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Infrastructure Technology Institute

ITI Helps CTA Evaluate Temporary Bridge Retrofits

ITI research engineers collaborate with CTA personnel to install sensors on a bridge in Chicago. Right: David Kosnik and Professor David Corr install sensors on the CTA bridge.

The Chicago Transit Authority (CTA) operates approximately 36 miles of elevated rapid transit passenger rail service in and around the city of Chicago. A significant portion of this service operates over several century-old structures built by a now-defunct predecessor to the CTA for freight service to and from downtown Chicago. Several of these reinforced concrete structures show signs of deterioration caused by corrosion of their reinforcing steel and subsequent spalling of their concrete columns. To address these issues immediately, the CTA has installed steel shoring to secure some of these bridges. The steel shoring is designed to pick up any share of the load that the deteriorating concrete columns cannot support.

In late 2009, the CTA enlisted the help of ITI to design a prototype continuous autonomous remote structural health monitoring system for one of these bridges. The goal of the research project is to evaluate the load bearing performance of the shoring to give CTA time to assemble and allocate limited financial resources for bridge replacement.

The monitoring system consists of several strain gages that can be used to determine the loads carried in both the vertical steel elements and in horizonatal reinforcing bars in newly-poured concrete footings supporting the shoring. As the concrete columns deteriorate, loads from the structure, roadbed, and trains will be transferred to the shoring, and strain gages will reflect the increased loading on the steel components. Strain gage measurements are monitored in real-time over the Internet by ITI and CTA engineers, who can assess trends in performance of the structural system.

After approximately one year, ITI and CTA engineers will identify which of several different experinmental strain gage configurations most clearly measures deterioration of the bridge. That configuration will be applied to shoring on deteriorating bridges of similar design throughout the CTA network, allowing CTA to make informed decisions about priorities for bridge replacement.