Cianbro Corp Creates Safer, Wider and More Efficient SM Long Bridge
The $170-million replacement for the Sarah Mildred Long Bridge, one of only three spans over the Piscataqua River linking Maine and New Hampshire, is on target for accepting road and rail traffic in less than a year.
"We've just passed the 65 percent mark, and we expect to have traffic on the bridge by September 2017," said Kaven Philbrook, Project Manager for general contractor Cianbro Corp.
The SM Long Bridge replacement project is a joint venture of the Maine and New Hampshire Departments of Transportation. MaineDOT is the venture lead, with Ron Taylor serving as Resident Engineer. Maine and New Hampshire officials negotiated a construction cost of $158.5 million with Cianbro to conduct the work, which began early in 2015. The project is part of a "Three Bridge Agreement" between the two states to address their three jointly owned bridges spanning the Piscataqua River. The other two bridges are the "high level" I-95 bridge, and the Memorial Bridge that links Portsmouth with Kittery, which was replaced in 2013.
Replaces Obsolete, Structurally Deficient Span
Originally known as the Maine-New Hampshire Interstate Bridge, the SM Long Bridge was built in 1940 to carry U.S. Route 1 Bypass over the Piscataqua River, with the goal of reducing traffic congestion in downtown Kittery and Portsmouth. In 1987, its name was changed to honor Sarah Mildred Long, an employee of the Maine-New Hampshire Interstate Bridge Authority for 50 years.
The existing SM Long Bridge is a 2,800-foot long span, with a major portion of it serving as a double-deck, combined highway and railroad movable bridge. It carries U.S. 1 Bypass on the top deck, and tracks for the Pan-Am Railroad on the bottom deck. A steel trussed lift span is supported at each end by twin steel towers. The bridge was closed to traffic in August 2016 due to a mechanical malfunction of the lift. But the entire bridge had already been slated for closing in November to allow construction of a modern replacement. DOT officials say the replacement will provide a safe, efficient and reliable crossing between Portsmouth and Kittery that meets the needs of highway, railroad and maritime transportation.
Carrying some 14,000 vehicles daily until it was closed, SM Long Bridge is too narrow, structurally deficient and was posted for just 20 tons. Today's standards call for a 36-ton load posting for this type of road. What's more, the old lift span provides only 175 feet horizontal clearance between tower faces - too tight for modern ships that need more than a 200-foot clear channel. In addition, the bridge's vertical clearance of just 16.7 feet above mean high water is insufficient and results in frequent openings and road closures so smaller vessels can pass through.
Taller Bridge, Wider Channel
The new bridge, designed by a joint venture of Hardesty & Hanover and FIGG Engineering, has many improvements and innovations. For example, its design requires 11 fewer piers in the river, and an alignment that straightens the channel, easing navigation. This lets larger ships access the port and shipyard. In line with this, the new 300-foot-long lift span creates a clear channel of 250 feet between tower faces. And this streamlined structural steel lift span has three elevation positions on the towers instead of just two.
In its normal operating, "resting" position, the bridge's lift span is at the middle level, providing a vertical clearance of 56 feet over MHW that enables motor vehicles to cross the river while allowing more ship traffic to pass underneath without opening the bridge. This higher clearance results in an estimated 68 percent fewer openings than the old bridge required. At its tallest setting, the lift span produces almost 140 feet of clearance, enough to allow passage of the tallest vessels. Conversely, lowering the span to the bottom setting permits trains to cross.
Other features of the new bridge are an improved collision system (fenders), and a wider deck (37 feet versus 29.9 feet) that provides safer shoulders for cyclists.
Precast Towers a First
The design team incorporated other important innovations as well for the bridge. Among these are the first application of precast post-tensioned concrete for the lift span towers; a lift span comprised of a steel box girder with an FRP wind fairing; and operating machinery located in the tower bases. The lift span has just one deck comprising both roadway surface and steel rails, with two additional seating positions on the towers to accommodate the double level highway and rail bridge.
