The first Northeast Extreme Tee (NEXT) beam has been awarded. The project in York, Maine, is a seven-span structure over the York River. It will include 28 NEXT beams, varying in length from 55 feet to 80 feet.
The project represents a significant milestone in precast concrete design. The Precast/Prestressed Concrete Institute (PCI) Northeast Bridge Technical Committee developed the NEXT beam in 2008 for the use in medium-span bridges.
Michael Culmo, CME Associates Inc., East Hartford, Conn., and Rita Seraderian, PCI Northeast, Belmont, Mass., described the committee's effort at the 2008 Concrete Bridge Conference.
The committee's objective was to develop a new standard NEXT beam. The goals of this new section are to provide a fast construction option for variable-width bridges with spans up to 90 feet. The section resembles a standard double-tee, except the stems are wider to handle the moment and shear demand for bridge loadings. The top flange of the beam is kept thin and provides a deck form for a cast-in-place concrete deck, saving substantial construction time.
There was a tremendous need for such a design in the Northeast. The notion of a typical bridge here is not realistic. The highway system in the Northeast is very old, and new bridges must be built to very specific widths to fit within the tight confines of the highway's constraints.
Another purpose for developing a double-tee section is the need to accommodate under-bridge utilities. Many streets have underground utilities. The current precast bridge system of choice for short to moderate spans is precast adjacent box beams or slabs. These do not have the ability to accommodate utilities since there is no space between the beams. The new NEXT beam will be able to support several utilities between the stems, eliminating the need for parapet attachments.
The NEXT beam offers significant advantages over typical stringer beam bridges in the construction process. The top flange of the beam is designed to support the weight of the cast-in-place concrete. There is virtually no installation or stripping of formwork required in the field.No diaphragms are proposed for the NEXT beam. Installing diaphragms on stringer bridges is also time-consuming. End diaphragms are proposed to support the free edge of the deck at the supports. It is possible to install these diaphragms as a secondary pour in the precast plant, which will further accelerate construction.
Copies of the paper are available at www.pci.org