CTA Main Street Viaduct Weekend Replacement
By James S. Szczupaj, SE, PE, Project Manager, Edwards and Kelcey
The Chicago Transit Authority (CTA) was confronted with a challenge for the
replacement of their viaduct over Main Street in Evanston, Illinois. The CTA needed to maintain
service during normal business hours for the thousands of customers on the Purple Line of its
mass transit rail system. To meet the challenge, the CTA employed rapid construction techniques
and permitted construction near their tracks during non-rush hour periods, including nights and
weekends, and during one very busy weekend shutdown in November 2005.
The Main Street Viaduct on this electrified third rail line carries two tracks, one each for
northbound and southbound traffic. The original 66 foot long viaduct consisted of four spans of
reinforced concrete deck slabs supported by three reinforced concrete piers and two unreinforced
concrete, gravity type abutments. Built nearly one hundred years ago in 1909 by the Chicago,
Milwaukee and St. Paul Railway, the viaduct had deteriorated significantly enough due the
effects of freeze thaw damage and needed complete replacement.
Because the limited time periods during which construction could take place and the
restricted use of typical planning and construction techniques for the bridge replacement, a
method was developed to construct a new, longer and wider, one hundred foot, two track, steel
through girder span adjacent to the existing bridge and roll it into place. The longer and wider
span allowed for the new substructure to be constructed behind the footings of existing
abutments. This enabled the installation of the four foot diameter drilled shaft piers on which the
abutments were founded to take place outside the clearance envelope of the transit cars during
non-rush periods in advance of the weekend shutdown. The steel superstructure was erected next
to the viaduct in approximately four weeks.
For a 54 hour window of time beginning in the evening of Friday, November 11, 2005, the
Chicago Transit Authority shut down service on its Purple Line to complete the replacement of
the viaduct. After service was halted and the tracks removed, the demolition of the existing
concrete viaduct began, and it was completely removed by early Saturday morning. Work
progressed continuously, and by Saturday night, the new 40 ton precast concrete abutment seats
were set onto the preconstructed drilled shafts. Early Sunday morning, remote-controlled, self-
propelled modular transporters lifted the new 210 ton span, rotated it more than 90 degrees to its
final alignment, and rolled it approximately 100 feet to its final position. Moving the bridge took
just 31 minutes, and the entire roll in, from the initial lift to final adjustments of the bearings, was
completed in four hours. The CTA tracks were reconstructed and service was restored to the
Purple Line in time for Monday morning’s rush hour period as planned.
The replacement of the Main St. Viaduct on the Purple Line was under contract with the Chicago
Transit Authority. Edwards and Kelcey was the Designer of Record for the project, and Kiewit
Western was the General Contractor.