Design of the Main Spans of the Chongqing Caiyanba Bridge
Man-Chung TANG
Chairman and Technical Director
T.Y. Lin International San Francisco, CA, USA
A member of US National Academy of Engineering and a foreign member of Chinese Academy of Engineering. Received Dr.-Ing. in 1955 and has been designing bridges or 40 years. He is the Technical Director of the Caiyuanba Bridge project.
John SUN
Associate T.Y. Lin International
San Francisco, CA, USA Technical Director T.Y. Lin International (Chongqing)
Born in 1963, he received Ph.D from University of California at San Diego, directed the design of several major bridges and has written a number of technical papers,. He is the Chief Engineer for the Caiyuanba Bridge project.
Summary
The new Caiyuanba Bridge over the Yangtze River in Chongqing will carry six lanes of highway and two pedestrian walkways on its upper deck as well as two tracks of monorails on its lower deck. It will serve as one of the transportation backbones for this ever-expanding city, connecting the two busiest business districts of the city: Yuzhong and Nanan. Because the bridge is located at the center of the city and is visible from most parts of the city, aesthetics is a very important matter. After careful study of the location, a slender tied-arch was selected to carry the double level bridge girder over the Yangtze River. Upon completion, its 420-m span will be the world’s longest tied-arch span for combined rail and highway traffic. The side spans are 102 meters long.
Keywords: Bridge, Long span bridge, Arch Bridge
1. Introduction
The mighty Yangtze and its tributary to the north, the Jialing River divide the metropolitan Chongqing into three parts, as shown in the project location map in Figure 1. The Caiyuanba Yangtze River Bridge is a key project connecting segments of the newly planned south-north corridor. Located 1.2 kilometers upstream from the existing “Chongqing First Yangtze River Crossing,” which was built in 1978, the Caiyuanba Yangtze River Bridge is situated in the heart of the town, and will be a vital transportation link for city of Chongqing.
The crossing is to carry six lanes of highway on its upper deck and two lanes of monorails on its lower deck. The project consists of a signature tied-arch with the record span of 420 meters, and the south and north approaches – the Sujiaba Interchanges connecting the main bridge to the southern shore, and the Caiyuanba Interchange connecting it to the northern shore.
The final design was completed in November 2003, and official ground-breaking for the project took place on December 28, 2003. When the bridge is completed at the end of 2005, it will be the longest arch span for combined rail and highway traffics in the world.
2. Development of Main Span Structural System
Because of its location, the bridge is visible from most parts of the town. Aesthetics is an important factor in the design. The general public of the City prefers an arch bridge. A panel of bridge experts appointed by the City subsequently accepted the recommendation of the designer, a joint venture of Chongqing Communications Research Institute, Chongqing and T.Y. Lin International, USA to build a half through tied-arch at this location.
The Department of Water Transportation determined that a bridge at this specific location must provide a minimum navigation clearance of 392 meters. This resulted in a main span of 420 meters.
Figure 1 Project Location
Figure 2 Renderings of the Tied-Arch
The water level of the Yangtze River in the Chongqing area varies significantly seasonally. The difference between the high water level and the low water level can be as much as 30 meters. The dry season is from about November to about May each year. During the dry season only a small portion of the river is navigable while in the high water season, the entire riverbed is under water. As a result, the piers of the main span should be sufficiently tall to put the superstructure above the high water level.
Most vessels on the Yangtze River are barges carrying cargos with a total weight of up to about 5,000 tons. To resist possible barge collisions, the piers and the lower portions of the arch rib are made of prestressed concrete.
Figure 3 Elevation
The portion of the arch above the deck is a pair of steel boxes. The box section was selected for its slender appearance, which is very important for a centrally located city bridge. The arches are further inclined inwards to achieve a more slenderappearance. This inclination also improves the stability of the structure.
Fig. 4 Plan and Elevation
The truss is 11 meters deep to provide sufficient clearance for the monorail, the loading of which is among the highest in the world. To further increase the slender appearance of the structure, the truss is spanning the end spans without intermediate strut supports. This is possible because the 11-meter-deep truss itself has sufficient stiffness to span the 102-meter end spans. Deletion of vertical struts in the end spans makes the bridge look much more graceful.
