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|Federal Highway Administration > Publications > Public Roads > Vol. 66 · No. 6 > A Lifeline Link|
A Lifeline Link
by Sybil E. Hatch
After the wake-up call of the Loma Prieta earthquake, which rocked the San Francisco Bay area in 1989, Caltrans is building what may be the strongest bridge in America.
The San Francisco-Oakland Bay Bridge officially opened on
November 12, 1936, with a 4-day celebration of one of the most remarkable engineering feats of its day. The grand opening was the culmination of 3.5 years of "firsts" in the world of bridge construction: the deepest bridge piers ever attempted, the longest crossing over water ever undertaken, and the world's largest bridge pier, among many other major engineering and construction advances.
"This marks the beginning of the greatest bridge ever erected by the human race," said President Herbert Hoover at the groundbreaking ceremonies in 1933. And, indeed, until October 18, 1989, the San Francisco-Oakland Bay Bridge was invincible, carrying hundreds of thousands of cars every day.
The collapse of a portion of the upper deck of the east span during the Loma Prieta earthquake, which registered 7.1 on the Richter scale, reinforced what many already knew: the critical need for seismic safety upgrades on all Bay Area bridges. The deck portion was repaired, but the California Department of Transportation (Caltrans) realized that the east span of the bridge was vulnerable to seismic shaking and the liquefaction that occurs when soils, especially saturated sands, turn into a liquid state.
A subsequent Caltrans study determined that replacing the existing east span with a new bridge would be more cost-effective than seismically retrofitting the existing structure. Thus began the process of creating, designing, and constructing the new east span from Oakland to Yerba Buena Island, which is roughly halfway between Oakland and San Francisco.
Although the new bridge will not boast the number of world records that the original structure did, says Rick Morrow, supervising bridge engineer for Caltrans, "it is equally monumental in its size, complexity, scope, and impact."
Genesis of a Bridge
During preliminary design of the new east span, Caltrans conceptualized a continuous skyway from Oakland to Yerba Buena Island, basically two elevated concrete segmental structures. The design was subsequently modified, based in part on extensive input from public hearings, public comment on the environmental document, and coordination with the Metropolitan Transportation Commission (MTC).
MTC created two project advisory committees: the Bay Bridge Design Task Force and its technical advisory committee, the Engineering and Design Advisory Panel. The task force reviewed the project from the initial stages of choosing the bridge design materials and alignment through detailed development of elements such as lighting and a bicycle and pedestrian path.
A design competition held in 1997 resulted in a number of creative approaches to a "signature span" for the segment over the shipping channel adjacent to Yerba Buena Island. The suggestions came from architects, engineers, and even local schoolchildren.
MTC recommended two combination skyway/cable-supported concepts, each of which included a skyway structure for approximately 80 percent of the entire bridge length. Thirty-percent designs were prepared for both of the concepts, which helped Caltrans and MTC effectively assess and select a final design for the replacement bridge.
Four Bridges in One
In December 1998, Caltrans announced its choices (listed here from east to west):
Two temporary steel structures will be designed and constructed at Yerba Buena Island to accommodate traffic detours during construction of the Yerba Buena Island transition structure. Alternatively, a double-deck detour structure may be constructed on the south side of the existing bridge.
What is a self-anchored suspension bridge? Because the geological conditions of San Francisco Bay cannot support economically cable anchor foundations where the new east span is located, the suspension span is self-anchoring, which means the main suspension cables are anchored in the superstructure itself.
A single suspension cable will wrap over the tower and underneath the western end of the span, then wrap over the tower again and anchor in both superstructure boxes at the eastern end. Suspenders will connect diagonally from this cable, which crosses over the roadway to the outside edges of the superstructure. Drivers traveling over the bridge will not only be treated to stunning views of the bay, but also, when reaching the suspension cable section, they will look up to see a breathtaking crisscross of cables overhead.
The new east span will consist of two side-by-side bridges, each with five lanes plus shoulders, and a bicycle/pedestrian path. In addition to addressing seismic concerns, the new bridge will provide critical operational and safety improvements over the current bridge, including shoulders and standard-width lanes. The existing bridge will be dismantled.
At $2.6 billion the new east span is the largest public works project in California history. Building it will take 90,700 metric tons (100,000 tons) of steel; 54 million kilograms (25 million pounds) of reinforcing steel; 60,600 meters (200,000 feet of piling); and around 334,000 cubic meters (437,000 cubic yards) of concrete, making it the largest ongoing bridge project in the Western Hemisphere. The bridge is scheduled for completion in 2007.
