When “I Lift NY,” formerly known as the Left Coast Lifter—one of the world’s largest floating cranes—hoisted a 645-ton crossbeam into place this February, it marked a milestone in the construction of the $3.9-billion replacement of the Tappan Zee Bridge over the Hudson River, 30 miles north of New York City. The four crossbeams, collectively weighing almost 3,000 tons, mark the halfway point in building two pairs of 419-ft-tall pylons for the pair of structures that constitute the New New York Bridge—and the halfway point for the five-year project.
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With substructure work substantially complete, “this year construction will be much more visible to the public,” says Jamey Barbas, project director for the New York State Thruway Authority (NYSTA).
The first crossbeam installation was postponed a week due to icy river conditions, but that’s negligible, considering the 30 years of discussion about what do to with the existing 61-year-old Tappan Zee Bridge, a deteriorating truss structure that has required millions of dollars in patchwork repairs since the 1990s.
After much controversy, bidding drama and a rocky start that included a collapsed silo on a barge-mounted concrete batch plant, the consortium of Fluor Enterprises, Granite Construction Northeast, American Bridge Co. and Traylor Bros Inc. is swiftly progressing on its $3.14-billion design-build job to construct the two 3.1-mile-long crossings with 1,200-ft-long cable-stayed main spans, between Rockland and Westchester counties.
Located just north of the existing bridge, which carries 140,000 vehicles daily, the 96-ft-wide new northern structure will begin carrying eight temporary lanes of traffic before the end of 2017. Once its southern twin is complete, each crossing will carry four lanes of traffic. The northern bridge will include a bicycling and pedestrian lane.
尽管现在公众可以看到即将到来的上层建筑的切实证据，但大部分的下层和环境努力都在视线之外。
桥梁基础
In 1985, New York state transportation officials fired the opening salvo in a long-term campaign to deal with projected traffic growth on the Tappan Zee Bridge (ENR 10/10/85 p. 12). A study suggested the most cost-effective approach would be to add an express bus lane to the six-lane bridge carrying increasing amounts of traffic on the Interstate 287 corridor.
But by 1997, it was clear that it would not be cost-effective to keep the existing bridge functioning. NYSTA launched a $120-million effort for temporary repairs (ENR 6/9/97 p. 19). Eight years later, planners rejected a tunnel and an alternative light-rail route and, instead, closed in on the idea of a new cable-stayed bridge (ENR 12/12/05 p. 13).
但是，环境审查过程滞后了多年 - 直到2011年，当一场关键事件的完美风暴（ENR 3/24/14p。10）允许州机构完成新的许可程序，获得15亿美元的联邦贷款并授予新利18备用Tappan Zee Constructors (TZC) consortium the state’s first design-build contract.
TZC’s design offered a shallow, streamlined superstructure profile with a composite deck and outwardly inclined towers. It proposed to trim the original $5.2-billion estimate by minimizing the amount and sizes of piles and dredging for the New NY Bridge, which has a 100-year design life.
“We did a lot of work to use as [many] precast elements as we could,” says Jeffrey Han, senior vice president with HDR, one of TZC’s main designers, along with Buckland & Taylor, URS and GZA. “The pile caps, pier caps, deck—all are precast. Some of the picks on the pile caps were 500 tons. Everything on this job is just large.”
关于工作的一切也很忙。与I Lift NY起重机和约30个较小的起重机一起，有100多个驳船，两家混凝土批处理植物，两个1,000英尺长的临时工作栈桥，休闲河流交通，每天140,000个越过旧桥的通勤者，一对受保护的人展览猎鹰，两种类型的濒危鱼类以及恒定的社区和媒体游览。“这是一条无情的河。在某些地区，这与沿海工作一样危险。” TZC副总裁Walter Reichert说。首先，团队进行了翻新设备，并开发了用于工作驳船的特殊梯子系统。这份工作没有重大伤害。
However, up until 2014, the team had to address, repeatedly, barges that became unmoored. Two boaters were killed in 2013, when they collided with a barge that was properly moored (ENR 4/7/14 p. 13). And recently, on March 12, three crew members died when their tugboat crashed into a construction barge. TZC is participating in an ongoing investigation.
Piling It On: Hudson River Geology
TZC won the bid in July 2012 and began work in 2013. No time was wasted: Even during the proposal process, the NYSTA had begun preliminary borings and cone penetration tests to investigate geological conditions. Soon after, TZC did supplementary borings as deep as 440 ft, says Bob Palermo, TZC lead geotechnical engineer.
这个概要文件包含有机淤泥一样厚200 ft, with intermittent sand layers beneath, then soft clays, glacial till and, finally, bedrock. The layers also vary in thickness. “On the west side, the third layer, of clay, goes down to 750 feet,” says Palermo. “We wanted to limit the number of piles and maximize their capacity.” That goal resulted in piers spaced out at 350-ft intervals, he adds.
The 43 piers—all but two in water—range from 60 to 260 ft long and are supported on steel-pipe piles as deep as 330 ft, says Palermo. The 1,100 piles, totaling 80,000 tons of steel, have diameters of 36, 48 and 72 in.
为了优化大约1,000个桩的后续设计和位置，该团队进行了广泛的负载测试程序 - 16个静态轴向负载测试，160次动态测试和三项静态侧向负载测试，最高可达700万磅。第一个桩测试的进行得很厉害。Reichert回忆说：“那是一堆6英尺的堆，每个人都在看着。”
The hammer damaged the tip of the pile, which resulted in meetings with pile-driving experts. The rest of the 6-ft-dia piles, which mostly support the main span, received 5-ft-long, 2-in.-thick welded steel reinforcements, and the team changed its pile-driving methodology, says Palermo.
