缅因州第一座斜拉桥的新创新.doc

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 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 delivery 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 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 WALDO-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, 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 effort 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 and 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 LIFTED 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, reservoir 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 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 sealing 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 record 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 environment.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 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 stays 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. Dywidag Systems International provided the force monitoring systems, which can accurately determine the force within 1% using a portable field laptop unit.The system is very rugged, requires no maintenance, has no moving parts and will provide monitoring throughout the projected service life of the bridge, allowing Department employees ease in regularly monitoring the forces in the cable as part of their inspection procedures, without the need for lift-off equipment or the utilization of special expensive techniques such as vibration measurements. With this monitoring system, the force in the cable is obtained in minutes without any interference with the traffic on the bridge. The leads from the sensors are in a gas tight box located inside the box girder just under the cable anchorage, maintaining the integrity of the sealed system of the gas monitoring system.FIGURE 3CONSTRUCTION TOOK PLACE IN THREE DIRECTIONS ON BOTH PYLONS TWO CANTILEVERS EXTENDING FROM THE PYLON AS THE PYLONS WERE CAST TO THEIR FULL HEIGHT OF 420. THE TOP OF THE WESTERN PYLON HOUSES A THREE STORY GLASS PUBLIC OBSERVATORY WHICH IS ACCESSED BY AN ELEVATOR THAT RUNS UP THROUGH THE CORE OF THE PYLON.As previously mentioned, the Penobscot Narrows Bridge also includes the use of a cable-stay cradle system (U.S. Patent No. 6,880,193) designed by FIGG, the engineer of record for the project; it is the second such installation. The cradle system allows for the use of continuous strands from bridge deck to bridge deck, carrying each stay through a stainless steel sleeve in the cradle assembly. The individual sleeves increase durability by eliminating the potential for strand-to-strand contact. The cradle system also eliminates anchorages in the pylon, reduces tensile forces in the pylon and allows for more streamlined pylon dimensions. Any given strand may be removed, inspected and replaced with a new strand at selected intervals to verify the condition of the stays, without compromising the bridges integrity. Six months after the opening of the new bridge to traffic on December 30, 2006, six reference strands were removed and replaced with carbon fiber strands. This is the first such installation in the U.S. Monitoring of the strands, both steel and carbon fiber provides feedback on the use of this innovativematerial and will help further increase durability as future bridges will be designed utilizing this technology.FIGURE 4A LARGE PLAZA GREETS VISITORS TO THE WORLDS TALLEST PUBLIC BRIDGE OBSERVATORY ENTRANCE, ALONG THE BANKS OF THE PENOBSCOT RIVER. THE ENTRANCE IS FRAMED IN GRANITE, IN CONCERT WITH THE BRIDGE AESTHETIC THEME OF GRANITE. DETAILS REFLECT ON THE ADJACENT HISTORIC FORT KNOX. AN ELEVATOR TAKES VISITORS TO THE OBSERVATORY AT THE TOP OF THE PYLON.The proactive and pioneering approaches encouraged by the Maine Department of Transportation have advanced standards for future cable-stayed bridge designs. There will be opportunities to analyze and evaluate the monitoring records from the stay system, which will result in continued advancements in understanding cable-stay bridge performance and enhanced long term durability. The Penobscot Narrows Bridge & Observatory is providing safe passage for travelers over the Penobscot River, while nearly 75,000 visitors in the first season have traveled to the worlds tallest public bridge observatory (420 above the river) to enjoy views of coastal Maine from the top of the western pylon.ReferencesThe Author of this paper, Christopher J. Burgess, P.E., S.E., signed and sealed the design plans for the bridge and relocated to the site for two years during construction of the bridge.结构2008：穿越边缘 2008美国土木工程师协会缅因州第一座斜拉桥的新创新作者：Christopher J. Burgess, P.E., S.E., FIGG, Denver, CO缅因州的沃尔多汉考克刚桁架桥是美国的1号旅客干线，它位于诺斯科特河附近的巴克斯波特，已近有74年的历史了。这非常重要的交通枢纽，这些年来通过该道路提供的车辆安全通道的货物，除了巨大的商业联系与区域造纸，花岗岩采石，造船，制造工业和民用住房还有货主运送的物品，如，家庭取暖用油。2002年，由于桥的南侧主缆恶化，比预期的更快些，一个翻修计划开始并于2003年开始，计划准备在每一个主索分成37个小的钢索。为了加快这一进程，利用缅因州运输部拥有较早的桥梁，以现有的桥梁同时开展对沃尔多汉考克桥梁加固工程和诺斯科特纳罗斯桥的设计，并将其与天文台于平行排列。缅因州决定用一个新的具有创新意识的业主对桥梁进行设计和建造，该斜拉桥于2003年12月破土动工。这是缅因州第一座斜拉桥图片1沃尔多的，汉考克大桥，新诺斯科特纳罗斯上空的诺斯科特河大桥及天文台台，两端连接这缅因州沃尔多和汉考克县。2006年12月30日新大桥建成通车，但仅仅42个月后就需要维修并更换新的零部件。其中的一个现浇混凝土桥梁，人们给予了相当大的关注，它的投资计划是要提供超过100年寿命的服务。其中易于维护的结构设计工作的关键因素。这座桥是第一个利用一个专为保护桥梁而做的现浇梁，第二个是受益于创新的摇篮制度，制定桥梁设计专利保护功能的独特组合。图片2吊篮与模板一起定位在上部吊架，图中为正在准备移动吊篮的下一节。吊篮是先制造在运送到工地，先定位同时在架构的地方测量，然后再吊装到设计好的地方。其中一个创新是氮气保护和监测系统提供在每个斜拉索加压惰性气体的密封环境。该系统的主要组成部分是气体，高密度聚乙烯护套，储水箱，锚固密封帽和监测硬件。氮气被选为保护气体是由于其非腐蚀性和易于储存，纯氮气环境的压力基本上消除了氧，氯化物、水分和所有潜在的腐蚀因素。该系统的一个主要目标是有效地创建一个不透气的密封系统，完全将索揽包围。安装后，缆索将注入清洁的水分和氮气用以清除封闭的环境中现有的空气，此时的氮加压至2磅每平方英寸。然后用一个密封帽将各锚具的尾部、钢绞线锚固端和所有硬件充分封装。再预留一个明显的端板可以直接目视检查的锚区。每个端部都提供了一个300立方英尺氮气水库补给施工中的压力下降。仪表会记录施工中的气体的所有波动压力，并作为该部分的数据，轻松监控系统的状态，并采取纠正措施。如有必要，电缆之间的环形空间和吊篮空间可以让气体流经在隔离的环境下。据我们所知，利用在新诺斯科特纳罗斯桥的斜拉索气体保护系统是世界第一。这种独特的系统提供了完全周围的惰性气体的环境，并通过气体压力自动监测水平和连续的两个附加保护层。交通监措施使系统中的任何潜在的威胁都可以及时通知给缅因州交通部。该系统有四嵌套级别的保护，共提供冗余