FHWA is acting in a technical support and review role and is providing $642 million of Federal funding from the Highway Bridge Rehabilitation and Replacement Program.
Major Seismic Upgrade
Because it is a vital link in emergencies and a major connector for other transportation hubs, the east span is classified as a lifeline structure, meaning that the bridge must remain serviceable after a major earthquake. Standard practice in California is to design structures for the maximum credible earthquake. On the east span, however, Caltrans made a huge step forward in seismic design, basing the design on a statistical approach to seismic hazards.
The statistical analyses defined a "safety-evaluation earthquake," which in this case corresponds to an earthquake that is so intense that it can be expected to occur only once every 1,500 years, and a "functional-evaluation earthquake," corresponding to an earthquake that would occur only once every 450 years. The safety-evaluation earthquake is the more stringent of the two criteria.
In the wake of a functional-evaluation earthquake, the bridge will provide full service almost immediately, and damage to the structure will be minimal. But the new east span is being designed also for the more stringent safety-evaluation earthquake. After a safety-evaluation earthquake, the bridge will provide full service almost immediately. Damage sustained—including yield of reinforcement, concrete spalling, and limited yielding of the structural steel—will pose only a minimal risk to functionality and should be easily repairable.
Seismic Safety Advances
The new east span incorporates important seismic safety advances, which will help the structure absorb shock and allow for differential movement between segments. For example, the skyway uses a hinge beam system, which includes 0.8-meter (2.6-foot)-diameter steel tubes placed between each segment of the superstructure. The steel tubes will allow superstructure segments to slide during expansion and contraction from temperature changes or an earthquake. Supplemental dampers will absorb the earthquake energy, preventing damage to the main structure.
The suspension span single tower design also will allow for greater movement. The single tower consists of four separate legs connected by cross beams. Like the hinge beam system in the skyway, the cross beams will absorb earthquake energy, thereby preventing damage to the tower legs. If one of the legs does sustain damage, the other legs will keep the bridge standing.
The transition spans between the skyway, the suspension bridge, and the Yerba Buena Island structure are connected with hinges, designed to allow the structures to move relative to each other in the longitudinal direction only. This restriction of movement helps ensure that the four elements of the bridge will maintain their different dynamic responses to a seismic event, thus improving the bridge's integrity.
Time for Construction
Caltrans held groundbreaking ceremonies for the new east span on January 29, 2002, nearly 70 years after the groundbreaking for the original bridge. California Governor Gray Davis declared, "This will be the strongest bridge in America and perhaps the strongest bridge in the world."
Discrete projects offer opportunities for smaller businesses to compete for work on the bridge, a request made by Governor Davis. Although breaking the overall project into smaller projects complicates Caltrans' contract management and coordination efforts, Rick Morrow believes that it is a good strategy for the project.
The construction contracts include the skyway, the geofill (a large mound of soil that acts as the embankment and helps to stabilize the underlying soil to create a roadway for the land portion of the bridge at the Oakland touchdown area), the self-anchored suspension bridge/Yerba Buena Island transition structures (broken up into five or six separate contracts), and the Oakland approach structures. In addition, Caltrans will pursue a separate contract to remove the existing bridge.
"There are many more firms able to bid on a $30 or $40 million contract," says Morrow, "than could bid on a $2 to $3 billion contract. This helps Caltrans to get more competitive bids, and it spreads the work around. It also helps us accelerate the schedule."
Caltrans has built a detailed schedule to manage the multiple contracts. The mega-schedule helps identify construction duration, staging area usage, and potential overlap or space conflicts between various contractors.
First to Bid
Placement of geofill on the Oakland touchdown area of the bridge was the first contract Caltrans put out to bid. The geofill is used to accelerate consolidation of the soft younger bay mud that underlies the site, thereby alleviating excess pore pressures and stabilizing and strengthening the soft clay.
A border riprap barrier for sediment control was constructed around the construction area, and then wick drains and a gravel layer were installed. The engineered fill was placed carefully to avoid overloading the bay mud.
Caltrans estimates that the settlement and pore pressure dissipation will take approximately 9 months. When completed, the Oakland touchdown area will be ready to support the new westbound approach and the relocated maintenance road.
The second major contract, awarded just 1 week after the geofill contract, covers construction of the skyway. Major milestones for the skyway construction include access dredging, cofferdam installation, pile driving, and installation of the superstructure.
