Construction EngineeringChicago, IL
The historic Fixed Trunnion Bascule bridge was constructed in 1915 along the Chicago River. Consisting of two leafs, the movable bridge opens to allow marine traffic passage over the river. Throughout the years, the structure has carried both vehicular and commuter rail traffic into the downtown area.
After a century of service, elements took a toll on the trusses located within the bridge pit that supported the trunnion girder. The City Department of Transportation (DOT) selected a Contractor to perform work and MKA as the Structural Engineer firm to design the repairs.
The primary challenge with the Chicago fixed trunnion bascule was determining how to support the bridge superstructure while the trusses, located in the Bridge Pit and supporting the trunnion girder, were rebuilt. To remedy this, the DOT designated loading of approximately 3.5 million pounds to be transferred from the temporary shoring to the pit corners.
Other challenges included designing and detailing not only supports, but also retrofitting existing century-old members to take the added forces applied during the temporary support conditions. Further complicating the work was the fact that the fixed elevated train structure, approaching the movable bridge, was also supported by the trusses that were being removed.
All repairs had to be conducted safely but on an aggressive schedule, without disruption to the vehicle, train or marine traffic.
MKA’s Engineers provided DOT with multiple options to support the bridge and transfer all loads to the four existing caissons. DOT selected a low-profile reinforced concrete beam design that allowed the bridge to open. When open, sections of structural steel were lowered down into the pit, for both the MKA designed shoring towers and replacement truss members.
Working closely with the Contractor and the Steel Erector, retrofits were designed to strengthen existing members that would see additional loading during construction. In addition, the team designed temporary tube supports that could be installed through the center of the trusses to support the Transit Authority trains above.
MKA’s Engineers successfully provided Engineering services to support the bridge and elevated train tracks above while repairs were made to the 100 year old support structure. In addition to providing a safe and efficient repair methodology and allowing the bridge to remain open, MKA’s Engineers also presented DOT with the process and methodology needed to accomplish similar repairs on other bridges.