Complex Middlesex Bridge Spans Railroad and Interstate
Beck & Bellucci Works at Tight Site to Build Curved Bridge Carrying Route 2 Over I-89 and NECR Train Tracks
General contractor Beck & Bellucci Inc. is on schedule to complete a difficult three-year bridge project for the Vermont Agency of Transportation (VTAOT), despite cramped quarters and unpredictable ledge on a site crossed by live railroad tracks and an interstate highway.
The Franklin, New Hampshire, bridge builder has a $9.6 million contract with the state agency to build a replacement for the existing bridge carrying U.S. Route 2 over the New England Central Railroad (NECR) and I-89 in the Town of Middlesex.
Beck & Bellucci is also responsible for demolishing the old structure once traffic is allowed on the new bridge. Key project personnel include: for VTAOT, Doug Bonneau, P.E., Project Manager, and Tom Mancini, Resident Engineer; and for Beck & Bellucci, Dale Sackett, Vice President and Project Manager, Christoph Schroeder, Project Engineer, Louie Menard, Project Superintendent, and Bill Frye, Bridge Foreman.
Shorter, Wider and Curved
Built in 1960, the existing bridge is in poor condition, according to VTAOT.
Its replacement is a shorter and wider structure – 512 feet by 40 feet – has just two piers and is horizontally curved with steel plate girders supporting a concrete deck.
Horizontally curved bridges are increasingly being built in the U.S., especially in densely populated urban environments where space is at a premium. Usually, curved bridges require fewer piers, conserving land and mitigating traffic congestion. And the curves actually improve the aesthetics of the structure.
Due to the complexity of the new Route 2 Bridge, it is being built on a new alignment near the old bridge, with vehicular traffic maintained on the existing structure during construction. To further reduce construction impact on traffic, much of the work including steel erection and bridge demolition has taken place between the hours of 8 p.m. and 6 a.m.
Beck & Bellucci’s work involves building both abutments, two hammerhead piers and the bridge superstructure on the new alignment. The new bridge is laid out as a 512-foot arc of a 1,000-foot radius circle, with the two piers dividing the superstructure into curved spans of 187 feet, 168 feet, and 151 feet, measured from abutment No. 1 on the east end to abutment No. 2 at the west end. Five 72-inch deep curved plate girders spaced 8 feet, 6 inches apart support the 8-1/2-inch-thick, high-performance concrete deck. The deck is paved with 3 inches of Superpave bituminous concrete.
The two concrete piers have tapered 3 feet, 6-inch thick shafts topped with 39 feet by 6-feet, 4-inch hammerhead pier caps. Beck & Bellucci performed the high-precision surveying needed for the project’s complex geometric layout. This involved, among other procedures, establishing a bridge major chord and working line stretching between the two abutments, radial stations running along the arc between abutments, and offset measurements between the major chord and the bridge centerline arc at strategic layout points.
Adding to the complexity of the bridge geometry, its two piers are laid out along radii of the circle, while the railroad tracks and I-89 travel lanes pass beneath the new bridge at a skew. Piers have stepped bearing seat pedestals to provide necessary super elevation for the curved bridge, and they allow a minimum clearance over the railroad tracks of approximately 25 feet.
The Importance of Cross Frames
High Steel of Lancaster, Pennsylvania, fabricated 1.1 million pounds of structural steel curved plate girders required for the job, plus structural steel for numerous cross-frames (lateral diaphragms between girders) and other bridge components.
Cross frames are set between girders and are spaced between 12 and 18 feet apart. Intermediate X-shaped cross frames were fabricated from WT5x15 shapes while truss-shaped abutment cross frames were made from heavier stock such as W16x26 for top struts.
Cross frames are critical bridge components because they brace girders against lateral torsional buckling and assist in distributing live loads to girders. This is especially true with curved bridge girders, since their dynamic behavior is more complicated than that of straight girders. Cross frames are usually detailed using one of three different “fits” pertaining to the assumed loads on the girders – no load, steel dead load, or total dead load.
For the Middlesex Route 2 bridge, cross frames are detailed to mate with girders for total dead load fit. This means they are detailed to fit to the girders in a plumb position in which the girders are deflected only vertically under the bridge total dead load. The choice of fit condition can affect how much force needs to be employed to assemble the steel together during steel erection. For a bridge with a small radius and thus a tight curve, it may be extremely difficult for the ironworkers to fit cross frames to girders. In the case of the Middlesex bridge, the 1,000-foot radius curve is flat enough so that it has had little effect on the ease of fit between girders and cross frames. More than 130 cross frames were fabricated by High Steel for the Middlesex bridge.
Polymer Rebar Protect Deck
Both flat and vertical concrete required for the job has been formed and placed by Beck & Bellucci workers. Carroll Concrete supplied about 1,350 cubic yards of ready mix, which was pumped to work crews by Independent Concrete Pumping using Schwing Pumps.
To help extend the design life of the new bridge, its concrete deck incorporates non-corrosive glass fiber reinforced polymer rebar (GFRP). Corrosion of steel reinforcement causes deterioration of concrete resulting in expensive repairs or replacement of bridge sections. The idea of substituting GFRP for steel rebar in certain components of a bridge project has reportedly been around since the 1960's, but advances in the field of polymers and production techniques, and growing acceptance by industry specifiers, have sparked a rise in the use of GFRP bars. For this project, Barker Steel supplied approximately 217,000 pounds of GFRP rebar for the deck, as well as some 119,000 pounds of steel rebar for other areas of the structure.
Ledge and RR Schedules
One unwelcome surprise confronting the contractor was the uncertainty of ledge location due to insufficient information. For example, a large layer of rock was encountered in the area designated for Pier No. 1. This rock had to be removed to a depth of more than 25 feet from existing grade. Unfortunately, this was also the area where NECR tracks crossed beneath the bridge, so subcontractor Maine Drilling and Blasting had to execute relatively small shots. MD&B blasted a total of 1,340 cubic yards of ledge for the project. The unpredictability of ledge location and depth also hampered Beck & Bellucci’s work to install piles to support Pier No. 2 and Abutment No. 2. More than 1,250 feet of HP12x74 and HP14x102 steel piles were driven.
Railroad schedules have been particularly difficult for the contractor to accommodate. When construction work is near the NECR right-of-way, and a train is approaching, work has to cease until the train passes. This happens frequently since the NECR is a busy line. Running from East Northfield, Massachusetts, NECR passes through Middlesex on its way to the state’s largest rail yard in St. Albans, which handles about 40,000 cars each year. Railroad traffic includes Amtrak’s “The Vermonter” passenger trains, general freight including lumber, chemicals and stone, and stacked rail cars.
Work began on the project in July 2016 with site work by subcontractor JA McDonald, and by July 2018, the bridge project was 90 percent complete and demolition of the old bridge by Beck & Bellucci crews was well underway.
Beck & Bellucci’s Dale Sackett commented on this progress:
“We’ve had a limited working area surrounded by a railroad, Interstate I-89, wetlands, and rock outcrops. Our work has had to be scheduled around active railroad schedules, and pile driving has been difficult because the bedrock is unpredictable. Also we had to erect 25 curved plate bridge girders ranging between 100- and 140 feet in length and weighing anywhere from 25,000 to 75,000 pounds apiece.
“So we have had some major and unusual challenges on this job, but we expect to complete the new bridge and remove the old one in September – right on schedule.”