The footprint of the future composite materials -the world's first large bridge relying on CFRP completely

In recent years, new materials have been the focus of bridge innovation. In Stuttgart, Germany, a new rod arch bridge for light rails is built, using carbon fiber enhancement composite materials (CFRP). The upper structure of the arch bridge is 130 meters long, the main span is 80 meters, the arch height is 8.5 meters, the two arch ribs are the narrowest distance of 8.5 meters wide, the widest point is 11.7 meters, and the total area of ​​the bridge covers an area of ​​1424 square meters. Compared with traditional bridges, the CFRP tie arch bridge is a very innovative and efficient structural system. This structural system not only reflects the unique design aesthetics and structural mechanical properties of the naked structure, but also has durability, safety and sustainability, but also reduces the interruption of construction and reduces the cost budget.

CFRP with lower maintenance costs

Researchers at the Swiss Federal Materials Science and Technology Laboratory EMPA have begun to promote CFRP in the 1980s and use it as the most appropriate tie rod component applied to bridge construction under extreme loads and harsh environmental conditions. For more than 20 years, the CFRP lever applied on the bridge has proven to be reliably maintained under the continuous stress of up to 1800MPa. However, contrary to the method of CFRP's external bonding reinforcement structure, there is no commercial success case so far. Initially, civil engineers did not believe the reliability of CFRP. For them, this was an unknown material. But now, at least ten years ago, thousands of engineers used CFRP reinforcement applications worldwide. Therefore, in the bridge engineering, reliability is no longer a reason to oppose CFRP. The only reason for the CFRP tie rod has not yet been promoted is its initial cost. For example, if CFRP is used in the new Query Bridge project in Scotland instead of steel structures, the total cost of the project will increase by about 20%. In principle, the initial cost is unfair. Responsible bridge authorities should consider that in the 100 -year bridge service life, a bridge with a CFRP lever, its maintenance costs will be lower.

In fact, in Germany, the CFRP boom is not the most advanced technology in bridge engineering. Because application cases applying for new technologies take a lot of time and money, the process approval cycle is long. In this case of light rail bridges, before construction began in 2016, there was neither sufficient time nor enough funds. The only feasible solution was to apply for "single case approval (ZIE)". Stuttgart City Public Transporter (SSB) requires EMPA researchers to conduct necessary experiments to simulate the bridge status during the service life of 100 years, and provide corresponding expert evaluation reports.

More suitable for railway bridges

At the end of the 19th century, while the classic vertical boom tie arch bridge was highly popular, Claus KOEPCKE designed a mesh sloping boom arch bridge in Risa, Germany in 1878, with a bridge span of 101 meters. A mesh sloping lever with multiple cross points can be axial pressure and tension like a truss.

Figure 1 Classic vertical boom tie arch bridge and mesh sloping boom tie arch bridge

Due to the pressure on the diagonal, KOEPCKE designed them as a carefully made framework. The complexity of this design is one of the reasons why there is only a relatively small tie bridge with a relatively few tilted components. In the near future, the German bridge designers use flat steel as boons in the mesh sliding bolt layout. Once such a boom is under pressure, it will relax. As part of the bridge panel, heavy concrete pressure can avoid this. However, a better and more economical solution to overcome this shortcoming is to prestress the boom. In this way, they will not be stressed anymore, nor need extra weight. Due to the low deformation related to the system under transportation load, the mesh diagonal boom is particularly suitable for railway bridges. However, so far, there are only about 20 such bridges in the world. This may be mainly due to the challenges brought by high fatigue loads of railway transportation.

Together with the smaller elastic modulus of CFRP, the elongation rate of the boom is greatly increased, thereby solving the problem of relaxation of the boom. By reducing the quality and increasing the relative prestress of the cross section of the boom (the same force is related to the obvious small cross sectional area), the inherent vibration frequency of the boom is significantly shifted upward. This avoids the frequency range of vibration caused by rain and wind. Therefore, in this case, the use of circular cross -section is not a disadvantage. In the past 25 years, in the application of marine ships, the vibration frequency of the CFRP rod has been above 10Hz. In the case of suitable connection design, because CFRP has high anti -fatigue performance, they will not reduce the service life. By using carbon fiber tensile components as booms, slender rods also make the bridge more beautiful. At the same time, through effective layout, the siphon is relaxed, and it will not affect the service life due to vibration.

Figure 2 The mesh tie arch bridge that has been installed and looks like it is in the night

Higher anti -fatigue performance and fire resistance performance

Through the research and exploration of the world for nearly a century, the design methods of the current steel structure and concrete structure have reached a very high level. However, CFRP has not been applied in engineering structure since 1991. Therefore, the design method has not been mature as steel and concrete structures. It needs to be used to provide a basis for the calibration and analysis of the finite element model with the help of full -size experiments. The researchers designed three sets of CFRP boom tests to simulate the fatigue effect of the crane during the 100 -year service life. After that, the load/elongation rate of the CFRP boom was loaded under the quasi -static loading, and the CFRP boom showed almost perfect linear elastic behavior throughout the load range. During the uninstallation process, a very narrow stagnation was generated without permanent deformation. The loading and unloading load/elongation curve of the CFRP boom A and B in the load load of 11.3 million times is consistent. Fatigue tests have a small impact on the stiffness of the CFRP boom.

Finally, the limit load of all three booms (A, B and C) is estimated. Table 1 lists the results of these tests.

The results show that even the test of this small base clearly shows that the impact of the century of the train operation on fatigue on fatigue is quite obvious. CARBO-LINK determines the limit load of 1800kn of unacceptable CFRP boom based on the finite element calculation.

Compared with the lower limit of the 95%segmentation of the limited load of the rod that is not carried out, under the limit state (ULS), some of the safety coefficient of the resistance is 1.72. Compared with the load of the rod design in the limit state, some of the safety factor of the resistance is 1.5.

