THE CONCRETE BRIDGE MAGAZINE

WINTER 2018

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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38 | ASPIRE Winter 2018 CBP Concrete Bridge Preservation The Franklin Avenue Bridge: Investigation and Rehabilitation Design by John S. Lawler and Arne P. Johnson, Wiss, Janney, Elstner Associates Inc.; Dan Enser, HNTB; and Paul Backer, Hennepin County Transportation Department The Franklin Avenue Bridge, constructed from 1919 to 1923 in Minneapolis, Minn., and listed in the National Register of Historic Places, crosses the Mississippi River with five open-spandrel, concrete arches. During the bridge's nearly 100-year history, harsh winters and aggressive deicing operations have resulted in advanced deterioration. Beginning with a condition investigation in 2007 and culminating in construction slated for completion in 2017, a rehabilitation project has been executed to restore this historically important structure. This Concrete Bridge Preservation article focuses on the investigation and rehabilitation design; an article 1 in the Summer 2017 issue of ASPIRE ® focused on the structural analysis and the accelerated bridge construction methods used to replace the deck, cap beams, and railing. Additional details about this project can also be found in two articles published in Concrete International. 2,3 The bridge spans consist of two parallel arch ribs ranging from 55 to 400 ft in length. The bridge has a total of five spans, an overall length of 1014 ft, and a 66-ft 4-in.-wide deck. The concrete arch ribs are reinforced using the Melan system— steel trusses composed of double-angle chords connected with riveted steel gusset plates and diagonal crossbraces. The steel trusses were erected between the piers, and then concrete was cast around the trusses forming the arch rib. No conventional reinforcement was included in the arch ribs. They were reinforced by the embedded trusses only. The historical concrete mixture incorporated gap-graded local aggregates. To accommodate the large (2½ in.), angular coarse aggregate, a high water-cement ratio (w/c) of about 0.50 was required in the non-air-entrained concrete mixture. Investigation The rehabilitation process began with comprehensive investigation of the condition, performance, and historic importance of the structure. A follow-up assessment was conducted during the repair design phase to refine the repair approach and update estimates of repair quantities. The scope of both condition assessments consisted of an overall visual examination of the bridge and subsequent detailed surveys, and nondestructive testing and sampling of materials at representative areas. CBP CONCRETE BRIDGE PRESERVATION Franklin Avenue Bridge during condition investigation by snooper truck. Photo: WJE.

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