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/319569
segmental design adapts to conditions and includes sensors to assess thermal loading for piers and girders The new Wakota Bridge that spans the Mississippi River near South St. Paul, Minn., connects Washington and Dakota Counties, giving the bridge its name. To meet a variety of challenges, the twin structures of the bridge were designed as cast-in-place, two-cell, segmental box girders. The bridge will benefit future projects, thanks to the embedment of sensors that will provide information to evaluate thermal loading in the piers and girders. The new side-by-side structures carry eastbound and westbound I-494 over the Mississippi River, as well as over railroad tracks, a city street, and a bike trail. With widths of 122 ft and 111 ft at the widened end spans, the twin bridges will each carry 5 lanes of traffic and are the widest in the state. The design required variable-width roadway and bridge geometry to accommodate merging entrance and exit ramps at both ends of the structures. The side-by-side structures replace an existing four-lane, steel, tied-arch structure that no longer could carry steadily increasing traffic volumes. T h e i n i t i a l p l a n w a s t o c o n s t r u c t two, four-lane bridges to double the volume, but a fifth lane was added to both structures early in the design p ro c e s s t o m a x i m i z e re p e a t a b i l i t y a n d re d u c e w i d t h v a r i a t i o n s . T h e w e s t b o u n d b r i d g e , w h i c h w a s c o n s t r u c t e d f i r s t , a l s o i n c l u d e s a pedestrian and bicycle trail with two scenic overlooks. Each structure features parabolically haunched soffits for efficiency and to gain clearance over the navigation channel. The slope of the outer webs was held constant, creating a dramatic width variation in the bottom slab along the haunched spans. segmental Design minimizes piers The span configuration was determined by the Minnesota Department of Transportation (Mn/DOT) bridge office during the preliminary design process. Navigational requirements dictated that the easterly most pier had to remain on the river bank, as the navigation channel along the outer easterly side of the river bends through this stretch. Placing Pier 4 at this location established it at the same spot as a pier from the old bridge. With rock elevations close to the surface on the east bank, placing a new pier here was feasible since spread footings were used for both the old and profile WAKOTA BRIDGE / WASHINGTo N AND D AKoTA c o UNTIES, MINNESoTA Br IDge engIneer: HNTB, Minneapolis, Minn. constructIon engIneer: Janssen & Spaans, Indianapolis, Ind. constructIon engIneerIng & InspectIon AssIstAnce: Parsons Transportation Group, Minneapolis, Minn. prIme contrActor: lunda construction co., Black River Falls, Wis. post-tensIonIng suppLIer: VSl, Grand Prairie, Tex. concrete supp LI er: cemstone, Mendota Heights, Minn. by Dustin Thomas, Minnesota Department of Transportation Looking to the Future The Wakota Bridge near South St. Paul, Minn., features twin cast-in-place concrete, two-cell, segmental box girders. Completed in July, the bridge is being monitored to evaluate thermal loading in the piers and girders. All photos: Minnesota Department of Transportation. 20 | ASPIRE , Fall 2010 ASP10-1704.indb 20 9/17/10 2:05 PM