With the development of urban construction, aluminum alloy has been paid more and more attention in the application of pedestrian bridge structure in municipal engineering because of its light weight, good durability, easy maintenance and easy processing.
The United States is the first country to apply aluminum alloy materials to bridges. Smithfield Bridge in Pittsburgh is the first aluminum alloy bridge in the world, which was built in 1933 and adopted aluminum alloy panels to bear the live load on the bridge. Between 1946 and 1963, nine aluminum alloy bridges were built in North America, of which eight still exist today. In Europe, the first aluminum alloy bridge was built in Sunderland, England in 1949. From 1949 to 1985, 35 aluminum alloy bridges were built in Europe.
After entering the 1990s, with the improvement of aluminum alloy processing technology and welding technology, aluminum alloy materials are favored by more and more countries, which opened the second climax of aluminum alloy bridge construction.
At present, the longest and heaviest aluminum alloy footbridge in China is located on the expressway in the north of Nongye Road, West Third Ring Road, Zhengzhou City. The structure of this footbridge is truss-type aluminum alloy structure with a straight plane. The span of the main bridge is 90 meters, the total length of the bridge body is 94 meters, the total weight is over 130 tons, and the total width of the bridge deck is 4.65 meters. There are ladders and ramps in the north-south direction at both ends, and the total width of the ladders and ramps is 3.34 meters.
2 Performance comparison between aluminum alloy and steel.
Because of its good mechanical properties and machinability, steel structure is often used as the first choice for long-span bridges, municipal landscape bridges and pedestrian bridges in traditional design. However, defects such as poor durability of steel structure and difficulty in later maintenance also appear. Compared with traditional steel structure, aluminum alloy has the following characteristics:
(1) high strength and light weight. The material characteristics of aluminum alloy and conventional Q235 steel are as shown in Table 1 below. The density of aluminum alloy is about 1/3 of that of carbon steel, and the strength of 6000 series aluminum alloy commonly used in engineering is higher than that of carbon steel Q235. The use of aluminum alloy instead of steel or concrete in bridge superstructure can effectively reduce the self-weight of the bridge and realize a larger span of the bridge. With the reduction of self-weight, the requirements for foundation are correspondingly reduced, and the construction cost of substructure is effectively reduced. The “processing” of larger components in factories becomes a reality, and the transportation and hoisting tend to be simple and cheap, thus reducing the construction period.
(2) Good corrosion resistance. Under the influence of atmosphere, a dense oxide layer can naturally form on the surface of aluminum alloy, thus effectively preventing the corrosion of aluminum alloy.
This excellent anti-corrosion performance greatly reduces the cost of anti-corrosion and maintenance of bridge structures. Different series of aluminum alloys have good applicability in various harsh environments. In northern China, salt is usually sprayed on the road to remove ice in winter, and steel bridges are prone to corrosion. The use of aluminum alloy can fundamentally solve the problem. (3) Aluminum alloy has special texture and luster, which can change the color and improve the visual effect after anodizing or electrolytic coloring.
(4) When the existing bridge is repaired and reinforced, the smaller weight is exchanged for the larger bearing capacity, which effectively increases the proportion of the bridge bearing live load. When aluminum structure is designed reasonably, prefabricated parts are used and installation is simplified, its original cost is not much different from that of steel structure.
(5) Easy to obtain and reusable. Aluminum is the most distributed metal element in the earth’s crust (about 7. 73%), which can be easily obtained by electrolysis. The aluminum alloy material is easy to recycle, high in recycling value, low in reprocessing cost, high in utilization rate, convenient to regenerate, capable of being reused and beneficial to environmental protection.
(6) The melting point of aluminum is low (6600C), so the processing does not need to consume a lot of power, saving energy compared with steel processing.
(7) Aluminum alloy can be extruded by pressure machine to produce profile products with different cross-sections which cannot be produced by the hot rolling method of steel, so as to meet different requirements under various conditions, and the design scheme is diversified.
To sum up, the aluminum alloy material has the characteristics of high strength, light dead weight, corrosion resistance, low maintenance cost and convenient recycling, which maximally meets the needs of urban development.
Structural analysis of aluminum alloy footbridge.
Main technical standards.
(1) when calculating the bridge truss, the crowd load value shall be implemented according to article 3.1.3 of the technical specification for urban pedestrian bridges and underpasses (cjj69-95); The crowd load of the flyover is taken as 5 kPa or 1. 5kN for local loading, and W=5X(20-4. 43/2) /20=4. 45kPa for overall calculation (the clear width of crowd load arrangement is 4.43m). (2) Phase II dead load: after the railings and canopy are collected, take the load of each vertical pole node: 2. 6kN, and the bridge deck pavement: 1. 0 kN/m.
(3) Stiffness requirement: The maximum allowable vertical deflection of the main bridge structure of the overpass under the action of live load of people is L/800; The vertical fundamental frequency of overpass structure should be greater than 3Hz.
All the main truss structures of footbridge are made of aluminum alloy, and 6082 T6 aluminum alloy is used for the stressed members. T6 aluminum alloy is used for this flyover.
4. Component fabrication and installation.
The aluminum alloy truss beam is prefabricated in sections and assembled on site. The fabrication and assembly of aluminum alloy trusses shall comply with the relevant provisions of Code for Design of Aluminum Alloy Structures. Truss structure shall be fabricated and assembled on the flat fetal membrane in the factory. Truss can be transported to the bridge only after being processed in the factory and qualified in pre-assembly. When installing the aluminum alloy truss, the three-dimensional coordinates of the control points should be accurately measured and correctly positioned, so as to keep the architectural line shape of the aluminum alloy truss smooth and the central axis position consistent with the design drawings. After the main structure is installed, the bridge deck pavement shall be carried out.
(1) Aluminum alloy has many advantages such as high strength, light weight, durability and low maintenance cost, which makes it more advantageous than steel bridge in both performance and cost.
(2) It is necessary to strengthen and speed up the research and implementation of relevant design, construction and acceptance specifications for aluminum alloy bridge structures, so as to provide theoretical support and normative support for the application of aluminum alloy structures.
(3) It is necessary to continue to improve the manufacturing process of aluminum alloy materials, and continue to promote the development of factory production, on-site assembly and rapid construction in depth.
(4) The application of aluminum conforms to the national policy of promoting low carbon and environmental protection. After the completion of the project, the main structure hardly produces redundant construction waste, and aluminum can be recycled. The local government can introduce a number of incentive policies to promote the application and development of aluminum alloy structures in municipal engineering.