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1、Structures 2008: Crossing Borders 2008 ASCENew Innovative Features of Maines First Cable Stay BridgeAuthor:Christopher J. Burgess, P.E., S.E., FIGG, Denver, CO, The steel suspension Waldo-Hancock Bridge carried U.S. Route 1 travelers over the Penobscot River, near the town of Bucksport, Maine for 74
2、 years. This vitally important transportation link provided safe passage for carloads of tourists through the years, in addition to significant commercial traffic related to regional paper-making, granite-quarrying, boat building,manufactured housing production and a host of freight items, such as d
3、elivery of home heating oil. In 2002, a scheduled renovation began and in the summer of 2003, the main suspension cables were unwrapped to prepare for a visual inspection of the individual wires in the 37 cables in each main cable. Deterioration of the main cable on the south side of the bridge was
4、much more advanced than expected, leading the bridge owner, Maine Department of Transportation, to simultaneously undertake a strengthening project on the Waldo-Hancock Bridge and the design of the Penobscot Narrows Bridge & Observatory on a parallel alignment to the existing bridge.FIGURE 1THE WALD
5、O-HANCOCK BRIDGE AND NEW PENOBSCOT NARROWS BRIDGE & OBSERVATORY SIDE-BY-SIDE OVER THE PENOBSCOT RIVER, LINKING WALDO AND HANCOCK COUNTIES IN MAINE. THE NEW BRIDGE OPENED TO TRAFFIC ON DECEMBER 30, 2006, JUST 42 MONTHS AFTER THE EMERGENCY REPLACMENT NEED WAS IDENTIFIED.In order to speed the process,
6、Maine undertook the replacement in an innovative owner-facilitated design build process and ground was broken in December of 2003 for the new cable-stayed bridge, a first for Maine. A key element in the design of the new cast-in-place concrete segmental bridge is the considerable attention and effor
7、t invested to provide a durable and easy to maintain structure with a planned service life exceeding 100 years. The bridge is the first to utilize a unique combination of features designed specifically to protect the bridge and only the second to benefit from a pioneering cradle system, developed an
8、d patented by the bridges designer.FIGURE 2ADJACENT CRADLES POSITIONED IN THE UPPER PYLON ALONG WITH FORMWORK, AS THE JUMP FORMS ARE BEING PREPARED TO MOVE UP AND CAST THE NEXT SECTION. THE CRADLES ARE FABRICATED, DELIVERED TO THE SITE, POSITIONED AND SURVEYED IN POSITION IN THE FRAMEWORK, THEN LIFT
9、ED INTO PLACE AND THE ENTIRE ASSEMBLY SURVEYED FOR CORRECT POSITIONING PRIOR TO CASTING.An innovative nitrogen gas protection and monitoring system provides an enclosed environment of pressurized inert gas around each cable stay. The main components of the system are the gas, HDPE sheathing, reservo
10、ir tanks, anchorage-sealing caps and monitoring hardware. Nitrogen gas was selected due to its non-corrosive properties and ease of availability.An environment of pressurized pure nitrogen essentially eliminates oxygen, chlorides and moisture, all potentially corrosive elements.A primary goal was to
11、 cost-effectively create a gas-tight sealed system that completely encloses the stays. After installation of the stays, they were purged of moisture and nitrogen gas injected to purge the existing air from the closed environment. The nitrogen was then pressurized to two pounds per square inch.A seal
12、ing cap covers the strand tails at each anchorage and fully encapsulates all anchorage hardware. A clear end plate allows direct visual inspection of the anchor area. Each stay is provided with a 300 cubic foot nitrogen gas reservoir to recharge the gas in the event of a drop in pressure. Gauges rec
13、ord all fluctuations in pressure and serve as a tool for the Department to easily monitor the status of the system and take corrective action, if necessary. The annular space between the cable stay strand and the cradle sleeves allows gas to flow freely through the stay system in an isolated environ
14、ment.To the best of our knowledge, the stay cable gas protection system utilized in the new Penobscot Narrows Bridge is a world first. This unique system provides two additional layers of protection by completely surrounding the stays with an inert gas, and through automatic andcontinual monitoring
15、of the gas pressure level. Monitoring allows the Maine Department of Transportation real-time knowledge of any potential compromise in the system. There are a total of four nested levels of protection, providing redundancy: Epoxy coating on the stay strands Outer layer of HDPE sheathing around the s
16、tays System filled with nitrogen gas to purge potential corrosives Sealed system with monitoring equipmentThis proactive approach allows maintenance crews to identify and address potential problems at an early stage, with minimal monitoring effort.An additional monitoring tool for the Penobscot Narrows Bridge is a series of force monitoring systems on each stay.
