First, the common diseases and causes of arch bridges in the western region
The density of expressway road network in the western region is low, and a large number of arch bridges in national and provincial trunk lines are under heavy operating pressure. Therefore, in the long-term high-load operation process, under the repeated action of various environmental factors, material deterioration, load (dead load or live load) changes and other factors, various diseases will inevitably occur, and the specific analysis is as follows.
1. 1, Overview of common diseases of arch bridges in western China.
Many arch bridges operating in the western region are investigated and summarized, and their common structural diseases are mainly manifested in the following three categories:
1, the vault sinks, the arch axis deforms, and the bearing capacity of the arch ring decreases;
2. The main arch ring is cracked;
3. The building on the arch is damaged.
1.2, analysis of common diseases
There are many reasons for various diseases of arch bridges. In view of the western environment and management status, this paper summarizes the common reasons, mainly in the following aspects:
1, overloaded vehicles act on the old arch bridge for a long time;
2. Uneven displacement of arch foot or pier causes damage to arch ring or transverse connection;
3. Insufficient material strength causes insufficient bearing capacity of arch rib or arch ring;
4. The arch bridge is unstable due to insufficient lateral connection;
5. The arch bridge foundation is destroyed by erosion;
6, weathering, harmful gas erosion damage to the durability of the arch bridge;
7, bridge deck and arch filler drainage caused by poor arch damage.
Second, the common methods of arch bridge reinforcement
According to the analysis of the above reasons, the actual situation of the structure in the western region and the summary of engineering practice, the following principles should be implemented in the reinforcement of arch bridges in the western region:
A, scientific and reasonable design, economic and environmental protection;
B, as far as possible not to increase the weight of the structure, not to damage the arch ring and other main load-bearing components;
C, reinforced bridge can meet the requirements of operating load, and have a certain safety reserve;
D, the construction is simple and quick, and the impact on traffic during the steel bar construction is minimized;
E, the reinforced bridge has strong durability and convenient maintenance.
In view of the above principles, when the bearing capacity, stability, integrity and durability of arch bridges are insufficient, the following methods can be selected to strengthen the existing arch bridges on the main lines of western countries and provinces.
2. 1, increase the reinforcement of arch ring section.
The method of increasing the cross-section reinforcement is to use the same material-reinforced concrete to increase the cross-section area and reinforcement of the original concrete structure, so as to improve the cross-section bearing capacity, stiffness, stability and crack resistance of the component, and can also be used for repairing cracks. It can be applied to the reinforcement of truss arch, rigid frame arch and hyperbolic arch bridge. Before increasing the cross section, the defects of the original arch rib should be repaired, and the joint surface should be chiseled to expose coarse aggregate. Then, steel bars are implanted in the arch ribs, vertical and horizontal main bars are erected, and then concrete is poured and maintained.
Aiming at the old stone arch bridge in the west, the technology of closing the main arch ring with reinforced concrete hoop is widely used to strengthen the main arch ring, in order to improve the mechanical integrity and bearing capacity of the original bridge and enhance the overall durability of the structure. Masonry arch bridge is strengthened by the method of set arch, which can be strengthened by integral reinforcement or ribbed reinforcement. Its remarkable function is to effectively thicken the section of the original arch ring and strengthen the transverse connection of the original arch ring, which not only enhances the bearing capacity of the main arch ring, but also effectively seals and handles the longitudinal and transverse cracks of the original main arch ring to prevent it from further weathering.
For masonry arch bridge, the arch ring can also be strengthened by bolting and shotcreting. The steel mesh is suspended by the anchor rod anchored by the original main arch ring, and then the concrete with an appropriate amount of accelerator is sprayed on the surface of the structure until a composite main arch ring is formed, which deforms in coordination with the original main arch ring and shares part of the live load, so as to improve the bearing capacity of the bridge.
2.2, paste high strength material reinforcement.
Strengthening by pasting high-strength materials mainly refers to pasting steel plates on the arch or pasting fiber composite materials to strengthen the arch ring or building.
Sticking steel plate mainly uses anchor bolts and grouting materials to stick steel plate on the surface of arch rib concrete, and uses anchor screws to fix steel plate with main girder concrete to achieve the purpose of local reinforcement. Bonding steel reinforcement technology has the advantages of good construction control, mature technology, easy procurement of materials and fast construction speed, which is more suitable for practical engineering application.
The basic principle of the method of pasting FRP sheets is the same as that of pasting steel plates, which is to paste reinforcement materials on the tensile edges or weak parts of concrete structures to form a whole with the structure, thus replacing the steel bars that need to be added, improving the bearing capacity of beams and achieving the purpose of reinforcement.
