PROJECT OVERVIEW
On August 23, 2016 the Sarah Mildred Long Bridge stuck in a raised position. The 1940’s green bridge had been scheduled to be shut down in November as part of the final phase of construction of a new precast concrete segmental structure. Ultimately the Maine Department of Transportation (DOT) decided that it was not worth the investment to reopen it to traffic for ten more weeks.
Foreshadowing this event years before, the project team of Maine DOT, Cianbro and Joint Venture of FIGG and Hardesty & Hanover had started the process of designing a $170 million replacement structure, where Cianbro provided constructability means and methods, project schedules and budget estimates through the process. The design, which incorporates four separate concrete lift towers, is the first of its kind in the United States.
The Bridge, which connects Portsmouth, New Hampshire, to Kittery, Maine, via the Route 1 Bypass carries 14,000 vehicles a day. It’s one of three bridges across the Piscataqua River which helped with the detour of vehicular traffic.
Multi-Modal Towers
The new two-level structure features an upper level that carries vehicles while the lower level provides rail access. It includes a 300-ft-long movable lift span supported by four 194-ft-tall concrete lift towers.
In the fall of 2014, construction began with installation of work trestles in the river to provide access for drilling and concrete placement of drilled shafts for all the foundations. According to Kaven Philbrook, Senior Project Manager, CIANBRO, given their prior experience working on the river, trestles were the best solution for safe and efficient access.
The Piscataqua is one of the swiftest rivers in the country. The depth and speed of the water precluded traditional foundation construction methods. An innovative precast segmental concrete structure for the lift tower foundation and shared pier foundations was developed that could be quickly erected to minimize the exposure of workers to the harsh conditions.
Two temporary trestle bridges and construction staging areas were installed on each side of the Piscataqua River. These structures provided a stable base for drilling operations that anchored piers into bedrock. "It was a very challenging site. The trestles were more stable than a barge, and allowed work to continue during the 10 foot tide cycles," described Philbrook. That stability was critical for operating the 250-ton cranes.
Cianbro chose to use precast and site cast concrete due to the tight timetable. The structural elements were sources from three different locations. Lift tower segments were precast adjacent to the site at the Port of New Hampshire. The vehicular and railroad approach precast concrete segments were manufactured by Unistress in Pittsfield, Massachusetts. Drilled shaft foundation tubs were precast at Coastal Precast in Chesapeake, Virginia.
Marine Life
The expansion of the Panama Canal in 2016, made harbor improvements essential to accommodate the larger ships traversing the NorthEast. In addition, another goal was to reduce the severity of the skew, as well as accommodate both vehicular and rail traffic. After looking at several different combinations and alignments the project team was able to reduce the skew to 15° and increase the span length to 300 ft. The new layout uses eleven fewer piers than the old bridge and the new alignment improves marine navigation by straightening the channel, allowing larger ships to access the port and shipyard. With a higher 56’ vertical clearance in its normal position, there will be 68% fewer bridge openings. Typically, the bridge’s lift span is at its middle level, allowing vehicles to cross the river.
Precast concrete segments trucked to the site, combined with the use of long span lengths and shared piers, minimized the number of piers required and reduced the construction impact to marine traffic and the public. The precast concrete towers support the 300’ long streamlined steel box girder lift span that enables the movable “hybrid” span to rise up for passage of tall vessels and lower for trains to cross.
CM/GC Delivery Method
The bridge is being constructed via a construction manager/general contractor (CM/GC) alternative delivery approach. This was done to better understand the risks involved in the construction process and address them during design, saving on the cost of construction. The method worked well for this project, collaboration was instrumental in its success.
“During design phase, a site for casting concrete segments adjacent to the bridge became available and thus made precast construction a viable option for the towers,” Philbrook said. Both precast and cast in place construction alternatives were considered and the precast system was selected as the preferred alternative. The schedule was developed with input from project team through the CM/GC delivery method in an effort to minimize impacts to the traveling public and surrounding communities.
Balanced Cantilever
The railroad bridge below and the vehicle bridge above were erected in a balanced cantilever style of erection. The railroad and vehicle bridge shared three piers. At the three shared piers, the vehicle bridge was integral with the columns with an extensive amount of post-tensioning to allow for the long cantilevers.
Kaven Philbrook, Senior Project Manager, CIANBRO recalls the challenge of the shared piers. The shared piers proved support for the vehicle bridge and the railroad bridge directly under it. The project team had to accommodate movement, displacement and stiffness criteria for both vehicular and rail loading.
Custom-made forms were used on site to create 80 segments, which were placed one on top of the other to create the four lift towers. The precast concrete segmental hollow sections were shaped to accommodate the internal workings of the lifting mechanisms.
The vehicle and RR precast deck segments were cast off site and delivered. When the oversize vehicular approach segments were too tall to haul by truck Cianbro and Unistress developed a lay down method over-size restrictions. Coastal Precast barged the large tub sections up to the Cianbro crane. The towers were erected from a barge mounted crane, and the lift span was assembled on a barge and floated into place.
Environmental concerns were also addressed to minimize the impact on the river. The majority of work was performed from temporary trestles close to the location of the new bridge. No work occurred over open water, and every effort was made to prevent debris from falling into the water. Demolition occurred in stages; the existing bridge was demolished and floated out in large sections.
Conclusion
This very unique dual-purpose structure in a lift bridge opened to traffic in March 2018. Construction of the Sarah Mildred Long Bridge presented many challenges that were overcome through thoughtful design and innovative construction techniques. The key challenges included: construction of concrete piers within a high flow tidal zone; construction of precast concrete segments for the lift span tower, which is a first of its kind application; fabrication and float-in of lift span. |