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Seismic Safety

The new East Span incorporates unique safety features that will help it withstand a major earthquake. How major?

According to Caltrans, the new bridge is designed to absorb the largest ground motions expected to occur at the bridge site once every 1,500 years. Engineers call this the "Maximum Credible Earthquake."

In Richter Scale terms, the bridge is designed to withstand an 8.1 to 8.2 magnitude quake as close as 3.2 miles away on the Hayward or San Andreas fault,. In comparison, the Loma Prieta earthquake measured 7.1 and was centered more than 30 miles away.

The likelihood of a Maximum Credible Earthquake happening during the bridge's expected 150-year life is 10 percent. Still, the bridge was designed in anticipation of that event.

The Skyway section is built to sway and slide during a quake. This is accomplished through a hinge beam system of expansion joints that uses 6.5-foot-diameter steel tubes placed in strategic deck segment joints. Acting like shock absorbers, the tubes will allow deck segments to slide during the expansion and contraction caused by a quake as well as by temperature fluctuations, preventing damage to the main structure.

Seismic safety was addressed deep beneath the Skyway as well. The Skyway foundation will consist of 160-8-foot diameter hollow steel piles that will support the section's 14 piers. These 365-ton piles will be driven 310 feet into the Bay mud at an angle, or battered, to obtain maximum strength and resistance.

The Suspension Section tower likewise will allow for movement. Rising 525 feet above mean sea level, the tower will be embedded in rock and will consist of four separate legs. These legs are connected by cross-beams, also known as "shear links" or "sacrificial fuses." In the event of a major earthquake, these beams will deform as they absorb earthquake impacts, sparing the tower from damage. These crossbeams are designed to be easily replaced. In all, the tower is expected to remain stable up to a displacement of approximately five times that of the maximum earthquake the bridge was designed to withstand.

The area where the bridge touches down in Oakland has its seismic design features buried under the roadbed. The challenge was that the new roadbed sits on fill. During an earthquake, the fill could absorb water seeping in from the bay, creating a devastating condition known as liquefaction. When soil liquefies, it acts like quivering gelatin, causing a host of settling and buckling problems.

As a preventative measure, hundreds of fabric wick drains and 4-inch flexible pipes were driven into the fill area to draw invading water up from the substrate into a gravel layer, through which the water would flow back to the bay.

(For an animation of how the wick drains were driven into the fill layer and more information on how the East Span touchdown area was built, visit the Geofill page.)

In addition to these features, some of the bridge piers with footings below the water will have access casings that will allow inspectors to walk down on the top of the pilings and check for damage after an earthquake.

A seismic monitoring system will also be installed at key locations on the bridge to help identify potential structural changes more quickly.

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