LONDON MILLENNIUM FOOTBRIDGE
GROUP 12
SHANE O’DRISCOLL
PATRICK BURKE
BARRY LYNCH
JEROME COUNIHAN
Description
The London Millennium Footbridge is the only pedestrian-only bridge crossing the
river Thames and the first new bridge built in London since Tower Bridge in 1894. It
spans 325 metres across the Thames linking Bankside at the Tate Modern, with the
City at St. Paul’s Cathedral.
After an initial competition for a design in 1996 and a winning design from architects
Foster and Partners, engineers Arup and sculptor Sir Anthony Caro. Construction
began on the “Blade of Light” design in 1998 with Monberg Thorsen and McAlpine
contractors.
The bridge is a shallow suspension bridge with a slender aluminium deck supported
by cables and prefabricated steel V-sections beneath the deck level. This design was
in accordance with height restrictions imposed on the project. The deck was four
metres wide and divided into three sections, of 81m, 144m and 108m in length from
North to South respectively, by two concrete Y-shaped piers.
The bridge was opened to the public on June 10, 2000 (2 months late) at a cost of
£18.2m (£2.2m over budget)
Upon opening the bridge had upwards of 90,000 pedestrians crossing it with around
2,000 crossing at a time. Unexpected lateral motion was noticed in the span,
predominantly in the Southern span, with the deck swaying and twisting in regular
oscillations in a way not predicted in the designs. Attempts were made to limit the
number of people crossing at any one time but this action failed. The bridge was
closed two days later on June 12 after extensive media reporting. The movement was
found to occur when large numbers of people crossed the bridge but were reduced
when the numbers were reduced. The movement was found to be a resonant response
to the movement of pedestrians crossing the bridge.
Problem and Cause
The Millennium Bridge phenomenon became known has “Synchronous Lateral
Excitation” and was due to the low natural frequency of the bridge which was
under1.3 Hz. Human comfort perception played a major role in the problem. The
initial oscillation in the bridge deck caused by the rising and falling masses of
crossing pedestrians induced the crossers to adjust their gait accordingly, spreading
their legs apart and moving their feet parallel to the lateral oscillation. This
positioning was found to de more comfortable for crossers, allowing them to predict
the movement of the swaying deck and keep their balance. At this adjusted gait,
footfalls just so happened to fall at the resonant frequency of the bridge. Crowd
behaviour acted to compound the problem. In their designs, the engineers had
assumed the movement of humans as being in a random pattern. However, the crowd
began to act as one, “locking – in” to the same adjusted pattern of movement. . This
action merely amplified the motion until the excitation exceeded the designed
damping capacity of the bridge causing the synchronous lateral excitation. Arup
performed extensive tests on the bridge. While these tests were being carried out the
bridge was closed to the public. This decision was taken both to allow for testing and
remediation but also out of pedestrian discomfort and fears for public safety. The built
structure was compared to the predicted design, the forces exerted on the bridge were
quantified and remedial solutions were proposed.
The above graphic illustrates how the bridge responded as predicted under a
pedestrian load of 156 but with the addition of 10 extra people crossing there was
sudden increase in lateral movement. These numbers were predicted using the
formula;
F=k .v
where
F is the average sideways force exerted by pedestrian
k is the lateral walking force coefficient
v is the sideways velocity of the bridge
This phenomenon had been reported before, notably on the Auckland Harbour Bridge
in New Zealand. However, most engineers were unaware of the problem. The
phenomenon had not been fully investigated with little literature available on it.
Remedial Action
In an attempt to limit dynamic excitation, two approaches were considered. ARUP
Engineers initially considered stiffening the structure so that the frequency of the
bridge and the footsteps of the public would no longer match, eliminating the
excessive swaying. However, this option was quickly disregarded when it was
discovered that the bridge would need to be at least ten fold stiffer laterally to move
its frequency out of the excitation range. The additional structure required to perform
this task would change dramatically the aesthetics of the bridge.
This left one option to repair ‘the wobbly bridge’ and that was to add damping to the
structure which would harness the movements of the structure in order to absorb the
energy. It was then a case of deciding which type of damping system should be used:
Active or Passive.
Active damping uses powered devices to apply forces to the structure to counteract
vibrations. These dampers are commonly used in other engineering fields such as
aeronautics. Although active damping systems have been used in buildings, no
previously designed systems were sufficiently developed for a more complex
multimodal system such as the bridge in question. There was also concern with
regards to the maintenance requirements. Following discussions with manufacturers, a
conclusion was reached that active damping was too complex, and production times
were too long for this to be a feasible solution.
Passive damping was therefore the solution of choice for ARUP Engineers. Two
forms of passive damping were deployed: Viscous dampers and Tuned Mass
Dampers. Viscous dampers function in a similar way to shock absorbers in that they
dissipate energy by the movement of a piston which extends and compresses through
a fluid. This leads to dissipation of the energy. These were placed under the deck,
around the piers and the south landing to control the lateral motions. Distinctive new
steelwork transferred the bridge movements to the under deck viscous dampers.
The tuned mass dampers were also placed beneath the deck in an attempt to reduce
vertical movements. In essence these inertial devices are effectively weights on
springs, and are attached to discrete points on the structure. While no excessive
vertical movement occurred on the bridge, these were added to the solution as a
precaution since researchers suggested that synchronous pedestrian vertical loading is
also possible and has been observed elsewhere.
After nearly two years of testing, the alterations were deemed a success and the bridge
finally reopened to the public in February 2002 at a cost of £5m.
What Was Learnt
The most important thing to learn from the London Millennium Bridge disaster is to
know how to prevent this phenomenon from occurring again in the future. There are
two ways of preventing this kind of behaviour. The deck can be stiffened, a method
which was not chosen for the millennium bridge as it was considered to be too
expensive and not suitable for the project. Another method is that the bridge can be
damped. The millennium bridge was modified to include dampers in both the
horizontal and vertical directions.
The London Millennium Bridge disaster also highlighted the fact that the British
Standard codes are just a guideline to designing structures. How new structures will
behave is difficult to predict, so it is necessary to think of all the possible
combinations of forces that could act on a structure. The Bridge was designed in
accordance with the codes, and the analysis that was carried out was correct. The
problem arose because the designers failed to see that their assumptions were not
justified. They failed to see that even a slight movement of the slender bridge would
cause people crossing the bridge to follow a walking pattern in accordance with the
movement of the bridge, therefore making their assumption invalid. Following
Arup’s investigation, the BS codes are currently being modified to cover this
particular phenomenon so that the same situation can be avoided in the future.
References:
http://www.arup.com/MillenniumBridge/
www.galinsky.com/buildings/ millenniumbridge/
www.compsoc.man.ac.uk/.../ millennium_bridge
http://www.taylordevices.com/papers/damper/damper.html