Imagine tuning a 77 million-pound harp suspended some 15 stories above San Francisco Bay's cold waters and you'll start to get the picture of what is happening now on the new Bay Bridge.
Contractors have begun lifting the suspension span's massive steel decks off temporary platforms and transferring the nearly unfathomable weight -- 35,200 metric tons, or the equivalent of nearly 1 million concert grand harps -- onto its mile-long cable.
Raise even one of the 200 uber-strong steel cable suspender ropes too high or too fast and it could twist the decks or throw the 525-foot tower out of plumb.
And because it is the world's largest self-anchored, single-tower suspension bridge, no one has ever before performed a load transfer on this scale. The lengths of the biggest self-anchored bridges today -- the Konohana Bridge in Japan and the Yeoungjong Grand Bridge in South Korea -- are slightly less than half the 2,047-foot new Bay Bridge.
There's no way around it, of course.
Lifting the deck off its nether supports and hanging it from the span's steel cable on vertical suspenders is a defining visual and structural feature of this and all suspension bridges: The combination of tension and weight compresses the roadway deck and allows the bridge to structurally support itself and the 280,000 cars and trucks that will use it every day.
"It's complicated, but my number one rule for the contractor is, 'Don't break my bridge,'" said Brian Maroney, principal engineer for the Caltrans Toll Bridge Program.
Caltrans and American Bridge/Fluor Enterprises have devised a complex balancing act based, in part, on hundreds of hours of computer modeling designed to predict the effects of the forces of weight and tension.
First, ironworkers tightened the main cable and cambered, or moved, the top of the tower 1½ feet to the west, explained Maroney.
Because the bridge is asymmetrical, meaning the tower is offset rather than centered within the span, the intentional lean is in anticipation of the extra deck weight on the longer side. The tower will straighten as crews add load, he said.
Next, ironworkers will begin connecting the bridge's 200 twisted-steel-strand suspender ropes -- 100 per side -- to the outside of the deck using a hydraulic jacking system capable of 400 tons of force.
The first, biggest and most dramatic phase will feature eight closely coordinated crews that will tension strategically selected ropes along the bridge.
Slowly, the bridge deck along the entire span will begin to lift 1 ½ feet off its temporary supports. At roughly rope No. 101, the span will transition into an officially self-supporting bridge.
It's a much faster and more nail-biting transfer than the segment-by-segment process used on a traditional suspension span like the Golden Gate or Carquinez bridges, which are anchored on two ends, Maroney said.
At the same time iron workers tension the ropes, they must tighten the main cable as it absorbs the load and stretches.
"During load transfer, the cable becomes like a living creature," Maroney described. "It starts to move and pick up momentum. We have to cater to its needs."
On the finished bridge, the mile-long looped main cable will act as a giant weight-bearing sling. So, the idea during load transfer is to allow the steel to stretch to the point of elasticity, or the amount at which it fully rebounds. If stretched too much, steel becomes plasticlike and brittle, Maroney said.
The contractor initiated a transfer dress rehearsal this week and, depending on the outcome of the analysis, workers will likely begin the first phase in early September, said Toll Bridge Program Manager Tony Anziano.
Properly tensioning all 200 suspender ropes and the cable could take three months or longer.
Lastly, crews will fine-tune the ropes for what engineers dub the profile or what motorists describe as the ride -- the glide, the bank and the feel of the bridge from behind the wheel.
The ironworkers' job of meeting Caltrans' profile requirements is further complicated by a unique design curve in the span and the fact that the westbound lanes sit higher than the eastbound ones, Anziano explained.
When the suspension cable, ropes, tower and deck are in structural harmony, the contractor will align and connect the bridge with the 1.2-mile skyway segment to the east and the transition structure linking it to Yerba Buena Island.
The $6.4 billion bridge is set to open Labor Day weekend of 2013.
Contact Lisa Vorderbrueggen at 925-945-4773, Twitter @lvorderbrueggen or www.facebook.com/lvorderbrueggen.
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