Unshaken: The Fortification of The San Francisco-Oakland Bay Bridge
By Stephanie Aurora Lewis
For the Wire Rope Exchange
Editor’s Note: The below is an excerpt from the July/August 2014 issue of Wire Rope Exchange. Since the original publication of this article not only has The San Francisco-Oakland Bay Bridge been completed, it was the recipient of two awards: The 2015 Project of the Year by the Post-Tensioning Institute and a finalist for the 2015 Outstanding Civil Engineering Achievement Award from the American Society of Civil Engineers.
“Where is the safest place to be in San Francisco when the soon-coming Big One strikes,” asks Ira Flatow, the host of NPR’s Science Friday, joking with his audience. Marwan Nadar, PhD, the Lead Design Engineer of the San Francisco-Oakland Bay Bridge answers Flatow’s question, “the safest place in the Bay area may indeed be sitting in your car on the new eastern span on the Self-Anchored Suspension (SAS) Bridge.” Marwan Nadar represents T.Y. Lin International who joined with Moffatt Nichol Engineers as a joint venture company to design the new SAS Bay Bridge for Caltrans, the California Department of Transportation. Of all the materials used in Nadar’s avant-garde seismic bridge design, wire rope is the key element.
Quake of ‘89
On October 17th, 1989, the San Andreas Fault shifted to produce the devastating 7.1 magnitude Loma Prieta earthquake which shook the Bay area long and great enough to cause billions of dollars of damage and a renewed fear for seismic movements. Many Americans remember this earthquake because it delayed the World Series Game Three that took place that year in San Francisco between the Oakland Athletics and the San Francisco Giants.
The original Bay Bridge was designed by the chief engineer Charles Henry Purcell in 1936. Later in 1955, the American Society of Civil Engineers named the Bay Bridge as one of the seven modern civil engineering wonders of the United States. When the Quake of ’89 shook, the eastern part of the bridge shifted seven inches, causing the bolts of one section to shear off, sending a 250-ton section of the bridge’s roadbed crashing down. The crash caused one car to its fatal plunge and bridge closure for 30 days.
The Future Bridge
Caltrans is the project’s client who set serious goals for the new Bay Bridge to make sure that history would not repeat itself as it did on that tragic day in 1989. First of all, the existing bridge of 1936 was a structural steel truss system that would have been too difficult to retrofit and repair for updated seismic prevention design. Secondly, the bridge needed to be able to survive for 150 years. Thirdly, the bridge was to withstand significant seismic events so that the roadway would remain open for emergency vehicles.
The entire Bay Bridge crosses from San Francisco to Oakland via several connecting parts including a tunnel and two bridge spans. The western suspension bridge was updated with extensive seismic retrofitting. The eastern section was completely replaced with the innovative Self-Anchored Suspension (SAS) Bridge. “What makes a SAS different from the average suspension bridge is that the main cable used to uphold the bridge is anchored into itself creating a type of structure like that of a hanging basket,” says Nadar.
“The construction team has a photo of the 1989 bridge failure in every construction trailer as a reminder of why they are doing the project.” — Jordana Jackson, Caltrans
Geological scientists predict that there is a 60 percent chance for a major earthquake to hit San Francisco within the next 30 years. Each part of the SAS Bridge, the decks, the towers, and the wire ropes will all move in an earthquake, but are designed to not fully break apart. Some concrete pieces are designed fail and then be easily repaired after the event without having to shut down the bridge’s traffic. Caltrans takes this project very seriously. “The construction team has a photo of the 1989 bridge failure in every construction trailer as a reminder of why they are doing the project,” says Jordana Jackson, a representative of Caltrans.
The Bridge’s Engineering Features
The SAS has just one tower and spans 2,047 feet across the eastern part of the Bay Bridge expanse, making it the longest self-anchored suspension bridge in the world. The singularly-long cable stay is anchored underneath one side of the bridge and then anchored into the roadway on the other side. 200 suspender ropes then hang off the main cable stay and fully support the roadway sections below.
The tower has four independent legs that rest on soft soils rather than on stone as most other bridges that have a more solid foundation. “Soft soils offer very little resistance naturally for the foundation. Therefore, it was critical to design the bridge to move and float during the shaking as different pieces. We designed the bridge by looking at every microsecond of how the structure would react during a seismic event through a computer program,” says Nadar.
When the tower supports move in an earthquake, there is a flexible connector that will be able to cushion the tower supports so that they will not break apart. Likewise, there are pins holding together pieces of the roadway so that the roadway can also flex back and forth, up and down during the seismic event.
