CATEGORY: Studies, Research and Consulting

ENTERING FIRM: HBM Engineering Group, LLC

OTHER CONSULTANTS: Hanson Professional Services, Dywidag, University of Illinois- Chicago, CTL Group

OWNER: Illinois Department of Transporation

DESCRIPTION: The existing structures S.N. 101-0133 (NB) & S.N. 101-0134 (SB) carry Interstate I-39 over the Kishwaukee River south of Rockford, Illinois. These twin bridges consist of five continuous spans, comprised of 156 single cell precast, post-tensioned segmental concrete box segments with a single shear key at each web. The superstructure has three interior spans of 250’-0” and two end spans of170’-0” totaling 1,190’-0” center-to-center abutment bearings. The roadway consists of two 12’-0”traffic lanes, an 8’-0” interior shoulder and a 10’-0” exterior shoulder for an overall out-to-out width of 42’-0”.The structures are of the “first generation” of post-tensioned precast concrete segmental bridges (PCSB) with segments erected by means of a launching truss and balanced cantilever construction. They were the first bridges of their kind built in Illinois and the first constructed by means of a launching truss in the United States. The twin structures have gained a great deal of notoriety since their construction in 1979 and 1980.During the balanced cantilever erection of the southbound structure, the epoxy applied to the segment joints did not properly harden creating excessive stresses in the single-key shear keys. A retrofit was carried out before the structures were opened to traffic involving placing stainless steel dowels set in epoxy across the overstressed joints. Over time the box girder webs began to display continuous cracking and crack growth. A structural investigation determined that the passive method used to remedy the shear capacity deficiencies had left large forces in the shear keys responsible for the stress cracking observed in the bridge causing continued deterioration. HBM Engineering Group, LLC (HBM) was retained by the Illinois Department of Transportation (IDOT) District 2 to perform a multi-phase physical and analytical structural evaluation of the northbound and southbound bridges. The overall objective of the project was to assess structures to extinguish any remaining uncertainties about the structural health and performance of these bridges by means of in-depth inspection, analytical modeling, instrumentation, and static load testing. The final objective of the project was to instrument the bridges with vibrating wire strain gauges, crack meters, LVDT deflection monitoring systems as well as fiber optics to evaluate the structures capacity during static load testing. Vibration based testing was also performed on external post-tensioning to evaluate the effective stresses in the tendons. A load rating tool was also developed for these structures to use for overweight permit vehicles. The tool incorporates all analytical and physical load testing results to provide the most accurate and current bridge response. After the project's completion, three scholarly articles stemming from this endeavor have been featured in the Journal of Civil Structural Health Monitoring. Additionally, two Ph.D. candidates utilized this project as a cornerstone for their doctoral research, culminating in their successful graduation.