The new east span is a bridge of monumental proportions. For example, 160 large-diameter steel pipe piles, up to 100 meters (328 feet) long and 1.8 to 2.4 meters (6 to 8 feet) in diameter, will support the skyway. Pile driving operations for the skyway are expected to take approximately 1.5 years, requiring the use of the largest hydraulic hammers in the world.
Caltrans conducted a pile installation demonstration project in late 2000 to provide invaluable information on construction techniques and possible vibration mitigation techniques.
Once the foundation is complete, the world's largest precast concrete segments—9 meters (30 feet) high, 24 meters (80 feet) wide, and 7.6 meters (25 feet) long—will be used to build the skyway superstructure. Huge cranes will be used to lift the three-story-tall precast segments into place. The bicycle path will be a prefabricated steel structure that will be lifted into place in segments and attached to the roadway with large anchor bolts.
An average of more than 270,000 vehicles travel across the Bay Bridge every day. Maintaining traffic flow during construction is one of Caltrans' key goals.
Because the new east span is being built on a new alignment, the vast majority of construction activities will not require lane closures on the existing bridge. However, there are several key junctions, including the traffic switchover at the Yerba Buena Island tunnel, that will require careful coordination.
Caltrans is still studying this critical switchover. During most of construction, traffic will continue to flow on the existing bridge. Temporary detour bridges, themselves more than 61 meters (200 feet) above mean sea level, will be constructed adjacent to the permanent structure.
The current plan is to detour westbound traffic onto the temporary bridge to the north of the existing structure, and, later, detour eastbound traffic onto the south temporary bridge. A portion of the existing bridge then will be demolished, creating the space to construct the permanent bridge tie-in to the existing Yerba Buena Island tunnel.
As an alternative, both westbound and eastbound traffic could be diverted to a temporary bridge on the south side of the existing bridge. This diversion is more difficult technically, but it would allow access to the tie-in area sooner.
Caltrans is going to great lengths to minimize impact to the flora, fauna, and habitats that share the bay with the Bay Bridge. For example, the underwater sound pressure waves caused by pile driving can cause trauma to fish, increasing stress and the risk of mortality. The vibrations also may disturb the sea mammals that rest on the western shore of Yerba Buena Island.
To reduce the effects of sound pressure waves on nearby fish, Caltrans, consistent with permit conditions, has required the contractors to use sound attenuation devices, either dewatered cofferdams, which the skyway contractor will use on the deepest piers, or a "bubble curtain system." The latter will enclose all permanent in-water piles with a continuous stream of bubbles during pile driving to disrupt propagation of pressure waves through the water.
Dredging, required for barge access, foundation construction, pile cap construction, and for dismantling the existing bridge, may affect fish adversely by increasing water turbidity and making attacks by predatory fish easier. Although most fish avoid areas with high turbidity, Caltrans is taking special measures to lessen impacts, including limiting dredging to specific seasons if feasible.
Dredging also may affect rare aquatic sites within the river estuary, including sand flats and eelgrass beds, which are critical to the overall health of the estuary because they help stabilize shorelines from tidal action and provide habitat to invertebrates and many species of fish. Sand flats and eelgrass also provide a foraging and roosting area for shorebirds.
Caltrans will conduct onsite measures at the Oakland touchdown area to restore eelgrass and portions of the sand flats. Caltrans also will fund $1 million for an eelgrass research project to improve eelgrass restoration methodologies.
Caltrans will take care not to disturb the steep slopes on Yerba Buena Island, which have varying thicknesses of topsoil, and are home to several protected plants. The island is also home to the U.S. Coast Guard and several Coast Guard historic buildings.
One of Many
Although the east span is a large and complex project, it is not the only project intended to improve the seismic safety of the San Francisco-Oakland Bay Bridge.
Additional projects include:
There are seven major State-owned and operated toll bridges round the Bay Area, excluding the Golden Gate Bridge, which is owned and operated by the Golden Gate Bridge, Highway, and Transportation District. Caltrans is performing major upgrades on four of these bridges.
Projects include the construction of a new westbound Benicia-Martinez Bridge, construction of a new westbound Carquinez Bridge, west trestle and fender replacement and deck rehabilitation of the Richmond-San Rafael Bridge, and widening of the San Mateo-Hayward Bridge.
Rick Morrow sums it up, "If you love bridges, the Bay Area is the place to be."
Sybil E. Hatch, P.E., is a contract writer for Public Roads magazine.
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