THE CONCRETE BRIDGE MAGAZINE

SPRING 2009

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.

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For more information on this or other projects, visit www.aspirebridge.org. A E S T H E T I C S C O M M E N T A R Y by Frederick Gottemoeller Economy and elegance have a fruitful convergence in Minnesota's Crosstown Recon- struction. The Minnesota DOT has learned, as New Mexico, Florida, and other states did before them, that concrete segmental construction provides both an economical and attrac- tive solution for fyover ramps in complex interchanges. Let's start with the economics. In these interchanges the ramp widths can usually be suffciently standardized to produce long lengths of bridges with similar widths. This means that there will be a large number of similar segments that in turn justifes the establish- ment of a casting yard. It also allows the standardization of the substructure, resulting in additional economy through the repeated use of a few standard pier forms. Once those economic basics have been met, the inherent advantages of the box girder come into play. The small footprint of the piers compared to a typical multi-column pier bent means there are more places to put the piers, a great advantage in a complex interchange. That, plus the fact that the critical vertical clearance point is often not on the bottom of the girder but on the bottom of the thin overhanging wing, usually eliminates the need for costly and unsightly straddle bents. Finally, balanced cantilever construction minimizes falsework and allows traffc to be maintained with minimal disruption, another savings. All of these points of economy have their aesthetic payoff as well. First, the box girder, with its wide overhang and deep shadow line, looks thinner than a typical girder bridge of the same depth. The piers occupy a much smaller part of the visual feld than typical multi- column pier bents. All piers are essentially the same, varying only in height. They don't have to be modifed or rotated from place to place, as multi-column pier bents often do, in order to ft into tight locations. The result is an interchange that is easy to see through and to understand, a great advantage to drivers trying to navigate it. Finally, the girders themselves smoothly and continuously parallel the curves of the ramps. They ft right into the inter- change. After all, an interchange is basically an assembly of curves. Mn/DOT will not likely specify duct couplers on future segmental projects until the couplers from each supplier a re f u l l y d e v e l o p e d , t e s t e d , a n d preapproved for use. Mn/DOT certainly wants to maximize corrosion protection of the tendons, but not at the expense of potentially reducing the service life of the bridge deck. The decks for these segmental bridges are critical to the longevity of the structures. Falsework. Precast balanced cantilever segmental bridges are constructed with minimal use of falsework. However, four- legged falsework towers used adjacent to the piers are critical for structural stability during construction. The AASHTO Guide Design Specifications for Bridge Temporary Works includes a diagram showing a 1-ft-minimum set- back clear distance behind a barrier for falsework leg placement. This is shown for a tangent section of highway with full shoulder widths. The Crosstown project has curving, nonstandard temporary alignments weaving through the construction area with minimum shoulder widths. Recent crash studies show that when a truck impacts a barrier, the upper portion of the vehicle extends beyond the barrier into what is termed as the "zone of intrusion." To maintain a safe work-zone, the falsework support legs cannot be allowed to be placed within this zone of intrusion. The contractor understood the risk and responded to this issue by casting a temporary concrete wall, taller than the "zone of intrusion." Two legs of the tower were then fixed to the top of the wall. Cold Weather Issues. The segment casting cycle allows a 3-day curing period after which the segment is moved into an outdoor storage area. Concrete test cylinders are kept with the segment during the initial curing period. When the segments are moved outdoors, the cylinders are put on an outdoor curing rack, which happened to be on the north side of the building. These control cylinders then become the basis for determining when the concrete reaches its required strength to erect the segment. During cold weather, the required 28-day compressive strengths were not achieved on several segments. Prior to erecting these segments in the early spring, the contractor moved them and their corresponding cylinders to a heated enclosure to help increase strength gain. When the initial 28-day strengths were not met, additional cylinders were tested or, as a final step, the segments were cored and the cores tested. All segments eventually attained their required strength. The curing process must be carefully monitored and coordinated with the erection schedule to ensure the proper concrete strength is attained prior to erecting the segments. Conclusion Precast segmental bridges are proving to be economical and a good choice for the Minnesota Crosstown project flyover ramps. Being highly elevated structures, they are especially aesthetically pleasing and are expected to provide the added lasting value of low, long-term maintenance costs. Reference 1. Tse, Joseph K., Paul J. Pilarski, Laura M. Amundson, and Keith Molnau, 2007, "Crosstown Segmental Design Standards," Proceedings of the PCI- FHWA National Bridge Conference, October 22-24, Phoenix, Ariz., 19 pp. ___________ Keith Molnau is bridge design unit leader with the Bridge Office of the Minnesota Department of Transportation, Oakdale, Minn., and Franklin Hines is resident engineer with FIGG, Tallahassee, Fla. ASPIRE , Spring 2009 | 33 Aspire_spr09.indb 33 3/11/09 4:17:28 PM

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