Rather than driving piles as hard as possible throughout, crews varied the forces at strategic points, Palermo notes, saying, “We had no more damaged piles after the first, except for one in which we hit a boulder that wasn’t supposed to be there.”
The team also had to make sure it didn’t damage the well-being of two types of federally endangered sturgeon. It decided to create bubble curtains around piles of 4 diameters and up, says John Duschang, HDR environmental compliance manager. Each curtain consisted of a series of aluminum rings through which pressurized air was pumped to create bubbles that mitigated sound waves emanating from the pile-driving.
The project also features what officials believe is the first-ever 24/7 public access to noise-monitoring data, available on the website. “We sited monitors for air quality, vibrations and noise … in sensitive areas,” says Duschang. The information can be played back to determine if the noise was construction-related or “a train, a landscaper or a kid at the park,” he says. Moreover, the NYSTA and TZC each chipped in $10 million for a grant program to which residents can apply, requesting soundproofing renovations.
Capping It Off: Precasting Operations
HDR的Han说：“从一开始，地下条件就推动了我们的思维”。为了最大程度地减少对基础的影响，“我们最终获得了Girder Substringer系统。主要大梁深12英尺。我们使用了五跨单元，每个单元长1,750英尺。为了支撑甲板并最大程度地减少其上的负载，我们在主梁之间使用了36英寸深的子弦乐器。”他说。汉补充说，通常，桥梁的主要梁将是这里的基弦的尺寸。
Unistress Corp., Pittsfield, Ma. precast almost 6,000 steel-reinforced concrete panels, each about 12 ft long and up to 45 ft wide. They are being placed on the steel-girder system, supported by 68 football-field-sized pile caps, the bottoms of which were precast by Cape Charles, Va.-based Bayshore Concrete Products. Crews placed the last of the 300-ton pile caps last summer, reinforcing each cap with galvanized steel and sealing it with up to 750 cu yd of concrete.
Chesapeake, Va.-based Coastal Precast Systems convinced TZC to let it precast the caps for the columns that sit atop the pile caps, says Paul Ogorchock, president of CPS. “They told us it would take two to three months to build one cap cast in place,” he says. On the other hand, CPS could precast and send eight caps at a time for them to install, all within the space of a few weeks.
CPS precast 59 caps, each 13 ft tall, 10.5 ft wide, up to 92 ft long and about 300 tons. Each cap is reinforced with 70,000 lb of galvanized steel rebar and prestressed with 28 galvanized strands, then filled with 150 cu yd of concrete, says Ogorchock. Subsequently, CPS precast the four crossbeams placed this year. To handle such gigantic components in its casting yard, “we have a 450-ton-capacity crane that we bought from NASA,” he notes. “We have a 350-foot-long pier and a 75-foot-wide barge.”
Last year, TZC crews began placement of the main span’s pile caps. In lieu of traditional cofferdams, the team used a system devised by VSL to lower 360-ft x 60-ft concrete-and-steel “tubs” onto the piles. Thirty-four computer-guided hydraulic jacks pulled on cables woven through each tub, lowering it slowly onto the piles. It took about eight hours to lower each 14-ft-tall tub by 9 ft, says Reichert. This alternative method alleviated risks associated with the Hudson River main channel’s strong currents and 6-ft tidal swells.
The two pairs of main-span towers are supported on the two foundations—in contrast to the rest of the two bridges, which sit on separate foundations. “Combining the main-span supports into a single pile cap will allow that foundation to resist ship impacts,” says Don Bergman, Buckland & Taylor’s vice president for major projects.
A driving force behind the main-span design was the need to accommodate a future Metro-North commuter rail line. “The foundations needed to support heavy rail without [crews] needing to work in the river,” says Bergman. The outward-leaning legs of the main-span towers are designed so that a third deck can be built between the twin structures to handle commuter rail, he says. “The towers are arranged such that the inner legs can be connected at the top and cable anchorages installed between,” he says.
The NYSTA invested $300 million to prepare the structures for future rail, but, when open, the new bridge will have room for dedicated bus lanes.
To handle the supersized aspects of the project, TZC’s key weapon is the I Lift NY crane, which arrived in fall 2014 (ENR 10/13/14 p. 9). With its 1,900-ton capacity and 328-ft boom length, the crane began work last spring by placing a 600-ton approach-span pile cap. Among its final tasks will be lifting out sections of the old Tappan Zee Bridge—up to 1,100 tons—once all traffic is operational on the twin spans in 2018.
人员使用小型起重机钢结构for the approaches. On the Westchester County side, crews connected individual girders into a single assembly and pushed it out over Metro-North Railroad. Subsequent assemblies kept cantilevering the structure out to the first bridge pier. The work trestles used on both banks, plus low-draft tugboats and prefabrication, reduced the need to dredge to less than 1 million cu yd—only half the amount the NYSTA had anticipated, says Reichert.
建造will continue this year on the 419-ft-tall towers. Crews are using self-climbing jump forms to build the towers in segments. Within the jump forms, workers assemble and encase in concrete steel rebar cages. Then, the forms are jacked up to build the next 12- to 18-ft segment. Crews also will begin placing 973 precast deck panels, provided by The Fort Miller Co., Schuylerville, N.Y., for the main span.
In 2015, the team did some $800 million in construction, and this year will be similar, says Reichert. “The plan is to be able to walk [across the northern bridge] by the end of the year.”
