PROJECT OVERVIEW
The $45M Rockingham Bridge 24N and 24S project replaces the existing bridges with two new structures. The current bridges were constructed in the early 1960s and are locally known as the High Bridges because they are more than 130 feet above the Williams River and Green Mountain Railroad.
VTrans developed a Base Technical Concept (BTC) for the project, consisting of cast-in-place segmentals. Reed & Reed won the best value Design-Build proposal I-91 twin bridge replacements in Rockingham, Vermont. The winning team’s plan converted the original BTC to four-spans with pre-stressed concrete, post tensioned bulb tee girders. Abutments and piers are supported on pile foundations. Concrete pier shafts range from 65 to 130 feet that support the superstructure.
Construction of a temporary bridge to maintain two lanes of traffic in each direction would have been cost prohibitive. The traffic volumes in this portion of I-91 are low enough to allow a reduction to one lane in each direction through the project site. Accelerated Bridge Construction (ABC) methods were not required. Work began in May of 2017 and is on schedule to be completed by fall of 2020.
The new structures are two distinct bridges, with individual substructures.
Prestressed Girders
Casting the prestressed bulb girders began in September 2017, with all pieces needed for the northbound bridge superstructure completed by May 2018. The main prestressed concrete girders range in length from 96-ft. to 147.5-ft., and vary in depth from 6 to 10 ft. They were transported to the site with trucks using trailers with a steerable dolly. The pieces range in weight from 144,920 to as much as 186,920 lbs.
The size of the girders were some of the largest manufactured by JP Carrara & Sons and maxed out the lifting capacity at the Middlebury (VT) plant. Carrara utilized their own professional drivers skilled at getting these massive girders to the job site safely. Using special steerable trailers, the delivery trucks can make turns that would otherwise be impossible.
Toting what VTrans classifies as a "super load," the rig measures end to end three to four times the length of an average tractor-trailer and are accompanied by escort vehicles.
According to Daryl Bassett, resident engineer of Vermont Agency of Transportation (VTrans) “the delivery required Carrara to obtain special permits to transport these massive beams to the site. These were some of the largest loads transported through the state and deliveries went well”.
Mega Cranes
Due to the weight of the girders, large size cranes with 440 ton capacity cranes were required to install them. To provide crane access under the bridge the contractor built a temporary trestle across the river. To handle crane access below the temporary trestle and its supports across the river had to have enough capacity to hold the weight of the cranes as well as the picks. The result was quite a substantial trestle to facilitate the erection between the two bridges.
Traffic Control
The three piers and two abutments needed for the new northbound bridge were constructed as soon as demolition work was complete. Traffic was on the new northbound bridge in early 2019. Both directions of travel were then put onto the new northbound bridge, and the process was repeated for the southbound bridge. Southbound bridge completion is scheduled for the fall of 2020, with project cleanup and completion by mid-summer 2021.
A series of rolling roadblocks on the interstate, which slowed and even stopped traffic at times, allowed for the delivery of the pre-cast concrete girders which were brought to the site and off-loaded by one of the giant 440-ton cranes and some picks required both cranes to handle the massive pieces.
The superstructure is comprised of 5 precast concrete bulb-tee girder lines. The new bridges are wider than the existing bridge with two 12-ft travel lanes, a 4-ft left side shoulder, and a 10-ft breakdown lane. Both Bridge 24N and 24S have abutments and pier substructures supported by footings on piles, with the exception of one pier on the southbound bridge bearing on bedrock.
According to Daryl Bassett, resident engineer of Vermont Agency of Transportation (VTrans), to facilitate the demolition process the contractor constructed several shoring walls, two work trestles and two shoring towers. In addition, the demolition requires several high-capacity tandem crane lifts to remove the superstructure. To dismantle the existing bridge, the contractor had to use precautions to avoid concrete slurry from getting into the river which is common to this type of construction.
Bassett said the project is only lagging 3 months schedule and on budget. The new bridges are 862 feet long, and soar 130 feet over the Williams River. The new bridges are a slimmer and sleeker version of the steel truss and concrete bridge. Crews worked all last winter, even in temperatures of 20-below zero. Much of the old bridge will be recycled, with the steel cut and formed into new steel and the concrete rubbleized for other uses.
Winter weather can be brutal in Vermont creating challenges to worker safety like working on snow covered girders. Cold weather also requires extra preparation and heating concrete and grout during placement as well as maintaining moist warm environment during curing. “It takes a lot of additional work on the part of the contractor to work with poured concrete in the winter and to do it successfully and get quality product”, describes Bassett.
Bassett explains “These girders have been designed for 100 years which exceeds the typical 75 year standard service life. The precast team performed an engineering analysis and determined a 106 year service life.” The design for longevity includes high performance 9,000 psi concrete mix design and the use of stainless steel for any steel that projected out of the girders into the deck.
The use of precast concrete girders minimized the use of bolted connections to certain bearing locations. “The lack of bolted connections makes it simpler for bridge inspection crews” predicts Bassett. Ultimately, the low maintenance aspect will pay dividends down the road.
Another unique project feature is the post-tensioning of the girders across the bridge. Bassett adds that there were concerns due to the long length of the tendon run of 860 feet. “With a 40-ft drop from one end of the bridge to the other, we had to pay special attention to the pumping and grouting operations.”
Conclusion
The new Rockingham Bridge built with prestressed girders is designed for long service life and will require less maintenance. On schedule and on budget, the new spans soar 130 feet high and are sleek replacement of the original version. |