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/657823
The state route (SR) 520 Evergreen Point Floating Bridge and Landings project is an 8643-ft-long project crossing Lake Washington near Seattle, Wash. The floating bridge portion— the Evergreen Point Floating Bridge— is 7710 ft long, making it the longest floating bridge in the world. It replaces the old SR 520 floating bridge that was opened to traffic in 1963 and that currently holds the record as the longest floating bridge. The Washington State Department of Transportation determined that the old bridge needed to be replaced to improve traffic congestion, maintenance access, and bridge performance under extreme events such as windstorms. Structure Types The project consists of four different structure types supported on floating concrete pontoons or traditional fixed foundations. At the eastern end of the project, a concrete, cast-in-place segmental twin box-girder bridge is supported on spread footings. The twin box-girder bridge is 627-ft-long with a main span of 320 ft. A 190-ft-long transition span connects the box-girder bridge to the floating bridge. At the western end of the project, another 190-ft-long transition span connects the floating bridge to the west-approach structure. The west approach is part of a different contract. The transition spans and their pinned connections accommodate differential displacements and the associated rotations between the floating bridge and the fixed approach structures due to changes in the level of Lake Washington. The support structure for the Evergreen Point Floating Bridge consists of 77 concrete pontoons joined together and anchored to the lake bed with large steel cables. For 1150 ft at the east end, and for 880 ft at the west end, reinforced concrete columns and bent caps are rigidly attached to the pontoons. They support simple-span, prestressed concrete girders with an 8-in.-thick, cast-in-place concrete deck. The typical span is 90 ft. These two regions of the floating bridge are referred to as the "high-rises." The east high-rise accommodates a 5% grade; the west high-rise accommodates a 3% grade. Between the easter n and wester n high-rise regions is a 5580-ft-long "low-rise" portion of the project. The low-rise consists of a precast concrete, segmental, ribbed-slab superstructure supported on circular columns that are rigidly connected to the pontoons. The low-rise has a straight plan alignment profile SR 520 EVERGREEN POINT FLOATING BRIDGE LOW-RISE SUPERSTRUCTURE / SEATTLE AND MEDINA, WASHINGTON BRIDGE DESIGN ENGINEER: International Bridge Technologies Inc., San Diego, Calif. PRIME CONTRACTOR: Kiewit, General Construction, Manson Construction (KGM)—a joint venture, Federal Way, Wash. PRIME CONSULTANT: KPFF and BergerABAM, partners as the prime engineering team, Seattle, Wash. POST-TENSIONING CONTRACTOR: Schwager Davis Inc., San Jose, Calif. Aerial view of state route (SR) 520 Evergreen Point Floating Bridge Low-Rise Superstructure under construction. Photo: KGM. A e r i a l v i e w o f s t a t e r o u t e ( S R ) 5 2 0 E v e r g r e e n P o i n t F l o a t i n g B r i d g e L by David B. Birrcher and Christopher M. Hall, International Bridge Technologies Inc. SR 520 Evergreen Point Floating Bridge Low-Rise Superstructure Precast concrete, ribbed-slab segments 26 | ASPIRE Spring 2016 P R O J E C T