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/297006
Rail transit projects have become more prevalent over the years as municipalities look to encourage rail transport to alleviate congestion. In a growing number of cases, segmental concrete bridges are creating the optimum solution for cost-effective, quickly constructed, and aesthetically pleasing structures. Today, 35 major metropolitan areas have passenger rail systems, with many of them elevated above surface streets and highways. These systems are often challenging to construct, with limited right of ways, obstructions to span, and small radius curves. They also are highly visible to the public, requiring more attention to detail, especially on the substructures and the underside of the superstructures that are more apparent to pedestrians and drivers. Segmental concrete bridges offer a variety of benefits that overcome these design and construction challenges. They provide an optimum span length of 100 to 150 ft but can extend to 350 ft or more when required. Segmental construction can be used in a variety of difficult site conditions. Piers can be set in tight footprints, and superstructures can go over and around community landmarks and roads. The segments also can be set from above, alleviating ground congestion and disruptions. Segmental designs allow tight radii for curved spans. Precasting, using shortline casting methods, allows segments to be cast and erected with speed and accuracy. Generally, no specialized transport e q u i p m e n t i s n e e d e d t o d e l i v e r segments to the site, where they can be picked and placed immediately, allowing faster on-site handling. Power supplies and other rail service requirements can be located inside the box girders leaving an uncluttered outside concrete appearance. The designs offer aesthetic versatility by allowing concrete to be cast into sleek geometric shapes from piers to superstructure and railings. Designs also can use formliners to create textures that fit with an existing neighborhood. Tr a n s i t p ro j e c t s h a v e b e e n u s i n g s e g m e n t a l c o n c re t e c o n s t r u c t i o n since the early to mid-1980s, and they continue to grow in number and diversity, as the following examples show. Atlanta's mARTA Completed between 1983 and 1985, two precast concrete, segmental, post-tensioned structures for the Metropolitan Atlanta Rapid Transit Authority (MARTA) were the first of their kind for a railway bridge in the United States. They showed transportation officials nationwide t h a t s e g m e n t a l t e c h n i q u e s c o u l d economically solve bridge construction needs in heavily congested urban areas. The project's original plan for using cast-in-place concrete box girders was value-engineered to a precast concrete segmental design erected using span- by-span construction with external post- tensioning tendons. The tendons were located within the box-girder void but external to the concrete. The first segmental structure is 5230 ft long and 30.35 ft wide, and was d e s i g n e d t o c a r r y t w o t r a c k s . I t consists of simple spans ranging from 70 to 100 ft in length. The box-girder superstructure segments are 10 ft long, 7 ft deep, and weighed approximately 30 tons. The second structure, which is 1900 ft long, has span lengths from 75 to 143 ft. It includes a four-span continuous unit. Continuity was accomplished by The first precast concrete, segmental, post-tensioned rail transit bridges in the United States were built for the Metropolitan Atlanta Rapid Transit Authority in 1985. Photo: FIGG. by William R. Cox, American Segmental Bridge Institute Overcoming challenges with cost-effective, low-maintenance, segmental concrete solutions Rail Transit Bridges 26 | ASPIRE , Summer 2013 Book_Sum13.indb 26 7/1/13 6:59 AM