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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Page 42 of 55

ASPIRE , Summer 2008 | 41 A T-shaped bridge provided the solution for all structural, thermal, seismic, geotechnical, and aesthetic design requirements. supported on single-column, "golf tee" concrete piers, provided the ideal solution to meet all structural, thermal, seismic, geotechnical, and aesthetic design requirements. Several structural options were initially considered for the bridge. Framing issues and thermal movement requirements for steel and concrete girder spans proved to be undesirable where the upper level drive entrance connected to C ASt-In-PlACE REInfORCEd COnCREtE / SAChS PROPERtIES InC., St. lOuIS, MO., OwnER BridgE ContraCtor: St. louis Bridge, St. louis, Mo. FiEld oBsErVation: Alper-ladd, St. louis, Mo. BridgE dEsCriPtion: A cast-in-place reinforced concrete bridge, t-shaped in plan view consisting of eight spans supported on six columns and three end bents the main structure. Use of a monolithic c o n c re t e s l a b b r i d g e e l i m i n a t e d structural discontinuities and complex superstructure framing details where the elevated structure arms converged. Pier caps supporting girders were no longer needed. Expansion joints on the structure could also be eliminated, with joints being used only at abutments and where the entrance drive joined the parking structure. Eric Neprud, project engineer for Harrington & Cortelyou, summarized the choice: "Selection of the voided slab bridge drastically simplified the detailing requirements where the access drive meets the main bridge." The use of 16-in.-diameter voids embedded within the 2-ft 3-in.-thick slab reduced weight and material requirements, allowing the thin superstructure to span up to 60 ft between pier support points. Drop panels varying from 6-in. to 10-in.-thick were required to distribute loads to the narrow columns. A 28-day concrete compressive strength of 4000 psi was used with the Missouri Class B2 mix design, allowing the bridge to carry AASHTO HS-20 live loading. A 5-ft-wide sidewalk was also cantilevered from the bridge. Chesterfield, located just west of St. Louis, Mo., is subject to Category B seismic requirements. The complex T-shape of the structure dictated that a three-dimensional analysis would be required for seismic considerations with a ground acceleration of 0.12g. Structural analysis utilizing a response spectrum analysis with mTAB Stress (SAP 386) software indicated displacements and loadings would be acceptable. The tall, slender piers supported on single drilled shafts performed well when analyzed for seismic loading. Conventional analysis of the bridge was made utilizing the Brass program. Field exploration consisted of four borings extending through the 41-ft to 46-ft-thick overburden and 25 ft into the rock below. Due to the geology of the site, a potential for slope instability in the earth fill required special analysis. a cast-in-place reinforced concrete bridge provided the elegant solution needed. Aspire_sum08.indb 41 6/24/08 1:45:35 PM

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