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/306894
Mendota Bridge, TH 55 over the Minnesota River. (Nineteen-span cast-in-place concrete spandrel arch bridge) TH 371 over the Mississippi River, near Brainerd. (Three-span prestressed concrete girder bridge) TH 57 over the South Branch of the Middle Fork of the Zumbro River, Mantorville. (Three-span, post- tensioned slab) ASPIRE , Winter 2007 | 41 S TAT E C o n c r e t e b r i d g e s h a v e b e e n a k e y component of Minnesota's transportation system for the past century. Numerous arches built in the early 1900s gracefully span the rivers of the state. While cast-in-place concrete bridges were frequently used, the advent of prestressed concrete resulted in the number of concrete bridges rising dramatically and becoming the predominant type of bridge. Today, precast, prestressed concrete and cast-in-place concrete bridges represent over 80% of the new bridges built on Minnesota's highways. Despite the severe Minnesota weather conditions, extreme temperature ranges and heavy use of deicing chemicals, concrete bridges are providing increased service lives due to improvements in materials and corrosion protection systems. Precast, Prestressed Concrete Bridges The first prestressed concrete beam (PCB) bridge in Minnesota was built in 1957 using precast, pretensioned AASHO*-PCI Type I beam sections. This was truly the beginning of a new era in bridge design. In the almost 50 years since then, over 2,800 PCB bridges have been built in Minnesota. During this time, the PCB bridge has emerged as the preferred choice for most situations due to its economical cost and low maintenance requirements. The first prestressed concrete beams ranged in depth from 36 to 54 in. and had narrow flanges and thick webs. Until about 1970, they also had end blocks (thickened webs at the ends of the beams). In 1986, new more efficient I-beam shapes labeled the "M" series were added. These beams have depths ranging from 27 to 81 in. with wider top flanges, narrower webs, and more room for prestressing strands in the bottom flange. The new shapes increased the available span range, expanding the use of prestressed concrete beams in Minnesota. In 2005, an even more efficient "MN" series I-beam was added with depths of 45, 54, and 63 in. The "MN" series beams are more robust and provide more area for prestressing, thus allowing for a reduced number of beams for a given span compared to the "M" series beams. Other factors have improved precast, prestressed concrete beam efficiency. For instance, concrete quality and strength have increased. Required concrete strengths at strand release have risen from the 4500 to 5000 psi range of the early years to a current value that can go as high as 7500 psi, and design strengths have been specified as high as 10,000 psi. Prestressing steel has also changed from the original 3/8-in.-diameter stress-relieved strands to ½-in. diameter and now 0.6-in.-diameter low- relaxation strands. Slab Spans Concrete slab spans, have always had a place in Minnesota's bridge history. Whether it is for short slabs between spandrel beams of concrete arch bridges, voided slabs for grade separations, or the modern post-tensioned slabs capable of spanning greater distances, these spans fill a niche in the appropriate structure type for a particular location. A r c h b r i d g e s , w h i c h a r e r a r e f o r n e w construction, many times require slabs to be rehabilitated for repair and greater traffic capacity. Often historical, they must utilize some of the original construction methods to retain the key classical features. Reinforced concrete slabs have evolved into post-tensioned slabs to accommodate longer spans. Normally, slab spans are chosen to provide a shallower superstructure than can be obtained with the typical beam-slab arrangement. An example is that of a bridge crossing the Zumbro River in the historical town of Mantorville. Here, the highway grade could not be raised due to its proximity to the town's main street. The river carries high flow rates during periods of fast run-off. Also, the townspeople did not want a standard utilitarian bridge to replace the high- Spanning the Land of 10,000 Lakes— Minnesota's Concrete Bridges by Dave Dahlberg, Kevin Hagen, Dave Hall, Brian Homan, Keith Molnau, and Arlen Ottman, MnDOT ______________________ * AASHO was subsequently renamed AASHTO ASPIRE_win07.indb 41 12/29/06 2:34:31 PM