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Stonecutters model would simplify solution

NewsIn Californian engineers must come up with an alternative tower design for the Oakland Bay Crossing's self-anchored suspension section

The choice of a complex 160m high, four steel box, single tower for the main span of San Francisco's eastern Oakland Bay Crossing was the main reason why the single bid received was, at $1.8bn, well over the original $1bn budget. Only one JV contractor was prepared to take the challenge involved in erecting the 67,000t of steel fabrications needed - 'enough to build 10 Eiffel Towers'.

The crossing's massive tower is needed to support its unique and challenging self-anchored suspension (SAS) span.

A SAS dispenses with separate anchorages for the suspension cables. In conventional suspension crossings, tensile loads from the main cables are transferred into the ground via rock anchors or massive concrete foundation blocks. On a SAS bridge the cables are anchored at the end of the bridge deck, putting the deck itself into compression.

This is analogous to the difference between a conventional and a bowstring arch span.

But, even though steel prices are rocketing, the alternative design will almost certainly be steel, albeit of lighter construction. The crossing's location, in one of the world's most active seismic zones, means the material's ductility will be needed to cope with earthquake forces.

A model could be the tower for Hong Kong's soon-to-be-built cable stayed Stonecutters Bridge.

The simple, 300m tall circular pylons on Stonecutters Bridge are designed to cope with a combination of aerodynamic and seismic forces of the same order as those in Oakland Bay.

Like Stonecutters, the deck design would be more conventional, in the sense that twin box girders widely separated for maximum aerodynamic stability and transverse stiffness are now an accepted solution for long spans, and this is unlikely to change.

The main span is 385m, and the back span 180m, relatively modest by the standards of modern conventional suspension bridges and cable stay spans. It would be the longest cable stay span ever built, however (see box).

As with all SAS bridge designs, constructing the deck for the Oakland Bay Bridge structure poses special challenges. The entire deck structure has to be temporarily supported during assembly, almost certainly by cable stays. And it is still unclear how the complex suspension cable would be installed.

Controversy over the Oakland Bay SAS bridge choice is not confined to its cost.

One local seismic expert, Professor Albolhassan Asteneh-Asl of the University of California, has claimed that the highly stressed deck would be vulnerable to terrorist attack, and that there is no proof that the basic SAS concept has the resistance to seismic events expected by the client.

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