Bridging the bay: the San Francisco-Oakland east span, a 1930s landmark, is being replaced by a seismically advanced structure designed to last for 150 years.
For years, officials with the California Department of Transportation (Caltrans) had been concerned about the aging bridge and its crack-susceptible eyebar design. Then, after the Loma Prieta earthquake in 1989 severely damaged the bridge's east span, Caltrans officials decided the time had come to replace that span with a new bridge that meets modern seismic and safety standards. The preliminary work started shortly after the Loma Prieta earthquake, and construction continues today as the new structure nears completion.
"For 75 years, the Bay Bridge has been the workhorse of the bay area transportation network," says Andrew B. Fremier, deputy executive director of operations with the Metropolitan Transportation Commission for the San Francisco Bay area. "When the spectacular new east span of the bridge opens next year, the Bay Bridge will be recognized around the world as both a workhorse and a show horse."
Here's a look at the past, present, and future of the Bay Bridge and its newest span, which is designed to withstand the shaking associated with an earthquake seismologists expect to occur only once every 1,500 years.
A History of Growth
In 1769, Spanish explorers discovered the entrance to the San Francisco Bay and its surrounding lands, which eventually would house the populous modernday cities of Oaldand, San Francisco, and San Jose. From the establishment of the 21 Franciscan missions along the coast between 1769 and 1823 to the discovery of gold in California in 1848, San Francisco's population boomed, increasing from 1,000 to 20,000 residents by 1850. The bay area's population was swelling on the eastern shores at Oakland as well. During that time, ferry service was the only mode of transport across the bay to San Francisco. By 1856, talks of building a bridge to connect the two cities had begun. However, technology to construct a bridge of such magnitude would not catch up for another 80 years.
Growth in the bay area continued to climb, with San Francisco's population soaring to more than 500,000 in 1920, overtaxing the capacity of the railroads and ferries in the region. When Herbert Hoover, a graduate of nearby Stanford University, was elected President in 1928, he was well aware of the need for a bay crossing. Soon after he took office, President Hoover and California's then Governor C. C. Young formed the Hoover-Young San Francisco Bay Bridge Commission, bringing together stakeholders to find a solution. The solution was a bridge.
The Original Bay Bridge
The first bridge crossing the bay between San Francisco and Oakland opened to traffic on November 12, 1936. The structure consisted of two back-to-back suspension spans, a tunnel on Yerba Buena Island, a cantilevered truss span, and several through and deck truss spans. Construction of the bridge, the longest in the world at the time, began in 1933 at a cost of $77 million, funded through the Reconstruction Finance Corporation, a former U.S. Government agency that lent money to facilitate economic development in the 1930s. At about the same time, the Golden Gate Bridge also was constructed.
The construction project employed more than 8,300 workers and took more than 3 years. The original configuration featured a double-decked design, which carried electric trains and trucks on the bottom level and passenger vehicles on the top level. Each level was bidirectional. In 1958 the electric trains ran for the last time, and, by 1963, the bridge was reconfigured to five westbound lanes on the upper deck and five eastbound lanes on the lower deck. The first year of service after the reconfiguration saw 9 million vehicles cross the bridge. Today, the Bay Bridge carries more than 102 million vehicles annually.
Earthquakes: A Force To Reckon With
Much of the bay area's written history is marred by earthquakes. Of the many recorded quakes, significant damage occurred during those in 1812, 1856, 1865, 1868, 1897, 1898, 1906, 1957, and 1989.
Injuries occurred in many of the seismic events, as well as fatalities in the 1868, 1906, and 1989 quakes. Notably, the 1906 earthquake was the most devastating, causing more than 3,000 fatalities.
The extent and type of physical damage have varied for each earthquake. Most of the destruction affected brick structures such as walls and chimneys, buildings and structures constructed along the bay on manmade ground (usually infilled soils excavated from the bay), and gas and water pipelines embedded in the ground. Fissures, some with water spewing from them, were noted in some areas, with gaps ranging from 1 to 6 inches (2.5 to 15.2 centimeters). Water levels in the bay also were affected during and after some of the events.
