With the development of China's national economy and the rapid growth of traffic volume, the traffic density and axle load on roads are increasing, which will inevitably lead to the increasing load of highway bridges. 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 2000, the total length of dangerous bridges in China has reached more than 20,000 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 way to restore and improve their bearing capacity, 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 reconstruction 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 and improper operation and maintenance. , make some bridges appear diseases, for maintenance. Bridge reinforcement, like bridge design, should not only meet the design specifications, 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 cannot 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 need to rebuild the bridge, after knowing that the existing bridge can meet the use requirements through reinforcement and maintenance, the bridge reinforcement design scheme is put forward;
4. When the main load-bearing members of bridges and extra-large bridges 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 design and construction of steel bars should not destroy the original structure as much as possible, and retain valuable components 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 old and new structures can work together.
7, reinforcement design should be calculated according to the actual damage of the structure;
8. During the reinforcement construction, the interference to 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 hidden parts of the original structure or related engineering structures, the construction shall be stopped immediately, and the research shall be carried out in conjunction with the reinforcement scheme designers, and the construction shall be continued only after effective measures are taken to deal with them;
10, during the reinforcement construction, safety monitoring measures should be taken to ensure the safety of personnel and structures.
In a word, in the design of concrete reinforcement, we must first make clear this reinforcement principle and make it "firm, durable, economical and applicable".
2, the method of bridge reinforcement
Simply put, reinforcement is to improve the bearing capacity and performance of components and even the whole structure through certain measures to meet the new requirements. These measures include directly aiming at the whole structure, such as external prestressing, changing the stress state of the structure to make it return to the original design state or adapt to the new requirements; Some measures are aimed at sections, that is, by improving the bearing capacity strength of a 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 reason, location and existing bridge type scheme of the bridge. At present, the commonly used reinforcement methods of bridge superstructure are: external prestress reinforcement, system transformation reinforcement, additional member reinforcement, steel plate reinforcement, carbon fiber reinforcement, deck reinforcement, etc. The common reinforcement methods of bridge substructure include enlarged foundation reinforcement and high-pressure jet grouting reinforcement. 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 to set prestressed materials in the tensile area of the lower edge of the beam, and eccentrically prestress the beam by stretching. Under this eccentric pressure, the beam arches, which offsets part of the dead weight stress, reduces the structural deformation and crack width, improves the structural stress, and can greatly improve the bearing capacity of the structure. Compared with the usual prestressed concrete structure, the prestressed tendons 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 through heavy vehicles, but also can be used as a permanent reinforcement measure to improve the bearing capacity of bridges.
The main applications of this method are as follows: when the prestressed steel bars or ordinary steel bars in concrete beams are seriously corroded, the bearing capacity of the structure is reduced; Need to improve the load grade of the bridge; Used to control the crack of beam and the fatigue stress amplitude of steel bar; It is suitable for structures under high stress, especially for the reinforcement of large structures.
At present, the commonly used external prestressing methods are through prestressed tie rod reinforcement method and external prestressed steel strand reinforcement method.
The main line bridge of National Highway 30 1 Panjin Interchange and the WH ramp bridge of Panjin Interchange are strengthened by external prestressing method, which improves the overall service performance of the bridge and prolongs its service life.
2. 1.2 system transformation and reinforcement method Changing the structural system to reinforce the old bridge usually means adding additional members or carrying out technical transformation to change the stress system and stress state of the bridge, thus reducing the stress of the load-bearing members, improving the performance of the bridge and achieving the purpose of improving the bearing capacity. This technology has the advantages of improving the bearing capacity of the structure, increasing the stiffness of the structure and reducing the deflection.
The commonly used methods are: converting arch bridge into beam arch construction method; Convert beam bridge into beam-arch combined system method; Simple support continuous method; The multi-span simply supported beam is transformed into a continuous system method of simply supported first and then deck; Methods of adding auxiliary piers, etc. The above reinforcement schemes are different in form, but the essence of reinforcement is the same, that is, the reinforced bridge adds new support points, shortens the calculation span of the beam and improves the bearing capacity of the structure.
Daxi Bridge in Yong 'an City, Fujian Province is a 9-span hyperbolic arch bridge built in the early 1970s. The bridge type is 22.10m+25.60m+7x22.10m continuous arch bridge, and the design load is automobile-13 and trailer -60. Due to the increase of traffic volume, the bridge has produced great diseases and loads during its operation. In 2002, Yong 'an City inspected the bridge. The inspection results show that the bridge can meet the requirements of automobile -20 class and trailer-100 load class by adopting reinforcement measures. The reinforcement scheme adopts the method of transforming continuous arch bridge into beam arch building system. During reinforcement and reconstruction, the side wall, arch protector and arch filler are removed first, 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 connect the high-abdomen arch piers and construct a simply supported bridge deck 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 reconstruction, after several years of use, the effect is proved to be very good. There are many similar examples, such as Tujiazui continuous arch bridge reinforced in Ezhou, Hubei Province in 2003 (L0=70m) and Jianglejishan continuous arch bridge reinforced in Fujian Province in 2005 (L0=30m).
