with the development of China's national economy, the rapid growth of traffic volume, and the increasing traffic density and axle load of vehicles on the road, the load of highway bridges is bound to increase day by day. In addition, some old bridges are aging, damaged or limited by the original design standards, which can no longer meet the requirements of modern transportation. By 2, the total length of dangerous bridges in China has reached more than 2, linear meters. If it is demolished and rebuilt, it will not only cost a lot of money, but also take a long time. If the existing old bridges are reinforced and rebuilt in a planned and step-by-step manner, and their bearing capacity is restored and improved, it will not only meet the needs of highway transportation in the new period, but also bring great economic and social benefits to the country.
1. Steps and basic principles of bridge reinforcement
Generally speaking, bridge reinforcement includes the reconstruction of existing bridges and the repair of diseased bridges. Among them, the transformation of existing bridges is generally due to the increase of traffic volume, the improvement of design load standard and the widening of highway subgrade width, which makes some bridges unable to meet the current traffic and transportation requirements. The repair of diseased bridges is generally due to the aging of bridge materials, improper operation and maintenance, etc., which make some bridges have diseases and carry out maintenance. Bridge reinforcement, like bridge design, should not only meet the design specifications and conform to the principles of technical feasibility, economic rationality and structural safety, but also go through certain procedures and steps, which is called reinforcement conceptual design.
1. When the bridge structure fails to meet the requirements of structural safety or normal use due to structural failure or damage, it must be reinforced. The content and scope of reinforcement design should be determined according to the evaluation conclusion and the requirements put forward by the entrusting party, which can include the whole bridge, designated sections or specific components;
2. Establish an economic analysis model for the maintenance, reinforcement and reconstruction of existing bridges, and through analysis and comparison, choose a scheme that is technically feasible, economical and reasonable, and has little interference to existing traffic, so as to ensure the safe operation of the reconstructed bridge;
3. According to the evaluation conclusion and economic analysis of the bridge that needs to be rebuilt, after learning that the existing bridge can meet the use requirements through reinforcement and maintenance, the design scheme of bridge reinforcement is put forward;
4. For bridges and extra-large bridges, when the main load-bearing components need to be reinforced, there should be ≮2 reinforcement design schemes, and the best reinforcement scheme should be selected through scheme comparison and economic evaluation;
5. The reinforcement design and construction shall not damage the original structure as much as possible, and the components with use value shall be reserved to avoid unnecessary removal or replacement;
6. The reinforcement design should be closely combined with the construction method, and effective measures should be taken to ensure that the connection between the new and old structures can work together;
7. The reinforcement design should be calculated according to the actual damage of the structure;
8. During the reinforcement construction, the interference to the passing vehicles and pedestrians on and under the bridge should be reduced as much as possible, and necessary measures should be taken to reduce the pollution to the surrounding environment;
9. In the process of construction, if serious defects are found in the structure of the original structure or hidden parts of related projects, the construction shall be stopped immediately, and the research shall be carried out in conjunction with the designer of the reinforcement scheme. After effective measures are taken, the construction can continue;
1. During reinforcement construction, safety monitoring measures should be taken to ensure the safety of personnel and structures.
In a word, in the concrete reinforcement design, it is necessary to make clear this reinforcement principle first, so as to achieve "firmness, simplicity, durability, economy and applicability".
2. methods of bridge reinforcement
simply speaking, reinforcement is to improve the bearing capacity and service performance of components and even the whole structure through certain measures to meet the new requirements. These measures include directly targeting the whole structure, such as external prestressing, changing the stress state of the structure, making it return to the original design state or adapt to the new requirements; Some measures are aimed at the section, that is, by improving the bearing capacity strength of a certain section (such as shear strength), the bearing capacity level of the whole structure can be improved.
