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1、It is said that necessity is the mother of all invention. If that is the case, then the need for striking, elegant cable-stayed bridges that can span greater lengths and be more easily maintained over the course of longer lives has yielded a significant new invention that may well benefit bridge des
2、igners the world over.Driven by a desire to push the design of single-plane cable-stayed bridges beyond their current limitsboth in span length and in aesthetic appealwhile still delivering an economic solution that is easy to construct and maintain, FIGG, an engineering firm based in Tallahassee, F
3、lorida, has created a novel system for routing stay cables from one end of a bridge deck, through the bridges pylon, and then down to the other end of the deck in a way that precludes the possibility of cable-to-cable interactions.The innovation has been employed on two new bridgesthe Veterans Glass
4、 City Skyway Bridge, which carries Interstate 280 across the Maumee River in Toledo, Ohio (see “Ohio DOT Endorses Design for Maumee River Crossing,” Civil Engineering, September 2000, page 12), and the Penobscot Narrows Bridge and Observatory, which carries U.S. Route 1 over the Penobscot River in s
5、outheastern (“down east”) Maine near the coastline (see “Observatory to Cap Maine Crossing,” Civil Engineering, April 2004, pages 1517).The cradle offers benefits both during construction and over the life of the bridges. Most important of all, it permits the use of the largest number of strands wit
6、hin a single stay cable in the world: 156, an increase of more than 70 percent over the second-highest number known to have been used in the United States. It also makes it possible to increase the distance between stay cables by approximately 50 percent when used with precast delta frames to facili
7、tate a single plane of stay cables, resulting in aesthetically superior designs. This is accomplished by having all the strands parallel as they travel from the anchors at the deck level through the cradle in the pylon and back to the deck. Individual sleeve pipes located within the cradle system en
8、able each strand to act independently of adjacent strands. This also permits the cables to be much larger and to be spaced farther apart, which translates into longer spans.Furthermore, the cradle system lowers initial costs by reducing the amount of materials and labor needed because no anchorages
9、in the pylon are required. This simplifies construction operations by allowing all of the cable-stressing operations to be performed at the bridge deck level rather than within the (often restricted) confines of the pylon, high above the bridge deck. The system also includes removable “reference” st
10、rands in each stay cable that provide a simple, reliable method for verifying the condition of the stay cables in the future. Because the cradle system does not require strands to be grouted, they can be individually removed, inspected, and replaced, even when there is traffic on the bridge. Bridge
11、owners can thus safely and accurately assess the conditions of the stay cables at any time over the course of the bridges life. In the case of the Maine bridge, new strand materials can be tested side-by-side with traditional materials in an actual setting, as opposed to a process of computer simula
12、tion.What is more, the cradle system makes possible the use of a wider variety of pylon designssome with much smaller and more elegant cross-sectional shapesthat can be constructed more economically. Engineers can thus design pylons that are far more unusual and aesthetically pleasing than has been
13、the case in the past.The cradle design works with the natural flow of forces because the forces transmitted through the cradle naturally compress the pylon in an efficient manner, the stresses being applied radially along the curve of the cradle (see the illustrations on page 41). In traditional sys
