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As bridges become more
extreme, the limiting factor is no longer the strength or durability
of the materials used. Dynamic response to wind, earthquakes
and traffic loads dominate the design process. Dave Parker reports
on how designers have dealt with dynamic challenges on three
extreme bridges. Record breaker Spanning 2.3km across the typhoon
haunted waters of the Bali Sea, the Java-Bali Bridge will be
the longest in the world – if it opens before the giant
Messina crossing.
There is still no certainty the
landmark structure will ever be built, but the concept design
has posed some interesting challenges for structural engineer
Flint & Neill, not least on the dynamic front. Flint &
Neill partner Ian Firth explains: “We chose a hybrid cable
stay/suspension bridge design because this effectively reduces
the suspended span to 1,600m. And
the cable stayed backspans with their multiple piers are very
stiff. So the natural frequencies of the deck are higher, amplitudes
of motion are lower and it responds much better to wind loads.”
Given the high probability of typhoons in the area the aerodynamic
characteristics of the structure were crucial. Deck design was
made no easier by the client’s decision that only two traffic
lanes in each direction were needed.A conventional single deck
girder would have been unfeasibly narrow for the record-breaking
span. To achieve the necessary lateral stiffness Flint &
Neill went for a cross-braced twin box design, with a 16m central
slot between the aerofoil cross-section steel boxes. “Messina
style slotted decks are very efficient aerodynamically,”
Firth points out. “And the separation helped us develop
a very efficient tower design with a distinctly Balinese flavour.”
Unusual they may be, but Firth insists the 300m high towers have
much greater transverse stiffness than conventional portal frame
designs. He also expects that once detail design begins the real
challenge will be fine tuning the cross sections of each box
in the wind tunnel. “Vortex shedding is the key issue –
predicting how curved soffit box girders will behave is really
difficult.” Aerolastic testing –modelling the dynamic
response of the structure to high winds – will also include
the effect of local topology, Firth adds. Lateral and torsional
motion on long span structures is a problem for which Firth sees
no simple solutions. Active damping and “intelligent”
aerodynamic control fail to impress. He sees the risk of power
or equipment failure as too high. But passive aerodynamic damping
– “flat plates paddling air” – is another
matter. Such appendages would make Java-Bali an even more dramatic
crossing.
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Had it been built with active
damping, the Royal Victoria Dock footbridge in London’s
Docklands would have weighed 25% less. So says structural engineer
Tekniker director Matthew Wells,adding: “The original intention
was to react to deflections caused by the passenger gondola which
will eventually run below the deck. “The system could in
theory have coped with all imposed loads – the problem was
defining which frequencies to damp.”
A mini transporter bridge design
was architect Lifschutz Davidson’s response to a design
brief from the then London Docklands Development Corporation
for a landmark 130m span pedestrian crossing over a stretch of
water which was being
developed as a regional sailing centre. A 13m minimum clearance
below the deck seemed to imply that pedestrians would need some
form of weather protection – but at the same time the structure
had to cause as little wind turbulence as possible. “Some
form of tube would have been a possibility,” says Wells.
“But who would want to cross it late on Saturday night?”
The transporter bridge alternative would allow pedestrians to
enjoy the views from the open top deck in fine weather, with
the option of skimming across the dock in a 40 passenger enclosed
car in less clement conditions.
An inverted Fink truss was the
chosen structural form, partly because of its inherent lightness
and flexibility, partly because it invoked both the dockyard
cranes and the forest of masts and cables that once filled the
dock.Five fabricated “whale backbone” steel box girders
linking the six masts were spigoted together around the mast
feet. This simple to erect pin-jointed arrangement also significantly
increased the structure’s inherent lateral damping.Beneath
this backbone runs the track for the passenger gondola, which
weighs 11t fully loaded. Wells says the gondola only increases
midspan deflection by a maximum of 75mm. “Dynamically, there
was no problem with the gondola swinging beneath the deck, as
its ‘bouncing’ frequency is very much lower than that
of the bridge.And in high winds it will be able to winch itself
close up under the deck during the crossing.” Doubts about
development time for an actively damped solution meant Techniker
went for a “static” solution at competition stage.
As design development began, tests revealed a potential aerodynamic
problem – the first torsional and first vertical frequencies
were very close together, less than 0.5Hz apart. “Normally
you would expect vertical to be around 1Hz with torsional three
or four times higher,” Wells comments. “When they’re
this close you could be facing a Tacoma Narrows
situation.”