different reinforcement methods should be adopted according to the reasons, parts and existing bridge type schemes of the bridge. At present, the common reinforcement methods of bridge superstructure are: external prestress reinforcement, system transformation reinforcement, adding members reinforcement, sticking steel plates reinforcement, carbon fiber reinforcement, bridge deck reinforcement and so on; The common reinforcement methods of bridge substructure include expanding foundation reinforcement method, high pressure jet grouting reinforcement method and so on. The above reinforcement methods are introduced as follows:
2.1 Bridge superstructure reinforcement
2.1.1 External prestress reinforcement method The reinforcement principle of external prestress method is that prestressed materials are arranged in the tension area of the lower edge of the beam, and eccentric prestress is generated on the beam by stretching. Under this eccentric pressure, the beam is arched, which offsets some dead weight stress, reduces structural deformation and crack width, improves structural stress, and can greatly improve structural bearing capacity. Compared with the usual prestressed concrete structure, the tendon and the original structure are only connected to the beam at the anchorage point, which is similar to the unbonded prestressed structure. This method can greatly improve and adjust the state of the original structure, and improve the stiffness and crack resistance of the structure under the condition of little increase in self-weight. This method is not only suitable for temporary reinforcement when passing heavy vehicles, but also can be used as a permanent reinforcement measure to improve the bearing capacity of bridges.
This method is mainly applicable to the following situations: when the prestressed reinforcement or ordinary reinforcement in the concrete beam is seriously corroded and other diseases cause the bearing capacity of the structure to decrease; Need to improve the load grade of the bridge; Used to control beam cracks and fatigue stress amplitude of steel bars; It is suitable for structures under high stress, especially for the reinforcement of large structures.
At present, the commonly used methods of external prestressing include the down-supporting prestressed tie rod reinforcement method and the external prestressed steel wire bundle reinforcement method.
The main line bridge of Panjin Interchange and the WH ramp bridge of Panjin Interchange on National Highway p>31 are reinforced by external prestressing method, which improves the overall performance of the bridge and prolongs its service life.
2.1.2 system transformation reinforcement method to change the structural system to reinforce an old bridge usually means adding additional members or carrying out technical transformation to change the stress system and stress condition of the bridge, so as to reduce the stress of the load-bearing members, improve the performance of the bridge and achieve the purpose of improving the bearing capacity. This technology has the advantages of improving structural bearing capacity, increasing structural stiffness and reducing deflection.
The commonly used methods are: the arch bridge is converted into the beam arch-over-arch construction method; The beam bridge is converted into beam-arch combined system method; Simply supported to continuous method; Multi-span simply supported beam is transformed into simply supported first and then bridge deck continuous system method; Add auxiliary pier method, etc. The above reinforcement schemes have different forms, but the essence of reinforcement is the same, that is, new support points are added to the strengthened bridge, the calculated span of the beam is shortened, and the bearing capacity of the structure is improved.
Daxi Bridge in Yong 'an City, Fujian Province is a 9-hole double-curved arch bridge built in the early 197s. The bridge type is a continuous arch bridge with unequal spans of 22.1m+25.6m+7x22.1m, and the design load is -13 for automobiles and -6 for trailers. Due to the increase of traffic volume, the bridge suffered great diseases during its operation, and the load level could not meet the existing transportation requirements. In 22, The reinforcement scheme adopts the method of transforming continuous arch bridge into beam arch building system. During the reinforcement and reconstruction, firstly, the side wall, arch protector and arch filler are removed, and the arch back of the main arch is completely exposed. Secondly, the arch back and arch foot of the main arch are reinforced, and the cast-in-place concrete is used to increase its cross section. Then, the high-abdomen arch pier is connected, and the simply supported bridge deck is constructed according to the beam-slab bridge, which not only reduces the load of the main arch, but also enhances the bearing capacity of the main arch. After reinforcement and transformation, it has been proved that the effect is very good after several years of use. There are many similar examples, such as the Tujiazui continuous arch bridge reinforced in Ezhou, Hubei Province in 23 (L=7m) and the Jiangle Jishan continuous arch bridge reinforced in Fujian Province in 25 (L=3m).
