Metro-North Railroad Bridge over Atlantic Street

Traffic congestion, railroad inefficiencies, low bridge clearance, roadway flooding, an aging bridge structure, and impacts on future economic development added up to a “perfect storm” of transportation infrastructure problems in Stamford, Conn., one of the Northeast Corridor’s most heavily traveled and densely populated areas.

The replacement of the Metro-North Railroad (MNR) Bridge along the New Haven Line provided some relief. But it also came with some challenges, the most prominent being how to maintain uninterrupted train service along the primary commuter route between Connecticut and New York while replacing the five-track structure.

The bridge crosses Atlantic Street, one of Stamford’s most important connectors to the downtown area. The thoroughfare is directly east of the Stamford Transportation Center, which houses the Stamford/MNR Station, the CT Transit bus station, commuter parking garages, taxi stands, and corporate shuttle facilities. Interchange access ramps to/from I-95 and multiple one-way east/west streets surround Atlantic Street in the area of the bridge, adding to congestion during peak commuter hours.

The original plate girder bridge, designed by W. H. Moore, was built in 1896. Over the years, most of the legacy railroad bridge crossings along the line have been replaced using traditional construction methods and staged construction, and Atlantic Street and its adjacent crossings within Stamford are the last on the line in Connecticut to be replaced. Incorporating accelerated bridge construction (ABC) techniques provided the confidence that the bridge could be replaced without creating major long-term reductions in service for all commuters. By combining these techniques with careful planning, the bridge was demolished and replaced over a nine-day span without disruption to train service.

The project involved the off-site construction of the new replacement bridge elements, construction of retaining walls to accommodate the future Track #7 and platform for local train service, and widening of Atlantic Street to accommodate new pedestrian walkways, bike lanes, and three northbound and three southbound lanes. Atlantic Street was lowered to increase bridge clearance to 14 ft, 6 in. to allow emergency and commercial vehicular travel. The roadway underpass was reopened to vehicles and pedestrians in a matter of months, whereas traditional construction would have caused years of service disruptions.

The permanent superstructure was divided into three sets of two spans, each carrying two tracks over the permanent structure. “Jump spans” (short temporary spans) were installed in the railroad embankment behind each of the original bridge abutments. These spans were framed with steel beams and supported on steel-encased micro-piles. With the spans in place, the railroad embankment was excavated and the new abutments were constructed beneath live rail traffic. Concurrently, each 700-ton span (750 tons with added ballast) of the new superstructure was constructed off-site at separate assembly areas north and south of the existing bridge. Over a nine-day period, the existing bridge was demolished and the new superstructure rolled in using self-propelled modular transporters (SPMTs). At least two tracks of rail traffic were maintained at all times during the roll-in.

Construction of the center pier also benefitted from an ABC mindset. While the micro-pile-supported, cast-in-place pier footing could be constructed ahead of time, the columns could not be installed until after the existing bridge was demolished. The columns would also be subjected to heavy loads from the superstructure and railroad almost immediately after installation. Steel plate columns were chosen since precast concrete columns would have required cure time for splice sleeves—time that was not available during the nine-day roll-in period. The steel columns were designed with bolted connections that doubled as leveling bolts, allowing vertical adjustment and plumbing of the columns to account for construction tolerances.

While replacing railroad bridges using SPMTs or other ABC techniques has become more common, the “bottleneck” location of the bridge within the line’s busiest interlock and in proximity to MNR’s Stamford Storage Yard presented its own challenges. Accommodating the traveling public on I-95, city roadways, and the railroad, including pedestrians and bicyclists, required innovations that considered the specific transportation needs of each affected group. Conventional construction techniques would have required more extensive closures and service disruptions. MNR operational impacts were of importance as more than 300 trains pass over Atlantic Street each day, and the bridge is located inside a critical interlock at CP 234, which contains five separate track crossovers between Stamford Station and the Stamford Storage Yard or ten individual switch tracks. These crossovers allow the railroad to control traffic in and out of the station. With operational capacity reaching limits during peak hours, each track out of service would restrict rail operations and limit access to the Stamford Storage Yard just east of the project limits. The project was required to maintain uninterrupted service during all phases of construction, including the bridge demolition and replacement stages.

Solutions focused heavily on planning the most invasive operations for periods of reduced usage, scheduling temporary construction to allow train service to operate during foundation and substructure work, and using off-site and on-site precast components to reduce assembly times. The project worked with railroad staff to identify periods of historically lower ridership, which has traditionally been the week of July 4, when ridership is about 25% lower than other times of the year. The nine-day period surrounding a midweek July 4, 2019, holiday was chosen years in advance of the actual bridge roll-in.

Project Team

• Steel Team

  • Fabricator: STS Steel, Inc. , Schenectady, N.Y. *AISC CERTIFIED*

  • Bearing Manufacturer: R.J. Watson, Inc. , Alden, N.Y. *AISC CERTIFIED*

• Owner: Connecticut Department of Transportation

• Structural Engineer: AECOM

• Construction Engineering/Inspection Consultant: Atane Consulting