2.3. Self-compacting concrete is injected into the external steel plate for reinforcement.
Self-compacting concrete, that is, self-leveling concrete, can self-level and self-compact by its own gravity without vibrating at a low water-binder ratio, and is filled with formwork and wrapped steel bars. In construction, higher requirements are put forward for the strength and stability of formwork system, so it is often used in conjunction with steel formwork with better firmness and sealing to form a "steel plate concrete structure". The process is as follows: after cleaning and roughening the surface of arch rib, the original structure surface is planted with steel bars and arranged with stressed steel bars, steel formwork is installed and welded, and then self-compacting concrete is poured. This method is mostly used to strengthen T-beam and other beams and ribs, and is also often used in arch bridge structures at present.
2.4, increase the transverse contact reinforcement.
The transverse tie beam or diaphragm in ribbed arch bridge or double-curved arch bridge can enhance the overall lateral stability of ribbed arch bridge and also play the role of lateral load distribution, which requires sufficient stiffness and strength. However, after long-term load response, various diseases often occur in front of the main arch rib, especially the hyperbolic arch bridge built in earlier years, and its transverse stiffness is weaker. At this time, even if the main arch rib has not been seriously damaged, the overall bearing capacity of the structure will decrease due to the deterioration of lateral integrity. Therefore, effective measures must be taken to increase and strengthen the lateral connection of the structure in order to restore the design stress mode of the structure. Generally, there are two ways: enlarging the original beam or adding partitions. This method has become one of the important technical measures to strengthen this kind of bridge, and can be implemented together with other reinforcement methods.
The main reinforcement of transverse tie beam should be set as long as possible. When the cross section of the arch rib is added to the bottom surface, the bottom surface of the transverse tie beam can be at the same height as the arch rib, which is convenient for the arrangement of long steel bars and ensures the integrity.
2.5, arch filler replacement and reinforcement.
Many arch bridges that have been built for many years, especially masonry arch bridges, usually use a large thickness of filler on the arch. In the calculation and analysis of this kind of arch bridge, the dead load often accounts for a large proportion, and most of the bearing capacity of the main arch ring needs to bear the dead load. With the increase of vehicle load, it is easy to exceed the bearing capacity of the bridge, leading to the cracking of the main arch ring and developing into a dangerous bridge. Replacing the filler on the arch with light material can not only adjust the pressure line by adjusting the dead load on the arch, but also make the pressure line of the main arch ring as close as possible to the axis of the arch and reduce the internal force of the bending moment in the arch. When the bearing capacity of the bridge is reduced and the bearing capacity of the bridge foundation is limited, which can not meet the requirements of strengthening the arch ring and improving the bearing capacity of the live load, we can also transform the arch bridge by reducing the self-weight of the buildings on the arch, reducing the requirements for the substructure and reducing the main arch ring.
2.6, adjust the arch axis reinforcement
Because the arch ring is stressed and the arch axis changes greatly, the actual arch axis often deviates from the pressure line for arch bridges with excessive arch ring deformation, resulting in various diseases. In this case, section reinforcement, defect repair and local reinforcement alone can not effectively improve the stress condition of arch ring. Therefore, it is necessary to adjust the arch axis and pressure line to match them as much as possible, improve the stress state of the arch ring, fundamentally solve the disease status of the arch bridge, and effectively play the role of reconstruction and reinforcement. In particular, the stress on the arch ring is closely related to the distribution of dead load on the arch and the shape of the arch axis. If it is necessary to adjust the thickness of the filler on the arch or strengthen the bridge deck to adjust the arch axis, the stress on the arch ring must be calculated and analyzed in detail to determine the reasonable scheme, and the dead weight should not be blindly increased.
2.7, concrete filled steel tubular arch bridge tube grouting reinforcement.
The biggest feature of concrete-filled steel tube (CFST) combination is the ferrule effect of steel tube and concrete. When the arch rib of concrete filled steel tube is loose in many places or in a large area, the internal force will be redistributed between the steel tube and the core concrete, which greatly reduces the bearing capacity and stability of the members and lays a safety hazard for the normal operation of the structure. For this kind of disease, the arch rib can be reinforced by grouting in the pipe to restore the bearing capacity of the structure.
Generally, polymer concrete is used to repair cavities, pressure grouting is used to repair cavities, and modified epoxy adhesive is commonly used. In order to pour concrete, a hole should be drilled in the steel pipe. If the aperture is large, steel plate can be used for repair welding, and the damage to the original structure should be reduced during welding. When the gap is small, it can be smoothed by pouring glue.
