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Bay Bridge Makes Record Squeeze Play

Friday, May 4, 2012

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The San Francisco-Oakland Bay Bridge East Span project should reach another milestone Monday (May 7), as crews wrap up the gargantuan task of compacting the structure’s record-setting SAS main cable.

Crews have been working for three weeks on compaction of the Self-Anchored Suspension Span’s (SAS) signature cable. Workers are using four hydraulic compaction devices to compress the 137 individual strands for the cable, which is nearly one mile long.

 Each of the 5,291-ton cable’s 137 strands holds 127 5mm steel wires.

 baybridgeinfo.org

Each of the 5,291-ton cable’s 137 strands holds 127 5mm steel wires; each wire is strong enough to support a military-grade Hummer, authorities say.

The 2.6-foot diameter cable is anchored into the east end of the roadway. The cable “travels up and over the single tower to wrap around the west end, before traveling back up and over the tower to anchor back into the east end,” acting “like a giant, unbelievably strong sling,” supporting the weight of the deck, bridge officials explain in a release.

Unlike traditional suspension bridges, whose cables are anchored into the ground, a self-anchored suspension bridge’s cable is anchored in the road decks.

The East Span’s SAS cable will be the longest single looped suspension bridge cable in the world, officials say.

Compaction: By the Numbers

Compaction of the 5,291-ton cable began April 14 at the top of the 525-foot-tall single tower. Workers moved the compactor down toward the road decks, with the assistance of winches, 1.5 meters at a time.

The compactor places temporary galvanized carbon steel seizing bands around the cable at the 1.5-meter intervals. The strands (each containing 127 high-tensile strength steel wires) are also compacted between the jacking and deviation saddles as they pass around the span’s western end.

 Workers are finishing compaction of the world’s longest single looped suspension bridge cable, on the new East Span of the Bay Bridge.

 NBC Bay Area / Joe Rosato Jr.

Workers are finishing compaction of the world’s longest single looped suspension bridge cable, on the new East Span of the Bay Bridge.

Each of the 17,399 5mm wires can support the weight of a military-grade Hummer, authorities say.

The cable is not compacted at the top of the tower; there, the individual strands pass through a cable saddle. Crews use a different compactor as the cable approaches the road decks at the east end of the span.

A Cable Compaction Fact Sheet and video of the compaction process explain more.

Gawker’s Dream

To offset the SAS cable’s natural pull toward its eastern anchor, crews used temporary cables anchored into the bedrock of Yerba Buena Island to pull the top of the permanent tower 20.4 inches to the west in September. Once the cable is in place, the tower will be released to stand up straight once again.

After compaction, workers will begin attaching the 114 bands that permanently hold the cable strands and serve as anchor points for the suspender cables on the main cable; the temporary bands will be removed as the permanent bands are attached.

In late 2011, workers began erecting a temporary footbridge that will travel the path of the SAS cable, so ironworkers will have “unfettered, if precarious, access” to the cable as it is being placed, bridge officials say. Once the SAS is finished, the suspenders connecting the cable to the bridge deck will be tightened and the bridge will be lifted off the temporary support structure, which will be dismantled.

The whole, dazzling task is unprecedented, and the work will be visible from the existing bridge, authorities say. However, they urge project gawkers to keep their eyes on the road while driving and view the progress later—safely—from a new interpretive display located on Treasure Island and online via construction cameras.

   

Tagged categories: Bridges; Construction; Steel

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