FRP (fiber-reinforced polymer) wind fairing is a critically important feature designed to improve the aerodynamic performance of the bridge. This is a composite consisting of glass-fiber skin surrounding a polyurethane core. Fairing prevents wind-induced vibrations and what engineers term flutter instability. The 1940 Tacoma Narrows Bridge, a suspension bridge in the U.S. state of Washington, opened to traffic on July 1, 1940, and dramatically collapsed into Puget Sound on November 7 of the same year. Some experts believe the cause of failure was aeroelastic flutter. The failure of the Tacoma Narrows Bridge sparked research in the field of bridge aerodynamics-aeroelastics, the study of which has led to such design advances as FRP wind fairing.
The design process for SM Long Bridge began in 2013, followed by the start of construction in January 2015. Traffic has been detoured to the high-level I-95 and Memorial Bridges over the Piscataqua River.
Temporary Causeways, Trestles, Aid Marine Work
Cianbro's contract involves construction of a 2,800-foot vehicle bridge generally oriented in a south-north direction at a skew to the northwest-to-southeast flow of the Piscataqua River at this location. There are a total of about 1,500 feet of approaches.
For design purposes, the bridge layout is numbered from south to north (Portsmouth to Kittery). The vehicle bridge has two abutments (Abutments 1V and 2V) and five piers (Piers PV1, PV2, PV3, PV14 and PV15), while the railroad bridge has two abutments (Abutments 1R and 2R) and six piers (Piers PR4, PR6, PR8, PR11, PR13, and PR14). Three piers support both vehicle and railroad bridges and are identified as "shared" (PS5, PS7 and PS12), and the two towers are supported by the largest piers (Piers PT9 and PT10).
To enable worker and equipment access for the marine work on the new bridge and demolition of the old structure, Cianbro has built temporary causeways and trestles in several locations, Causeways were constructed of washed riprap placed on geotextile fabric, while trestles were built on driven pilings.
The new bridge was designed with spread footings for piers PV1, PV2, and PV3 and drilled shafts for PR4, PS5, PR6, PS7, PR8 and lift-span Tower 9 (Portsmouth side). The existing bridge and abutment are scheduled to be removed and existing piers removed to various depths depending on location. The remaining piers and the foundation for lift-span Tower 10 are being built on drilled shafts.
Tower Segments Cast at Job
The innovative towers are constructed of concrete segments cast by Cianbro workers in a casting yard set up on nearby land. More than 350 precast segments are required for the towers.
In contrast, Unistress Corporation, in Pittsfield, Massachusetts, manufactures the roadway and railroad box girders. Both types of girders are roughly trapezoidal in cross section, measure more than 20 feet across the top, between 8 and 9 feet deep, and are 8 to 10 feet long. The heaviest segments weigh approximately 100 tons, according to Philbrook. Segments are carried by barge to the bridge site, where they are raised and installed using the balanced cantilever method. A variety of cranes are being used to raise bridge components, ranging from medium-sized hydraulic and lattice-boom cranes to the largest rig, a Manitowoc Series 3 Ringer.
Seacoast Ready Mix is supplying an estimated 18,000 cubic yards of ready mix concrete to the job with all of it being pumped in place by Independent Concrete of Wakefield, Massachusetts.
Tide Helps Place 2.25 Million Pound Lift Span
The 300-foot steel lift-span for the bridge is being fabricated in sections by Casco Bay Steel Structures, Inc. of South Portland. According to Philbrook, the sections will be shipped by barge to the project site and joined together. He said the assembled span, which will weigh approximately 2.25 million pounds, will be barged to the tower location at low tide. As the incoming tide raises the barge, workers will connect the span to the low railroad position on the towers, and the towers' lift mechanisms will take over.
The new Sarah Mildred Long Bridge opens to traffic September 2017, while the entire project is scheduled to be finished by June 2018.