2.8, replace the derrick steel bar
When the main structure of the arch bridge is still good, but some of its main stress components, such as suspenders or tie bars, will be seriously ill due to fatigue load or other reasons, and the bearing capacity of some components can no longer meet the normal operation requirements, the structure can be strengthened by replacing or strengthening these components.
The replacement of suspenders and tie bars is suitable for half-through arch bridges. In the process of lifting and tensioning the suspenders, the stress of each part of the bridge changes gradually. Generally, graded tensioning is adopted to make the load transfer stable. Otherwise, it is easy to produce stress concentration and destroy the bridge structure. The suspenders are mostly finished cables, and the deck elevation is used to control the deformation when replacing the suspenders.
2.9, change the structural system reinforcement
If conditions permit, the self-weight distribution on the arch can be adjusted to improve the stress on the arch ring and achieve the purpose of reinforcement, that is, to change the structural system of the arch bridge. At present, the soffit arch and soffit section of hollow double-curved arch bridge are often demolished and converted into continuous slab arch buildings, so as to reduce the dead load, adjust the stress state of the main arch ring, improve the lateral stress performance of the bridge and improve the bearing capacity of the bridge. This reinforcement method should be carefully implemented.
Third, the engineering example
3. 1, stone arch bridge reinforcement
3. 1. 1, bridge section
A stone arch bridge is 1 hole 16m mortar slab stone arch bridge, which has been in operation for many years. Some diseases have been found in the maintenance, which are in urgent need of reinforcement because they are adjacent to the first-class highway reconstruction. According to the relevant inspection report, the main structural disease of the bridge is the oblique longitudinal penetration cracking of the arch ring. The crack width is 10 ~ 15 mm, extending to the foundation; Moreover, there are many transverse cracks in the arch ring, all of which are serious water leakage.
Figure 2 1- 16m Masonry Arch Bridge Overview
3. 1.2, main reinforcement method
1, arch ring reinforcement:
Aiming at the main diseases of oblique longitudinal cracking of the arch ring of this bridge, the reinforcement scheme of the rear arch ring is formulated.
(1) Open V-shaped groove, carry out pressure grouting, repair cracks and improve the integrity of arch ring.
(2) Jacking reinforcement method: After the original pier structure is reformed, a new arch ring is added under the ventral surface of the original main arch ring, and the new arch ring is poured close to the bottom surface of the original arch ring, which will bear the upper load. In order to ensure the quality of concrete, self-compacting vibration-free concrete should be used when pouring concrete. The specific method is shown in the figure below.
2. Foundation reinforcement: the foundation is reinforced by foundation grouting.
Fig. 3 Schematic diagram of arch reinforcement method
3. 1.3, evaluation of reinforcement effect
At present, the bridge has passed the completion acceptance, and the reinforced arch bridge is in good operation under the condition of dense traffic of heavy construction vehicles.
3.2, hyperbolic arch bridge reinforcement
Bridge profile 3.2. 1
A double-curved arch bridge will be built at 1973. The highway grade is Grade III, and the design load grade is Grade Steam-15 and Grade Hangzhou -80. The total length of the bridge is 86m, the clearance under the bridge is 30m, and the span is 1× 60m. The bridge deck is 8.7 meters wide and the guardrails on both sides are 0.5 meters wide. Asphalt concrete is used for bridge deck pavement. The substructure of the bridge adopts gravity abutment.
There are transverse cracks in the arch rib of arch ring vault, with a width of 0.3mm, arch wave seepage, abdominal arch ring seepage and cracks. Main causes of bridge diseases:
1, the early bridge design load standard is low, only steam -l5 level. Can't stand the current large-tonnage vehicles.
2. The main arch ring composed of arch rib, arch wave and arch plate has poor integrity and weak stiffness.
3.2.2, the main reinforcement methods
Due to the serious water seepage disease of double-curved arch bridge, and under the action of water seepage, the internal reinforcement of the original structure may have rusted, and the stress section of the structure has been greatly weakened. Therefore, the weakened main arch ring should be strengthened by wrapping concrete on the arch rib and adding diaphragm. Through empirical analysis, it is determined that the reinforcement scheme of this bridge is to add 10cm and 20cm thick concrete to the side and bottom of the original arch rib respectively, and at the same time add 15 new beam. The reinforced arch rib section of the bridge is 70cm wide and 90cm high.
