Vinod Patel, SE, PE And Hemal Patel, SE, PE 2024-01-06 00:42:26
Tight urban spaces and strict displacement tolerances were overcome in designing and erecting a flyover at a critical CTA interchange.
THE CHICAGO TRANSIT AUTHORITY (CTA) left no area uncovered in its plan to update its main North-South artery.
The Red and Purple Modernization (RPM) Project is a $2.1 billion project, the city’s largest capital project to date. It involves completely reconstructing two miles of elevated track on Chicago’s north side, most of which was originally built more than a century ago. The Red Line, going north-south through the city, is the CTA’s busiest route and carries approximately 20% of all daily CTA riders. The Purple Line shares the same route but runs express during rush hour periods.
The RPM is being delivered as a design-build project and is slated for completion in 2025. One recently completed piece of it, the Red-Purple Bypass (RPB), has helped reduce congestion near the bustling Belmont station at a junction where a third route, the Brown Line, joins the Red and Purple lines. The three lines share the same set of tracks heading into the Belmont station before the Brown Line diverges to head west.
The old junction had four mainline tracks 20 ft above street level and was supported by a 120-year-old riveted steel structure. The northbound Brown Line traveled on the easternmost track and diverged by crossing three mainline tracks at the same level. It caused a major bottleneck, sparked frequent delays, and limited the overall capacity of the Red and Purple lines.
The RPB involved a total reconstruction of the old junction’s track structure starting just north of the Belmont station and spanning three city blocks. Its signature component, though, is a new single flyover track that carries the northbound Brown Line over the four mainline tracks and eliminates the bottleneck. Designing the flyover demanded stringent displacement tolerances, and it had to be erected in a tight space that included alleys with multi-story buildings on both sides. In addition to the flyover, a temporary elevated track structure called the RVT was built to carry southbound Brown Line trains through the junction for construction staging.
The new permanent flyover and RVT provide two extra tracks, which allows for the demolition and reconstruction of the fourtrack mainline structure in two stages. The RPB will increase the Red and Purple lines’ speed and capacity, including the ability to add more trains during rush hour periods and accommodate 7,000 additional daily commuters.
Flyover Structure
The flyover bridge is a closed deck structure consisting of four parallel steel plate girders that are composite with a 10-in.-thick cast-in-place concrete deck varying in width from 14 ft, 6 in. to 16 ft. The top of the rail is approximately 46 ft above ground at the flyover’s highest point, where it is supported by a straddle bent.
The total flyover length is approximately 1,800 ft, with spans varying from 45 ft to 130 ft and a radius varying from 750 ft to as tight as 425 ft to allow the bridge to snake through the Lakeview neighborhood’s dense urban environment. The structure consists of two-span continuous units each less than 200 ft long, except for a signature 420 ft long, four-span continuous unit that’s centered on the straddle bent. All structural steel was galvanized to ensure 100-year service life.
Cross frames spaced at approximately 12 ft along the entire length create a unified system where all four girders work as a single unit 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 supports or shoring.
A combination of AREMA and AASHTO codes allowed for a comprehensive and effective design approach of a curved rail bridge with continuous structural units. AASHTO was used for guidance on curved girder analysis, dead load fit, and cross-frame designs. Meanwhile, AREMA provided allowable stress limits for girder plate sizing and governed the design of intermediate web stiffeners, bolted girder splices, and slip-critical bracing connections.
CTA trains are sensitive to minuscule magnitudes of structural movement and vibration, so an integral part of the design was ensuring adequate structural stiffness and frequency. That was especially important for the longer spans because it governed the girder sizing over strength requirements. The structure’s stiffness also plays a vital role in limiting the stress in the rails, which was crucial at structural expansion joints, where differential displacements and rotations tend to cause localized tension and bending in the rails.
