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/306897
43 | ASPIRE , Spring 2007 S TAT E T he State of Texas has a long history of successful implementation of concrete bridges, from simple I-beam spans to graceful segmental elevated freeways. Texas itself has a wide variety of geographic areas—plains, m o u n t a i n s , r i v e r s , a n d c o a s t l i n e — e a c h demanding different construction methods and durability considerations. With 33,000 on- system and 17,000 off-system bridges, the Texas Department of Transportation (TxDOT) finds it imperative to maintain economy, durability, and function. Using precast concrete, Texas continues to report one of the lowest bridge costs in the country. TxDOT has been in the precast, prestressed concrete arena for over 50 years. The first bridge of this type, the San Bernard River Bridge in Austin County, was a post-tensioned, cast-in- place slab span built in 1952. This was followed by the first precast concrete beams on the Corpus Christi Harbor Bridge in 1956. Notable structures now abound all across the state. Standard Prestressed Concrete Elements Precast, prestressed concrete beams are the predominant element used in Texas bridges. This is a reflection of the durability, low cost, and adaptability of prestressed concrete. A key factor in TxDOT's widespread use of precast, prestressed concrete beams is cross-section standardization, facilitating economical mass production of these bridge elements. No one cross-section is optimal for all bridges, leading to variations of beam type and size, each targeted to address specific bridge geometries and construction challenges. I-beams are the most frequently used beam section due to their adaptability to a wide variety of span lengths, skew angles, and bridge curvatures. TxDOT uses five specific cross sections—its own Types A, B, and C beams along with AASHTO Types IV and VI. TxDOT's I-beam bridges are a case study in simplicity— the beams rest on elastomeric bearings, no permanent diaphragms between beams are used, and a deck slab, formed with precast sub-deck panels, is placed continuously over a number of spans, forming multi-span units. This simplicity results in TxDOT's low bridge costs. Box beams are employed by TxDOT when the section depth of an I-beam exceeds specific bridge constraints and on rapid construction projects. These beam sections are TxDOT's own and have widths of 4 and 5 ft. They can be made with four depths ranging from 20 to 40 in. Placed side by side on bent caps and set normal to the roadway, the large shear keys are typically filled with concrete and the beams are then topped with either a concrete deck or an asphaltic concrete pavement (ACP) overlay. Transverse post-tensioning is applied only to the beams that will be topped with ACP. Similar to box beams, TxDOT uses nonvoided slab beams, without shear keys and with a cast- in-place concrete deck. Details are provided for beam widths of 4 and 5 ft, allowing them to be fabricated on box beam precasting beds. They are available in depths of 12 and 15 in. Slab beams are excellent for short span bridges and especially when high span-to-depth ratios are necessary. The TxDOT standard beam section best suited for rapid construction is the double tee. Details are provided for three depths ranging from 22 to 36 in. and widths of 6, 7, and 8 ft. Beam to beam connections have evolved over the years and the current connection utilizes a longitudinal bar welded in a v-groove formed by steel plates in the flanges. When speed of construction is imperative, the beams are topped with an ACP overlay; otherwise the beams are covered with a concrete deck. The most unique standard sections are U- beams, developed in the mid 1980s by TxDOT in close collaboration with industry. These beams are tub-shaped with sloping webs and provide a more aesthetic option than I-beams. Standard depths are 40 and 54 in., with maximum span lengths of 105 and 120 ft, respectively. Although more expensive than I-beams, U-beam bridges require fewer beams—due to their high structural efficiency—which can result in an economic advantage. U-beams are being used in urban settings and, when coupled with an aesthetic substructure, present an attractive, clean appearance. Prestressed concrete panels (PCPs) used as stay-in-place forms for bridge decks have become the main forming system for most beam-type bridges built in Texas. Approximately 85 percent of the prestressed concrete I-beam bridges use stay-in-place PCPs. This currently amounts to over 4.5 million ft 2 per year. The panels effectively replace the bottom half of the bridge deck and act as a safe and convenient work platform. The use of PCPs, first researched by TxDOT in the early 1960s and with widespread use in the early 1980s, represents a major improvement in the speed, cost, and safety of superstructure construction. Concrete Bridges in Texas by Randy Cox, David Hohmann, Amy Eskridge, Michael Hyzak, Gregg Freeby, Lloyd Wolf, Brian Merrill, and John Holt, TxDOT TxDOT builds one segmental bridge every two years. ASPIRE_spring_2007.indb 43 3/7/07 12:34:37 PM