When discussing the fire resistance of the CFRP boom, some people think that when an oil tank accident occurs on the highway, the prestressed concrete board of the mesh arch bridge will protect the boom from the fire. In scientific literature, there are no related articles about CFRP material fire resistance. However, the report given by EMPA conducted a fire resistance performance test of reinforced concrete beams, which involves CFRP materials, which can analogize the CFRP boom.

In the test, a CFRP layer pressure plate with a width of 74mm and a thickness of 1mm was used to strengthen the two reinforced concrete beams with 400mm wide and 300mm deep and 300mm deep. The permission load was loaded at a four -point bending method at a span of 5.2m. The CFRP layer pressure plate must withstand the fire temperature that meets the ISO 834 standard. After 10 minutes, the temperature reached 660 ° C, and some surfaces on the CFRP layer pressure plate were on fire in a short period of time. When the average temperature at the bonding joint reaches 200 ° C, the bonding of the concrete begins to fail. However, the CFRP layer pressure plate continues to play the role of prestressed anchor because the adhesive still plays a role in the support area. The length of the layer pressure plate is 20cm, which is protected by rock wool to prevent the heat loss at the support. After 20 minutes, at the temperature of 765 ° C, the surface of the layer was completely on fire. Individual fibers hang down and burn. The carbonization process on the combustion surface is carried out step by step on the thickness of the layer pressure plate. Because carbon fiber has almost no strength and rigid loss on the remaining cross -section, the CFRP layer pressure plate continues to be prestressed. After 45 minutes at a temperature of about 865 ° C, the remaining CFRP layer pressure plate began to loosen due to the "anchor" of the CFRP tendon, but the remaining cross -sectional area was still about 50%.

By changing the behavior of the above 1mm thick CFRP layer pressure plate, the following conclusions can be drawn: the ring on the CFRP boom is very critical, assuming that the 8.5mm thick ring layer pressure plate is at a temperature of about 860 ° C, burning for 45 minutes at a temperature of 45 minutes. The cross section will lose 1mm wide and 1mm thick. However, about 85%of the cross -section will not be damaged. This means that the possibility of a crane in 45 minutes is unlikely.

In the 1990s, for people who opposed CFRP for the first time in bridge construction, it was a very important reason for CFRP rods to be destroyed. For this reason, EMPA researchers simulated the destruction of the CFRP boom in the laboratory. Experiments show that it is very difficult to cause real damage. The opponent of CFRP further claims that the prestressed boom may be shot down, and then falls to the bridge with a large number of concrete, causing traffic accidents. Therefore, EMPA experts began to study prestressed CFRP rods. But the shooting did not drop the boom, but the cross -sectional area of ​​the boom was reduced according to the size of the bullet. Because the stress CFRP rod is one -way, there is even no stress concentration at the shooting hole.

Environmental protection and sustainability

In the case of Stuttgart City Railway Bridge, the competent authority did not require the CFRP boom to conduct a full life cycle assessment (LCA). However, on the 130 -meter span Odhe Railway Bridge, whether it is a CFRP boom or a steel boom, a full life cycle assessment must be performed. For CFRP, the relatively high energy consumption of carbon fiber is particularly important. The production temperature within 1 hour is about 1500 ° C. The basic raw material is a fiber of oil -based polyacryne polymer.

According to the EN15978 specification, the system boundary of LCA analysis includes the material production phase (module al ~ A3), the construction phase (module A4 ~ A5), the use phase (module B1 ～ B7), the scrap stage (module C1 ～ C4), and module D, and module D, The module D distributes the benefits and loads brought by the recycling, recycling or reuse of the material. In this standard, only AL to A3 modules are mandatory. In addition, Module D is considered to be beyond the LCA system boundary. Even if a complete life cycle analysis is performed on the structure, its use is also available. The determination of the life cycle list (LCI) data of the CFRP and titanium components of the boom is relatively accurate, and the envisioned process is clear. It is difficult to determine the LCI data of steel, because in this case, the energy portfolio and the production process are not clearly defined. Therefore, only standard experience can be adopted. However, these uncertainties have not fundamentally affected the results. For the production phase of AL to A3, the carbon dioxide emissions of the steel variant are about three times that of the CFRP solution, and the energy consumption of steel is more than doubled. When installing the structure (A4 ～ A5 stage), the CFRP boom is hardly affected by any environment. Because the weight of the boom is very light, there is no need for a lifting equipment. One or two workers can easily move the boom to the scene with their hands. Compared with the steel boom, it does not need to be welded. Just fix the bolts with bolts. In the use phase (B1 ～ B7), CFRP can play excellent corrosion resistance and anti -fatigue performance. If the abandoned phase (C1 ～ C4) occurs after 100 years or earlier, the bridge is no longer needed, and the CFRP boom can continue to be used as a stretch parts in a heavy machine structure. So far, CFRP can be crushed and added to the asphalt as a reinforcement material for heavy -duty road pavements. For five years, Britain has been recycling fiber from CFRP and re -used it in a small structural engineering.

In the case of Stuttgart's mesh sloping boom tie bridge, the CFRP boom defeated steel for the first time in competition based on initial costs. CFRP is the most matched with the specific materials required for this type of bridge. This makes the economic span of the future -shaped sloping scroll rod arch bridge greatly extended to more than 300 meters. This is the first large bridge to rely on the CFRP boom completely, which will establish more and wider recognition for the application of CFRP in the subsequent structure.

This article is published / "Bridge" magazine

2022 Phase 4 Total Issue 108

Author /URS O. Meier, etc.

Author Unit /Swiss Federal Materials Science and Technology Laboratory

Source /SAMPE European Conference 2020 Thesis Collection

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