The design team performed a rail-structure interaction (RSI) analysis to quantify the thermal movement of the structure relative to the rails and resulting force transfer between the two. Nonlinear springs were used to represent the rail fastener clips, which clamp the rails to the flyover deck. The analysis determined rail stress at expansion joints, longitudinal shear in the piers, and any force transfer at the flyover ends into the existing CTA structure.
Originally, the project’s technical requirements preferred continuous welded rail (CWR) for improved ride quality. However, CWR on the tightly curved alignment resulted in large transverse thermal forces and movements that could cause issues at the ends of the flyover, where it transitioned back into the existing CTA tracks. The design team and CTA ultimately decided that jointed rail with 80-ft rail segments would provide the best balance of serviceability and ride quality.
The rails were hung at their final elevations above the bridge deck, and plinth concrete was poured 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 were met for the track elevation. They also raised the rails off the concrete deck to protect them from rainwater. The 10-in. structural deck followed the vertical profile of the low rail, while the plinth thickness varied from 4 to 6 in. to accommodate the track’s changing radius and superelevation.
The closed concrete deck contributed to noise reduction compared to the original open deck configuration that had timber ties. Additionally, 4-ft-tall precast concrete walls running along the deck edges helped to contain the noise. Their outside surface incorporated form liner relief, enhancing the overall aesthetic appeal.
Flyover piers are single-column hammerhead type with a specialized architectural form liner relief. Each pier is supported by a single concrete caisson belled in hardpan about 90 ft below grade, minimizing the foundation footprint within the limited right-of-way. However, given the relative flexibility of the free cantilever system, plus the weak nature of fill and silty clay along Chicago’s lakefront, extra care was needed to size the caissons to limit deflections at the track level.
Straddle Bent
One of the flyover’s significant features is the “straddle bent” that supports it at the highest point where it crosses the mainline tracks. The straddle bent is supported on two columns and minimizes the foundation footprint needed in a dense area.
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 from center to center of bearings. The bearings are HLMR urethane discs, which allow rotation about each axis. Each includes a high-strength steel shear pin that transfers horizontal forces into the concrete columns and caissons.
The beam is a fracture-critical member and was one of the most scrutinized design elements of the project. It includes two bottom tension flanges for redundancy, though only one is needed for strength. The tension flanges are bolted to the web plates with L6×6 angles. The L6×6 angles are not included in the section’s flexural capacity for additional redundancy in the tensile region. Meanwhile, the top compression flange, which is not a fracture risk, is fillet welded to the web plates.
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 ft on center within the box. The interior features lighting to accommodate future maintenance and inspections.
The design process considered if the straddle beam plates, particularly the welded top flange-web assembly, could be fabricated as one continuous piece. However, at 80 ft long and 6 ft deep, the fabricator’s galvanizing tub could not accommodate the entire beam, even with double dipping. Instead, a bolted splice was added near midspan and the two segments were connected on site.
The straddle beam’s box components were shipped to the site and assembled on the ground adjacent to the tracks. Among them were T-shaped plates nicknamed “ears” that were bolted to the webs of the straddle box. These ears facilitated an easy connection of the straddle box to the longitudinal girders, resembling a conventional bridge field splice.
The CTA allowed a 48-hour weekend mainline closure to install the straddle beam and adjoining longitudinal girders. During it, the straddle beam and structural steel in the adjacent spans were erected. The 117-ton straddle beam was the most critical pick of the project and required a crane with a 158-ft boom length, 50-ft radius and 237-ton counterweight.
RVT Structure
The RVT structure, built to temporarily carry southbound Brown Line trains into the Belmont station, allowed the contractor to stage the demolition and reconstruction of the mainline. The RVT needed to be constructed and made operational while the existing mainline was active. Given the limited right-of-way, the main challenge was fitting the structure into a 16-ft wide alley flanked by residential and commercial buildings. It also had to clear utilities on the west edge of the alley and the mainline structure and its maintenance platforms on the east edge.
