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Example of existing open-deck steel elevated structure at CTA. Trains on the Red-Purple Line often create noise levels exceeding 95 dBA near similar structures.


Chicago Transit Authority Red-Purple Modernization Program


Chicago Transit Authority

The CTA Red and Purple Modernization Program is envisioned as a series of improvements to the North Red Line and the Purple Line to increase core capacity and modernize the system into a state of good repair. As part of the EA, ATS performed detailed noise and vibration assessments of the existing and planned future structures that will be used for the Brown Line Red-Purple Bypass flyover structure and the Lawrence to Bryn Mawr Modernization projects. Because existing noise levels are high, based on the FTA noise impact criteria, the relatively small increases in noise exposure due to increased capacity and higher speeds could cause severe noise impacts. In addition, because a number of residential building are within 50 ft of the existing elevated structures and the structures will need to be widened to achieve ADA requirements for the station platforms, levels of groundborne vibration are likely to increase and generate impacts near elevated structure columns.


The analysis performed by ATS has included detailed testing and computer modeling of existing CTA elevated structures to understand the noise and vibration characteristics of the structures, modeling of the proposed designs of the bypass and the modernization structures, and recommending mitigation measures to avoid noise and vibration impacts. Mitigation measures that have been considered include structural modifications to reduce the noise radiated by steel I-beams, use of concrete deck structures in place of open-deck steel structures, structural modifications to reduce vibration transmitted into the ground, continuously welded rail, and use of tuned vibration dampers to reduce the vibration of the rails and of I‑beams.

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