Only rarely can a construction project legitimately be described as unique.
But the San Francisco-Oakland Bay Bridge – a single tower, asymmetric, self-anchored suspension structure – truly is a world first.
Click here for map of Oakland Bay Bridge
A suspension bridge involves slinging cables over one or more towers. Cable ends are usually anchored to the ground close to the bridge's abutments, where deck meets land. The bridge deck is suspended from the main cables with resulting tension forces resisted by the anchors.
By contrast, on the Bay Bridge a single cable will be looped over the tower and around the deck ends. Tension in the suspension cable will be resisted by compression along the length of the deck. Although other bridge types would have been viable, a suspension bridge solution was preferred for its visual impact, says client Bay Area Toll Authority (BATA) project manager Peter Lee. The site itself required the self-anchored structure: out of reach of land, there is nothing other than the deck to anchor cables to.
The bridge replaces a section of existing San Francisco-Oakland causeway that suffered partial collapse during the 1989 Loma Prieta earthquake. It forms a key element in a wider £5.7bn (£2.9bn) programme of seismic upgrade works on bridges in the San Francisco Bay area. It will be an all-steel structure with a 385m main span and 180m back span, supported by a 160m-tall tower. The lighter steel structure reduces seismic forces and the ductility of steel enables it to cope better than concrete with earthquake-induced movement.
Getting the bridge from drawing board to construction has been far from easy. A joint venture between Hatch Mott MacDonald and URS known as Bay Area Management Consultants (BAMC) participated on a technical review team, brought together by the California Department of Transportation, Caltrans, which has played a pivotal role in realising the project.
The design was carried out by TY Lin and bids for construction were invited early in 2004, but the project was put on hold later that year when only one contractor responded and anticipated construction costs soared. To get the project back on track, BATA and Caltrans established an owner's management team and appointed BAMC to provide project oversight, review the design, advise on buildability, monitor the delivery programme and keep track of costs.
"The range of technical, logistical and procurement proposals have cut £206M off the projected delivery cost," says BAMC programme manager, Hatch Mott MacDonald's Ted Hall. Now a keen eye must be kept on the construction schedule as the overarching risk on this project is not late delivery, but cost escalation. And locals know that the question is not if, but when, another quake will strike.