Figure 5 Typical Girder Cross Section
3. The Girder
The 11-meter-deep steel truss is similar to a Warren Truss. The hangers are spaced at 16.00 meters on center so the truss is configured accordingly. In the mid span, the hangers are anchored to an extension of the floor beams. The anchor points coincide with the centerline of the arches. A diagonal strut at each side of the girder carries the hanger load to the bottom of the truss.
The truss is continuous over five spans. In the main span, the truss is suspended by cables from the inside legs of the rigid frame. The approach spans are concrete box girders.
With six lanes of highway and one pedestrian path on each side, plus barriers and space for hangers, the total width of the bridge deck is 43 meters out to out. The deck is a steel orthotropic plate with floor beams spaced at 4.00 meters on centers. The ribs are trapezoidal and are 8 mm thick. The deck plate is 16 mm thick.
Box sections are used for the bottom chords. Most diagonals have an H section. Shop splices are mostly welded and field splices are bolted using high strength bolts.
At the lower level, the bottom chords are horizontally braced. Transverse floor beams at 16-meter spacing supports the steel box shaped rails for the monorail. The deck will have pavement of either epoxy asphalt or SMA.
4. The Arch
The lower portion of the arch is a concrete frame. Concrete is selected to provide better resistance to possible barge collisions. Aesthetically the heavy concrete frame also offers a more sturdy appearance for the structure. The outside legs of the concrete frame are anchored vertically with vertical tendons that are adjustable so that a known vertical tie down force can be assured. The front legs of the concrete frame penetrate above the deck so the connection between the concrete legs and the steel box ribs is located above the deck level. Horizontal ties are placed at the deck level to stress the legs together. These ties are also adjustable.
The concrete frames are prestressed using strand tendons.
Fig. 6 Structural System of the Concrete Frame
The upper portions of the arch ribs are steel box sections, 2.40 meters wide and 4.00 meters deep, constant along the entire length. Diaphragms inside the ribs are provided to stiffen up the hollow box section. Longitudinal stiffeners are simple plates. The box ribs are to be delivered in sections and will be welded together at the site.
It is intended to keep the humidity sufficiently low to prevent corrosion inside the box by mechanical ventilation.
5. The Hangers and Ties
The hangers are parallel wire strands with Hi-Am type sockets. The upper end of the hangers penetrates the bottom plate of the arch rib and is anchored at a diaphragm inside the box rib. The bottom end of the hangers is provided with threads and is anchored at the floor beams of the deck. The hangers are located in the same center plane of the arch ribs.
The ties are made up of seven wire high strength stands. They are individually sheathed. Each tie is divided into three sections. The center section anchors at the inside legs of the concrete frames, while the other two connect the inside and outside legs. Each tie can be individually stressed. They can also be individually replaced without interrupting traffic.
Both hangers and ties are made to meet the same standard for stay cables in cable-stayed bridges and must provide the same level of safety and durability.
6. Foundation
The soil is generally soil rock. The foundations are supported by 3.00-meter-diameter short caissons.
7. Construction
There are three major difficulties in the construction of this structure: the variable water level, the restrictive city area for material transportation and the short construction schedule.
During high water season, the Yangtze River is very rough. It can rise up to 30 meters above the low water level. Building the foundation during high water would have required very heavy cofferdams, the price of which would be prohibitive. Therefore, the foundation must be completed during the dry season, which usually runs from November to about early May. To expedite the construction, all caissons are hand dug at the same time. Fortunately, this is not a major problem in China at the moment as labor is relatively readily available.
It is anticipated that all piers will be completed to a level above the high water level before high water arrives. Then the rest of the construction will be free of the interference of the high water.
Due to the restrictive city traffic, most materials, including the steel sections of the girder and the arch will be delivered to the site by barges. However, the large difference in water level makes docking and handling very difficult. In addition, the navigation channel is located on one side of the river during dry season so it is difficult to access from the other bank of the river. Consequently, it was decided to use a heavy-duty double high line to lift and to erect the steel sections directly from the barges.
The arch ribs will be erected using the high lines and a temporary cable-stayed system with its pylons located above the main piers. The advantage of the high lines is that they can lift the steel sections from a barge at the navigation channel, which is near the southern bank, and transport them to any place of the main span. The girder will also be erected by the same high lines after the arch ribs are completed.