Aerodynamic damping via a perforated
balustrade proved to be the solution to this particular problem.
Meanwhile, Techniker worked up two alternative active damping
designs, using adapted Formula One self-levelling suspension
technology applied to the backstays This promised a massive weight
reduction and a cut in the maximum depth of the box girders from
1,650mm to 1,250mm. There was a downside, however. Wells explains:
“With full active damping it would be impossible to use
the bridge in high winds if there was any equipment failure.
Then there was the ongoing cost of maintenance. But the real
problem was the time needed to decide on the sampling
window – which motions to monitor and to correct.”
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Between midnight and 5
am on 05/08/00 the bridge was lowered 10mm and moved southwards
10mm. There will be another 24 hour closure on 12/08/00 for bearings
to be jacked into place and a final closure on 26-27/08/00 to
put the bridge onto the bearings.
Lifting jacks have supported
the bridge for seven months while new concrete piers have been
constructed. The bridge , built in 1970, is one of Europe's busiest,
carrying an average 150,000 vehicles a day - five times original
estimates. . The £31.5m operation began after the discovery
of major defects in the bridge in 1990. The north pier wall near
the bridge's support piers was found to be bulging.
Extract from the Scotsman
newspaper (31/07/00):
MSP calls for fresh assessment
of Kingston Bridge , Andrew
Murray-Watson
A GLASGOW MSP has called for an independent assessment of the
Kingston Bridge after concerns that it will not be able to cope
with ever-increasing levels of traffic. Strengthening work, being
undertaken has so far cost £33 million. Despite assurances
from the Scottish executive, the SNP’s local government
spokesman, Kenny Gibson MSP, wants independent checks made on
the bridge to guarantee its long-term future. Mr Gibson said:
"Having spoken to several civil engineers, I am worried
about the executive’s apparent confidence in the remedial
work being carried out on the bridge.
"The experts I have consulted
say that there is no guarantee that the work will restore the
bridge to a condition one would expect it to be in after 30 years
of life. However, the fact that there were severe structural
design problems in the first place means that its rate of deterioration
in the future is likely to be faster than accounted for, particularly
given the fact that it was built to handle only a fraction of
the traffic now using it."
The bridge is due to be closed
on four separate occasions. On Saturday (05/07/00) evening all
traffic will be diverted for 24 hours while the bridge is lowered
in order for essential work to be completed. Mr Gibson added:
"Sarah Boyack, the minister responsible, can happily say
that she expects the bridge to live out the balance of its engineering
life, because all they have are estimates, they don’t actually
know for certain. I will therefore be asking the executive if
they intend to keep publishing the results of the structural
monitoring of the bridge after the current work is finished.
If yes, then fine. If not, then they obviously have something
to hide."
An Executive spokesman said that
the bridge had a long future: "Strengthening work and normal
maintenance will mean that the bridge will be able to see out
its 120 year life-span -even given the additional traffic."
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Bad vibrations
Update: Aug 2000:
Arup's investigation results
will be available in August, with remedial work starting in September,
possibly overseen by the City of London's engineers. The bridge
will remain closed to the public.
Extract from New Scientist
08 July 2000.
How could the designers of a
revolutionary bridge miss something so obvious?
Engineers who designed London's
wobbly Millennium Bridge admitted last week that the computer
simulations they used to model its behaviour couldn't cope with
the effect of people walking across it. The blunder, say observers,
resulted from an out-of-date standard for testing bridges and
poor dissemination of information in the industry about previous
incidents.
The 320-metre footbridge over
the Thames, which cost £18 million, was closed on its opening
weekend last month, when it was found to wobble more than expected.
Since then
Arup, the engineering firm responsible for the bridge, has
been analysing its response to vibrations.
The engineers used shaking machines
to send vibrations through the bridge. They found that horizontal
vibrations at 1 hertz (one complete cycle per second) sent the
bridge into the kind of S-shaped lateral wobble that was seen
when it opened.
This was a clue to the source
of the problem, says Pat Dallard of Arup, structural adviser
on the bridge project. "Normal walking pace is about two
strides a second, so you produce a vertical force at around 2
hertz," he says. But the horizontal frequency is half that.
"As we walk, one foot pushes left, then the other pushes
right, so you have a 1-hertz horizontal force," he says.
Ian Sample
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