2.1.3 increasing the component reinforcement method mainly refers to adding longitudinal beams to improve the bearing capacity or widening the reconstruction, and adding diaphragms to strengthen the lateral connection. When the pier foundation has good safety performance and bearing capacity, and the superstructure is basically intact, but its bearing capacity can not meet the requirements, and the bridge deck is required to be widened, new longitudinal beams with high bearing capacity and stiffness are generally used to connect the old and new beams to each other and bear the same force. For those who need to be widened, the piers and abutments need to be widened.
The commonly used methods can be divided into: adding longitudinal beams for reinforcement (without widening the bridge deck); Adding side beams for reinforcement; Unilateral widening technical transformation; Bilateral broadening of technological transformation; Add auxiliary beam reinforcement.
Doumen Bridge in Shaoxing is a rigid-framed arch bridge (L=4m) built in 198s. After more than 2 years' use, many diseases have occurred, especially when heavy trucks cross the bridge. Through the investigation and analysis of the bridge's diseases in 24, it was found that the beam was too thin (originally designed as a hollow thin-walled beam), which was the main reason for the shaking of heavy vehicles crossing the bridge. During the reinforcement and reconstruction, eight reinforced concrete solid beams were added to each main arch leg and four reinforced concrete solid beams were added to the diagonal. After the transformation, the bridge is still intact, and the bridge no longer shakes when heavy vehicles cross the bridge, and the operation is completely normal.
Tongbai Huaihe River Bridge in Nanyang, Henan Province adopted the technical scheme of strengthening and widening by adding large side ribs, which widened the upper and lower structures at the same time and improved the bearing capacity of the structure. Since the reinforcement of the bridge was completed, it is in good condition and has a good visual effect.
2.1.4 bonded steel plate reinforcement method bonded steel plate reinforcement is a kind of reinforcement method which uses adhesive to bond steel plate to the tensile edge or weak part of reinforced concrete structure, so that it can form a whole with the structure, thus improving the bearing capacity of the beam. If anchor bolts are used to anchor the steel plate on the beam, it is also called anchor bolt steel plate method, and then the steel plate can be appropriately thicker. Fixing steel plate on the surface of tensile concrete can increase the bending stiffness of concrete structure, reduce the deflection of the structure and limit the development of cracks. Moreover, steel plates can be cut according to the design requirements during construction, so that the bending, compression and shearing properties of bonded steel members can be effectively exerted, the stress is uniform, and the phenomenon of stress concentration will not occur in concrete. In addition, this method has the advantages of simple and rapid construction, no influence on the structural shape, low reinforcement cost, no reduction in bridge clearance and little increase in load. The disadvantage is that the quality and durability of binder are the main factors affecting the reinforcement effect.
Guangzhou Dongpu Bridge is a super-large bridge across the Pearl River, which was completed and opened to traffic in 1998. The main channel bridge type is 16.6m+2x16m+16.6m, and the auxiliary channel bridge type is 51m+3x8m+51m. In 23, it was found that there were oblique cracks in the end web of the main channel bridge, and later the owner adopted the technology of sticking steel plates to reinforce it. The concrete measures are to repair the concrete surface of the repaired part to make it smooth; Scrubbing the concrete surface and the treated steel plate surface of the repaired part with acetone or xylene; So as to remove grease and dust on the bonding surface; Evenly coating epoxy-based liquid adhesive on the bonding surface of steel plate and concrete; Use square timber, angle steel, and fixing bolts to evenly press the steel plate; Keep in good health for the required time, and remove the materials used for pressing; Brush curing paint on the surface of the steel plate. The reinforcement project of the bridge has been completed now, and the reinforcement effect is good.
