Spring 2019

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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on the crest of a vertical curve, which resulted in a sump at each abutment just past the end of the bridge. The City of Takoma Park requested that drainage improvements be made to alleviate flooding from these sumps. During design, the bridge design engineer found that the existing sump inlets were severely undersized as the result of increased development over the years. The inlets were therefore replaced with larger, more efficient curb-opening-at- grade inlets to minimize the potential for flooding in the future. M a i n t e n a n c e o f t r a f f i c d u r i n g reconstruction of the cast-in-place reinforced concrete bridge was a significant challenge. Unlike a typical r e d u n d a n t , m u l t i - g i r d e r b r i d g e arrangement, removing one of the two arches during a staged construction scheme would render the bridge unstable. After this inherent problem with the structure configuration was explained to the stakeholders, all parties agreed that closing the bridge during construction and detouring traffic was the only feasible construction solution. However, this decision presented a problem for pedestrians because there was no alternative way to safely cross the ravine on foot. Therefore, MDOT SHA required the contractor to provide a 295-ft-long temporary pedestrian bridge along the east side of Carroll Avenue. A major structural concern involved m a i n t a i n i n g a b a l a n c e d - l o a d i n g condition during the sequence of demolition and build back to avoid overstressing the existing arches that were to remain. The project leaders were especially mindful of this issue because three workers had died on this site in 1932, when a previous reinforced concrete bridge built in 1904 collapsed during demolition prior to construction of the arch bridge. 1,2 In the rehabilitation project, every precaution was taken to ensure safe conditions. The contract plans included requirements for a 22-stage systematic construction sequence working from the center of each arch outward in a concentric fashion about the crown of the arch. A structural analysis was performed using software under each condition of demolition and build back to verify that the arches were not overstressed during any of the stages. T h e r e c o n s t r u c t i o n p l a n s w e r e painstaking in recreating and replicating the details of the existing bridge, which included extensive architectural features for the spandrel arches, floor beam overhangs, and columns. These architectural details were matched identically. However, details could not be exactly matched for the existing open-balustrade railings, which were significant to the Maryland Historical Trust. Fortunately, the Texas Department of Transportation Traffic Railing Type C411, which was crash tested and met the test requirements for the design speed and classification of Carroll Avenue, closely resembled the existing railings. This barrier received approval without comment. Construction The primary construction challenge for the project was access. Steep side slopes at each end of the bridge precluded the provision of construction entrances from Carroll Avenue. The solution was to provide an entrance off Sligo Creek Parkway; however, this location complicated matters because the parkway also served as the temporary detour route. Flaggers were employed daily to facilitate traffic flow while construction vehicles entered and exited the site. One of the construction innovations that made the project such a success was the prime contractor's use of a work platform at the elevation of the arch supports. This work/demolition platform consisted of steel beams and timber flooring supported by shoring towers 15 to 20 ft above the floodplain of Sligo Creek. This platform solved MARYLAND DEPARTMENT OF TRANSPORTATION STATE HIGHWAY ADMINISTRATION, OWNER BRIDGE DESCRIPTION: Three-span, 220-ft-long, open-spandrel concrete arch STRUCTURAL COMPONENTS: Repair of substructure units and arch ribs and replacement of columns, floor beams, and deck using 712 yd 3 of concrete MDOT SHA Mix No. 6 (28-day compressive strength of 4500 psi); 64 yd 3 of concrete MDOT SHA Mix No. 3 (28-day compressive strength of 3500 psi); 172,000 lb of epoxy-coated reinforcing steel; and 15,000 lb of black reinforcing steel BRIDGE CONSTRUCTION COST: $9.3 million AWARDS: American Council of Engineering Companies Maryland 2018 Outstanding Project Award; Portland Cement Association 16th Concrete Bridge Awards (2018) Award of Excellence Debris accumulation on the demolition shield /work platform. Polystyrene protects the arches from debris. All Photos: Johnson, Mirmiran & Thompson. Concrete being placed for new floor beams at the top of one of the arches. ASPIRE Spring 2019 | 13

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