News

With the continuous development of economy, the construction of urban viaduct, light rail and other bridges has gradually become the primary content of infrastructure construction in China. Under this premise, the introduction of foreign advanced construction skills, speed up the construction speed, reduce the impact on the environment and traffic, and improve the level of civilized construction has become a primary issue in bridge construction. Segmental bridges have been widely used in the construction of PC-connected beams, rigid frames, cable-stayed bridges and arch bridges. China has also made a lot of experience in planning and construction, and has also made considerable progress.
Prefabrication and Assembly Method of 1. Segmental Bridge
First of all, the duration can be greatly shortened. Compared with the cast-in-place box girder, the segment can be prefabricated while the substructure is constructed. If factory precast and concrete low temperature steam maintenance skills are selected, the production cycle of each segment can be shortened to 1 day. In addition, because the new special bridge machine equipment is selected for the segment span-by-span assembly, the erection time of a span box girder is about 7 days, which greatly shortens the construction period.
Second, reduce the impact on the environment. The above-ground roads occupied by the segment assembly are less, and the interference to the above-ground traffic and pedestrians in the construction process is also small. It is especially suitable for the construction of elevated bridges within the city. In addition to taking necessary safety measures, the original traffic operation can be maintained., Improved the degree of civilized construction.
Again, the segmental prefabrication assembly process can ensure that the bridge line is beautiful. This is because the close-fitting and matching pouring method is selected for prefabrication, and each segment of the whole beam is poured from the same set of formwork, thus ensuring the consistent line type of the beam body. There are still some limitations in the segmental prefabrication assembly process, and reducing these constraints should be the goal of improvement and progress today.
Suspension Assembling Method of 2. Segment Bridge
There are many construction methods for segmental bridges, but as far as urban bridges are concerned, the cantilever assembly method is a competitive plan, I .e. the main girder is prefabricated in sections at the prefabrication site, with prestressed holes reserved, and after the construction of the substructure is completed, the girder segments are transported to the site for assembly, while the required steel beams are tensioned. The structural system of the whole process is the cantilever structure first, and the connection system is formed after the closing. The segment length of the segmental bridge can be divided flexibly according to the stress requirements of the structure and the construction equipment.
In terms of planning, high-grade concrete can be used, prestressed systems can be varied, and the use of accounting machines has made the structural analysis and deflection control of the bridge very simple. Compared with the full support method, the structure from simply supported to connected, there is the problem of structural system transformation, prestress and creep caused by the secondary internal force can not be ignored. The steel beam can be placed in the web plate, but also can be placed in the roof and floor, the structure from the serious big drop, across to strengthen.
Construction, can save a lot of support, steel and formwork, concrete quality can be ensured. For urban bridges, it is not necessary to use hanging baskets for tensioning steel beams and other operations, but only a simple mobile bracket. The prefabrication of the segment can be carried out at the same time as the substructure, which greatly speeds up the construction progress, on the other hand, it can reduce the negative impact of creep and give full play to the high strength performance of the reinforcement. The device of the segment can make full use of mechanized equipment and organize it in the period of small traffic flow, which has less impact on traffic. Segmental bridges have high skill requirements, many factors affecting the structure, and strict construction control. For small-span bridges, because the beam height is low and there is no working face tensioning and connecting reinforcement, it is not suitable to choose segmental bridges.
Key issues in planning and construction of 3. segmental bridges
The key factors affecting the planning and construction of segmental bridges are secondary internal forces, connections between segments, system transformation and elevation control. The prestressing tendons of segmental bridges can be divided into cantilever tendons and connecting tendons. The secondary internal forces of the structure are primarily the secondary internal forces of prestress onset and the secondary internal forces of creep. Cantilever tendons only attack creep secondary internal force, but not elastic secondary internal force, continuous tendons attack elastic secondary internal force, when there are repeated system transformation, will also attack creep secondary internal force. The net effect of the secondary internal force (moment) adds the negative bending moment at the central fulcrum and the positive bending moment across the span, which adversely affects the structural attack. However, the secondary internal force is not completely unfavorable. In the continuous structure, the secondary internal force can be used usefully, and then the economy of the structure can be added. In fact, in the cantilever assembly stage, the negative bending moment at the fulcrum increases gradually with the addition of the cantilever length, and the positive bending moment in the span is very small when the cantilever is just closed. It is the redistribution of internal forces caused by the creep secondary internal forces that reduces the negative bending moment of the fulcrum, while the addition of the positive bending moment across the span, with the increase of time and the addition of other loads, the overall stress of the structure tends to be reasonable. The secondary internal force of prestress attack can be calculated by force method or equivalent load method. It is difficult to accurately account for the internal forces of the creep, which is related to the large discrepancy between the relevant assumptions and practice in the accounting process, but no matter what method is chosen, even if the deviation of the accounting results is large, it will not have a serious impact on the structural attack.
The beam of the segmental bridge consists of two parts: cantilever reinforcement and connecting reinforcement. The cantilever reinforcement is placed in the top plate, web plate and upper stem rib. The steel beam in the top plate is usually arranged in a straight line and directly anchored on the segment assembly surface. The steel beam in the web plate is generally in a curved shape, so that the steel beam can bear part of the shear force. However, some long-span bridges are only bunched in the roof and not bent into the abdominal cavity plate, which brings convenience to the construction. The continuous reinforcement is arranged in the bottom plate, and the sawtooth block is left on the bottom plate in the box to tension and anchor the steel beam. For cantilever-assembled segmental bridges, the steel beam is generally arranged into short beams, and the bending reinforcement has only 2 bending points. From prefabrication to device, most of the segment creep has occurred, thus the loss of prestress is greatly reduced, which can effectively increase the immortal prestress and give full play to the high strength performance of the steel beam.
In the cantilever assembly stage, many cantilever beams need to be installed on the beam body, but after the system is completed, there are not so many steel beams. In order to avoid waste, temporary beams are often required. The external prestressing system is a useful attempt. Because the steel beam is placed in the box (box beam) can make the section thinner, after controlling the tension, it is useful to control and adjust the deflection and pre-camber in the construction. However, the ultimate bearing capacity of the external prestressed structure is slightly lower, and how to use it reasonably needs further research.