Grafting a new artery

November 2000

Bridges Article December 28, 2000
Printer-friendly version

The first time some people ever heard of the San Francisco-Oakland Bay Bridge was when part of it collapsed during the Loma Pri

The first time some people ever heard of the San Francisco-Oakland Bay Bridge was when part of it collapsed during the Loma Prieta earthquake that shook the area on October 17, 1989. The Bay Bridge, like the many other bridges in the Bay area, lives in the shadow of the Golden Gate Bridge. It only stepped out of that metaphorical shadow because of the strongest earthquake on the San Andreas fault since the San Francisco earthquake in April 1906.

The Loma Prieta earthquake killed 63 people and caused an estimated $6 billion in property damage. One person was killed on the Bay Bridge when bolts holding a section of the upper deck on the truss section sheared, causing a portion of the deck to unhinge and fall onto the lower deck.

After the Loma Prieta earthquake, it was decided that the west span of the Bay Bridge could be seismically retrofit but that the east span would be safer and more economical if it were replaced with a new structure rather than retrofit.

The California Department of Transportation (Caltrans) has settled on a $1.5 billion preferred alternative for the east span. The plan is to build a new bridge located alongside and a little north of the existing bridge. Caltrans is the owner-operator of the Bay Bridge, as well as the toll plazas at each end, as part of I-80.

T.Y. Lin International, San Francisco, is taking the lead in a joint venture with Moffatt & Nichol Engineers, San Francisco, to design the new bridge.

The east span connects Yerba Buena Island on the west end to Oakland on the east. The preferred alternative calls for a single-tower asymmetric self-anchored suspension bridge over the navigation channel near Yerba Buena Island and a segmental concrete haunched girder skyway structure near the Oakland shore.

Traffic will travel on side-by-side decks, with five eastbound lanes of traffic on one and five westbound lanes on the other, with a provision for a future light rail line. The decks will have standard 10-ft shoulders and, on the south side of the eastbound structure, a pedestrian and bicycle path.

Unlike a regular suspension bridge, where the cables are anchored in rock at the ends of the bridge, the cables of the new self-anchored suspension bridge will be anchored in the steel orthotropic deck itself. The long side of the suspension bridge will stretch to the east 385 meters. On the other side of the tower, the suspension bridge will stretch 180 meters.

The suspension tower will stand 160 meters tall, anchored in bedrock below, with a vertical clearance of 52 meters between sea level and the underside of the bridge deck.

Building a self-anchored suspension bridge is somewhat more difficult to erect than a regular suspension bridge, according to Rafael Manzanarez, an engineer at T.Y. Lin International and design manager on the Bay Bridge east span reconstruction project, "because the deck has to be in place before you erect your cable." He explained that temporary towers would support the deck while the cables and hangers were being assembled, and then the weight of the deck would be transferred to the cables.

The skyway structure, with 160-m spans and three or four piers per frame, will cover 2.4 km from the suspension bridge to the Oakland shore. The skyway will rest on steel tubular piles 90-100 meters long driven to the lower Alameda Formation. Including intermediate structures and shore approaches, the east span will cover a total of about 3.6 km.

Spanning the century

When the existing San Francisco-Oakland Bay Bridge opened in 1936, the world was a different place.

The bridge was conceived in the Gold Rush days, but at that time the techniques did not exist to economically bridge a channel that deep and that wide. When the automobile became popular enough to be mass-produced, California authorities decided the time was right for an automobile bridge to replace the fleet of ferry boats that were operating on the bay. In 1928, ferries carried 46 million passengers across the bay between San Francisco and Oakland, according to a report by Caltrans. The passengers were delivered to the ferry ports in Oakland by a system of streetcars.

Spanning the 1.78 miles between San Francisco, on the west side of the bay, and Yerba Buena Island, in the middle of the bay, was a monumental engineering and construction feat. The water is 100 ft deep in some places. The combination of water depth and soil conditions required great ingenuity and some new techniques to place the foundations. California State Highway Engineer Charles C. Purcell, who was in charge of the design and construction, eventually decided to build two suspension bridges joined in the middle by a massive anchorage. The design was based on plans conceived by Daniel E. Moran, at the time the top expert on deep-water foundations.

The east crossing, from Yerba Buena Island to Oakland, was only slightly less difficult to construct. It involved several record feats, according to Caltrans, such as the 10,176-ft cantilever bridge, the longest bridge of its kind at the time, with the world’s deepest bridge pier, sunk 242 ft below water level. Constructing the bridge consumed more than 6% of the country’s steel output in 1933.

The east and west spans met at Yerba Buena Island in a tunnel said to be listed in the Guinness Book of World Records as the largest diameter bore tunnel in the world, measuring 76 ft wide and 56 ft high.

Traffic on the bridge grew quickly. Almost as soon as the bridge opened, traffic exceeded predictions for 1950. By 1958, traffic was so great that California reconfigured the bridge to remove the railway system on the lower deck and put five lanes of westbound car and truck traffic on the upper deck and five lanes eastbound on the lower deck.

