China's bridge capital Chongqing will open another world-record bridge later this year, reports Man-Chung Tang
Chongqing, a city with a population of 32 million people, was part of the Xichuan Province in the mid-western region of China until about ten years ago, when it became an independent city with the same rank as a province. Most of Chongqing is mountainous and intersected by several big rivers: the Yangtze and the Jialing, which divide the metropolitan area, the Wujiang River and so on. Consequently, development of the area requires the construction of many major bridges.
In 1981 the city's first major bridge, the First Shibanpo Bridge over the Yangtze River, was completed. This multi-span prestressed concrete girder bridge has a main span up to 173m. Since then, dozens of bridges have been completed, and the city has numerous projects that are under construction or in the planning stage. These include six suspension bridges, the biggest with a span of 600m; 14 cable-stayed bridges up to 460m span; seven arch bridges up to 552m span; 15 prestressed concrete girder bridges up to 330m span, and four steel truss bridges up to 192m span. Among them, the Second Shibanpo Bridge, which was opened last year (Bd&e issue no 44), is the world's longest girder bridge span and the Chaotienmen will be the world's longest arch bridge span. Not surprising then that Chongqing is the city in China with the most major bridges, hence it is often called the bridge capital of China.
Befitting this status, at the end of 2003 a groundbreaking ceremony was held in the city to celebrate the start of two world-record span bridges; the Caiyuanba Bridge and the Second Shibanpo Bridge - on the same day, in the same city, on the same river, for the same owner.
The Caiyuanba Bridge over the Yangtze River will be a vital transportation link between Yuzhong, the central district, and Nanan, the south bank. It is located in the heart of the town and is about 1.2km upstream of the First Shibanpo Bridge. It will carry six lanes of highway on its upper deck and two monorails on its lower deck. The 420m span will be the longest arch span for dual highway and rail traffic when completed.
But the entire project is significant. As well as the tied-arch main spans, there are the south and north approaches: Sujiaba Interchange to the south and Caiyuanba Interchange to the north. The main span length requirement was determined by the Waterway Department of the Ministry of Communications through a study of navigational needs in the area; but at this point the main navigation channel is on a curve, hence the span required is much larger than that of the Shibanpo Bridge.
Since the Caiyuanba Bridge will be visible from almost every part of the town, aesthetics was an important factor in the design. In the preliminary stage, various bridge types were studied, but in the end, the general public of the city preferred an arch bridge. A panel of bridge experts appointed by the owner, the Chongqing City Construction Investment Company, subsequently accepted the recommendation of the designer, a joint venture of Chongqing Communications Research & Design Institute and TY Lin International, to build a half-through tied-arch at this location.
Along with the factors above, the configuration of the final structure was driven by several other important criteria. The city has a hilly landscape and the water level of the Yangtze River at this location varies seasonally by more than 30m, requiring special considerations for barge collision and corrosion protection. The bridge girder must be 10m deep in order to accommodate the monorail on the lower deck, and to satisfy these requirements, a Y-shaped concrete frame was chosen for the lower portion of the bridge. This raises the arch to a higher level, and the use of concrete will ensure better protection against corrosion and barge impact.
One particular type of structure that was considered at the preliminary stage was a concrete-filled steel tube arch bridge, many of which have been built in China. However, aesthetically, it was considered that a bridge in this sensitive location should be as light as possible so as not to hinder the view of the city. Consequently, the designers decided that the arch ribs should have a box-shaped cross-section. The arches are also inclined inwards to achieve a more slender appearance and to improve the stability of the structure.
To further improve the slender appearance of the structure, the truss has no intermediate strut supports across the 102m-long end spans. This is possible because the deep truss itself has sufficient stiffness.
Chongqing is well-known for suffering from thick fog almost all year round. For this reason yellow or orange-red, which are easily visible in fog, were proposed as the colour of the bridge, and the owner chose orange-red. Chongqing is not a seismically-active area; hence seismic/wind effects were not significant for this structure.
Another concern was that a 10m-deep girder span across the skyline of the Yangtze Valley would look bulky, which led to the decision to use a steel truss instead of a box girder, making the bridge look much lighter. Also, a truss will allow passengers in the monorail a more pleasant outlook when crossing the bridge. To improve the appearance still further, a trapezoidal cross-section was selected for the truss girder, since the tapered edges make the structure look thinner.
The resulting structure is a Warren-type truss with hangers at 16m centres. At each hanger point, a diagonal strut brings the vertical load from the hanger back to the truss. The hangers are anchored to an extension of the floor beams below the deck.
The truss is continuous over five spans and at the intersection with the arch ribs, it is suspended by cables from the concrete frame. The approach spans are concrete box girders.
With six lanes of highway, one pedestrian footway on each side, plus barriers and space for hangers, the total width of the bridge deck is almost 40m. The deck is a steel orthotropic plate with16mm-thick deck plate and 8mm-thick trapezoidal ribs. The floor beams are spaced at 4m centres. Box sections are used for the bottom chords and most diagonals have an H-shaped cross-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 16m spacing support the steel box-shaped rails for the monorail.
The lower portion of the arch frame is made of concrete, to provide better resistance against 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 with adjustable vertical tendons so that a known vertical tie-down force can be assured. The front legs of the concrete frame rise above the deck so the connection between the concrete legs and the steel box ribs is located above the deck level. Adjustable horizontal ties are placed at the deck level to stress the legs together, and the concrete frames are prestressed using strand tendons.
The upper portions of the arch ribs are steel box sections, 2.4m wide and 4m deep along the entire length. Diaphragms inside the ribs stiffen up the hollow box section while the longitudinal stiffeners are simple plates. The box ribs were delivered in sections and welded together at the site.
The hangers consist of parallel wire strands with hi-am type sockets. The upper end of each hanger penetrates the bottom plate of the arch rib and is anchored at a diaphragm inside the box rib while the bottom end is anchored to the deck with an eye socket.