8. Design Specifications
The bridge is designed according to the Chinese specifications. For the orthotropic deck, the design is also supplemented by AASHTO specifications. For hangers and ties, the design is supplemented by PTI’s “Recommendations for Design of Stay Cables.”.
Chongqing is not a seismically active area. Seismic effect is not significant for this structure.
9. Acknowledgement
The owner of the bridge is Chongqing City Investment Company. The designer is a joint venture of Chongqing Communications Research Institute and T.Y. Lin International. The contractor is Zhongtie Major Bridge Construction Company.
邓文中 美国加利福尼亚州旧金山林同炎国际公司董事长兼技术总监，美国国家工程院院士、中国工程院外籍院士。1955年取得工程学博士学位，已从事桥梁设计40多年。菜园坝大桥技术总监。
孙峻岭 美国加利福尼亚州旧金山林同炎国际公司 (重庆)技术总监，1963年出生，他在美国加州大学圣地亚哥分校取得了博士学位，参加了几个主要桥梁的设计，并写了很多的技术论文，菜园坝大桥总工程师。
重庆菜园坝大桥横跨长江，主桥为六车道行车道、双侧人行道（上层桥面），双线城市单轨轻轨（下层桥面）。它将作为这个正在发展中的城市的交通骨干之一，连接两个的最繁忙的商业区：渝中区和南岸区。由于大桥位于城市中心，从全市重要地位上来说，对这座桥的美学有很高的要求。经过认真的方案必选，确定使用系杆拱拱式桥和组合刚构桥跨越长江。工程建成后，主跨为420米，将是它成为世界上最长的系杆拱组合刚构公路轻轨两用大桥。它的边跨为102米。
关键词：桥梁，大跨度桥，拱桥
奔腾不息的长江及其北边的支流嘉陵江把大都市重庆分成了三个部分，工程为位置如图1所示。重庆菜园坝长江大桥是一项重点工程，连接新规划的南北走廊。位于从现有的“重庆长江第一大桥（建于1978年）”的上游1.2公里处，重庆菜园坝长江大桥位于城市中心，将成为重庆市的交通运输作出重大的贡献。
通过这座桥上层为六车道的公路，下次为两个在单轨铁路。该项目包括世界纪录为跨度420米系杆拱拱桥，以及南北方向的引桥，南岸连接到苏家坝立交，向北到与菜园坝立交。
该桥的最终设计完成于2003年11月，官方的动工仪式2003年12月28日举行。当这座桥于2005年底完成时，它将是世界上最长的公铁合建拱桥。
由于这座桥处于城市的重要位置，美学必然成为桥梁设计的重要因素。因为广大市民喜欢拱桥，所以重庆交通科研设计研究院和美国林同炎国际公司组成了一个联合体，任命了一个由城市桥梁专家组成的小组完成这个设计。
水路运输部门规定，在这个特定位置的桥梁，必须提供一个392米的最短通航净空。这导致了主跨应为420米。
重庆地区的长江水位季节变化明显，最高水位和最低水位差异可以高达30米。旱季从大约十一月至下一年五月，在干旱季节，河道只有一小部分可通航，而在高水季节，整个河床下的水。因此，主跨的桥墩应足够高，保证在高水位时上层建筑在水位之上。
长江上的船只多为驳船，运载重量高达约5000吨货物。为了抵御可能的驳船碰撞，桥墩和拱肋的下部使用预应力混凝土。
在桥面之上的拱肋是一对钢箱。钢箱被设计成细长轻盈的外观，这一点对于城市中心的桥梁非常重要。拱肋进一步向内倾斜，实现了更加苗条的外观，这种设计同时也提高了结构的稳定性。
