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/296997
U.S. Highway 2 is the northern-most, east-west highway crossing the United States and the Rocky Mountains. It runs along the southern edge of Glacier National Park and provides access for visitors to both the east and west entrances. On the eastern side, just outside the community of East Glacier Park, Mont., U.S. 2 crosses the Two Medicine River and the gorge that the river has carved through the ages. Rising majestically from the steep banks of the Two Medicine River are two "twin column" piers reaching high above the stream bed to support the new, graceful, variable-depth, segmental box girder bridge. This beautiful structure is the first of its type in Montana. The box girder unit has spans of 290, 520, and 350 ft, while the three east approach spans have precast, prestressed concrete girders made continuous over the intermediate bents and span lengths of 120 ft. Planning D u r i n g t h e p l a n n i n g s t a g e s o f the project, the prices of steel and concrete were quite volatile making it difficult to estimate the cost of the structure or to reliably predict which material would result in the most cost-effective alter native. This was part of the reason that two different s t r u c t u re s w e re d e s i g n e d f o r t h e replacement: a concrete box girder and steel deck truss using weathering steel. Both alternatives were included in the bidding plans and contractors were encouraged to bid on either or both structures. A second aspect of the bidding process required a price for the construction cost as well as a number of days of construction to complete the project. The number of days multiplied by a specified cost per day combined with the construction cost constituted the total bid. As a result, the Montana Department of Transportation (MDT) was able to get the best value in capital cost as well as minimize the inconvenience to the traveling public. Substructure Concer n for the slope stability of the steep river banks prompted a significant geotechnical exploration program and the borings into rock dictated careful consideration for the location of the supporting piers and bents. All substructure members are founded on drilled shafts 6 to 8 ft in diameter taken down and socketted into rock. The main piers (piers 2 and 3) feature four, 8-ft-diameter shafts with a 36 by 36 by 12 ft cap, with heavy reinforcing steel and draped post-tensioning to help resist the loads from the columns located between the shafts. T h e t w i n c o l u m n m a i n p i e r s a re rectangular in cross section (6 by 15 ft) with tapered corners on the long sides and rustication running vertically up the center. They rise higher than the trees, then splay o u t w a r d f r o m t h e c e n t e r l i n e o f t h e p i e r a s t h e y a p p r o a c h t h e superstructure and transition smoothly to meet the bottom of the box section with its sloping webs. Both main piers are fixed piers so the connections profile THE TWO MEDICINE RIVER BRIDGE / EAST GLACIER PARk, MONTANA BRIDGE DESIGN ENGINEER: Jacobs Engineering, Seattle, Wash. PRIME CONTRACTOR: Ralph L. Wadsworth Construction Company, Draper, Utah CONSTRUCTION ENGINEERS: Jacobs Engineering, St. Louis, Mo., and Nutt, Redfield and Valentine, Orangevale, Calif. PRECAST CONCRETE SUPPLIER: Montana Prestressed Concrete, Billings, Mont.— a PCI certified producer POST-TENSIONING AND FORM TRAVELER SUPPLIER: Schwager Davis Inc., San Jose, Calif. MODULAR EXPANSION JOINTS AND DISC BEARINGS: D.S. Brown Company, North Baltimore, Ohio The Two Medicine River Bridge by Bruce Kates, Jacobs Engineering Segmental box-girder bridge complements surroundings, improves access to Glacier National Park 24 | ASPIRE , Winter 2013 P R O J E C T AspireBook_Win13.indb 24 12/28/12 11:39 AM