2.1.5 carbon fiber reinforcement method sticking carbon fiber reinforcement technology refers to sticking carbon fiber cloth on the surface of building structures with high performance adhesive. When the structural load increases, the two work together to improve the bearing capacity of components, so as to achieve the purpose of reinforcement. The mechanical characteristic of fiber composite is that its stress strain is completely linear, and there is no yield point or plastic zone. Because carbon fiber has excellent physical and mechanical properties such as high strength, light weight, corrosion resistance and fatigue resistance, as well as the advantages of fast construction speed, short construction period and easy guarantee of bonding quality, it is an ideal material for strengthening old bridges. The performance of bonding material in carbon fiber reinforcement method is the key to ensure that carbon fiber and concrete work together, and it is also the weak link in the way of force transmission between them. Therefore, the bonding material should have enough stiffness and strength to ensure the transmission of shear force between carbon fiber and concrete, and at the same time, it should have enough toughness to prevent brittle bond failure due to concrete cracking. Compared with other strengthening methods, strengthening the old bridge with carbon fiber can change the stress distribution of the original structure to the minimum extent, and ensure the same force as the original structure within the design load range.
There are many cracks at the bottom of the cast-in-place special-shaped concrete hollow slab beam of Futian Interchange Bridge on Guangzhou-Shenzhen Expressway, which leads to the corrosion of the steel bars in the slab and the decrease of the bearing capacity of the bridge. After discovery, the scheme of sticking carbon fiber sheets was adopted to reinforce it. The specific measures are to treat the concrete surface at the cracks, remove the loose part of the reinforced surface until the concrete structure layer is exposed, and polish it flat. The surface dust is thoroughly removed by using a strong blower or acetone to make it dry and clean. Then apply primer on the treated concrete surface in strict accordance with the mixture ratio and process requirements provided by the manufacturer, and cut and paste carbon fiber cloth according to the design requirements. After reinforcement, the bearing capacity of the bridge was restored, and the reinforcement effect was good after observation. There are many similar examples, such as the reinforcement of Chunshen Road overpass on Shenfengjin Expressway and Yangyonghe Bridge on National Highway 17 (Shenzhen section).
2.1.6 Bridge deck reinforcement method Bridge deck reinforcement method is a common and effective method to increase the effective height and compressive section of the main girder, increase the overall stiffness of the bridge deck and improve the bearing capacity of the bridge by laying a layer of reinforced concrete on the top of the beam (bridge deck) to make it form a whole with the original main girder. In order to reduce the dead load of the reinforcement layer, the original bridge deck pavement is often chiseled away, and the new and old can be well combined, and the * * * is stressed together.
At present, in many bridge reinforcement and reconstruction, several different methods are adopted for different parts, different components and different reconstruction reasons of the same bridge. For example, the reinforcement of the Jinsha River Bridge in Mamingxi, Yibin, adopts the methods of adding members, sticking steel plates, carbon fiber reinforcement and bridge deck reinforcement; The reinforcement of Nimu Bridge in Tibet adopts the methods of sticking steel plate, carbon fiber reinforcement and bridge deck reinforcement. The reinforcement of Doumen Bridge in Shaoxing adopts the methods of adding members, deck reinforcement and so on.
2.2 Bridge pier and foundation reinforcement
2.2.1 Enlarged foundation reinforcement method The reinforcement method of bridge foundation to enlarge the bottom area is called enlarged foundation reinforcement method. This method is suitable for the case that the bearing capacity of the foundation is insufficient or buried too shallow, and the pier is a rigid entity or foundation of masonry or concrete. When the foundation of the structure has large uneven settlement and the foundation soil is relatively solid, the enlarged foundation method can be used for reinforcement. For the problem of insufficient foundation bearing capacity at the bottom of the enlarged foundation, a certain number of piles can be driven under the enlarged foundation to improve the foundation bearing capacity, and the parameters of the piles are selected according to the foundation deformation calculation. Expanding the foundation reinforcement method, the construction is relatively simple. The disadvantage is that it must make the new and old base