ASPIRE Summer 08 - Aspire - The Concrete Bridge Magazine

The site geometry called for a bridge to span the roadwayand provide access to the parking structure.The tall, slender piers supported on singledrilled shafts performed well when analyzed forseismic loading.Any change in natural conditions of thehighly plastic shaley clays could leadto instability. Drilled shaft foundationswere designed for an additional 40kips of lateral load, in addition to otherlateral loads on the piers, as a result ofpotential slope creep in the 45-ft-thicksoil mass overlying the limestonebedrock. Drilled shafts were socketed2.5 diameters into the limestone belowthe fill to provide additional lateralThe substructure consists of single pierson single drilled shafts.resistance. Since falsework and shoringwere used to build the new structure,recommendations were also providedfor allowable bearing pressures forfalsework support footings.The bridge was built over a 10-monthperiod. Following installation ofthe drilled shafts and pier columns,the voided slab superstructure wasconstructed on falsework with concreteplaced in a sequential manner. Positivemoment sections were placed first,followed by 30-ft-long segments overthe pier supports. Forms were camberedto account for dead load deflections andremoved when the concrete strengthreached 3000 psi.The cast-in-place concrete bridgeprovided an economical and attractivesolution in this combined commercialand residential setting while the complexgeometry of the site and shallowstructure requirements made the castin-placeoption ideally suited for thisapplication._____________________________Kevin Eisenbeis is a principal withHarrington & Cortelyou Inc., Kansas City,Mo.For more information on this or otherprojects, visit www.aspirebridge.org.AESTHETICSCOMMENTARYby Frederick GottemoellerThis structure is an excellent resolutionof a complicated structural andaesthetic problem: the bridge with abranch. Designing a bridge like this withtypical steel or concrete I-beams alwaysresults in a confusion of beam layoutsand awkward connections. The cast-inplacevoided slab, on the other hand,adapts readily. All of the connections andpoints of force transfer are inside theslab. The exterior shows only a smoothcontinuous surface along the main linesof the structure. The bridge seems to beall one piece. The eye effortlessly followsits edges from abutment to abutment.The cross section of the superstructurealso has enough torsional stiffnessto allow single column supports. Thisminimizes the number of columns andallows streets, planting areas, and viewsto flow through under the bridge. Thespace under the bridge remains a pleasantplace to be. Finally, the flares at thetop of the columns make clear the flowof forces from the superstructure to theground. They make this small but complexstructure memorable.42 | ASPIRE, Summer 2008