CTA Red-Purple Bypass
The CTA’s Red and Purple Modernization (RPM) is a $2.1 billion program to reconstruct two miles of elevated track structure originally built more than a century ago. The Red Line is Chicago’s main north-south artery and carries approximately 20% of all daily riders. The Purple line shares the same route but runs express during rush hour periods. Vinod was PM and Hemal was the lead structural engineer on the Red-Purple Bypass (RPB), a signature component of the RPM project. The RPB is situated at the junction between the Brown, Red and Purple lines, just north of Belmont Station. The three lines share the same set of tracks heading into Belmont station before the Brown diverges to head west.
Photo: CTA
Photo: CTA
Proposed Bypass
The old junction carried four mainline tracks 20 ft above street level and was supported by a 120-year-old riveted steel structure. The three lines intersected at the same level with trains stopping for cross traffic. This caused a major bottleneck and limited the system’s overall capacity.
The RPB involved complete reconstruction of the old junction’s track structure starting just north of the Belmont station and spanning three city blocks. Its signature component is a new single flyover track that carries the NB Brown Line over the four mainline tracks and eliminates the bottleneck.
In addition, a temporary elevated track structure (RVT) was built to carry SB Brown Line trains through the junction for construction staging. The new permanent flyover and RVT provide two “extra” tracks, which allows for the demo and reconstruction of the four-track mainline structure in two stages.
The "Flyover"
The Flyover is a tightly curved, closed deck structure consisting of four parallel steel plate girders composite with a concrete deck. At the flyover’s highest point, the top of the rail is approximately 46 ft above the ground and the structure is supported on a straddle bent. With 1800’ length and radius varying from 425 to 750 ft, the Flyover snakes through the dense neighborhood of Lakeview.
Photo: David Burke Photography
Cross frames spaced at 12’ along the entire length, tie the girders together to counteract the torsional effects of the tight curvature. Lateral bracing between the interior girders was provided to limit the displacement of the girders during erection, which was accomplished without any temporary shoring. Both AREMA and AASHTO codes were used for a comprehensive design approach.
The flyover is supported on single-column concrete piers aesthetically flared at the top. Each pier is founded on a single concrete caisson belled in hardpan about 90 ft below grade, minimizing the footprint within the limited R.O.W.
The "Straddle Bent"
A significant feature of the flyover is the "straddle bent" that supports it at the highest point. The cross beam of the straddle bent is a simply supported steel box girder that is 4 ft wide, 6 ft deep, and 80 ft long. The beam is a fracture-critical member and includes two bottom tension flanges for redundancy, though only one is needed for strength.
Photo: David Burke Photography
The box’s components were shipped to the site and assembled on the ground adjacent to the tracks. Among them were T-shaped plates (“ears”) that were bolted to the webs of the box to facilitate quick bolted splices to the longitudinal girders.
Photo: CTA
The installation of the straddle beam and adjoining longitudinal girders was accomplished within a weekend closure that CTA allowed of the mainline tracks.
The longitudinal girders frame directly into the straddle beam’s webs with bolted shear connections. A continuity tension plate is placed over the top of the box, connecting the girder top flanges on either side. Diaphragm plates with access openings are spaced 6’ on center within the box.
Photo: CTA
The Track System
The rails were hung at their final elevations above the bridge deck with pipe jacks clamped against the curbs. Then the plinth concrete was poured up to the bottom of the fastener base plates. As a controlled secondary pour on top of the 10-in deck, the plinths ensured that strict 1/16-in. tolerances for the track elevation were met. They also elevated the rails 4-6 inches off the deck to protect them from rainwater.
