THE CONCRETE BRIDGE MAGAZINE

SUMMER 2008

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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A E S T H E T I C S C O M M E N T A R Y by Frederick Gottemoeller This structure is an excellent reso- lution of a complicated structural and aesthetic problem: the bridge with a branch. Designing a bridge like this with typical steel or concrete I-beams always results in a confusion of beam layouts and awkward connections. The cast-in- place voided slab, on the other hand, adapts readily. All of the connections and points of force transfer are inside the slab. The exterior shows only a smooth continuous surface along the main lines of the structure. The bridge seems to be all one piece. The eye effortlessly follows its edges from abutment to abutment. The cross section of the superstruc- ture also has enough torsional stiffness to allow single column supports. This minimizes the number of columns and allows streets, planting areas, and views to fow through under the bridge. The space under the bridge remains a pleas- ant place to be. Finally, the fares at the top of the columns make clear the fow of forces from the superstructure to the ground. They make this small but com- plex structure memorable. 42 | ASPIRE , Summer 2008 The substructure consists of single piers on single drilled shafts. The site geometry called for a bridge to span the roadway and provide access to the parking structure. Any change in natural conditions of the highly plastic shaley clays could lead to instability. Drilled shaft foundations were designed for an additional 40 kips of lateral load, in addition to other lateral loads on the piers, as a result of potential slope creep in the 45-ft-thick soil mass overlying the limestone bedrock. Drilled shafts were socketed 2.5 diameters into the limestone below the fill to provide additional lateral resistance. Since falsework and shoring were used to build the new structure, recommendations were also provided for allowable bearing pressures for falsework support footings. The bridge was built over a 10-month p e r i o d . F o l l o w i n g i n s t a l l a t i o n o f the drilled shafts and pier columns, the voided slab superstructure was constructed on falsework with concrete placed in a sequential manner. Positive moment sections were placed first, followed by 30-ft-long segments over the pier supports. Forms were cambered to account for dead load deflections and removed when the concrete strength reached 3000 psi. The cast-in-place concrete bridge provided an economical and attractive solution in this combined commercial and residential setting while the complex geometry of the site and shallow structure requirements made the cast- in-place option ideally suited for this application. _____________________________ Kevin Eisenbeis is a principal with Harrington & Cortelyou Inc., Kansas City, Mo. For more information on this or other projects, visit www.aspirebridge.org. The tall, slender piers supported on single drilled shafts performed well when analyzed for seismic loading. Aspire_sum08.indb 42 6/24/08 1:45:41 PM

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