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/306888
I-270 Ramp over I-76: 2282 ft long, 230-ft-long maximum span, 764-ft-minimum radius. 64 | ASPIRE , Fall 2008 S TAT E C olorado has benefited from a practice of utilizing a broad range of structure types for its bridges. The Colorado Department of Transportation (CDOT) designers and its consultants have generally been given wide latitude to use the structure type that is optimal for the conditions at a particular bridge site. Consequently, CDOT has a relative large inventory of structural steel, precast concrete, and cast-in-place concrete bridges, with a wide variety of different sections used for these three different materials. Since the 1960s, precast, prestressed concrete beams have become the structure type used most often because of the typical economy and general construction advantages of these beams. Making the beams more efficient and easier to construct has been pursued aggressively. However, precast concrete beams tended to be used in the past for simpler bridges; the exceptions being the seven segmental box girder bridges used for I-70 over Vail Pass and the nine segmental Long-Span Precast U-Girders in Colorado by Michael L. McMullen, Jamal I. Elkaissi, and Mark A. Leonard, Colorado Department of Transportation bridges through Glenwood Canyon. These were especially large and topographically unique projects. Precast girders were generally regulated to spans less than 140 ft and to bridges with little or no curvature. This began to change in 1992 and has increased more in recent years. This year alone, CDOT will construct four precast concrete bridges with span lengths over 140 ft using horizontally curved girders. These will be in addition to the four existing bridges built since 1995 that meet this description. The recently completed I-270 bridge over I-76 is one of these. Although not the first constructed, it was the first bridge designed using yard fabricated, long-segment trapezoidal U-girders for a bridge with significant horizontal curvature. "Long- segment" is used here to differentiate these girders from those conventionally referred to as "segmental concrete box girders" which are typically built with shorter segment lengths. The broadened applicability of precast, p r e s t r e s s e d c o n c r e t e g i r d e r s h a s b e e n accomplished by adopting new beam shapes, increasing maximum shipping lengths, utilizing post-tensioning for splicing, and employing curved segments. In 1992, CDOT transitioned from precast I-beams with 5-in.-thick webs to bulb-tee beams with 7-in.-thick webs, permitting the use of post-tensioning ducts. The first precast concrete long-segment girder spliced with post-tensioning built in Colorado was the Buckley Road Bridge over I-76. This bridge was constructed in 1992 using 72-in.-deep bulb-tee beams with 96-in.-deep haunched bulb-tee pier segments. Before the Buckley Road Bridge was built, CDOT had already begun developing plans for using precast, trapezoidal box girders for long- segment construction. The Park Avenue Ramp over I-25, built in 1995, and the I-225 Ramp over Parker Road, built in 2000, were the first long-segment box girders designed in Colorado Buckley Road over I-76: 515 ft long, three spans, 183-ft-long maximum span. ASPIRE_fall08.indb 64 9/15/08 4:11:20 PM