August 13, 2007 2:11 PM PDT
A seismic shift for San Francisco Bay Bridge
It turned out, of course, that it wasn't a truck but rather the Loma Prieta earthquake, a 7.1 temblor that killed dozens of people in the Bay Area and caused a section of the eastern span of the San Francisco-Oakland Bay Bridge to collapse.
In the years that followed, there was a lot of hand-wringing about the seismic safety of the Bay Bridge, the Golden Gate Bridge and many others in the area. And as a result, starting in the 1990s, it was decided that the eastern span of the Bay Bridge needed to be replaced.
After a lot of public discussion and debate, the powers that be chose an innovative design for the project: A 525-foot, self-anchored suspension (SAS) news:link> bridge that, upon its expected completion in 2013, would be the world's largest such span.
A self-anchored suspension bridge is one that, instead of having the main cable anchored down into the ground, has the main cable anchored to the roadway itself. The theory, in this case, is that by anchoring the cable to the roadway, it will strengthen the entire construction and, hopefully, stand up to a large earthquake.
For most people who live in the Bay Area, the sight of the construction that followed has been a regular component of the drive from Oakland into San Francisco, though the view of the work, off the north side of the bridge, is stunted at best.
So, as the last stop on Road Trip 2007, I dropped in on Bart Ney, the public information officer assigned to the bridge project for Caltrans (California's transportation public works organization), and got a personal tour of the construction, including a boat ride out to where workers are laying the foundation for the SAS section.
And as someone who loves bridges, I have to say, this was quite a treat. I had been wanting to get a close-up look at the work for a long time, but since I don't have a boat, and you can't really see much of the work being done when you drive across the existing Bay Bridge, this was the first opportunity I've ever had to get a sense of what's really happening.
For example, I didn't even know until the boat got close to the construction that the new skyway is actually two different roadways, one for eastbound traffic, and one for westbound. And it will be comprised of four separate sections, only one of which uses the SAS approach.
One thing I remembered about the process of getting the construction going was that several years ago, when the final design was still up in the air, there were several roadblocks standing in the way of the SAS approach. First, some were concerned that such a design wouldn't be seismically safe in an area where it is virtually certain that there will be at least one major earthquake within 30 years. Second, there were major budget worries, especially since the expected cost of the bridge, if done with SAS, would come out at $5.487 billion.
Labor Day bridge makeover
The busy Bay Bridge will be closed as a portion of its eastern span is destroyed. Renderings help explain.
Ney told me that Caltrans administration initially didn't want to go with SAS. Instead, they wanted a giant skyway.
But "that didn't take into consideration what would work best for this area aesthetically," Ney said.
Indeed. The fact is, the project incorporating the SAS section will be one of the world's great bridges, and easily a worthy counterpart to the glorious and famous western side of the Bay Bridge, which connects San Francisco to Treasure Island. Even better, it will be visible largely from the East Bay, an area that tends to get short shrift in great civil engineering projects.
Ney explained that the SAS section's main cable will begin on the eastern side of the bridge, extend up to its tower and then down to its western end, go underneath the roadway, wrap around, head back up to the tower, and then back down again to the opposite side of the bridge from where it begins. The theory is that this creates tension across the entire span of the SAS section that is self-reinforcing and will then be strengthened against an earthquake.
He also pointed out that no known construction method is guaranteed to hold up against a major quake, but that the SAS design is thought to be as strong as any other approach, and therefore should serve the Bay Area well.
One reason for that, Ney explained, is that there are several points along the bridge construction that are intentionally designed to take the force of a big quake. The idea is to direct the force of a quake to these areas specifically because, if they fail, they can be easily repaired without the bridge collapsing or suffering catastrophic damage, as recently happened in Minneapolis.
One such application of this method, Ney explained, is the use of what are called "shear link beams" in the SAS tower, which is really four separate pillars linked together. The shear link beams are designed to move independently of each other in a quake, and can be unbolted and repaired if necessary.
Similarly, the west end of the SAS section has what are known as isolation casings, the largest such casings in the world, Ney said, which are big holes cut out around the columns coming up from the SAS section's foundation. Because the casings are not tied to the soil itself, they can move in an earthquake.
One other example of this design methodology is the use of hinge pipe beams, which connect the eastern skyway section of the bridge to the SAS section. It's sort of like two large dowels, the inside joints of which are designed to allow for some parallel movement in a quake. The hinge pipe beams are actually 60 feet long and 6 inches in diameter. At the beams' center is a "fuse" that can bend in a quake.
"So if the bridge starts moving," Ney said, "that's the weak link. We can replace the fuses with new ones and get the bridge going" again quickly.
As part of my tour, Ney took me out on a boat and we circled around the western end of the new construction. This was great. We moved slowly around the bridge's different sections, coming to a halt right where the new skyway--which will connect to the SAS section--ends, and where the SAS section itself will be built.
There, on a barge, are 13 "gazebos" inside of which the SAS foundation is being created. This is done through 13 "piles," each of which is 10 feet in diameter and 200 feet deep. I couldn't see inside, but I imagined peering down into the depths of the bay.
After this, we turned around and began our trip back towards the Caltrans dock on the eastern side of the bridge. But first, we had to stop and check out what is surely the money shot of the construction.
There, with the existing eastern side of the bridge on one side, the curved double-skyway sits majestically with the East Bay area behind it and the bay in the foreground. For someone who loves bridges, this was a view for all time.
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