Without question, the Loma Prieta earthquake in 1989 had the most impact on the bay area's transportation structures. The quake damaged more than 80 bridges, with 10 needing temporary shoring and another 10 closed due to severe damage. On three bridges, one or more spans collapsed. The most severe damage occurred to older structures on less stable ground, such as the Cypress Street Viaduct, the collapse of which resulted in 42 fatalities. One death also occurred when 50-foot (15-meter) spans failed on the Bay Bridge.
The 1989 quake revealed how critical the bridges crossing the bay area had become in keeping the region functioning. It underscored the need for bay area bridges to be able to withstand strong earthquakes. Given the frequency and severity of such events in the region, today's bridge standards require every structure of significance to be constructed to withstand considerable seismic accelerations. To meet this need, Caltrans was required to perform seismic retrofits to bring the bay area toll bridges up to current standards. The task quickly became a high priority in the bay area and throughout California.
Retrofitting the Inventory
Today, Caltrans owns and operates 7 bay area crossings, which include 10 bridges at Antioch, Benicia-Martinez (2 bridges), Carquinez (2 bridges), Dumbarton, Richmond-San Rafael, San Francisco-Oakland Bay Bridge (2 bridges), and San Mateo-Hayward. Necessary seismic retrofits of these bridges have been completed or are currently under construction through the State's Toll Bridge Seismic Retrofit Program.
In the aftermath of the Loma Prieta quake, Caltrans' engineering staff examined each bridge to determine what retrofit work needed to be done based on traffic loads, expected remaining life, cost of higher post-earthquake performance levels, and other considerations. Each retrofit was designed to a level that, at a minimum, ensures that the bridge, although possibly damaged, will remain standing during an earthquake. For example, the Bay Bridge is located between and within approximately 10 miles of two fault lines, the Hayward Fault that lies to the east and the San Andreas Fault that lies to the west. These faults are capable of producing 7.5 and 8.0 magnitude earthquakes, respectively. Caltrans designed the new bridge to be in service within 24 hours of an earthquake generating motions that are expected to occur once every 1,500 years.
Caltrans gave special consideration to the Bay Bridge and the Benicia-Martinez Bridge, which the California State Legislature designated as "lifeline structures" because of their locations along transportation corridors crucial for emergency relief and economic revitalization following a major earthquake. Based on this distinction, the retrofit strategies for these two bridges incorporated some design elements that exceed the requirements of standard seismic bridge design.
Specifically, Caltrans determined that a seismic retrofit of the Bay Bridge's east truss span would not be cost effective, and after much deliberation of engineering analysis and costs, and public involvement, the agency decided replacement was the best option.
The total current estimated cost for the scheduled seismic retrofit projects is $9.1 billion, which includes the Bay Bridge's east span replacement at approximately $6.3 billion. Toll revenues paid for most of the seismic retrofit work, which was performed through several contracts.
East Span Replacement Project
Replacing the Bay Bridge's east side span has occurred in several phases. In 1998, Caltrans selected a single-tower self-anchored suspension (SAS) design as the signature span for the bridge. The SAS span, a design preferred by stakeholders and the public because of its appearance, has one continuous suspension cable and two anchorage points built into the superstructure. Comparatively, a typical suspension bridge has two suspension cables, each with two anchorages into the ground.
Because the new bridge connects to an existing tunnel and the west side's suspension bridge, both of which are limited to five lanes in each direction, Caltrans did not increase the capacity for the new span. However, the design included a pedestrian/bike path mounted on the south side of the bridge, with access to Yerba Buena Island from Oakland.
After an extensive environmental review process, construction began in 2002 on the segmental concrete box viaduct, or Skyway section of the bridge. The Skyway is the easternmost section and stretches 1.2 miles (1.9 kilometers) from the Oakland touchdown structures to the SAS span. The design is two parallel precast concrete segmental box structures made of 452 segments on 28 pier columns. Caltrans completed the Skyway section in 2008.
In 2010, the agency completed construction of the foundations and spans for the Yerba Buena Island detour, which carries traffic over the east side of the island during construction of the transition spans. The SAS span tower foundation and east pier were completed in 2008. The Oakland touchdown detour, completed in 2012, carries traffic just south of the construction area so the touchdown can be completed concurrently with the SAS and the Yerba Buena Island transition spans.
