I. Introduction of Suspension Bridges
Suspension bridge refers to a bridge with tension cable as the main load-bearing component, which is composed of main cable, bridge tower, anchorage, suspender (suspender) and deck structure. The main cable is hung on the bridge tower through the saddle, anchored at both sides (or both ends of the bridge) as the main load-bearing component, and is generally formed by extruding multiple steel wires, each of which is composed of multiple steel wires; The geometric shape of the main cable is determined by the balance condition of force, which is generally close to parabola. The bridge tower mainly bears the pressure of the main cable, generally using concrete, and some bridges using steel towers; The top of the pylon is provided with a saddle to support the main cable.
Anchorage is the general name of main cable anchorage, which consists of concrete anchor block (including steel bar), bracket, anchor rod and saddle (loose cable saddle). The main cable enters the anchorage through steering, unfolding, anchoring and other parts. A sling is a kind of cable that hangs from the main cable and is used to hang the bridge deck. The sling is connected with the main cable through the cable clamp to transfer the gravity of the bridge deck structure to the main cable; Stiffening beams are usually arranged between the bridge deck and the sling, and form a combined system with the cable to reduce the deflection and deformation caused by live load. Suspension bridges can be divided into single span, double span, three span and multi-span according to cable system. According to the anchorage mode of main cable, it can be divided into ground anchor type and self-anchor type.
Second, the brief introduction of suspension bridge construction technology
1. anchorage construction. Anchorage is mainly composed of anchor block, anchor rod and saddle. The main function of the anchor block is to accommodate the anchorage system of the anchorage and transmit the larger cable tension to the rock mass, which can be divided into gravity type and tunnel type; If there is a solid rock layer near the anchorage surface, it may be economical to build tunnel anchors; However, the disadvantage of tunnel anchor is that the force transmission mechanism is unclear. The geological conditions of anchor field suitable for tunnel anchor construction should have the following characteristics. (1) The geological conditions of the anchorage zone should be regional stability. There should be no regional geological disasters such as landslides, collapses, toppling bodies and interlayer sliding in the anchorage, and no deep and large fault zones should pass through. (2) The rock mass in the anchorage zone should have strong integrity. There should be no more geological structures such as cracks and bedding in the rock mass in the anchor site area, which reduces the integrity of the rock mass and is extremely unfavorable for controlling the displacement of the tunnel anchor. (3) The rock mass in the anchorage zone should have high strength. Because the bearing capacity of tunnel anchor is closely related to the strength of rock mass, it is required that the rock mass in the anchorage zone should have high strength to meet the bearing capacity requirements of tunnel anchor.
If gravity anchor is used for the anchor block, in case 1, if there is solid bedrock near the ground in the anchor site area, the anchor block should be embedded in the bedrock, so that the bedrock in front of the anchor block can resist the tension of the main cable by bearing pressure. For example, Shantou Bay Bridge in Guangdong Province uses rock formations on both sides to resist the tension of the main cable. If the solid bedrock in the anchorage zone is located below the bridge deck and the depth is only 30 ~ 50m, an anchor pier directly located on the bedrock can be built; If the solid foundation is buried deeply and the design intention is to completely transfer the load to the bearing stratum, it is necessary to set up exploration foundations such as open caissons, caissons and large-diameter piles (including inclined piles), which are relatively expensive. The main function of anchor is as the initial support of excavation, strengthening the connection between anchor body and rock mass, improving the strength of excavation disturbance area around anchor hole, and at the same time using anchor hole to complete grouting of surrounding rock of anchor body. Its setting should be determined comprehensively according to the overall structural continuity of the surrounding rock of the anchor hole and the thickness range of the loose ring commonly existing in the surrounding rock of the anchor hole, combined with the mechanical analysis results. The saddle directly bears the pressure exerted by the main cable on the loose saddle and transmits it to the bedrock.
2. Cable tower construction. Generally, the tower body is poured in sections by turnover formwork method, and attention should be paid to the reserved reinforcement and formwork support embedded parts at the connecting plate of the main tower. The construction control of cable tower is mainly verticality monitoring. After each section of concrete construction is completed, when the temperature is relatively stable at 8:00 ~ 9:00 the next morning, the perpendicularity of the tower is monitored by the total station, and the concrete construction of the tower is adjusted. It is necessary to avoid testing in the period of drastic temperature change, and at the same time, observe the quality of concrete at any time and adjust the concrete mixture ratio in time. After the tower body is poured, check the elevation of the top surface. After it meets the design requirements, clean the surface and prepare to install the cable saddle. The cable saddle can be hoisted as a whole or assembled after being hoisted in sections; Cable saddle installation should strictly control the cable saddle lateral axis deviation and elevation deviation. It is also required that the bottom surface of the saddle body is closely attached to the base, and the gaps around it are filled with butter.
