Figure 23 - Kishwaukee River Bridge, Illinois. (Photo courtesy of Parsons Transportation Group.) Figure 24 - Anchorage congestion at upper box corners. (Photo courtesy of Parsons Transportation Group.) Longitudinal layout of the Kishwaukee River Bridge (Drawing courtesy of Parsons Transportation Group).

The Kishwaukee River Bridge Fig. 23 is a post-tensioned precast segmental concrete box–girder bridge opened to traffic in 1980. The bridge has five spans with lengths of 170 ft + 3 x 250 ft+170 ft (51.8 m+3 x 76.2 m + 51.8 m). As the first-generation of segmental structures, the Kishwaukee Bridge engineers chose the design of a single shear-key joint usually located close to the centroid of the cross-section. The overall length of the deck is 334 m (1096ft). The deck was built by the balanced cantilever method. Each cantilever consisted of seventeen segments 2150mm (7'-3/5") long and one pier segment 1067mm (3'-6") long. Cast-in-place closures have a length of 984mm (3'-2 3/4").

The original construction team was comprised of Edward Kraemer and Sons (EKS) who subcontracted with Dywidag Systems International (DSI) as the Post Tensioning designer/supplier. During the original construction contract in the late 1970’s the bridge’s post tensioning system was redesigned by DSI using Dywidag Threadbar® post tensioning systems.

In the early 2000’s the owner; Illinois Department of Transportation (IDOT) decided on a strengthening program to extend the service life of the bridges. The strengthening design developed by Parsons Transportation Group required an addition of a total of twenty four 12-0.6" external post tensioning tendons of various lengths in each bridge. In 2007 IDOT awarded the contract through competitive bidding to the same team as 28 years previously: EKS who again subcontracted with DSI to supply and install the post tensioning. The Owner’s schedule required all work to be completed within a very short time as the owner wanted to keep bridge closure to a minimum due to I-39 being a high traffic route especially for Chicago area residents travelling to Wisconsin during the weekends.

Challenges:
1970’s design practice did not require the inclusion of provisions for future additional PT tendons, therefore new deviators had to be constructed throughout the bridges as well as new PT anchorage zones at the diaphragms and abutments. The new anchor zones and deviators were cast in place within the segmental box and then post tensioned. Instead of using steel pipes at the deviators, continuously curved voids (diablos) were cast in the deviation diapraghm to allow large deviations from the theoretical tendon profile. In order to meet the higer standards of today’s post tensioning technology the PT was designed with a continuous air tight HDPE pipe from anchor to anchor grouted with high performance grout and equipment. Due to the diablos and continuous HDPE pipe requirements; specially designed PT anchors were installed at the faces of the existing diapraghms. The location of the new anchors was made more difficult by the tight constraints of the existing reinforcement and PT.

EKS, IDOT, PTG, and DSI worked very closely throughout the project due to the complex 3D geometry and challenges of accommodating the tendon paths throughout the existing bridges. To assist with this task Ground Penetrating Radar was used to locate the existing PT and reinforcing in the diapraghms and abutments so crews could follow closely behind with coring of the holes (in 3D) for the tendon paths. After casting, the PT install crew quickly went to work installing the new anchors and the HDPE pipe; overcoming the challenges compounded by the largely deviated tendon paths and longer tendons of over 770 feet. The tendon stressing was difficult due to the location of the tendons in the upper corners of the box, along with the congestion of the new tendons and new anchors. (see Fig. 24)

The Southbound Bridge was completed in May 2008 and the Northbound Post Tensiong portion of the project was succesfully completed in early August 2008 with final project completion coming shortly within the Owner’s schedule.

Owner: Illinois Department of Transportation
Designer: Parsons Transportation Group
Contractor: Edward Kraemer & Sons
Construction Engineering Inspection: HDR
Post-Tensioning Materials: Dywidag Systems International (DSI)
Epoxy Supplier and Prepackaged Grout: Sika