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Tuned mass dampers could give answer to Oakland Bay Bridge crisis

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TUNED MASS dampers could be the key to a simpler, cheaper main tower for the crisis-hit suspension span on California's Oakland Bay Bridge, British bridge engineers have suggested.

The most obvious alternative would be a circular steel tower similar in appearance to that chosen for Hong Kong's record breaking Stonecutters Bridge.

'But to get the seismic resistance you could find yourself needing extremely thick walls, ' said independent bridge consultant Jolyon Gill.

'Adding a tuned mass damper at the top of the tower could be the only cost effective answer, ' he said.

Tuned mass dampers - heavy weights suspended inside tall structures - would swing out of phase with earthquake induced sway and damp it out.

A panel of Californian structural engineers is advising the California Department of Transportation (Caltrans) on options for keeping the unique self-anchored suspension span design alive.

California governor Arnold Schwarzenegger has refused to fund the sole bid for the span.

Caltrans hopes to cut the cost of the span from ú1bn to ú600M without compromising the self-anchored suspension (SAS) concept by coming up with a cheaper tower that can still meet a completion date of 2010.

The complex original design, featuring four tapering pentagonal steel boxes connected by sacrificial shear links, combined distinctive aesthetics with seismic performance.

But at least one UK expert doubts that changing the tower design alone will solve the cost problem.

Flint & Neill partner Ian Firth said that he saw major problems with the design of the main cable. This is 780mm diameter, made up of dozens of preformed strands locked together.

It is designed to run continuously from an anchorage at an outer corner of the longer western span, over the top of the tower to the equivalent corner on the eastern span.

From there it runs round a saddle to the opposite corner of the eastern span, round another saddle and back up to the top of the tower.

It would then run down to a second anchorage on the outer edge of the far corner of the western span.

'No-one has yet explained exactly how this cable is going to be formed, ' Firth said.

'It would be a massive logistical challenge involving extremely long strands.'

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