3.2.3, reinforcement effect evaluation
Strengthening the double-curved arch bridge by increasing the cross section has a relatively short construction period, and only needs to block traffic for a short time, which has little impact on social benefits. After reinforcement, the span structure of the double-curved arch bridge shows great stiffness and strength, which conforms to the current structural stress characteristics, and its bearing capacity and workability meet the design standards and use requirements of reconstruction and reinforcement, and it is running well at present.
Fig. 4 General situation of reinforced bridge
3.3, concrete filled steel tubular arch bridge reinforcement
3.3. 1, bridge section
An arch bridge is a 2× 120m half-through concrete-filled steel tubular arch bridge with a total width of 13. 1m and a clear width of 9m. The main arch ring is dumbbell-shaped with a total height of 2.1m. The upper and lower chords are made of φ 820× 12mm A3 steel plate, and the webs are made of 16Mn steel plate. There are five I-shaped cross braces above the deck of each hole, and a K-shaped wind structure below the deck. The specification of steel pipe is φ. The suspender consists of110 Φ 5 mm high-strength carbon steel wire, which is protected by glass fiber cloth and steel wire mesh slurry. Deck system is a simply supported system, which is mainly composed of suspender beam and carriageway T beam. The design load grade of the bridge is automobile -20 and trailer-100.
On June, 2002165438+1October, it was found that the bridge mainly had the following diseases:
1, the deck deflection is serious, the maximum deflection is 27cm, the ratio of deflection to span is 1/444, and the structure is in a bad state;
2. At the same time, the lateral deviation of the upstream and downstream main arch rings, the maximum lateral deviation 13cm, is about11000 of the span, which has exceeded the allowable value of the quality inspection evaluation standard;
3. The steel wire mesh sheath is used for the suspender, so its water tightness and air tightness are difficult to guarantee, which does not meet the requirements of the current code. After on-site inspection, the sheath has been cracked or damaged in many places, and the internal steel wire is seriously corroded;
4. There are many unconsolidated areas in the steel tube of the main arch ring, which reduces the bearing capacity of the bridge span structure to some extent;
5. There are many vertical cracks in the suspender beam, the maximum crack width is 0.3mm, and the stirrup in the suspender area at the beam end is cut off during construction, which seriously reduces the shear bearing capacity of the beam.
3.3.2, the main reinforcement methods
1, concrete filled steel tubular arch rib
Because the concrete in the arch rib tube is not dense, the bearing capacity and safety factor of the structure are reduced, and grouting treatment measures are taken for the arch rib steel tube. In order to achieve better grouting effect, epoxy resin slurry is used as grouting material. Before grouting, determine the range of unconsolidated area by manual tapping, mark it with chalk on site, and then open a certain number of grouting holes according to the range of unconsolidated area. When grouting, the grouting pressure is about 0.2MPa.
2. Crane
After reinforcement, the original bridge suspender is replaced by a new cable body with low stress and corrosion resistance, which is fully waterproof, galvanized high-strength steel wire, oiled and wrapped with two layers of PE sheath. The beam end is the tension end, the arch rib end is the fixed end, and the upsetting anchor is used for anchoring.
3. Bridge deck structure
The deck system of the main bridge is simply supported, with suspended beams and longitudinal carriageway T beams, with a height of 50 cm. Reinforce the beam by sticking steel plates and increasing the section near the suspenders. In order to avoid the further corrosion of steel bars in the structure and the further development of cracks, the cracks in the beams are sealed by grouting before reinforcement.
4. Transverse tie beam and wind structure
Considering that the arch rib of the bridge has a large lateral deviation in the same direction, the stability coefficient against pushover is low, and the original I-shaped cross brace is only welded with the web, so the deformation resistance is weak. In this reinforcement, K-shaped braces are added to the steel tube arch rib to enhance the lateral stability of the bridge span structure, that is, a 45-degree inclined wind structure is added on the basis of the original I-shaped cross brace, and the steel tube of the fresh air structure is φ500×8mm, which is connected with the upper and lower chord steel tubes of the original arch rib respectively.
3.3.3, reinforcement effect evaluation
Through the above targeted reinforcement, the traffic capacity and driving conditions of this bridge have been greatly improved. The reinforcement has been running for nearly 5 years, and the whole bridge structure is in good working condition.
Four. conclusion
In recent ten years, highway transportation in western China has developed rapidly. With the increase of the service life of a large number of arch bridges, the task of arch bridge reinforcement will be very heavy in the future. The method introduced in this paper can guide the maintenance and reinforcement of common diseases of general arch bridges. In engineering practice, how to analyze the arch bridge diseases and their causes on the spot, and comprehensively take various reasonable reinforcement measures to restore the bearing capacity and service performance of the arch bridge still needs further summary and research by engineers and technicians.
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