Innovative Scheduling Solutions
As the largest bridge project in California's history, the Bay Bridge's east span replacement required the monumental task of monitoring the progress of 21 contracts. Caltrans had to keep track of each contract to prevent costly delays and to analyze the potential risks. Should one contract be accelerated or should another be delayed? What are the costs, the effects on schedules, and impacts on the other project contracts? To address these questions and assist management with decisionmaking. Caltrans developed a Corridor Schedule Team.
The Corridor Schedule Team made a number of recommendations along the way to mitigate potential risks to the construction schedule. In particular, two suggested measures significantly improved the project schedule: the Yerba Buena Island detour and the Oakland touchdown detour.
Construction of the Yerba Buena Island detour structure limited disruption by shifting traffic off the original roadway over Yerba Buena Island and onto a temporary detour. Crews then could demolish the existing spans west of the cantilever truss and construct the transition structures on Yerba Buena Island concurrent with the construction of the SAS spans.
In the first of two phases, the single-level west tie-in structure was precast and rolled into place during a weekend bridge closure over Labor Day in 2007. Caltrans then constructed the remaining portion of the detour structure and prepared the existing structure for removal. Two years later, during another Labor Day weekend closure. Caltrans removed the existing double-decked truss span and slid the east end of the detour into place, completing the detour. These measures helped to keep traffic disruptions between San Francisco and Oakland to a minimum.
The second strategy involved redesigning and advancing the construction of the Oakland touchdown by building a shoofly detour (a short, temporary bypass) to divert traffic just south of the existing roadway. Caltrans completed most of the work prior to the 2012 Presidents Day weekend closure. Before the detour went into effect, construction of the new eastbound lanes was blocked by the original bridge's westbound lanes. This detour has enabled Caltrans to build the permanent eastbound touchdown at the same time as the SAS spans and the transition structure, shortening the total project timeline by an estimated 4-6 months. The detour also means Caltrans can switch both directions of traffic simultaneously when the new bridge is complete.
"Using an innovative risk management program has enabled us to analyze risks to multiple contracts and make modifications that minimize impacts to budget and schedule," says Tony Anziano, toll bridge program manager with Caltrans. "The work performed on Yerba Buena Island and on our Oakland touchdown will give us the ability to open the entire bridge at once ahead of our previous schedule."
Status of SAS and Transition Spans
Construction on the signature section of the bridge. the SAS span, began after completion of the tower foundation and the east pier in 2008. Caltrans started the SAS span by constructing a complex structure of steel truss falsework to support the deck sections.
In January 2010, a shear-leg barge crane built specifically for this project began placing SAS deck sections at the west end of the previously installed steel truss falsework. The deck sections were supported by the falsework until the suspenders could transfer the deck load to the suspension cable. However, before the transfer, the workers had to construct many additional bridge elements. For example, the tower foundation awaited tower legs, several deck sections needed to be installed and welded together, the suspension cable needed to be strung, and suspender clamps and cables needed to connect the deck sections to the suspension cable.
The westbound transition superstructure, which connects the SAS section to Yerba Buena Island, consists of cast-in-place, post-tensioned concrete box girder spans. By June 2011, Caltrans had nearly completed formwork for the westbound transition structure and, by August, much of the westbound transition span was cast.
"Construction progress has been great for both the SAS and the transition structures," says FHWA California Division Administrator Vincent Mammano. "When the tower went up and the four suspension cable catwalks were draped from the tower to the deck in June 2011, it was like the raising of a flag. The bridge became visible to the bay area."
On October 28, 2011, Caltrans reached a major milestone when it placed the last deck section at the northeast corner of the SAS span. This section houses one of the anchorages for the suspension cable strands and signaled the start of placing the suspension cable. Caltrans' next step was to install the cable haul assemblies needed to carry each of the 137 suspension cable strands from spools up to the tower, down and around the three saddle plates of the west end, back up over the tower, and finally to the southeast anchorage.
The suspension cables of typical suspension bridges are built by stringing a single wire back and forth between anchorages until the required number of wires is met. When a spool runs out of wire, the wire of the next spool is connected by a splice. The SAS suspension cable strands have no splice locations because they are constructed with continuous 1-mile (1.6-kilometer)-long wires that run from anchorage to anchorage. Strand installation is much faster than single wire installation. The SAS suspension cable has 137 strands, each with 127 wires, for a total of 17,399 wires.
