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The Ministry of Transport issued a new version of the Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts (JTG3362-2018) on July 16, 2018.
The “Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts”, published in September 2018, further clarifies the mechanism and deepens the understanding and understanding of the new code.
The new specifications and guidelines add a number of provisions on the design method of prefabricated bridges, the construction details of prefabricated segment joints, and the external prestressing technology closely related to them.
This paper further discusses the theoretical improvement and future development of the bridge prefabrication technology in combination with the relevant scientific research of the bridge engineering department of Tongji University participating in the new specification and the research practice of the bridge prefabrication technology in the past two decades.
With the rapid urbanization of China, prefabricated segmental box girder bridges and rapid construction prefabricated bridges (ABC) are being actively promoted at the industry level.
In fact, the construction concept of ABC Bridge originated from the bridge construction in France.
E.
Freyssinet adopts the method of longitudinal precast beam segment (1945) and matching joint (1952) for the construction of prestressed concrete bridge.
J.
Muller (1979) made great progress in technology by combining prefabrication with modern mechanical technology in the design of two long bridges (LongKey and Seven Mile) in Florida, USA.
Since then, segmental prefabricated and assembled prestressed concrete bridges have been widely used around the world, such as the Mid-Bay and Garcon-Point Sea-crossing Bridge in the United States (1990), the Inner Ring Road in Seoul, South Korea (1996), and the Manna High-speed Viaduct in Bangkok (2000).
Figure 1 shows that precast segmental construction bridges in China have only been used sporadically before the 21st century.
Until after the 21st century, precast segmental construction methods and external prestressing technology have received more and more attention and affirmation in China.
Shanghai Liuhe Bridge (2001) adopted the span-by-span construction method of externally prestressed precast segments, and used the upward-type bridge erecting machine for the first time in domestic highway bridges.
Subsequently, this technology was also used in the design of the second phase of the Shanghai Humin Line (2003).
In the approach bridge with a span of 75m in the deep water section of Sutong Yangtze River Highway Bridge (2003), the precast segmental cantilever assembly construction method is adopted, as shown in Figure 2.
This bridge is the first large-scale bridge constructed by precast segments in China.
Later, this construction method has been adopted in the approach of many river-crossing bridges in China.
Figure 2 The stress nature of precast segmental bridge in the approach of the deep water section of Sutong Bridge The stress nature of precast segmental bridge in the precast segmental construction can be seen as a reinforced concrete structure with several plain concrete sections inserted, that is, the continuous reinforced concrete section (R section) with longitudinal ordinary reinforcement and the plain concrete section (C section, that is, the joint) with longitudinal ordinary reinforcement broken, as shown in Figure 3.
Figure 3 Schematic Diagram of the Essence of Segmental Beam Joints (Section C) is the feature and weakness of precast segmental bridges.
Although there are many engineering examples of bridges constructed by precast segments in the world, because the longitudinal reinforcement is completely cut off at the joint, the joint itself and its impact on the structural stress are very complex, so far many international mainstream specifications have not reflected it.
Since the vicinity of the joint (section C) is actually just mortar, a large number of research results show that the crack development of the prefabricated segment structure is basically near the joint, as shown in Figure 4.
In fact, the seam is like the preset crack location, as shown in Figure 5.
Its stress situation has been separated from the cornerstone of section method in structural design – all sections meet the plane assumption.
Figure 4 Crack development near the joint Figure 5 Deformation after joint cracking Since the advent of the segmental construction method, a large number of studies and experiments have been carried out at home and abroad to study its stress performance.
Two methods are generally used in theoretical research.
One is the structural method, that is, the whole process elastoplastic finite element analysis method is adopted, but there are many studies on bending performance and few studies on shear performance.
The main reason is that the shear reinforcement itself is a difficult problem in the field of concrete structures, and there is no complete consensus method in the world.
The second is to use the traditional section method, which is a simplified and representative method in coordination with the standard method.
At present, the only one is the “Guiding Code for Design and Construction of Segmental Concrete Bridges” issued by the American Association of Interstate Highway and Transportation Workers (AASHTO).
It divides the segment joints into two types, namely, the wet joints and glue joints between prefabricated segments are called Class A joints, and the dry joints between prefabricated segments are called Class B joints.
The following table 1 shows the bending reduction coefficient and shear reduction coefficient of the segmental construction concrete structure with different prestressed reinforcement forms and different joint types in AASHTO standard.
The coefficient with 0.05 as the extreme difference in the table has some experience, but its advantage is that the section method adopted in the current specification can be directly used.
The structural bearing capacity of the bridge constructed by continuous precast segments with longitudinal common reinforcement needs to be calculated by section method multiplied by coefficient.
Although it can be coordinated with the specification, it can be seen from Figure 5 that its basis is not sufficient.
The elastic-plastic finite element method or test method considering the whole process, including the redistribution of internal force, is incompatible with the section method adopted in the specification, that is, its safety is different from the section method.
At the same time, most of its research methods focus on bending resistance.
Because there is no unified understanding of the shear bearing capacity of concrete beams, the shear resistance of segmental beams is more complex.
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