Many Americans remember the Loma Prieta Earthquake of 1989 that caused $6 to $7 billion in damages. Research from the U.S. Geological Survey suggests that there is a 62 percent probability of another magnitude 6.7 or greater quake causing widespread damage in the Bay region of California by 2031.
This realization has transportation officials and engineers working diligently to reinforce the area’s structures against a future catastrophe. The term for this reinforcement is seismic retrofitting.
Over the past few decades, seismic retrofit strategies have been developed with the introduction of new seismic provisions and the availability of advanced materials like fiber-reinforced polymers (FRP), fiber reinforced concrete and high strength steel.
Currently in the Bay Area the San Francisco Oakland Bay Bridge East Span, the Dumbarton Bridge and the Antioch Bridge are undergoing concurrent retrofits,” said Effie Milionis, public information manager and spokesperson of the California Department of Transportation (Caltrans).
California’s Toll Bridge Program was organized by Assembly Bill 144 in 2005 to implement retrofits/replacements for five of the seven state-owned toll bridges.
The Dumbarton Bridge is one of the structures being retrofitted. The original 1927 drawbridge was the first to carry vehicular traffic across the Bay. It was replaced with a new structure in 1982. The bridge connects the city of Newark on the eastern side of the San Francisco Bay to the borders of the cities of Menlo Park and East Palo Alto on the western side.
The Dumbarton Bridge provides access for approximately 61,000 trips across the bridge between Alameda and San Mateo County each day. It spans 8,600 ft. (2,621 m) accommodating three lanes of traffic in each direction, as well as an 8-ft. (2.4 m) wide bicycle/pedestrian pathway. The five main components of the bridge include: A main channel crossing at the middle of the bridge, an approach structure at each end of the main crossing, and a trestle structure at the end of each abutment of the bridge.
Work on the bridge began in fall 2010. Shimmick Construction, under contract with Caltrans, is performing the work. The $46 million contract is scheduled for completion in 2013. Implementing nighttime lane closures have allowed the contractor to access the interior and underneath of the bridge deck to complete work while traffic continues to cross the bridge. There will be two weekend-long closures required later this year for replacement of the two largest expansion joints.
“A key component to this retrofit is the installation of pendulum bearings at 16 mid-section pier caps,” noted Milionis. “In a seismic event, the bearings allow the deck and the piers to move independently of each other within the bearings’ expanded range.”
Similar retrofits are being added to the nearby Golden Gate Bridge, which is operated by the Golden Gate Bridge, Highway and Transportation District. Like the Dumbarton Bridge, The Golden Gate Bridge is also affected by activity along the San Andreas and Hayward faults.