ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/306900
20 | ASPIRE , Summer 2007 P R O J E C T In the San Francisco Bay area, two world-class segmental concrete bridges are nearing completion. Last winter's issue of ASPIRE ™ featured the San Francisco-Oakland Bay Bridge Skyway (Skyway). The second is the 2.5-km (1.5- mile) -long, 22-span Benicia-Martinez Bridge to be completed this summer. Both are built to withstand earthquakes in high-seismic zones and are considered to be lifeline structures that must remain open to emergency traffic immediately after a major earthquake. The new Benicia-Martinez Bridge was built to relieve congestion, unlike other toll projects in the Bay Area that were retrofit for seismic needs. When the Benicia-Martinez Bridge is completed, it will carry five lanes of northbound I-680 traffic across the Carquinez Strait between Martinez and Benicia, California. Despite corridor expansions, the existing bridge creates a severe bottleneck for traffic, which the new bridge will relieve. The Benicia-Martinez Bridge was designed using SI units. Conversions are included for the benefit of the reader. Because of their similar construction and time frames, the Skyway and Benicia structures invite comparison. They're both designed as segmental concrete box girder bridges built to withstand substantial earthquakes. The Skyway is renowned for the size of its precast segments, which weigh up to 800 tons apiece, compared to 40 to 60 tons for a more conventional bridge. Benicia- Martinez, though, offers its own unique aspects. 'Lifeline Standards' Used T h e C a l i f o r n i a D e p a r t m e n t o f Transportation (Caltrans) required that the Benicia-Martinez Bridge be built to "lifeline standards" because the Green Valley Fault is situated approximately 3 miles east of the bridge. Its location controlled the seismic design. Although both bridges are constructed primarily using the balanced cantilever method, the Skyway features a precast concrete segmental design and the Benicia-Martinez Bridge is cast-in-place with sand-lightweight concrete. The sand-lightweight concrete uses normal weight sand as fine aggregate and profile BeniCia-MaRtinez BRiDge / CARquInEz StRAIt, CAlIfoRnIA EnginEEr: A joint venture of t.Y. lin International, San francisco, and CH2M-Hill, Sacramento, Calif. Caltrans designed the northern elevated approach spans. Pri ME ContraCtor: Kiewit Pacific Co., Martinez, Calif. ConCrEtE SUPPLiEr: Kiewit Pacific Co. and RMC lone Star for the northern elevated approaches LightwEight aggrEgatE SUPPLiEr: Carolina Stalite Company, Salisbury, n.C. rEinforCing StEEL SUPPLiEr: Regional Steel Corporation, tracy, Calif. PoSt-tEnSioning SUPPLiEr: Schwager Davis, Inc., San Jose, Calif. When Lighter is Better Bay-area bridge features sand-lightweight concrete segments with long spans and a seismically resistant design ASPIRE_Summer_2007.indb 20 5/15/07 11:40:29 AM