Bay Bridge Field Trip
Heather Rowe and Jordona Jackson, California Department of Transportation—October 23, 2009
By Jean Hetherington, from the February 2010 newsletter
Photos by Mark Detterman
On Friday, October 23, 2009, almost a week to the day after the 20th anniversary of the Loma Prieta earthquake (LPE), 25 NCGS members and guests had the pleasure of a field trip to the underbelly of the construction site of the new Bay Bridge. This trip was a bit unusual in that members only looked at rocks from afar (actually, only one exposure, perhaps 300 yards away). No standing and arm waving on outcrops. Instead, attendees were treated to a boat cruise on calm bay waters to view the 73-year-old work horse next to its up and coming replacement, a graceful arc of steel and concrete rising out of the bay.
Participants arrived at 1 pm at the San Francisco-Oakland Bay Bridge Seismic Safety Project building in Oakland, where they were greeted by members of the Public Information Team. Heather Rowe gave an informal presentation to the group. With images of the construction project on the screen, taken primarily from the project website, we were told the story of why the new bridge construction is being done, and some of the highlights of recent events for the project.
New bridge construction from Yerba Buena Island to the Oakland “touchdown” is being done because of the significant damage that happened to the old bridge during the LPE. Although the old bridge was designed to be usable for 150 years, its rigid design and substandard seismic elements make it inappropriate to withstand the seismic shaking that is likely to happen during a significant Bay Area earthquake. Heather confirmed that support piers for the old bridge are indeed made of timber. She also said that the bridge failed during the LPE at the place that engineers considered most vulnerable: at the tower where the causeway transitions to the trussed portion of the bridge. Here, the bridge is very rigid and resisted the motion of the earthquake. Failure occurred as the old steel fatigued. Heather suggested that if a Bay Area earthquake of LPE magnitude occurs before the new bridge is finished, the entire section west of the causeway would likely tip over and fall into the Bay. The new bridge is designed to move with earthquake shaking, not resist it. A number of cutting-edge seismic innovations will allow portions of the bridge to displace up to a meter as seismic waves roll through. While the experience would be an “E-ticket” ride for those on the bridge, the span will remain standing. For example, hinge pipe beams have movable sleeves that will slide and absorb the seismic energy by deforming their middle “fuse” sections. The iconic single tower will have four separate steel legs, connected by shear link beams, which will allow the legs to move independently during shaking. Damaged beams and fuses can then be quickly replaced after the earthquake. Overall, the new bridge has been designed for a maximum credible earthquake with a recurrence interval of 1500 years. On the website, this translates to an event on the San Andreas Fault of magnitude 8, or an event of 7.25 magnitude on the Hayward Fault.
Questions arose about whether supports for the bridge were anchored in bedrock. For the Skyway, which will replace the existing causeway, 160 steel and concrete piles are driven into Bay Mud and older sediments that rest on Franciscan bedrock. The angled orientation of these piles provides support in a fashion similar to a tripod. For the tower, which will support the suspension portion of the bridge, concrete and steel piles have been inserted into “rock sockets” drilled into the Franciscan bedrock on the bottom of the bay near Yerba Buena Island. We later learned that one of the piles on the tower’s foundation will have accelerometers affixed to provide shaking data during earthquakes.
Heather was on-site during the Labor Day closure of the Bay Bridge. To appease the media’s appetite for information about progress during the closure, while ensuring safety during construction, Caltrans media people took video as work progressed and supplied broadcast media with downloads, for nearly continuous updates. Heather was very proud of this unprecedented approach to providing information to the public, along with Tweets (twitter.com) that were generated, as well. Public response to both of these approaches was very favorable.
At around 2:15, the information session ended, and the group got outfitted with hard hats, safety glasses and life jackets. The group was then met by Jordona Jackson, who was to be the guide on the boat tour. We walked outside to find a perfect Bay Area October afternoon of warm sun, no wind, and calm, fog-free water on the bay. After boarding the boat, we motored to Yerba Buena Island, where we viewed the “T1 foundation” for the soon-to-be-built tower for the Self-Anchored Suspension (SAS) portion of the bridge. The boating portion of the tour was a challenge. Where is the best place to be on a small boat with 28 people? Near the railing for the best photo-ops, or near Jordona to hear her wealth of information about the project?
With the old / existing bridge in the foreground, the W2 pier can be seen in the background along with a portion of the temporary scaffolding that is used to slide the new road sections in to place. The bedrock in the foreground was conclusively determined to be our outcrop for the day.
At the first “stop”, Jordona described three of the designated Environmentally Sensitive Areas (ESA’s). An ESA for aquatic life is designated here to protect eelgrass from being harmed by the construction. Buoys outline areas of growth. On the tip of Yerba Buena Island, directly under the new construction, is a structural ESA, to protect a torpedo bunker dating from World War II. Further onshore is a cultural ESA, where artifacts of Ohlone Indians, including 4” long obsidian spear points and shell beads, have been recovered. From this vantage, we were able to see rusty, steel scaffolding which was used over Labor Day to slide sections of the old bridge out of the way, so that the “S curve” could be emplaced, diverting traffic so that construction on the new tunnel approach could be started. We also viewed the W2 pier, anchored in Franciscan bedrock on Yerba Buena Island. One of the unique designs of the new bridge involves the use of a single suspension cable that will be anchored within the decks of the bridge at the eastern end. This single cable will wrap around and be held by the W2 pier. A short distance away, we got an excellent view of the T1 pier and foundation, where the SAS tower will stand.
With the old / existing bridge in the foreground, the temporary scaffolding almost obscures the E2 pier.
E2 on the left, with temporary scaffolding supporting the new bridge.
The next “stop” was by the E2 pier, where both ends of the single suspension cable will be anchored. Here we learned of another unconventional aspect of the construction. With typical suspension bridges, the suspension cables are hung first, and the sections of roadbed are attached afterward. For this bridge, the steps are reversed. Scaffolding is being built to support sections of the roadbed as they arrive (by ship). They will be lifted into place to sit on top of the scaffolding. Next, the single cable will be hung and connected to the bed sections. Then, the scaffolding will be removed. Essentially, “two bridges are being built to get one.” From this vantage, we also could see a cross section of the prefabricated sections of roadbed that have already been put in place. A large open space 30’ tall will run the length of the bridge, through its interior. In addition to providing access for repair to the fuses in the hinge pipe beams, there are water and power conduits, restrooms and storage facilities for equipment, all inside the bridge. Fantastic! We were also able to view wire perches hung from the underside of the new span, for the cormorants that will be displaced from their current accommodations on the old bridge, when it’s torn down.
The cracked I-Bar and the temporary fix that failed the day after our trip
Taking it all in!
The next “stop” provided an excellent view of the section of the old bridge damaged during the LPE. Piston-like elements have been attached diagonally across the upper and lower deck, to repair and strengthen the section. Steel salvaged from the repair by construction workers was artistically fashioned into a goblin, about eight inches tall. It was secretly attached to the side of the upper deck, a signature, of sorts, inscribed by the workers. From our vantage, it looked like a smudge of grease. But a photograph was passed around to show the details of the little dancing devil.
Bay Bridge Goblin
Motoring back to the project building allowed time to reflect on the complexity of this giant undertaking, the seismic aspect being only one part. The beauty of this iconic span may render this complexity unappreciated by most, but not by those who participated in the trip this day. Many thanks to Tridib Guha for arranging the trip for the NCGS, and to Heather and Jordona who guided us.