桁架是高11米，使单轨铁路有足够的空间，它的载重量也是位居世界前列。为了进一步增加结构的苗条外观，桁架跨越无中间支撑边跨。这是可行的，因为11米高的桁架本身具有足够的刚度跨越102米。边跨去掉了垂直支撑，使桥变得更加优雅。
11米高的钢桁架类似于华伦式桁架。吊杆以间距为16米逐一在桁架配置。在桁架的两边，吊杆锚固在向外扩展的斜支撑上，在拱中心线上，吊杆锚固在桁架的底部横梁上。
该桁架连续五跨。在主跨中，桁架被固定在支撑架上通过绳索。引桥为混凝土箱梁。
六车道的公路、每边人行道，加上障碍和锚固空间，桥面总宽度为43米。桥面是一个正交异性钢桁架梁，间距为4.00米，肋骨梯形为8毫米厚钢板，桥面为16毫米厚的钢板。
底部弦杆用的是箱型梁。大多数对角线有H型钢。大多数的接头是焊接和高强度的螺栓现场拼接。
在下层桥面上，下弦杆是水平放置的，横向的梁以间距为16米支撑钢箱形的单轨铁路。
公路路面使用环氧沥青或沥青马蹄脂碎石混合料。主梁
拱的下部是混凝土框架。在可能遇到的驳船碰撞下，这种构造能提供更好的保护，从美学的观点来看，笨重混凝土框架使结构更加坚固美观。混凝土框架外侧腿垂直锚在垂直筋上，在已知的拉力上是可以调节的。混凝土框架前腿穿透桥面以便混凝土结构的腿和钢箱肋骨连接在桥面以上的位置。横向联系放置桥面里，保证双腿的并拢。这些联系也可根据需要调整。
预应力混凝土框架结构的使用是钢绞线筋。
拱形的肋骨上使用的是钢箱梁节段拼接，2.40米宽和4.00米高，沿整个长度截面不变。里面的隔板，使中空箱形截面更加牢固，是简单的纵向加劲肋板。钢箱梁节段组成的肋骨在工厂预制，并将在现场焊接在一起。
为了达到保持足够低的湿度，以防止内部腐蚀的目的，对钢箱梁进行机械通气。
吊杆采用与Hi- AM型插座平行钢绞线。吊杆上端穿透的拱肋底板，并在锚固在钢箱内肋隔板上。吊杆下端通过相应的路程并锚固在下层路面的横梁上。吊杆是位于同一拱肋平面内。
钢绞线是由七根高强度钢丝组成，每根钢丝单独通过护套保护。每个钢绞线分为三个部分。锚固在在混凝土框架内两腿腿中间的部分，和其他两个连接内外的腿的部分。每一根钢绞线可以单独拉伸。 也可以单独更换，而不会中断交通。
吊杆和钢绞线必须达到于斜拉桥中斜拉索相同的标准，并必须提供安全性和耐久性相同的水平。
土壤为常见的岩石土壤。基础是由直径为3.00米短沉箱构成。
这个工程的建设有三个主要的困难：多变的水位，城市中对材料运输的限制和很短的施工工期。
在汛期，长江流域是非常可怕。与最低水位相比，水位可以上升30以上。如果在汛期进行基础施工，需要非常沉重围堰，其费用是昂贵的。因此，基础必须在旱季完成，一般情况下从十一月到下一年的5月上旬。为了加快建设，所有沉箱都是人工在同一时间开挖的。幸运的是，目前在中国，这不是重要的问题，因为劳动力相对容易获得。
根据计划，所有桥墩在汛期到来之前，施工完成的高度必须超过高水位。然后，其他的结构施工将不受水位影响。
由于城市交通的限制，大多数材料，包括梁的型钢和拱肋将用驳船运送到现场。然而，比较大的水位差，使对接和处理非常困难。此外，当处于的枯水期，航道变窄，驳船很难从河的一边到另一边。因此，决定使用重型双高线升降机，直接从驳船上竖直吊装材料。
拱肋的施工是通过高线和一个临时支撑在桥墩上方的斜拉桥挂架系统。高线的优点是可以摆脱在航道的限制，从靠近南岸的驳船上吊取型钢，并运送它们到主跨的任何位置。在拱肋建成之后，主梁架设也将通过高线来完成。
这座桥的设计是依据中国的规范。对于正交异性板的设计，参照以美国国家公路与运输协会标准规范。对于吊杆和钢绞线的设计，参照“斜拉桥的设计建议”。
重庆菜园坝长江大桥业主：重庆市城市建设投资公司；
重庆菜园坝长江大桥设计单位：重庆交通研究设计院/林同炎国际公司设计联合体；
重庆菜园坝长江大桥施工单位：中铁大桥局集团建设公司。
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