3. Main cable structure. The main cable is the main force-bearing component of suspension bridge, which is generally formed by extrusion of multi-strand steel cables. In order to ensure the uniform stress of the main cable, each steel cable must be parallel to the reference cable, and the main cable must be properly protected during erection so as not to damage the steel wire of the main cable. During the construction of main cable, it is necessary to erect circulating cable as the traction power of main cable strands and catwalk as the operation platform for the construction of main cable. Generally, the main construction procedures are: (1) establishing a traction system and erecting a catwalk; (2) traction of main cable strands; (3) manufacturing a single-ended cold-cast anchor head; (4) plastic surgery; (5) alignment adjustment; (6) finalize the design of the main cable; (7) Install cable clamps and slings. Main cable erection methods are divided into overhead wire feeding method (As method) and prefabricated steel strand method (Pws method).
No matter which erection method is adopted, it is necessary to set a reference line (or reference strand) to adjust the verticality of other wires. Lay a good foundation for the finalization, linear adjustment and finalization of the main cable; In China, Shantou Bay Bridge, Humen Bridge, Xiling Bridge and Jiangyin Yangtze River Bridge in Guangdong are all erected by precast strand method. The preliminary forming of the main cable should be carried out at night when the temperature is stable. When reshaping, first determine whether the arrangement of steel wire bundles is different, and whether the main span 1/2 and 3/4 are parallel to the steel wires at the side span 1/2. If so, adjust it in time. Then use steel belts to pack and bind. At first, the binding spacing is large, and then use dichotomy to encrypt it to 2.5m~5m ~ 5 m ~ 5 m. After the initial setting, the main cable needs to be tightened with a tightener. Tensioning starts from the two main towers to the mid-span, and then from the main tower to both sides, with the tensioning interval of1m. After compaction, tie the spare steel belts before and after compaction with a spacing of about 0.5m After the main cable is extruded, the section and void ratio of the main cable should meet the design requirements. After the main cable completes most of the dead load, wrap the main cable and conduct anticorrosion.
4. Strengthen the beam structure. Steel truss is often used as stiffening beam of suspension bridge, and its erection method is the same as that of steel truss bridge. After each beam segment is assembled, connect it with corresponding slings immediately, so that its own weight can be transferred from the slings to the main cables. Cable cranes, cable cranes and large floating cranes are generally used to erect stiffening beams of suspension bridges. Cable crane is composed of main beam, end beam and various running and hoisting mechanisms. The crane runs and works on the main cable, so the span of the main beam is the center distance between the two main cables, and the crane's operating mechanism must be able to cross the cable clamp obstacle. When the crane erects a beam near the tower, because the main cable is inclined, a mechanism that is relatively fixed and tightly held with the cable clamp should be set to bear the sliding force generated during lifting; Cable crane is mainly composed of lifting trolley, load-bearing cable and traction cable.
Before erecting the beam of the crane, it is necessary to erect the load-bearing cables and traction cables required by the crane on the cable towers on both sides. The load-bearing cable bears the gravity of the lifting trolley and the stiffening beam, and the traction cable bears the sliding force when the beam is hoisted, which drives the crane to walk. Compared with the three erection methods, due to environmental factors and navigation conditions, large floating cranes are rarely used for erection. Cable crane needs to erect a large number of load-bearing cables and traction cables before erection, which greatly increases the erection cost. Cable cranes are widely used to erect stiffening beams of suspension bridges because they are directly supported on the main cables, which not only saves costs, but also facilitates the erection of beams. However, attention should be paid to the protection of the main cable when erecting beams.
Suspension bridge has become the first choice for long-span bridges because of its good mechanical properties, strong spanning ability, light weight, beautiful appearance and convenient construction. Since the early 1990s, China has started to develop modern long suspension bridges, such as Shantou Bay Bridge, Humen Bridge in Guangdong and Haicang Bridge in Xiamen. It not only provides valuable technical experience for the construction technology of suspension bridges in China, but also lays a foundation for the further development of the construction technology of suspension bridges in China.
The above suspension bridge construction technology is the content of Zhong Da's consultation.
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