FALL 2012

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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the up-station and down-station deck segments. N e w g r a d e b e a m s a r e t y p i c a l l y 5 ft wide by 4 ft deep, but increase to 5 ft by 5 ft at expansion joints to a c c o m m o d a t e i n c re a s e d t o r s i o n a l loading from the hammerhead columns. Adjusting the column locations required reconstruction of the existing grade beams, which were supported on belled shafts extending approximately 60 ft below the surface to a hardpan clay layer. The revised column arrangement caused some existing shafts to bear substantial additional loading. In certain locations, pressure-meter testing was conducted to justify increasing the allowable bearing pressure from the 12,000 psf, used in the original design, to upwards of 20,000 psf. All told, 254 of the 264 existing 4-ft-diameter belled shafts were able to be reused, and only eight new shafts drilled. Another main goal of the reconstruction was to upgrade the alignment of both the upper and lower levels. Re-aligning the Lower Wacker Drive service and travel lanes proved to be the greatest geometric challenge because it affected the locations of the columns previously m e n t i o n e d . T h e m a i n g e o m e t r i c modifications to the Upper Wacker Drive Viaduct involved reducing the number of access points between Upper Wacker Drive and Lower Wacker Drive to improve and control the flow of traffic. Another key constraint of the Upper Wacker Drive geometry was that the sidewalks on the edge of the viaduct had to be level with adjacent building plazas. This made for very tight vertical constraints, while still ensuring the viaduct would drain efficiently during a rainstorm. Bridge Deck Design The viaduct deck is a cast-in-place concrete slab that is post-tensioned CITY OF CHICAGO DEPARTMENT OF TRANSPORTATION, OWNER BRIDGE DESCRIPTION: Cast-in-place concrete slab, post-tensioned in both horizontal directions. This portion of the viaduct is 1400 ft long by 140 ft wide and is split into seven segments with 44 spans in the longitudinal direction and five bays in the transverse direction. The average longitudinal span length is about 32 ft. The beam slab is supported on pot bearings that sit on individual columns. STRUCTURAL COMPONENTS: High-strength, high-performance, 6 ksi concrete, post-tensioned with 0.6-in.-diameter, Grade 270, low-relaxation, post-tensioning strands and 2-in.-thick, latex-modified concrete wearing surface BRIDGE CONSTRUCTION COST: Total project cost $300 million; North Viaduct Cost: $60 million ($306/ft 2 ) View from adjacent skyscraper looking down on the existing Madison Street intersection prior to completing deck demolition. Photo: McHugh Construction. ASPIRE , Fall 2012 | 19 AspireBook_Fall12.indb 19 9/18/12 8:58 AM

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