THE CONCRETE BRIDGE MAGAZINE

Spring 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

Issue link: http://www.aspiremagazinebyengineers.com/i/1093138

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34 | ASPIRE Spring 2019 C O N C R E T E B R I D G E T E C H N O L O G Y by Joseph Orlando, Cianbro Corporation Sarah Mildred Long Bridge Tower Foundation Construction The new Sarah Mildred Long Bridge, which spans the Piscataqua River from Portsmouth, N.H., to Kittery, Me., opened on March 30, 2018. The structure has two-level precast concrete segmental box- girder approach spans with a 300-ft-long lift span. Additionally, the 200-ft-tall towers of the lift span are vertically post-tensioned segmental towers, a first in the United States. The foundations for the lift span towers are 125 ft long, 65 ft wide, and 15 ft thick. The design of the foundation includes a precast concrete tub used as a cofferdam and form. An allowance of 18 in. for the tub-wall thickness and 3 ft for the bottom thickness was included in the design. Eight 10-ft-diameter drilled shafts socketed 35 ft into competent rock support each foundation. The top of the foundation is at elevation 8 ft, about 2 ft above the highest astronomical tide expected during construction. Each of the structural footings contains 3200 yd 3 of cast-in-place concrete and 500,000 lb of reinforcement along with 14 post-tensioning ducts for the segmental towers. The project was jointly sponsored by the Maine and New Hampshire departments of transportation. The project delivery method was the construction manager/ general contractor method, and the Cianbro Corporation of Pittsfield, Me., was the general contractor (for a profile of Cianbro, see the Spring 2017 issue of ASPIRE ® ). FIGG Bridge Engineers d esigned the approach spans, and Hardesty & Hanover designed the lift span and towers as a joint venture. Tower Foundation Design Challenges The Piscataqua River is a deep and fast- flowing tidal river, one of the fastest in the United States. This factor and others presented many challenges to the construction of the tower foundations. The following were among the primary consid erations that influenced the u l t i m a t e m e t h o d o f f o u n d a t i o n construction: • The tidal current could exceed 4 knots. • The maximum crane capacity at working radius using a Manitowoc MLC300 Lattice Boom Crawler, which was the largest crane the contractor had in its fleet, on a 72 ft by 200 ft deck barge was 180 tons. • The foundation had to be submerged 13 ft at the highest astronomical tide. • The water was 65 ft deep, with little overburden on the river bottom. • There were significant construction and environmental loads, such as water pressure due to the tidal current alternating in direction. • Worker and diver safety had to be ensured. Others have approached the construction of these types of foundations using the following methods: • Casting a complete tub and setting it on drilled shafts using a large capacity crane • Casting and launching a complete tub, floating it over the drilled shafts, and then submerging it into place • S u s p e n ding a p r e c a s t c o n c r e t e segmental soffit above high water, building a traditional cofferdam on the soffit, and using synchronous jacks to lower the system into place Cianbro analyzed these options and ruled them out because it did not have sufficient crane capacity to lift 1100 tons, and a dry dock or launching system was not available to fabricate and launch such a large floating tub. Additionally, the Piscataqua River's extreme current and a slack tide of only 30 minutes made the prospect of floating and accurately positioning a large tub over the drilled shafts too risky. Similarly, slowly jacking a tub down into the current was not ideal. Cianbro Solution A precast, sand-lightweight concrete, segmental post-tensioned tub addressed the design challenges. The MLC300 crane could quickly erect the tub. The combination of strength and density of the sand-lightweight concrete was critical to the success of the design because loads were close to crane capacity. Additionally, each piece could be set quickly to the design elevation and secured during slack tide. The tubs were free draining, which gave workers two hours to work inside the tubs at low water. Workers grouted and bolted the tubs from the top. There was no need for divers to go beneath the tubs, thereby improving safety. Each precast concrete segmental tub, one for each tower, was composed of nine segments: five drilled-shaft units and four fill units. The drilled-shaft units used strongbacks with hangers to support the segments from the top of the drilled shafts. For erection, the fill Sarah Mildred Long Bridge approaches, lift span, towers, and tower footings. Photo: Cianbro/Richard Hopley.

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