Surprisingly, Nadar was studying civil engineering in San Francisco and was actually working within his academic building in 1989 when the earthquake hit. “That experience had a huge impact on me,” he says regarding his work on the new bridge.
The Main Cable Stay
The entire length of the cable is one mile long, has a 2.6-foot diameter, and weighs 5,291 tons (10.6 million pounds) with 127 high-tensile strength wires contained within each strand totaling 17,399 wires. Each strand of the cable took the construction crew two entire days to install. The total sum of the main cable stay’s composition equals over 118 miles of 2 ½ inch diameter steel strands and more than 17,000 5mm diameter wires. Each 5mm wire alone can singly support the weight of a military-grade Hummer.
Once the cable was tensioned to support the weight of the deck pieces below, it was protected with a series of materials to guard it against 150+ years of corrosion. The cable was covered with a zinc paste, and then enclosed by interlocking galvanized steel wires, or S-wire. Lastly, the cable and suspender ropes were painted with a highly elastic noxide paint that would further protect the cable and allow for necessary movement.
The SAS Construction Process
The bridge’s temporary supports were constructed to support the new bridge in parts. “If it seems like we built a bridge in order to build another one, you would be correct,” says Jackson of Caltrans. The bridge deck pieces were constructed off-site and brought to the bridge supports one-by-one. Overall, there are 28 deck sections of lengths spanning from 60 to 229 feet long and the weights vary from 559 to 1,669 tons each. Each roadway piece was then slid via a tram of rollers to their respective locations. The main cable’s tower was then constructed.
Once the tower was completed and the main cable’s saddle, the main cable stay was then strung into place and installed wire by wire. Cable bands were used to temporarily bind together the wire bunch along the length of the entire cable. Suspender ropes were then hung from the main cable and sufficiently attached to the main cable. The bridge’s weight was then transferred from the temporary supports to the main cable via the tensioning of the suspender ropes. In December of 2012, the temporary supports were in the process of being removed. Once finished, the main cable will be covered and protected, leaving the work complete.
A giant crane barge called the Left Coast Lifter was a massive shear leg crane barge used to build the SAS. The crane’s boom weighs 992 tons, runs 328 feet long, and can lift up to 1,873 tons, which is an amazing feat for a barge to do from the water. The crane’s primary purpose was to bring steel to the site for the temporary support structure for the SAS. Secondarily, the crane helped to lift and place the deck segments from the eastbound and westbound roadways of the SAS.
Four hydraulic compaction devices were used to compress the suspender ropes along the main cable. Compaction of the main cable stay began at the top of the 525-foot tall tower. The compactors were then moved downward along the cable at 1.5 meters at a time. At each step, temporary galvanized carbon steel bands were wrapped around the cable’s strands. Permanent cable bands were then attached in place of the temporary bands and were used as the permanent attachment locations for the suspender ropes. The pressure of the strand compaction process went up to 9,350 pounds per square inch. Each steel compactor machine contained six hydraulic jacks and weighed 30,000 lbs.
Manufacturing the Suspender Ropes
WireCo WorldGroup provided the entire shipment of the suspender wire ropes, the handrail cables, wire rope for the cat walk assemblies, and the tower tie back strands for the SAS. The suspender ropes are galvanized steel of varying types and sizes including some with 6 strands and others with 8 strands. “Working with Caltrans was an efficient and smooth process. A full-time inspector from Caltrans stayed at our manufacturing plant for a full year to oversee the quality of the wire ropes,” says Richard Humiston, Global Market Director of WireCo WorldGroup.
While Caltrans developed a video of the construction of the bridge, they included footage of WireCo’s manufacturing process for the suspenders. This footage will soon be available via a movie that is soon to be released about the construction of the SAS. The San Francisco-Oakland Bay Bridge project was the largest shipment that WireCo WorldGroup had made at the time. A little bit later, WireCo started a larger order of suspender ropes for the Manhattan Bridge reconstruction project. Humiston says, “There was a time when we were shipping ropes to the East and West coasts at the same time. It started to get hectic, but we were able to handle the heavy load and to supply each bridge successfully, even ahead of schedule in some instances.”
Humiston summarized the Bay Bridge SAS by highlighting an interesting fact about bridge construction. For the older Manhattan Bridge, the wire ropes WireCo manufactured were designed as exact replacements for the product that was originally installed on the bridge. For the revitalized San Francisco-Oakland Bay Bridge, Humiston says, “This is a new project with revolutionary engineering that has now become the first, longest SAS Bridge in the world that can withstand the next Big One.”