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. The hangers are located in the same centre plane of the arch ribs.

Ties are made of high-strength, seven-wire strands that are individually sheathed. Each tie is divided into three sections; the centre section is anchored at the inside legs of the concrete frames, while the other two connect the inside and outside legs. Each tie can be individually stressed and can also be individually replaced without interrupting traffic.

Both hangers and ties are manufactured to meet the same standards as those on cable-stayed bridges and must provide the same level of safety and durability.

The foundation, supported by 3m-diameter short caissons, added some complications to the project. While the ground conditions are mostly solid rock, during high water season the Yangtze River is very rough and can rise more than 30m above the low water level. Building the foundation during high water would have required extremely heavy cofferdams, the price of which would have been prohibitive. Hence the foundations had to be completed during the dry season, and to expedite construction, all caissons were dug by hand at the same time. Fortunately, this type of labour is relatively readily available in China.

Zhongtie Major Bridge Construction Company ultimately became the winning contractor for the project after the original low bidder was disqualified. Unfortunately, as this happened after the ground-breaking ceremony, it meant that construction of the main span was delayed, pushing the construction of the concrete Y-frames into the high water season which required extensive structural support to resist the fast-flowing water. CCS Industrial Corporation was hired by the owner to perform full inspection of the construction.

The topography of the area hindered transportation through the city, and practically all materials, including the steel sections of the girder and the arch ribs, had to be delivered to the site by barges on the Yangtze River. However, the large difference in water levels made docking and handling extremely difficult. In addition, the navigation channel is located on one side of the river during dry season so it was difficult to access the materials from the other bank of the river. As a result, a heavy-duty double highline with a design capacity of 420t was used to lift and erect the steel sections directly from the barges. Due to a change in the construction procedure, the actual weight that had to be lifted was less than 350t.

The arch ribs were erected using the highlines and a temporary cable-stayed system with its pylons located above the main piers. The highline lifted the steel sections from a barge at the navigation channel near the southern bank and transported them for placement on the main span. The erection of the arch ribs went very swiftly, averaging three days for each pair of 16m sections. The deviation is less than 40mm between the opposite sections at the closure, which was easily adjusted to a perfect fit.

After the arch ribs had been erected, a temporary tie was installed to equalise the horizontal thrust of the arch ribs. The girder was then erected using the same highlines, cantilevering from both ends and suspended immediately from the arch ribs by the hangers. Truss sections were bolted together in the field and the contractor averaged a construction cycle of about seven days for each pair of 16m sections. The closure of the girder at mid-span was also very smooth and took place in March, earlier this year. Once the truss had been completed, the ties were installed and stressed. The bridge is scheduled to be open to traffic in October of this year.

The Caiyuanba Bridge was designed according to Chinese specifications, with AASHTO-LRFD specifications supplementing the design of the orthotropic deck. The Post Tensioning Institute’s recommendations for design and installation of stay cables were used for the hangers and ties.

Man-Chung Tang is chairman of the board at TY Lin International

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