Structural steel was an ideal choice for RVT because it facilitated construction in such a tight space and provided flexibility for changing field conditions. The RVT structural system is comprised of longitudinal rolled stringers that frame into steel double-column bents supported on spread footings. The base of each column was considered pinned in both directions. Transverse sway was limited by the frame action of the double-column bents. Longitudinal sway was limited by truss bracing provided in every third span.
The design features skewed bents to avoid existing obstacles along the east and west edges of the alley. Simple bolted connections between stringers, cross bents, and bracing allowed for quick erection. In addition, steel afforded a relatively lightweight structure and made spread footing foundations feasible within the alley’s limited footprint, affording the contractor savings in cost and schedule.
Various other considerations were made to address the geometric challenges. The mainline’s maintenance platforms were relocated (including systems and communications equipment) while the overhanging track timber ties were trimmed. Aerial utility lines were relocated to an existing underground duct bank in the middle of the alley, and the supporting utility poles were either trimmed or removed altogether. All adjacent buildings were potholed to identify the bottom of their basement walls. The spread footings were extended to the bottom of the basement walls to avoid applying track surcharge loads to the walls.
The RVT steel bents and columns were positioned in the narrow gaps between the adjacent buildings along the west edge. The corners of several steel bents were also coped to make way for the adjacent proposed mainline beams along the east edge. The sway bracing on the west edge of the structure was placed at a higher elevation to maintain adequate clearance to the adjacent garages. Sway bracing on the east edge was placed at a lower elevation to provide clearance to the proposed mainline bents.
South and West Connections at Flyover Ends
The design-build team faced a challenge when tasked with connecting the south end of the flyover to the existing mainline structure at Belmont, one of CTA’s busiest hubs. Any construction in its proposed final location would have required months of mainline track closures and triggered extensive system-wide impacts. CTA could only allow for a two-week closure window of the easternmost mainline track for making this connection.
In response, the contractor erected the new structure on false-work to the side of the existing alignment and then rolled it into place during the closure. The proposed structure was designed to match the type and span layout of the existing Belmont structure. It consists of longitudinal rolled stringers bolted to transverse welded-plate cross girders. The stringers are continuous through the cross girder with top and bottom splice plates. The floor system was also made composite with the cast-in-place concrete deck and welded shear studs.
Once the two-week closure window began, the original structure supporting the easternmost mainline track was demolished. Shortly after, the new structure was moved on Self-Propelled Modular Transporters (SPMT) into the final alignment in a mere six hours. Next, the new cross girders were connected to the existing cross girders with bolted field splices. The benefit of easy bolted connections to the existing structure saved time for installing and adjusting the track furniture and supporting equipment on top of the deck during the remainder of the closure window.
At the other end of the flyover, the west connection ties the structure into the existing Brown Line tracks and is threaded through a tight alley thanked by the existing Brown Line and residential buildings. The flyover transitions from closed deck to open deck to match the existing Brown Line track system.
The structural system consists of longitudinal rolled stringers framing into doublecolumn steel bents. Each column is supported by a reinforced concrete footing on steel micropiles. Micropiles were chosen as the foundation type due to limited access to the alley, tight horizontal and vertical clearances, as well as the need to minimize vibrations on the adjacent buildings. Due to access requirements to adjacent properties, cross-bracing of columns was not feasible. Instead, the structure uses rigid moment frames in both directions to limit sway.
Work is ongoing around Belmont. Crews have completed two new mainline tracks and are demolishing and reconstructing the other two. All work is slated for completion in 2025. The flyover, though, has already made an impact.
Owner
Chicago Transit Authority
Structural Engineer
EXP (Flyover and RVT), Stantec (Mainline)
Architect
EXP
General Contractor
Walsh-Fluor Design Build Partners
Steel Team
Fabricators/Detailers
Hillsdale Fabricators
Waukegan Steel, LLC
Erector
S&J Construction Co. Inc.
Vinod Patel (vinod.patel@exp.com) is the Vice President/Midwest Bridge Department Manager at EXP. Hemal Patel (hemal.patel@10-4eng.com) is the President of 10-4 Engineering, PLLC.
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