748 North Tonawanda, NY (716) Phone (716) Fax