(1) adopts steel truss arch and cast-in-place with support;
(2) Steel or concrete-filled steel tubular rigid skeleton is adopted and cast-in-place without support;
(3) Tower stay cable method, cable-stayed suspension casting and cast-in-place construction without support.
The fourth section of this chapter introduces the content of cast-in-place construction of concrete filled steel tubular rigid skeleton. The following are the common construction methods mentioned above.
The number of cast-in-place construction of through reinforced concrete arch bridges in China is small. The famous ones are as follows:
Bridge 3006 in Dukou City, Sichuan Province (1). The span of the bridge is 146m, the deck width is 15m, the section is box-shaped, the arch axis is catenary with equal section, and the arch ring is 2.5m high. Built in 1972.
(2) 3007 Bridge in Dukou City, Sichuan Province, with single span 170m, deck width 12m, box section, arch axis as catenary with equal section, arch ring height of 2.8m, built in1979;
(3) Dongguan Bridge in Dongguan City, Guangdong Province, with 6 spans of 68m and a deck width of 12m, was built at 1984.
The above three bridges all adopt steel truss arch structure. The following introduces the construction method of 3006 Bridge in Dukou City. The construction procedure of cast-in-place arch bridge with support mainly includes arch frame installation, arch ring pouring and arch building pouring.
(a) the arch installation
Cantilever installation method is often used to install the arch frame of long-span arch bridge.
1. Structure type of arch frame
Steel truss arch is adopted, and the arch structure is mostly assembled truss arch. Arch frames are composed of standard joints, vault joints, arch foot joints and connecting rods connected by steel pins, and then several arch frames are connected into a whole by longitudinal and transverse connection systems, which can be used as support for pouring arch rings or arch ribs. The curvature of the arch axis curve is obtained by changing the length of the connecting rod. The height of the assembled truss is 3m, and the internode length of the upper chord is1.45m. The structure of the arch is shown in Figure 10-2.
2. Arch installation
(1) Arch lifting
The installation arrangement of the arch is shown in figure 10-3. Before installation, the arches need to be assembled into an installation unit in the form of a frame, the length of which can include two or three arches, and the maximum lifting weight of a frame is 200kN. During installation, the arch foot and vault are symmetrically arranged on both sides. There are eight arches in total, four in the middle are installed first, and two in the upstream and downstream are installed after the arches are closed and lifted. Portal pylon is installed on the arch frame.
The four arches in the middle can be directly hoisted into place. When the arch transport track is fixed in the middle of the tower gate, the arches on both sides can be hoisted in place by exchange hoisting method. The hoisting procedure of the exchange hoisting method is as follows:
(1) After the frame is transported to the cantilever workbench under the tower, hang the front and rear racing blocks, as shown in figure 10-4. Among them, pulleys No.2 and No.3 are pulleys for installing shafts, and the transmission cables of pulleys 1 and No.3 must be interchanged, and the transmission cables of pulleys 1 and No.2 must be cross-bolted.
(2) Start blocks 1 and 2, raise the frame, and block 3 will tighten it (but it will not be stressed). The frame will be hoisted to a height that can be separated from the suspension platform, and then transported out of the suspension platform.
(3) High-altitude exchange, directly lifted by blocks 2 and 3, and transported to the installation position.
④ Install in place, drive the steel pin and install the lower chord.
⑤ Adjust the elevation of the axis, install the wind hook, set the wind cable and tighten the stay cable.
(2) Arch sealing and hoisting
When cantilever is used to install the arch, it is more suitable to seal the arch at low temperature and hoist it at high temperature. Lifting should be carried out symmetrically and circularly from vault to arch foot. Don't relax too much at a time, and don't relax the cable basket bolts more than 5cm at a time.
(3) Precautions for hoisting, arch sealing and crane unloading
(1) Before hoisting, all preparations should be made and trial assembly should be carried out.
(2) Before arch sealing, the arch axis and node elevation must be adjusted, and all crosswind cables should be tightened.
(2) After the arch is closed, tighten all the air hook bolts once before unloading the crane.
(2) Box-section arch ring pouring and arch frame unloading
1. Segmented casting of arch ring
For the arch ring of long-span arch bridge, in order to reduce the shrinkage stress of concrete and avoid cracks caused by arch deformation, the segmentation degree should generally be 6 m ~15 m, and both sides of the arch must be uniform and symmetrical when dividing the arch segments. Spaced warps should be reserved at the cutting point. The width of the interval joint is suitable for construction and steel bar connection, generally 50cm to 100cm. For the concrete in the interval section, in order to prevent the time of arch ring closure and arch frame demolition from being delayed, semi-hard concrete with a label one rope higher than that of arch ring can be used.
2. Box-section arch ring casting
Box-section arch ring generally adopts the method of double-ring segmental casting. Subrings are generally divided into two rings or three rings. When the second ring road is divided, the bottom plate is poured in sections first, and then the ribbed wall, partition wall and top plate are poured in sections. When pouring in three rounds, the bottom plate shall be poured in sections first, then the ribbed wall and partition wall shall be poured in sections, and finally the top plate shall be poured in sections. Two different closing methods can be used in circumferential segmental casting, that is, circumferential closing interval seam and final closing interval seam after the completion of full arch. When the seam is closed, the closed annular layer can be used as an arch frame. When pouring the upper ring layer, the arch ring load can be reduced, but the construction period is longer than one-time closure. When the final closing method is adopted, it is still necessary to pour ring by ring, but it is not closed ring by ring, but the gap joints of each ring are filled together and finally closed. At this time, the interval between the upper ring and the lower ring should be corresponding, and its width is generally about 2m, and the interval seam with reinforced joints is about 4 m. Figure 10-5 shows the schematic diagram of the box arch ring of the 3006 bridge with three rings and nine sections.
3. Unload the arch frame
Long-span arch bridge adopts cast-in-place construction of arch frame, and the unloading of arch frame is very important. Only after the arch ring concrete reaches a certain strength can the arch frame be removed. In order to make the gravity of the arch ring supported by the arch frame gradually transfer to the arch ring itself to bear, the arch frame should not be unloaded suddenly, but should be carried out according to certain procedures. In order to ensure that the arch frame can fall evenly according to the design requirements, special unloading equipment must be adopted. For long-span arch bridges, sand drums and jacks are commonly used for unloading equipment.
(1) sand tank
Sand slips are generally made of steel plates, with dry sand in the cylinder and a piston (made of wood or concrete) inserted into the upper part, as shown in Figure 1o-6.
Unloading depends on the sand flowing out of the sand discharge hole reserved at the lower part of the cylinder, so the sand in the cylinder is required to be dry, uniform and clean. Fill the space between the sand cylinder and the piston with asphalt to prevent sand from flowing out easily due to moisture. The unloading height of the arch can be controlled by the amount of sand discharge, so that the arch can be unloaded several times by opening and closing the sand discharge hole, and the arch can descend evenly without vibration. The diameter of steel sand cylinder used in China's 170m reinforced concrete arch bridge is 86cm, and the application effect is good.
(2) Jack
The removal of arch frame by jack is often carried out at the same time as the adjustment of internal force of arch ring. Generally, a space for placing jacks is reserved in the vault. Jack is used to eliminate the internal force of shrinkage, creep and elastic compression of concrete, so that the arch ring is separated from the arch frame.
(3) Buildings above the arch
The arch construction of long-span arch bridge should be symmetrical and uniform. The pouring procedure and condensation amount in construction should meet the design requirements. In the process of building construction on arch, the internal force and deformation of arch ring and the displacement of pier and abutment are cut and controlled in situ.