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/297000
A significant number of bridges in Nebraska must cross waterways or railroads where vertical clearance is critical and the span length required is less than 80 ft. The sparsely populated area located outside of the Omaha- Lincoln metropolitan area has a flat terrain and many small streams that have to be crossed, which requires shallow superstructures. Inverted Tee System First introduced in 1996, the inverted-tee (IT) system is structurally efficient, rapid to build, and economically competitive. Over 100 IT bridges have been designed and constructed by Nebraska Department of Roads (NDOR) and other agencies. The IT system consists of precast, prestressed concrete inverted-tee beams with a 6-in.-thick, cast-in-place (CIP) concrete deck. IT beams are extremely simple to produce and light to handle. The topping reinforcement is a single layer of welded-wire or mild-steel reinforcement located at mid-depth. The system can be used for a span-to-depth ratio of up to 35, making it shallower than other available precast concrete products. The Nebraska IT series was designed in SI units and ranges from 12 to 32 in. deep (designated IT300 to IT800) in increments of 4 in. The IT beams are set at a standard 37-in. spacing, center to center, and can span up to 80 ft. This bridge system is cost-effective for short-span bridges, despite the use of what appears to be "too many" girder lines. The IT beams are extremely simple to fabricate with standard strand and welded-wire reinforcement (WWR), and no tied reinforcing bars. The deck is formed with 32-in.-wide, ¾-in.-thick plywood sheets, which are placed into ¾-in.-wide, 1-¼ in.-deep ledges notched on each side of the web, requiring few field labor skills. The IT shape has no top flange and has a constant web width, and its simple shape allows the use of only one set of forms for various beam depths. Applications T h e I T b e a m s y s t e m h a s v a r i o u s applications, such as: • New construction, where the superstructure depth must be kept to a minimum. This may include high flood-elevation areas and nearly flat road overpasses over flat terrain. • N e w c o n s t r u c t i o n , w h e r e conventional on-site formwork cannot be used due to existing restrictions. • Bridge superstructure replacement, where a greater load capacity i s r e q u i r e d f o r t h e s a m e superstructure depth. • Bridges where structural continuity over the support is required. Fabrication The two components of the concrete b e a m s h a p e — b o t t o m f l a n g e a n d web—are standardized such tha t t h e b o t t o m f l a n g e w i d t h i s t h e same for all beam depths. The web height is easily adjustable within the standardized design at 4 in. increments. Self-consolidating concrete makes this small and efficient shape relatively easy to cast. The only types of reinforcement used in the section are straight, ½-in.- diameter strands and WWR. The beams are relatively lightweight reducing some of the heavy lifting and transportation requirements typically associated with AASHTO or bulb-tee beams. The standardized element shape, with a range of beam heights and only two types of reinforcement—combined with high-quality concrete mix designs and fabrication specifications required by NDOR—provide a superior beam system for the bridges that is both efficient to manufacture and lightweight to install. Design Because the IT beams are not connected in the transverse direction, except at the diaphragms and at abutment turndowns, they should behave similar General cross section of roadway. Figure: Nebraska Department of Roads. Reinforcement details of inverted-tee beam. Photo: Nebraska Department of Roads. Typical inverted-tee in the casting yard. Photo: Concrete Industries. Nebraska's Inverted Tee SHORT- SPAN Bridge System by Fouad Jaber, Nebraska Department of Roads IT system proves quick and efficient C L Project symmetrical about this line Stay in place formwork 34 | ASPIRE , Spring 2013 Nebraska_Spr13.indd 34 4/1/13 11:45 AM