The SF-Oakland Bay Bridge: Is the New Bridge Ready for the Next Earthquake?

After nearly 12 years of construction and $6.4 billion dollars later, the old East Span of the Bay Bridge was closed forever to make way for the new East Span.Procession_optimized

The unveiling on September 3 signaled the end of a story that started 24 years ago, when the 1989 Loma Prieta earthquake caused a 250-ton section of the Eastern Span to collapse, killing one driver and seriously injuring the passenger of the vehicle.

The bridge was repaired within a month of the earthquake, but a panel of seismic experts concluded that the entire bridge required seismic safety improvements–the San Francisco side of the Bridge (the West Span) would require seismic retrofit work, and that the entire Oakland side of the Bridge (the Eastern Span) should be completely replaced. Groundbreaking on the new Eastern Span occurred in January of 2002.

This Labor Day weekend, crews spent five days finishing the new Eastern Span–the largest self-anchored suspension bridge in the world. Seismic retrofit on the West Span and the West Approach (the 1-mile stretch of highway connecting San Francisco to the bridge) was completed in 2004.

The San Francisco-Oakland Bay Bridge is one of the country’s busiest bridges, with an average of 280,000 vehicles using the Bridge every day to travel between San Francisco and the East Bay. Bridge construction began in 1933 and finished in 1936–becoming a true engineering feat to the dismay of cynics who believed that the bridge would be impossible to build because of the potential impact of turbulent waters and winds. There were unique design challenges in developing a bridge to span eight miles across the San Francisco Bay. It was not feasible for a suspension bridge (a bridge in which the roadway deck is suspended from cables that pass over two towers; the cables are anchored at either end of the bridge) to span the entire distance between San Francisco and Oakland, and so the result was to build a bridge that combined the best elements of several different designs. The West Span, from San Francisco, is comprised of two suspension bridges that connect to Yerba Buena Island and then to the original East Span of the bridge, which features a truss-cantilever design (a cantilever is a projecting structure supported only at one end). Connecting the East and West Spans at Yerba Buena Island is the world’s largest-diameter bore tunnel, at 76-feet-wide and as tall as a four-story building.

But the Bridge was not designed to sustain the impact of a large earthquake, as seen by the damage from the 7.1 magnitude Loma Prieta earthquake. In the 1930s, engineers assumed that the area’s high winds posed a greater threat than earthquakes, despite the proximity to two major fault lines.

The Bay Bridge was then set to undergo a major transformation to ensure that it could survive the next major earthquake. In order to make the bridge seismically safe, the work was divided into numerous projects with their own unique challenges. Construction began in 2002 to retrofit the West Span and replace the entire East Span with a new design. Replacement of the East Span included the construction of a 1.2-mile Skyway, a Self-Anchored Suspension span consisting of a 525-foot tower supporting a bridge deck connecting the Skyway to the Yerba Buena Island Transition Structure (the tunnel), and the Oakland Touchdown (the east end of the Bridge connecting to the toll plaza).1_Corridor_Arial_0_1

Safety concerns were raised earlier this year after 32 out of 96 high-strength anchor rods holding two seismic safety devices in place failed. The devices, known as shear keys, are located beneath the eastern end of the Self-Anchored Suspension. There are four shear keys beneath the road decks at this end, and four other seismic devices called bearings. The bearings allow the road decks to move slightly during an earthquake, while the shear keys stop the decks from excessive movement. The rods snapped in March when workers tightened them to connect two shear keys. The 96 rods cannot be replaced because they are embedded in concrete directly underneath the decks. To address the problem, workers installed large steel saddles over the two seismic devices that will provide the equivalent clamping force as the original bolt design.

The rods failed because hydrogen contaminated the steel and caused them to crack (hydrogen embrittlement). However, the California Department of Transportation (Caltrans) launched tests early this month to predict the reliability of hundreds of steel rods and bolts on the East Span. The results of the tests have given way to cautious optimism that the parts will not need to be replaced, according to agency engineers. These tests were done to assess the steel parts’ reliability in a marine environment. Some of the tests involved immersing the rods in saltwater tanks in order to simulate a decade of weather conditions, and then slowly pulling them apart to simulate their susceptibility to environmental forces.

Seismic retrofit details

The seismic retrofit of the Bay Bridge involved addressing sections of the West Span of the Bay Bridge and completely replacing the East Span. One of the biggest difficulties was performing work on the bridge while accommodating daily traffic. On September 3, the new East Span was revealed after crews spent Labor Day Weekend taking the old East Span out of service. Retrofit of the West Span included projects addressing the area from the West Approach to the Yerba Buena Island Transition Structure.

West Approach

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The West Approach is a 1-mile stretch in San Francisco bordered by 5th Street and the Anchorage at Beale Street. Seismic safety retrofit work involved completely removing and replacing this stretch that originally had one foundation support system for the upper and lower deck configuration from 3rd Street to Beale Street. Now, each deck has its own independent column and foundation support system, which is crucial to making it seismically sound.

