RESEARCH AND ENGINEERING PRACTICE OF CONCRETE FILLED STEEL

ARCHES AND TRUSSES IN CHINA

Bikun FAN, email: 3042793@qq.com Ling KANG, email: 370236201@qq.com

Sichuan Highway Planning, Survey, Design, and Research Institute, Chengdu, China

ABSTRACT

Over the last 30 years, concrete filled steel tubular structures has gained popularity in China, with applications as arches, truss, and piers. Such structure type is used as an Accelerated Bridge Construction method, as well as an economical solution for projects in high seismic areas.

DEVELOPMENT OF CONCRETE FILLED TUBULAR STRUCTURES IN CHINA

The first significant concrete filled steel structure in China is the Wangchangdong bridge, which was constructed in 1990, with a main span of 115m. It utilized 8mm thick steel tube with C30 concrete. Concrete filled structures have demonstrated as a structure type which is effective use of materials, light weight and quick to construction. Concrete filled structures become particularly popular at the southwest portion of China, a mountainous area difficult to access and with height seismic loading.

Over the last 30 years, Chinese bridge engineers have conducted extensive research and testing on such structure type. Topics such as concrete creep and shrinkage, possible delamination between steel and concrete, and construction detailing to ensure composite action, have been investigated both theoretically and experimentally. A national standard “Specifications for Design of Highway Concrete-filled Steel Tubular Arch Bridges” has been developed. Research and testing have confirmed that on average concrete-fill steel tubular structure can reach a capacity significantly higher (1.4 times on average) than the sum of both materials while acting alone.

Concrete filled steel tubular structures offer the following advantages:

1. Lighter structure for erections. It is significantly lighter than typical steel only structures, and it is much easier to erect.

2. Steel tubular structures act as the form for concrete, which reduces field works and improve construction speed.

3. The truss type section typically associated with steel tubular structures are efficient and light-weight, which results in better seismic performance.

CONCRETRE FILLED ARCH BRIDGES

Arch rib is the most popular application of concrete filled tubular structures, with about 460 completed arch bridges in China with such structure type. The arch rib section can be a single steel tube (spans to about 110m) or a truss type section with multiple tubes. Long span arch ribs typically consist of a truss type section with four steel tubes. The four main longitudinal tubes are filled with concrete and the secondary truss members (bracing and diagonal members of the truss) are not filled with concrete. Pingnan III bridge is currently under construction with a span of 575m. Figure 1 shows Hejiang Yangtze River Crossing, the longest completed concrete filled steel tubular arch, with a main span of 530 m. Each arch rib is a four-tube truss section with a width of 4m and a height various from 16m to 8m (midspan). Steel ribs are erected with a temporary stayed tower. A typical rib section for erection was 45m long and weighted about 200 tons. Concrete filling of each main tube member (800 cube meters) was conducted in one continuous operation.

Figure 1, Hejiang Yangtze River Crossing, Sichuan, China. Main span 530m.

CONCRETE FILLED TRUSS BRIDGES

Steel filled concrete tubular sections has been used as truss for superstructure. Such structure type is preferred in the southwest region of China, a mountainous area leading to the Himalayan. The remoteness of the project, challenging foundation, and high seismic loading all strongly favor a light-weight structure solution.

Figure 2 shows the Yaxi Ganhaizi Highway. The total length of the elevated structure is 1800 m, with spans ranging from 45m to 62.5m. The roadway width is 24.5 m, carried by a pair of triangle trusses. The truss was continuous over piers and was launched incrementally during construction. The top and bottom chords of the truss were filled with concrete while bracing and diagonals were not filled. The bridge deck was constructed as cast-in-place with conventional forming.

Figure 2. Yaxi Ganhaizi Highway Truss

Figure 3, Wenchuan Kegu Highway

Figure 3 shows the superstructure of Wenchuan Kegu Highway. The 12.25 m wide highway is supported by two planes of concrete filled steel truss, at a span-by-span layout. The total length of elevated structure is 6400 m, with two standard spans of 30m and 40m. The truss plane is prefabricated offsite in about 15m sections. The onsite fabrication plant them assembled the prefabricated truss sections into the superstructure for one entire span, consisting of two planes of truss and bracing members in between. A 6mm thick permanent steel deck formwork was also installed in the on-site prefabricated plant. The complete superstructure unit was then erected with the erection gantry. Deck concrete was poured after the superstructure was in place.

CONCRETE FILLED PIERS

Concrete filled trusses have proved to be an economical solution for tall piers. The tallest pier height of our complete projects is 183m, with a 196m pier currently in construction. There are two types of concrete / steel composite piers. For short piers (less than 60m typically), the pier is constructed as conventional concrete filled steel tubular truss. For tall piers, the concrete filled truss is eventually encased by another layer of concrete.

Figure 4. Composite Tall Pier

Figure 5. Yaxi Ganhaizi Highway Piers

Figure 5 shows a typical section of Yaxi Ganhaizi Highway, where pier height ranges from 50m- 80m. In this project, the piers were mostly concrete filled steel tubular truss. For piers taller than 60m, the lower part of the pier was encased by an external concrete layer.

For 183m tall piers in Laba Bridge, the concrete filled steel tubes were encased with external concrete, creating a concrete box section overall. The concrete filled tubes acted as the forming and lateral load support for the exterior concrete construction.

SUMMARY

Engineering practice in China have shown that concrete filled steel tubular structures is an efficient structure type, as well as an ABC method. Benefited from the accumulated research and engineering experience, Chinese engineers have been pushing the envelope for this structure type, both in structure efficiency and application scales. The advantages of concrete filled steel tubular structures are light-weight, eliminating field forming application, construction speed and seismic performance.

Figure 6, 183 m tall pier (Laba Bridge)