Rendering Courtesy of New York State Thruway Authority

The winning design for the new Tappan Zee Bridge will use more steel and less concrete than the other two short-listed designs.

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The New York State Thruway Authority has named HNTB as owner's engineer for the Tappan Zee Bridge replacement project. Gov. Andrew Cuomo (D) on Dec. 17 announced the winning design, a $3.142-billion bid from Fluor-led consortium Tappan Zee Constructors in what is touted as the largest-ever design-build project in the state.

In addition to Fluor Corp., Tappan Zee Constructors comprises American Bridge Co., Granite Construction Northeast and Traylor Bros., with HDR acting as lead designer. The bid beat out two other short-listed teams, offering a lower price and significantly less dredging than the other proposals.

It is not uncommon for the owner's engineer to come from a firm on another short-listed team, but HNTB was in an unusual position.

The firm had been part of a fourth short-listed team that, at the eleventh hour, chose not to submit its project bid. Rochester, N.Y.-based Bergmann Associates has been subcontracted by HNTB for environmental monitoring and 3D modeling oversight.

The new Tappan Zee Bridge project is the largest design-build effort in New York state since the 2011 passage of a design-build law; the job has a 62.5-month project schedule.

"The design-build process produced a savings of at least $1.5 billion compared to the amounts estimated by the Federal Highway Administration and our own original estimates," said Thruway Authority Chairman Howard P. Milstein in a statement.

While the Thruway Authority expects work to begin in early 2013, the approval process is not yet complete. The plan will be reviewed by the state attorney general, and the state comptroller has approval authority over all authority contracts over $50,000. The comptroller's office has 90 days to review the contract.

The three submitted designs all call for cable-stayed dual spans with a 100-year design life, but the winning proposal favors steel over concrete far more than the other two.

In a statement, Fluor said its choice "reduced the weight of the structure and minimized the number and size of foundations required." It has a "shallow superstructure and a 350-ft-long span design to minimize the number of piers and provide a sleeker-looking bridge." A "Buy America" provision requires all steel to come from the U.S.

While the design for the three-mile-long bridge does not include a mass-transit component, the Thruway Authority stated that the bridge will have the "strength and capacity provisions to accommodate various mass-transit modes."

One of the project's major challenges is the composition of Hudson River Basin soils. The bridge, crossing at one of the river's widest points, has geology marked by significant silt buildup and soft-clay substrates.

A source familiar with one of the other short-listed bids but bound by a non-disclosure agreement notes that the soil quality in the river is very poor and will require regular dredging just to get barges to the bridge site. TZC may be able to avoid some dredging by using innovations such as barges with shallower drafts, the source says.

A related issue is the amount of test-pile data needed to build in such poor soils.

"Confirmation of the geotechnical assumptions and foundations capacities in the soft clays below the riverbed will be critical, and an extensive geotechnical testing program is anticipated, commencing in the new year," says Mark Roche of Arup, the project's longtime consulting engineer. "Major challenges and obstacles for the design team include confirmation of the capacity and deflection characteristics of the proposed foundation solutions in the deep varved clays in the causeway area, transitions from soft to hard founding substrates and compliance with performance requirements for the potential future addition of commuter rail."

The existing Tappan Zee Bridge is technically a floating bridge, with its main span supported by massive concrete boxes that were sunk to the riverbed during construction and pumped out.

Details of TZC support design have not been released, but Roche is optimistic. "This is really a foundation project—once the [team] is out of the ground, the project should be relatively straightforward."