91av

PROBLEMS with simulating the way London鈥檚 Millennium Bridge behaves when in
use may be the reason the radical new design sways alarmingly, sources say. As
91av went to press, engineers were checking the closed bridge
to find the fault that terrified many people who walked across it on its opening
day.

The 拢18 million, 320-metre footbridge over the Thames is essentially a
flat suspension bridge. It rests on two concrete piers in mid-river, while four
cables running along each side take some of the load and stiffen the structure.
鈥淎 very light structure like this with a long span is going to be flexible,鈥�
says John Dickens, a civil engineer at Loughborough University. 鈥淏ut it looks as
though the movements induced by crowd loading are in excess of what they
辫谤别诲颈肠迟别诲.鈥�

But how could the engineers have got it so wrong? 鈥淭he problem is that this
is a new type of bridge,鈥� says David Parker, technical editor of New Civil
Engineer. 鈥淭hey鈥檙e pushing technology to its limits and they鈥檙e having to
make educated guesses about certain factors,鈥� he says. Using computers to
estimate how the bridge will behave when crowded is particularly difficult.
鈥淭here are so many forces acting on a bridge like that at one time, that trying
to simulate the interaction between them all is extremely difficult,鈥� he says.
Dickens agrees: 鈥淓ven the most sophisticated simulation programs have
assumptions built-in and until you build the whole thing you鈥檝e still got a
degree of uncertainty.鈥�

Project engineers at international consultancy Arup must now work out exactly
why the bridge is swaying. 鈥淭hey can either stiffen it鈥攚hich means welding
on more metal鈥攐r use tuned mass dampers,鈥� says Parker. The dampers are
essentially small weights that can be suspended from the bridge. The idea is
that as the bridge wobbles, the weights wobble out of phase, cancelling out the
motion.