14、tems, anchorages within the pylon required large tension ties to resist the high splitting forces that would be generated. Using the cradle system eliminates this requirement and further enhances the elegance of the pylon shapes.The development of this new technique began in early 2000. The Ohio Dep
15、artment of Transportation (ODOT) and the Toledo Metropolitan Area Council of Governments had formed what was called the Maumee River Crossing Task Force Design Committee to assess and communicate the communities perspective on the developing design. The task force chose glass as the theme for the ne
16、w cable-stayed bridge. Meetings involving a diverse cross section of the community had made the publics views clear: the bridge was to feature glass in a very visible and striking way in recognition of Toledos industrial heritage as a leader in the glass industry. Many families in the area had worke
17、d in the local glass industry for generations, and it was important to them that the crossing be a symbol of the “Glass City,” as Toledo has come to be known. The public was also of the opinion that the bridge design should champion a product that had been of the utmost importance to the areas econo
18、my. The task force further determined that residents wanted the bridge design to be light, simple, and elegant.FIGG led two community workshops, or design charettes, during which the public was presented with a variety of aesthetic options. Community voting showed a clear preference for a single-pyl
19、on design. The consensus was that by creating a single tall pylon403 ft (123 m)using glass on the top 196 ft (60 m) and on all four sides, the new bridge would be visually stunning. In keeping with this directive, the top of the pylon takes on a prismatic shape, its panels of treated glass reflectin
20、g sunlight during the day on all four sides. Behind the glass are light-emitting diodes (LEDS) that enable the pylon to stand as a beacon at night. (The led fixtures are controlled remotely and capable of literally millions of color combinations. In fact, various color schemes have been preprogramme
21、d, some schemes marking major holidays and others exhibiting team colors for statewide sporting events.)有人说需求是一切发明的母亲。如果真的是这样，那么引人注目的，优雅的斜拉桥能够横跨更大的距离和使得桥梁在更长的使用寿命中的维护变得更简单将是一项可以让全球桥梁设计者都受益的重要的新发明。在推动单面斜拉桥超越现有极限（在跨度和美观两方面）的需求的驱使下，同时用一种在建造和维护过程中都经济的解决方法，FIGG，一家位于Florida首府Tallahassee的工程公司，发明了一个新颖的体系。该体
22、系的斜拉索的传力路径是将力从桥面板的一端通过桥塔传下至桥面板的另一端。这种方法就可以排除斜拉索间互相作用的可能性。这种创新的方法被应用在两座新桥上。一座是老兵玻璃城航线大桥，这座大桥连接着280洲际公路横跨流经Toledo, Ohio的Maumee河；另一座是Penobscot 纽约湾海峡大桥和天文台，这座大桥连接着美国1号公路横跨位于靠近海岸线的Maine 洲东南面的Penobscot河。这个支架为桥梁的建造过程和其使用寿命中都带来了好处。最重要的是，它使得一根拉索中绳的股数达到了世界范围内的最大值：156。这个值比美国境内用过的第二多的股数多了70。也使得当它用于预制的三角形框架（可使得单
23、面斜拉索更方便）的斜拉索的距离增加大约50成为了可能。这样就可以实现美学上的非凡设计。这是通过在绳股从桥面板水平面的支座穿过桥塔中的支架然后返回到桥面板过程中使得所有的绳股平行来实现的。位于支架系统内的独立套管使得每股绳都能不受相邻绳股的影响而单独作用。这也使拉索可以做得比以前大得多并且拉索间的距离也可以更大，也就是说桥梁可以横跨更大的距离。而且，由于桥塔内不需要支座所以材料的用量和人力的使用都减少了，从而支架系统降低了桥梁的初始成本。由于拉索的张拉操作都在桥面板上进行而不是在高高的桥塔上那么局限的范围内进行所以施工操作也变得简化了。这个体系还包括在每个斜拉索中都有可移动的参考绳股，这样为将来
24、检验拉索的使用情况提供了一个简单可靠的方法。因为支架系统不要求用水泥浆添塞绳股，所以即使是在桥上有车辆和行人的情况下它们可以被单独地移除，检查和替换。这样一来桥梁的业主在桥梁的使用寿命期间随时都可以安全地，准确地对拉索的情况进行评估。在Maine桥的案例中，新的绳股材料和传统的材料可以在实际中进行并行的测试而不需要用电脑仿真。而且，支架系统使得桥塔设计的多样化成为了可能，有些可以采用小得多的更优雅的截面形式，这样施工也更经济。工程师在将来也就可以设计出不寻常的，更美的桥塔了。支架的设计应该和力的自然传递结合起来因为力通过支架力以一种有效的方式挤压桥塔。应力沿着支架的曲线辐射地分布。在传统的系统
25、中，桥塔内的支座要求连接有很大的拉力以抵抗可能产生的突发的力。支架系统就没有这个要求而且进一步加强了桥塔形状的美观性。这项新技术的开发始于2000年年初，俄亥俄交通部和托莱多城市地区政府委员会成立了一个称为Maumee 河跨越任务力学设计委员会的机构对委员会的深化设计的效果图进行交流和评估。任务力学设计委员会把玻璃选作新型斜拉桥的主题。包括各种各样的横截面的委员会会议让大众的观点明朗起来：桥将会以醒目的和震撼人心的方式成为玻璃的象征。这座桥代表着托莱多在玻璃行业中领先的工业传统。很多家庭好几代人都在当地玻璃工业中工作。随着托莱多的出名，让这座桥梁成为玻璃之城的象征是很重要的。人们还认为桥梁的设
26、计应该使得一个产品成为冠军产品对地区经济的发展来说是最重要的。任务力学设计委员会还进一步决定居民们希望桥的设计应该轻盈，简洁和优雅。FIGG成立了2个公众车间或者说设计研讨组，在此期间向大众展示各种外观。大众投票的结果清楚地显示人们想要的是只有一个桥塔的设计。大家一致认为应该建造一个高为403英尺（123米）的桥塔，并在其上面的196英尺（60米）部分和桥塔的四周都使用玻璃。这座新桥将会看起来惊艳无比。为了达到这个目标，桥塔的顶部采用了棱镜的形状。在白天，经过处理的玻璃面会从四个方向反射太阳光。在玻璃里面放置了发光二极管（LED灯），这样在夜间桥塔就可以像一个灯塔般屹立。（LED设备由遥控控制，它可以形成数百万种渐变的颜色组合。有一些组合标志着主要的节日而另一些则表示洲内体育赛事团队的颜色。）