The San Francisco Bay Area recently became the proud parents of the world’s largest Self-Anchored Suspension Span (SAS) bridge. This momentous occurrence happened November 20, when crews successfully completed a highly complex load transfer, which shifted the weight of the new $6.4 billion San Francisco – Oakland Bay Bridge from temporary supports to a single, nearly 1-mile long main cable.
The process, which took just under three months, shifted the weight of the 35,200-ton bridge decks from the temporary steel that supported them for the past few years, onto the tower, suspender ropes and main cable, giving the Bay Area an iconic 2,047-ft-long engineering marvel.
But now that the shiny new SAS bridge is in place, with all its steel components swinging in the damp San Francisco Bay environment, what steps will be taken to fight one of a bridge’s biggest enemy’s – rust and corrosion? This pesky chemical reaction attacks steel from the moment it arrives onsite to the end of a structure’s life.
On the existing 75-yr-old West Span, the cables have had their share of exposure, but on the new span, crews are protecting the main cable from day-one. In fact, I just watched a very interesting video on the Bay Bridge website, www.baybridgeinfo.org, which discussed this very topic.
Beginning on the main cable, where more than 17,000 individual strands of 5mm wire are strung together to support the bridge, it is vital to keep it safe from corrosion. This started with each strand being carefully galvanized before woven into the massive main cable. Once all the wires are together and the cable placed into position, the entire cable is then coated with a zinc-based paste.
After it’s coated with the past, the cable is banded with wire wrap and then the bridge is painted with two coats of a noxyde-based paint that almost forms a gasket around the wire wrapping.
Then there is the all-important main cable saddle section, where the cable comes up to the top of the bridge and loops around itself. In this area, where the cable and wires are exposed to elements, the project team is using large dehumidifiers in the tower legs and around the length of the cable, where it is not protected with paints and covering. The dehumidifiers suck moisture away from the steel and then dry air is pumped in.
The operation to transfer the load to the main cable began in mid-August with crews using hydraulic jacks (which exert up to 400 tons of force) to gradually tension the 200 suspender ropes that connect the main cable to the decks. Once 104 of the 200 ropes were tensioned, the bridge was self-anchored and self-supporting. As the suspender ropes were tensioned, they pulled the main cable toward the deck causing it to move down about 16 ft and out about 30 ft. This caused the decks to lift up approximately 1.6 ft from their temporary supports.
A primary focus of the workers was balancing the various forces that were at play throughout the operation. While tensioning the suspender ropes, crews engaged a jacking saddle at the western end of the bridge to maintain the superstructure’s equilibrium while simultaneously releasing the tower from its 1.5-ft westward pull, allowing it to regain its vertical stance after being loaded with weight from the cable and decks.
Since the SAS cable is anchored into the eastern end of the roadways, the cable will naturally pull the tower to the east, so crews pulled the tower west toward Yerba Buena Island using steel strands that anchored into the island’s bedrock to hold the tower’s position.
Following the initial phase of load transfer were a series of steps that involve tensioning the remaining suspender ropes, completing connections between the decks and cable, and installing a steel shroud, or cover, to protect the cable at the western end of the SAS, where it wraps around the deck.
The main cable now acts like a giant sling, supporting the weight of the deck. Unlike traditional suspension bridges where the cables are anchored into the ground, a self-anchored suspension bridge’s cable is anchored in the road decks. For more information visit: www.BayBridgeInfo.org/projects/sas.