I-91 Bridge Replacement in Brattleboro, Vermont

Cantilever Construction Speeds Through The Winter

The I-91 Brattleboro Bridge Improvement Project located in Brattleboro, Vermont includes the replacement of four bridges with two new bridges, called Bridges 8 & 9. This design/build project is designed by FIGG and built by PCL Civil Constructors, Inc. for the Vermont Agency of Transportation (VTrans). At $60 million, this project is VTrans’ largest bridge project to date. The centerpiece of the project is Bridge 9, a new three-span, 1,036-foot arching concrete bridge over the West River. It is being built using cast-in-place balanced cantilever construction techniques. Standing 100 feet above the scenic valley, the Bridge 9’s graceful 515-foot main span will form an open gateway anchored by curving, open twin wall piers. When complete, this postcard-worthy bridge will provide a unique experience to drivers and sightseers in this beautiful valley. It will feature viewing platforms at the base of each pier overlooking the West River and surrounding mountains. The piers will feature Vermont-inspired, stone-formed concrete that compliments its environment. The concrete cantilever construction process is the most sustainable, environmentally-friendly and mobility-maximizing method for this project. It avoids impacts to Vermont’s busy Route 30, the West River, and popular West River Trail. PCL Civil Constructors, Inc. is nearing completion on the Pier 1 cantilever segments using cast-in-place construction and form travelers. To continue casting through the winter, the form travelers were enclosed with a custom-made tarp system and the exterior forms were insulated. Glycol heater hoses and forced air heaters were used to ensure the forms and the enclosure were above specified temperatures for pouring and curing segments. The enclosure and heating system allowed segments to be poured with outside temperatures as low as 15 degrees Fahrenheit. The segments are 104′-8″ wide, 14′-8″ to 16′-0″ long and vary in height from 26′-11″ at the pier table to 12′-7″ at mid-span. Pier Table 2 construction was recently completed with the last lift poured in January.

The 104′-8″ wide, 30′-7″ tall and 56′-0″ long pier table was poured in five different lifts using EFCO form systems. In March, the forms and falsework were lowered to the ground for disassembly using a DSI strand jack system. Form travelers will be disassembled and transported to Pier 2 this spring for segment construction this summer and fall. The bridge is scheduled to be complete this coming winter.

Innovation of Design and/or Construction

The major changes proposed by the Contractor-Engineer team included revising the pier table design, segment layout and post-tensioning specifications. The design modification called for an unbalanced design (22′ x 14′ from centerline of column). This required less falsework, and only two temporary supports during construction on a reduced schedule.
Temporary shoring for the prop drilled shafts which were incorporated into the pier table falsework.
The transverse and longitudinal post-tensioning was modified to utilize 4 strand tendons at 2’9-1/2″ spacing. This modification saved on duct, heads, grout, caps and allowed for smaller stressing anchors in some areas.
While reducing the length of each segment required more segments, the process of pouring each segment using the form travelers was optimized, and required less labor-intensive falsework to be built.
Cofferdams could not be used due to hard limestone rock at the bottom of the lake, close proximity to the existing bridge, and the need to maintain an open water channel for boating traffic. The design modification called for drilling shafts into the rock riverbed and lowering an on-site precast concrete footing form to accommodate the forms and the work platform.

Rapid Construction

The structures were built with two form travelers using balanced cantilever construction, with end segments constructed on falsework.
The modified pier table length allowed for a significant reduction in falsework. The unbalanced design also eliminated the need for a stability prop on both sides of the pier. This reduced approximately 12 weeks off the construction schedule and maintained the horizontal clearance envelope throughout construction.
Since it was desirable to limit the drilling in the water, the Contractor’s Engineer developed an innovative approach for the pier table falsework and stability prop design. The prop also served as the support for the main pier table falsework beams.
The Contractor’s Engineer used Bridge Information Modeling (BrIM) which made it possible to develop details quickly to meet the demanding schedule.

Aesthetics/Harmony with Environment

The public was very sensitive to the aesthetics of this bridge since the lake is also used for recreation.
The variable segments feature a unique tapered boat hull design in the bottom slab, an aesthetic treatment that matches the community’s focus on recreational boat racing. A flared column design with a seamless transition between the pier and pier table required a custom built form poured in two pieces, with 6′ of column base, 3′ under the normal water level.
To maintain a minimal footprint, traditional footing was not used. Pier table falsework was designed with bolted connections to ease installation and removal. Drilled shafts were designed to stop at the mud line. The props incorporated sand jacks to aid in removal by slowly relieving the 1000 kip per leg load.

Cost Competitiveness

The Contractor secured two form travelers that met the specifications for this project which resulted in cost savings of $750,000 and several month’s reduction to the schedule for design and fabrication of new travelers. These and other design modifications met all the goals of the project providing approximately $2 million savings, 5% below the TxDOT estimate.

Cost per square foot is $206.74

Minimization of Construction Impact

Since the closest river crossing was over 30 miles away, this bridge crossing had to remain open continuously, requiring phased construction.

Reused form travelers from another project resulted in cost and time savings. Highly aesthetic shape in an area with heavy recreational boat traffic. Innovative precast footing box allowed footings to be placed below the waterline without cofferdams. The perfect fit to meet this project’s diverse demands of the traveling public, local environment, and surrounding community. The boat-shaped hull bottom slab, tapered piers, absence of visible foundations, and lighting have combined to produce a beautiful bridge day or night.