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/622975
CONCRETE BRIDGE TECHNOLOGY Steel Forming of Precast, Prestressed Concrete I-Beams by Dr. Maher K. Tadros, e.Construct.USA LLC Precast, prestressed concrete I-beams are the most common girder shape used for bridges in the United States. They can span between 30 and 350 ft, the widest range of spans of any precast concrete girder stringer bridge product. The American Association of State Highway and Transportation Officials (AASHTO) I-beam series has been in use in the United States since it was adopted in 1957. More recent developments are given in the PCI Bridge Design Manual2 and various state departments of transportation websites. Developments include the AASHTO-PCI bulb tee and the deck bulb tee in the 1980s, the Nebraska (NU) I-beam and the Washington State wide-flange girder in the 1990s, and a number of other state shapes in the 2000s and 2010s. As the shape of the I-beam evolves with time, general trends are observed. The new shapes generally have a large bottom flange to allow for placement of more strands and thus an increase in span capacity. The top flange is generally wider and thinner than the original AASHTO beams, with flange width as large as 10 ft, to reduce or eliminate cast-inplace concrete deck forming and thus provide accelerated bridge construction. A number of owners are still using older shapes due to the capital investment involved in creating new forms and the corresponding modifications that might be necessary in the prestressing beds. Yet, another group of owners are seriously considering updating their shapes with new forms. In both cases, it is helpful to understand the features of forming and producing prestressed concrete I-beams. This understanding would help with the decisions needed to modify existing forms and the features desirable for acquiring new forms. This article gives an overview of the historical development of I-beam shapes and the method used to produce I-beams. It discusses the desirable features in optimizing I-beam shapes. Finally, it presents options for possible modification of existing forms. Understanding I-Beam Production I-beams are often produced in prestressing beds that are 350 to 600 ft long. The forms are primarily two-sided forms that are moved in toward each other. They are tied at the bottom and at the top. At the bottom, a flat steel plate runs the full length of the bed. It forms the bottom face of the I-beam and is called the pallet. The pallet is elevated above the foundation to allow for tying of the side forms and for strand draping, if this option is utilized. The tops of the two side forms are tied together with ties at about 30 in. spacing, called the yokes. Because the forms are continuous for the length of the bed, they are made of 40-ft-long sections that are bolted tightly together to ensure straightness for their full length