And that is how the bridge stands today. Average daily traffic across the bridge is about 270,000 vehicles.

Steve Hulsebus, Roadway Design Manager for Caltrans, said the new Bay Bridge will have a traffic metering system similar to what is on the existing bridge. The system ensures good traffic flow on the bridge by holding cars at the toll plazas and controlling how those vehicles enter the bridge.

"We have cameras on the bridge as well," commented Hulsebus. "Somebody in the toll booth or the administration building adjacent to the toll plaza can monitor traffic." The existing bridge has no shoulders, so any incident, such as a breakdown or a collision, causes a major disruption in the flow of traffic. The new bridge will have shoulders, so an incident can be cleared off to the side and traffic can continue.

By the end of the year, Caltrans plans to have an electronic toll collection system installed on all of its toll bridges in the bay area. Such a system has been working on the agency’s Carquinez Bridge toll plaza for several years. A transponder on the vehicle’s windshield sends a signal to sensors at the toll plaza. The sensors identify the vehicle and automatically register payment of the toll.

Forging ahead

Hulsebus calls the plan for the new Bay Bridge the "preferred alternative." Caltrans has consulting agreements with companies such as T.Y. Lin International and Moffatt & Nichol to provide design and engineering plans, but cannot go ahead with actual work until it has an approved environmental evaluation in accordance with the National Environmental Policy Act.

Hulsebus said Caltrans hopes to break ground in the middle of next year but first must get an environmental evaluation approved, acquire land, acquire construction permits and award contracts. "Our environmental staff is working extremely hard with all the other regulatory agencies that have permitting authority over the bridge . . . to make sure that everybody’s happy with the way it’s going to proceed once we do go to construction."

Hulsebus mentioned several issues to be resolved in an environmental evaluation. Dredged sediment will have to be reused or disposed of. The new bridge is in a different location from the existing bridge, and it occupies a different volume in the water. The existing bridge must be removed in an environmentally sensitive way. The new bridge will inevitably displace some aquatic resources, and that displacement will have to be mitigated.

The environmental evaluation process is usually finished before starting the design process, Hulsebus said, but, "In our case, we’re doing them in parallel because of the seismic importance of getting a new bridge out there for the public safety. We felt it was worth the risk to do this in parallel instead of sequentially to save time ultimately."

The Loma Prieta earthquake registered a magnitude of 6.9 on the Richter scale, the same magnitude as the 1995 Kobe, Japan, earthquake, which killed 6,000 people and caused $100 billion in damage. The difference is that the Loma Prieta quake was centered farther from an urban area.

Based on research since the Loma Prieta earthquake, the U.S. Geological Survey predicts that there is a 70% probability of at least one magnitude 6.7 or greater earthquake striking the San Francisco Bay region before 2030. Such a quake would be capable of causing widespread damage.

The design for the new San Francisco-Oakland Bay Bridge is based partly on lessons learned about seismic protection since the Loma Prieta earthquake.

Four towers in one

The central tower of the suspension segment of the new Bay Bridge looks deceptively like a single tower, but it is actually four towers, which show the results of seismic design. The four legs of the steel tower are connected by shear links, steel elements that tie the legs together, "but the main function of these links is to dissipate energy during a seismic event so that the towers remain elastic," explained T.Y. Lin International’s Manzanarez. "These links can be replaced in case they suffer significant yielding, without detriment to the tower and without disrupting traffic significantly after the seismic event."

The shear links will be connected to the towers using a high-performance steel (HPS-70) to "make sure that the shear links yield before the connections do," said Manzanarez.

The design for the two side-by-side decks of the new Bay Bridge also minimizes the number of expansion joints because, as Manzanarez observed, "these are locations where you typically have more damage during a seismic event. What happens is that the spans fall off the seats." The frame length between joints on the new bridge will be 600 to 700 meters.

The new Bay Bridge will incorporate components borrowed from research done for the seismic retrofit of the Golden Gate Bridge. "On the Golden Gate there was a testing program that was done on expansion joints for dynamic seismic loads," said Manzanarez. "That test is completed, and it was very successful, and we intend to use those joints on this bridge." These expansion joints can handle the rapid movements experienced in an earthquake even though they are typically used to handle the slow movements experienced in temperature changes.

Seismic planning also is going into the piers. Manzanarez said the team is designing the piers to minimize strains in the event of an earthquake. "We’re making sure that the piers are ductile," he said, "that they can take a lot of energy without any damage to the pier."

These components and others for the new Bay Bridge continue to be tested at several universities, including UC San Diego, UC Berkeley and the University of Nevada at Reno.

Caltrans does similar research at universities as part of a continuing program, and directly funds some of this research.

About the author: 
Zeyher is an associate editor with ROADS & BRIDGES
Overlay Init