West Span

The West Span lies between Yerba Buena Island and San Francisco, and consists of two complete suspension bridges connected at a center anchorage. It required extensive retrofit work to strengthen these twin suspension spans, while also allowing for a wider range of movement in the case of an earthquake. Crews replaced half a million rivets with twice as many high-strength bolts, added new bracing under both decks, and replaced all the “laced” diagonal crossbeams connecting the upper and lower decks with perforated steel. In all, 17 million pounds of steel were added to the West Span.

Crews also installed 96 viscous dampers that act as shock absorbers to isolate, absorb, and diffuse seismic energy. And to allow for uniform movement during an earthquake, the span’s main suspension cables were fastened by cable bands to the deck, while concrete keys were cast into the bridge supports to keep the road deck from slipping sideways. And most astonishing of all–crews lifted the entire 3-million ton span to install massive bearings or rollers between the roadway and bridge supports. This way, if an earthquake occurs, the bridge deck can roll and glide on top of the supports without major damage to the bridge.

The major structural components of the new East Span are, from West to East, the Yerba Buena Island Transition Structure, the Self-Anchored Suspension Span (SAS), the Skyway, and the Oakland Touchdown. The Yerba Buena Island Transition Structure connects the SAS to Yerba Buena Island and transitions the new East Span’s side-by-side road decks to the upper and lower decks of the Yerba Buena Island tunnel and the West Span.

Self-Anchored Suspension Span

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The signature element of the new East Span of the Bridge is the world’s longest Self-Anchored Suspension Span (SAS) at 2,407 feet. The nearly 1-mile long, single main cable now supports the weight of the bridge. The East Span has a single 525-foot-tall tower featuring a self-anchoring suspension with one cable, unlike traditional suspension bridges such as the West Span that have towers with two main cables that tie into anchorages in the ground. Though it may appear that there are two main cables on the SAS, there is actually just one main cable that is anchored within the decks at the eastern end. Because the SAS is self-anchoring, the roadway had to be built first, and therefore a temporary bridge was required first. Then once the single tower, main cable, and 200 suspender ropes were in place, workers began an intensive load transfer process, transferring the weight of the bridge to the main cable that supports the weight of the deck.

The single tower is made up of four separate legs that are connected by shear link beams, which are designed to move separately and absorb most of the shock during an earthquake. If the beams are damaged, they can later be removed and replaced. There are two very large marine foundations that provide a solid base of support for the tower and the eastern end, while two land-based structures on the Yerba Buena Island support the western end of the SAS, where the tower’s main cable wraps underneath the roadbed. The marine foundations include 13 piles of concrete that extend 196 feet below water to anchor into the bedrock, and 16 piles in the eastern support that extend almost 340 feet to reach the bedrock. The western support reaches down 80 feet through the island’s solid rock.

Skyway

The Skyway was the first section to be completed and is the longest part of the East Span at 1.2 miles long.

skyway_2

The Skyway’s parallel bridge decks will accommodate 5 lanes of traffic each, and provide motorists with open views of the San Francisco Bay. The decks are made up of 452 precast concrete segments supported by pier tables on top of massive pier columns. Steel piles measuring 8.5 feet in diameter and weighing about 365 tons each are driven up to 300 feet below the water into the Bay’s soil. The piles are welded onto pile caps that are underneath the pier columns. For additional stability and resistance in an earthquake, the piles are driven into the soil at an angle (a process called “battering”). In contrast, the original bridge’s wooden pilings extended 85 to 200 feet below the water’s surface.

SAS_base2

Twenty hinge-pipe beams (60-foot long devices) also provide extensive seismic resistance–they are positioned between deck segments as well as where the Skyway and the SAS meet. The hinge-pipe beams are designed to move within their sleeves during expansion or contraction of the decks due to changes in temperature or during an earthquake. They also are specifically designed to absorb the energy of an earthquake by deforming in their middle or “fuse” section, in order to minimize damage to the bridge’s main structure. If necessary, the beams can be quickly replaced. The hinge-pipe beams can also be found where the SAS connects to the Yerba Buena Island Transition Structure, and at the other end of the East Span where the Skyway connects to Interstate 80 in Oakland at the Oakland Touchdown.

Oakland Touchdown

In addition to hinge-pipe beams, the Oakland Touchdown features seismic innovations in its foundations. Columns are pinned at the top to reduce severe motion from being spread to the foundation and to the road-decks. These foundations also use flexible vertical piles to absorb seismic forces.

Take-away

So there you have it–all of the seismic innovations I could find about the new Bay Bridge–the biggest public works project in California’s history. So do you think the Bay Bridge is ready for the next “big one?” The replacement span was designed to withstand the strongest earthquake estimated by seismologists to occur at the site over a 1,500-year period. So I think we can safely say “yes.”

Not only do the seismic innovations of the Bridge protect it from potential earthquakes–they also will provide important data to better understand earthquakes. The Bridge contains monitoring equipment (accelerographs) that collects information on ground motion for the California Geological Survey. After an earthquake, the sensors will transmit the data to create a digital map showing the location of seismic activity and ground motion, and people can use the maps to determine potential damage to infrastructure.

If you have the reached the end of this article, I thank you and give you hearty congratulations!

Resources:
Fact Sheet: Seismic Innovations & Enhancements
http://www.baybridgeinfo.org

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