When installing the cables, the workers have to push each strand between saddle plates at the seven saddle locations along the SAS span. To ensure that there are no twists in the strands, two wires are colored in each strand, one blue and one red, which are inspected at each saddle. Each wire is galvanized and has an application of grease to help prevent corrosion. When all the strands are in place, the workers then compress them into a round shape. After being shaped, the cables receive an application of a zinc-rich paste to prevent corrosion, are wrapped with S-wires (an interlocking flat wire with an "S" cross-section), and are coated with an elastic primer and paint.
The Caltrans construction crew installed the first strand in December 2011. By June 2012, Caltrans had installed all of the 137 strands. The workers used timber blocks to wedge the plates and to press the strands deep into the saddles. Each strand was numbered and had a designated location at each saddle and at the anchorages.
In February 2012, Caltrans completed the Yerba Buena Island westbound transition spans including post-tensioning, and progressed with removal of the falsework. Simultaneously, crews were installing falsework and forms in preparation for casting the eastbound transition spans. The transition spans are scheduled to be ready for the estimated 2013 bridge opening.
"When this project opens to the public, they will be awed by what they behold," says Kenneth Terpstra, Caltrans' project manager for the east span replacement. "Personally, I'm struck by the passion of the myriad of disciplines involved: engineers, seismologists, architects, contractors, environmentalists, and administrators. These professionals are producing a project that solves the most difficult engineering, seismic, and architectural challenges."
Remaining Steps To Completion
With the installation of the suspension cable complete, the construction crews are clamping suspender cable hardware onto the cable at strategic locations in preparation for transferring the weight of the deck to the suspension cable. After the deck weight is transferred to the suspension cable, Caltrans will remove the falsework trusses. At the same time the SAS falsework is being removed, the eastbound transition spans are schedule to be completed.
The opening day for traffic on the new bridge is planned for Labor Day of 2013, with demolition of the old bridge beginning soon after.
"The replacement of the east span of the San Francisco-Oakland Bay Bridge is a historic project," says San Francisco Mayor Edwin Lee. "Not only does it ensure the safety of the hundreds of thousands of commuters, residents, and visitors who cross the bridge every day, but it is also an example of innovation, architecture, and engineering that the San Francisco Bay area is known for throughout the world. This replacement represents an investment in our critical infrastructure and creating jobs, and I am looking forward to the opening of the new east span next year."
For more information, visit baybridgeinfo.org or contact Greg Kolle at 916-498-5852 or firstname.lastname@example.org. The authors would like to thank the Caltrans District 4 project staff their consultant designers. the Bay Area Toll Authority the Toll Bridge Program Oversight Committee, the city of San Francisco, and the contractors' staff for taking the time to accommodate site visits for this article.
RELATED ARTICLE: Facts and Figures for the East Span Replacement Project
* The original east span of the Bay Bridge suffered damage in the Loma Prieta earthquake in 1989 when 50-foot (15-meter) spans failed.
* Estimated at $6.3 billion, the east span replacement is one of the largest bridge construction projects in the United States.
* The east span replacement is designed to withstand the shaking associated with an earthquake seismologists expect to occur only once every 1,500 years.
* The Skyway is the longest section of the new east span at 1.2 miles (1.9 kilometers).
* The Skyway design is two parallel precast concrete segmental box structures made of 452 segments on 28 pier columns.
* The SAS suspension cable has 137 strands, each with 127 wires, for a total of 17,399 wires.
* The SAS span is the largest of its kind in the world and has only one tower and one main cable.
* The opening clay for traffic on the new bridge is planned for Labor Day 2013.
Nancy E. Bobb was the major project oversight manager and director of State programs with the FHWA California Division Office before retiring in 2008. For 6 years, she served as the major project oversight manager of the project to replace the east span of the Bay Bridge. She holds a B.S. in civil engineering from the University of Nevada, Reno, and an M.S. in civil engineering from the University of Calitbrnia, Davis.
Greg A. Kolle is a bridge engineer with the FHWA California Division Office. Since 2008, he has served as the major project oversight manager for the project to replace the east span of the Bay Bridge. After serving in the U.S. Air Force from 1972-1976, Kolle received his B.S. in civil engineering from North Dakota State University at Fargo.
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|Author:||Bobb, Nancy E.; Kolle, Greg A.|
|Date:||Sep 1, 2012|
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