Source: CP staff
Advanced precast/prestressed production and hauling techniques, coupled with the prevalence of design-build delivery and value engineering, have led to new length records for single-piece, road-delivered concrete girders. Compared to structures of earlier practice, new precast/prestressed bridges can tend toward a) fewer girders at optimized spacing; b) shallower girders for height-sensitive, new or existing roadway conditions; or, c) longer girders bearing on fewer piers.
A December Concrete Products report tracks delivery of record-length girders—up to 205 ft.—to the sites of six state department of transportation-approved bridges. The standardization of higher strength concrete (> 8,000 psi) and 0.6-in. diameter prestressing strand, plus state DOTs’ use of more efficient girder cross sections than those of legacy AASHTO designs, have contributed to engineers’ ability to stretch out single-beam spans, according to Dr. Maher Tadros. As Department of Civil Engineering professor, he pioneered the University of Nebraska’s “NU” bulb tee girder on which much of the new generation of long-span (> 170 ft.) beams is based.
“The myth of going to steel specs if you need a shallower depth or longer spanning girder doesn’t hold anymore. If an agency needs a bridge of a given span and width, I can design it with concrete girders of the same or shallower profile than steel girders. I couldn’t make that statement 20 years ago,” affirms Dr. Tadros, now principal of e.construct.USA LLC, Omaha-based structural engineer, and University of Nebraska professor emeritus.
The precast/prestressed producers behind the record-length girder projects in 2010–2011 are:
• Concrete Technology Corp., Tacoma, Wash., Alaska Way Replacement Viaduct, Seattle (205 ft. girders)
• Hanson Structural Precast, Salt Lake City, Interstate 15 & Beck St. (195 ft.)
• Knife River Corp. Northwest, Harrisburg, Ore., Cornelius Pass Bridge, Washington County, Ore. (184 ft.)
• Standard Concrete Products, Tampa, Interstate 275 & S.R. 686, Pinellas County (175 ft.)
• Cretex Concrete Products, Maple Grove, Minn., Highway 61 at Mississippi River, Hastings, Minn. (174 ft.)
• Northeast Prestressed Products, Cressona, Pa., Lehigh River Twin Bridges, Carbon County, Pa. (168 ft.)
• Bexar Concrete Works I Ltd., San Antonio, S.H. 114 & S.H. 121, Irving, Texas (166 ft.)