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17 September 2008

One off wonder


A major procurement and buildability review has moved a spectacular new suspension bridge from the realm of desire to reality.

(Article taken from our customer magazine, Momentum)


Only rarely can a construction project legitimately be described as unique. But the $5.7 billion (£2.85 billion) San Francisco- Oakland Bay Bridge – a single tower, asymmetric, self-anchored suspension structure– truly is a world first.

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 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. Though other bridge types would have been viable, a suspension bridge solution was preferred for its visual impact, says Peter Lee, project manager for client Bay Area Toll Authority (BATA). The self-anchored structure was required by the site itself: 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 programme of seismic upgrade works on bridges in the Bay area. It will be an all-steel structure with a 385 m main span and 180 m back span supported by a 160 m tall tower. The relatively light weight of steel reduces seismic forces and its ductility 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 of Hatch Mott MacDonald and URS known as Bay Area Management Consultants (BAMC) has played a pivotal role in realising the project.
"The bridge will be the signature structure of the new east crossing, echoing the form of the west span but making a bold, 21st century statement"

Design was carried out by TY Lin. 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 California’s Department of Transportation established an owner’s management team and appointed BAMC to oversee the project, review the design, advise on buildability, monitor the delivery programme and keep track of costs.

A range of technical, logistical and procurement proposals have shaved $400 million (£200 million) 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 cost escalation but late delivery. Locals know that the question is not if another quake will strike but when.

Key issues

International procurement

The initial ‘buy American’ policy would have been great for US industry. BAMC pointed out that components for major infrastructure projects can be manufactured more cheaply and, potentially, to higher quality, elsewhere in the world. Inviting bids internationally increased competition and enabled contractors to procure components from a global marketplace. To encourage competition, bidders were awarded a stipend to cover the considerable costs involved in putting their bids together. Construction was ultimately awarded to the original bidder American Bridge, which re-tendered having engaged a multi-national team of subcontractors and fabricators.

Variations in tolerance must now be carefully controlled to ensure a good fit between different elements, says Hall.

The tower

The tower will consist of four legs, each measuring 8m by 10m at the base and tapering to 3.7m by 3m at the top. The legs will be linked by shear connections. Prefabricated sections will be bolted together to form the tower. BAMC advised on how to reduce the number of interfaces and achieve the bolted connections most economically to ensure that the tower – the bridge’s principal visual element – remains visually elegant. The arrangement of internal stiffeners was simplified to improve access for welders, helping achieve quality and delivery targets. Construction of the tower is on the project’s critical path. Fabrication started in spring 2008. Shipments and erection will begin in spring 2009.

Tower foundations

When procurement of the self-anchored suspension bridge was halted in 2004 foundation work was already under way. On any project the unpredictability of ground works can be a source of delay. To get construction restarted and ensure the foundations would be completed ready for start of work on the superstructure, BAMC provided independent cost estimates to compensate the contractor for the impact of the suspension. BAMC also drew up a proposal to incentivise early completion, achieved in spring 2008.

Decks

Fully welded steel box girders carry ten lanes of traffic. BAMC advised on arrangement and detailing of internal stiffeners and is helping fabricators to achieve required weld quality.

Main cable

The cable will be composed of 5mm diameter wires made up into strands of 127 wires apiece. The cable will comprise 137 strands in total. The cable has a complex geometrical form with large deviation angles at the western end of the bridge where it crosses under the deck from one side of the bridge to the other. Cable construction will require precise installation of the preformed strands to ensure that none of the strands and wires within each strand are twisted.

Saddles

On conventional suspension bridges the cables are supported on saddles at the tower tops and restrained by anchorages at the ends of the bridge. For this structure the ends of the suspension cable will be anchored at the outer edges of the deck at its eastern end. The cable will rise symmetrically to the top of the single tower and then pass around large deviator saddles at the western end of the deck.

Tower-top saddle

This measures 7m long by 6m wide and 6m high. Because the cable planes are inclined the saddle must deal with large opposing forces, requiring thick steel sections. Its size and complexity require highly specialist casting capabilities. To deal with forces in the saddle BAMC recommended inclining the yokes and webs in line with the cable planes. This would reduce the size of stiffeners, simplifying casting and saving on steel. BAMC also recommended breaking the saddle down into smaller elements to reduce the complexity of the individual castings. This will also make the saddle easier to lift and position. Even so, castings will weigh 60-80 tonnes apiece.

Deviator saddles

These will guide the cable through more than 90° as it makes the transition under the western end of the deck. BAMC recommended that each should be manufactured in three sections to reduce the individual weight of components.

Yerba Buena Island viaduct foundations

F oundations for the Yerba Buena Island viaduct structure are in difficult ground. Because of potential for delays, BAMC recommended that work be undertaken outside the main contract and accelerated to take it off the critical path.

Pipe beam fuses

Where the suspension bridge deck meets the adjoining concrete skyway there are special dowelled connections – two per deck. The 2m diameter pipe beams, rolled from 100mm thick high grade steel, prevent differential movement between the two structures. But they are designed with seismic ‘fuses’ that will rupture in the event of an extreme earthquake. By allowing the skyway and bridge deck to move independently when an earthquake strikes the fuses will limit damage to either structure. Fuses are constructed of thinner, lower grade steel and are designed to be replaceable. BAMC reviewed the manufacturing capabilities and practices of the selected subcontractor to help meet Caltrans’ tight performance and quality specification.

Key innovation facts

  • Major savings delivered through procurement rethink
  • Simplification of fabrication processes
  • Construction programme redrawn to remove risks from critical path
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