Self-Anchored Suspension Span (SAS)
CONSTRUCTION UPDATE (November 20, 2012):
The vision of the world’s largest Self-Anchored Suspension Span (SAS) is a reality. The single, nearly 1-mile-long main cable now supports the weight of the bridge. Workers successfully completed the highly complex process called load transfer, which shifted the weight of the 35,200-ton decks from the temporary steel that supported them for the past few years and onto the tower, main cable and suspender ropes and main cable, which is the longest single looped suspension bridge cable in the world; the process took just under three months. The SAS, at 2,047 feet, is the world’s longest SAS and the signature element of the new East Span.
One signature element transforms the San Francisco-Oakland Bay Bridge into a global icon – the Self-Anchored Suspension Span (SAS). The SAS is the largest bridge of its kind anywhere in world (2,047 feet), one of many firsts. This engineering and construction marvel raises the bridge building bar to new heights.
The bridge’s single 525-foot-tall tower echoes the height of the West Span’s towers. The tower’s placement closer to the west end of the SAS creates a distinctive asymmetrical design; the single mile-long main cable will present a sharper angle on the west side but a more sloping appearance on the east.
The prime contractor building this landmark is American Bridge/Fluor (A Joint Venture). The two engineering firms bring a combined 210 years of experience to this unprecedented project. American Bridge is no stranger to the Bay Bridge; the company helped build the original bridge in 1936.
Building a Bridge to Build a Bridge
Traditional suspension bridges, such as the West Span and Golden Gate Bridge, have towers with two main cables that tie into anchorages in the ground. After the suspender cables are attached to the main cable, the deck is hung from the cables.
Building a different kind of suspension bridge requires a different kind of construction strategy. Because the SAS is self-anchoring with one cable, the roadway must be built first. But with nothing to hold up the roadway, a temporary bridge is required first – crews built a 20,000- ton temporary bridge to build the approximately 35,200-ton permanent span. With the falsework in place, crews build the road-decks and tower first, and then the cable is anchored into the roadways.
SAS Road Decks: Heavy Lifting
The SAS’s east and westbound roadways are each composed of 14 individual sections. These deck sections vary in weight from 559 tons to 1,669 tons, as well as in length, from 64-feet-long up to 229-feet-long. The roadways are connected by 19 crossbeams.
Helping with the heavy lifting is a massive 400- by 100-foot barge equipped with a 1,900 ton shear leg crane sporting a 328-foot-long boom. The crane’s boom weighs 992 tons, and is 328 feet long; the crane can lift up to 1,929 tons. The crane barge, dubbed the Left Coast Lifter, was custom built for this project and is the largest of its kind on the West Coast.
After a deck section is floated on a crane barge to the work site, the crane lifts and places the section onto a cradle on the temporary supports. The cradle, which slowly carries the section into place, is outfitted with ski-like appendages that are pushed by two horizontal hydraulic jacks along raised, PTFE-capped pedestals greased with a special solution, moving about 10 feet with each push. This process was only used for the west end of the SAS, where the waters were too shallow for the crane to put the deck sections directly into place on the temporary steel.
To connect one deck section to another, 4,500 bolts are used, with a one millimeter variance; 350,000 bolts will be used to connect all 28 deck sections.
Load Transfer: A Massive Balancing Act
Once the single tower, nearly 1-mile-long main cable and 200 suspender ropes were in place, workers began the time- and labor-intensive load transfer process. Load transfer is not a straight-forward linear process but an incredibly complex endeavor of a multiple steps taking place simultaneously and affecting each other as they occur.
The operation 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 feet and out about 30 feet. This caused the decks to lift up approximately 1.6 feet from their temporary supports.
Crews also engaged a jacking saddle at the western end of the SAS to maintain balance on that part of the bridge while simultaneously releasing the tower from its 1.5-foot westward lean. This allowed the tower 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.
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.
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. Crews are currently painting the suspender ropes and have started wrapping the main cable – which entails slathering the cable with a zinc-based paste and encasing it with tightly interlocking steel strands, or S-wire. Once the cable is wrapped, it will be painted.
Foundations for the Future
Construction of the SAS began in 2005 with the two water and single land-based foundations.
The eastern SAS foundation (Pier E2) is comprised of twin structures to support the separate east and westbound bridge decks. Because of the geology east of the foundation in the Bay, 16 piles were driven nearly 340 feet to reach bedrock. Reinforced concrete was poured into the steel casings to make up the cast in steel shell piles. Two concrete pier columns rest on each footing and reach approximately 120 feet above the water, where they are linked by a cap beam beneath the SAS bridge decks.
The western support (Pier W2) is made up of two massive piers on Yerba Buena Island, where the main cable for the suspension span extends down from the tower and wrap under the western end of the road decks.
The foundations extend down approximately 80 feet through the island’s solid rock. Building the western supports required 2,000 tons of steel reinforcement and more than 12,000 cubic meters (15,700 cubic yards) of concrete. Pier W2 contains the world’s largest isolation casing.
The tower’s marine foundation consists of a steel footing box welded to steel shells surrounding each of the 13 piles. The footing box was fabricated in Texas and shipped by barge to the job site via the Panama Canal. The piles extend 196 feet below the waterline and are anchored into bedrock. Connecting the foundation into solid bedrock posed a considerable challenge, which was overcome through a process called “rock socketing.”
The piles are comprised of two parts. The lower portion of the pile is made up of heavily reinforced concrete and placed within shafts, which are drilled into bedrock. These shafts are referred to as rock sockets; they anchor the piles deep within the bedrock. The upper portion of the pile is a permanent steel shell, about eight feet in diameter, and filled with more heavily reinforced concrete. This shell is welded to the footing box. The entire steel footing box is encased in concrete to make up the tower foundation, which measures 85 feet long, 73 feet wide and 21 feet thick.
East Meets West
Shanghai Zhenhua Heavy Industry (ZPMC) is the prime steel fabricator for the SAS, including the deck and tower sections as well as most of the steel for the temporary supports. ZPMC is the largest heavy-duty equipment manufacturer in the world. It produces more than 75 percent of the world’s shipping port cranes, as well as steel for other bridges including the Golden Ears Bridge in Vancouver, Canada.
As a major supplier of port cranes for the Port of Oakland, ZPMC is an established entity in the Bay Area. The SAS sections are fabricated at ZPMC’s facility on Changxing Island, where 30acres are dedicated to this project, 20 acres of which are inside fabrication shops. ZPMC also custom built lifting cranes, cradles and rotisseries to fabricate the SAS sections. The SAS requires 51,800 tons of steel for its deck sections and tower.
- Signature span of the bridge
- Has only one tower and one main cable
- side by side steel roadways
- Spans between YBI Transition and Skyway
- Length: 2,047 ft
- Height of tower: 525 feet
- Largest span of its kind in the world
- Main cable is approx 1 mile long and made up of 17,399 steel wire strands