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/297030
42 | ASPIRE , Summer 2012 S TAT E N ebraska is not a stranger to concrete bridges. State inventory figures reveal that there are approximately 5000 cast-in- place concrete (mostly slab) bridges and 1500 precast, prestressed concrete girder bridges. A look at some of these bridges demonstrates how rapidly Nebraska's bridge technology has evolved over the years and why the state is among the pioneers in concrete bridge technology. The earliest concrete bridges in Nebraska date to the early 1900s; these modest concrete bridges, including concrete arch and box culver t s, appear throughout the state. By the second decade of the twentieth centur y, concrete gained favor as a material for a bridge's superstructure. Concrete plans were standardized and concrete slab bridges and concrete arch bridges became increasingly common into the 1920s and beyond. In 1911, state legislation created the State Aid Bridge Fund, which led to incr eased funding and building of bridges. A shortage of funds during the Depression forced Nebraska to phase out the popular program, with the last appropriation occurring in 1933. During the fund's existence, a total of 97 bridges at 80 locations were built or purchased. These bridges included designs popular during the period, including steel trusses, stringers and transverse-joist girders, concrete arches, and girders. A surprising number of well-preserved concrete bridges survive from this period. Jointless Bridges Emerge In the 1930s and 1940s, concrete bridges continued to become more economically feasible and increased in number. In the 1950s, Nebraska was among the first to build precast, prestressed concrete bridges, with four of these bridges located along the Sherman Reservoir in central Nebraska. These were simple-span girder designs without diaphragms at the interior supports. This design evolved in the following decades to simple span for dead load and continuous for live load by adding diaphragms at the interior supports starting in the 1970s. Thus, the concept of "jointless bridges" was born. The jointless bridge system eliminates all expansion joints over the entire length of the bridge superstructure, and limits joints to locations at the junction between the bridge approach slab and the pavement. With this system, the opportunity for joint leakage over the abutments and pier bearing areas is eliminated. This is not only an important durability issue, but also a very significant aesthetic issue. Having expansion or separation joints through the deck over the supports creates the potential for stains, cracking due to freezing and thawing, and spalling. Performance of bridges designed using the Nebraska system have stood the test of time. A number of structurally sound, 40-year-old bridges are being replaced due to Interstate realignment. The girders being removed appear to be as good as new, despite Nebraska's harsh weather and use of deicing salts. This validates the Nebraska Department of Roads' (NDOR's) design philosophy, which results in significant savings in prestressing levels and better control of cambers. university Influence N e b r a s k a i s t h e b i r t h p l a c e o f t h e N U (Nebraska University) hard SI I-girder series, developed in 1992. The NU girder series, NU900 to NU2000, vary in depth from 900 to 2000 mm (36 to 79 in.). These new shapes, despite the national retreat from the SI system, proved to have significant advantages compared to the previously used AASHTO girders. They: • are more stable during handling and erection, • house more strands, resulting in longer span capacities for the same depth, and • take advantage of the improvements in concrete technology such as high- strength, self-consolidating concrete that allows for thin, cross-section geometry. This new series of girders was selected as the standard girder I-shape by NDOR, providing a clear direction for consultants, suppliers, and contractors. The ability of the 38-in.-wide bottom flange to hold up to sixty 0.6-in.- diameter strands creates the potential for a relatively shallow structural depth. A 5.9-in.- thick web allows ample shear capacity while keeping the girder weight to a minimum. A 48-in.-wide thin top flange reduces the cost of deck forming, improves buckling stability, and minimizes the girder weight. Since its introduction, the NU girder series has been exclusively used in Nebraska for spans up to 206 ft. The 204th Street Skyline Bridge in Omaha, completed in 2004, was the first bridge to achieve this span length. It used NU2000 79-in.-deep girders at 9 ft spacing. This highly efficient design was possible t h r o u g h t h e u s e o f a c o m b i n a t i o n o f pretensioning and post-tensioning of three segments with lengths of 28, 150, and 28 ft. The prestressing consisted of 46 pretensioning strands and 45 post-tensioning strands. Self- consolidating, 10,000 psi compressive strength concrete was used for the girders. The bridge's main 206-ft-long span featured three girder segments per girder line with two cast-in-place concrete splice joints. This is thought to be the longest simple span with the greatest girder span-to-depth ratio of any bridge Pioneers in Concrete Bridges by Fouad Jaber, Nebraska Department of Roads The photo shows the historic Hall-Hamilton County Bridge in 1926. All photos: Nebraska Department of Roads. Nebraska bridges evolve from 1900s to 2000s...and beyond ASPIREBook_Sum12_R02.indb 42 6/29/12 12:39 PM