2. 1.3 Increase the reinforcement method of members. The methods of increasing the reinforcement of members mainly refer to increasing the longitudinal beams to improve the bearing capacity or widening the reconstruction, and increasing the diaphragm to strengthen the transverse 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 needs to be widened, the new longitudinal beam with higher bearing capacity and stiffness is generally used to connect the old and new beams and bear the same force. If it needs to be widened, the pier needs to be widened.
Common methods can be divided into: adding longitudinal beams to strengthen (without widening the bridge deck); Adding side beams for reinforcement; Technical transformation of unilateral widening; Bilateral widening of technological transformation; Add auxiliary beam reinforcement.
Doumen Bridge in Shaoxing is a rigid arch bridge (L0=40m) built in 1980s. After more than 20 years of use, many diseases have appeared, especially when heavy trucks cross the bridge. Through the investigation and analysis of the bridge's diseases in 2004, it is found that the thin cross beam (originally designed as a hollow thin-walled beam) is the main reason for heavy vehicles to sway over the bridge. In the process of reinforcement and reconstruction, 8 reinforced concrete solid beams were added to each main arch leg, and 4 reinforced concrete solid beams were added to the diagonal. The reconstructed rear axle beam 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 adopts the technical scheme of strengthening and widening the side ribs, which widens the upper and lower structures at the same time and improves the bearing capacity of the structure. After the completion of self-reinforcement, the bridge is in good condition and has good visual effect.
2. 1.4 bonded steel plate reinforcement method bonded steel plate reinforcement is a reinforcement method that uses adhesive to bond steel plate to the tensile edge or weak part of reinforced concrete structure to form a whole with the structure, thus improving the bearing capacity of the beam. If the steel plate is anchored to the beam with anchor bolts, it is also called anchor bolt steel plate method, and then the steel plate can be appropriately thickened. Fixing steel plates on the surface of tensile concrete can increase the flexural rigidity of concrete structure, reduce the deflection of the structure and limit the development of cracks. Moreover, the steel plate can be cut according to the design requirements during construction, so that the bending, compression and shear properties of the bonded steel members can be effectively exerted, and the stress is uniform, and the phenomenon of stress concentration will not occur in concrete. In addition, this method also 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 on 1998. The main channel bridge type is106.6m+2x160m+106.6m, and the auxiliary channel bridge type is 51m+3x80m+51m. 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; Press the steel plate evenly with square timber, angle steel and fixing bolts; Keep healthy for the required time and remove the materials used for pressing; Brush curing paint on the surface of steel plate. The reinforcement project of the bridge has been completed 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 structure with high performance adhesive. When the structural load increases, the two work together to improve the bearing capacity of the components, thus achieving the purpose of reinforcement. The mechanical property of fiber composite is that its stress and strain are 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 materials 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 rigidity 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 bonding failure caused by 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 stress as the original structure within the design load range.
There are many cracks at the bottom of cast-in-place special-shaped concrete hollow slab beam of Futian overpass bridge on Guangzhou-Shenzhen Expressway, which leads to the corrosion of steel bars in the slab and the reduction of bridge bearing capacity. After it was found, it was strengthened by sticking carbon fiber cloth. The concrete measures are to treat the concrete surface at the crack, remove the loose part of the steel bar surface until the concrete structure layer is exposed, polish it flat, and thoroughly remove the surface dust with a powerful blower or acetone to make it dry and clean. Then, in strict accordance with the mixture ratio and process requirements provided by the manufacturer, the treated concrete surface is painted with primer, and the carbon fiber cloth is cut and pasted according to the design requirements. After reinforcement, the bearing capacity of the bridge is restored, and the reinforcement effect is good. There are many similar examples, such as the reinforcement of Chunshen Road Overpass on Shenfengjin Expressway and Yangyonghe Bridge on National Highway 107 (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 compression 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 surface 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 off, and the old and the new can be well combined and stressed together.
At present, in many bridge reinforcement and reconstruction, several different methods are adopted for the same bridge according to different parts, different components and different reconstruction reasons. For example, the reinforcement of the Jinsha River Bridge in Mamingxi, Yibin, adopts methods such as adding members, sticking steel plates, carbon fiber reinforcement and deck reinforcement; The Nimu Bridge in Tibet is strengthened by sticking steel plates, carbon fiber reinforcement and deck reinforcement. The reinforcement of Doumen Bridge in Shaoxing adopts the methods of adding members and deck reinforcement.
2.2 pier and foundation reinforcement
2.2. 1 Enlarged foundation reinforcement method The reinforcement method of enlarging the bottom area of bridge foundation is called enlarged foundation reinforcement method. This method is suitable for the case that the bearing capacity of the foundation is insufficient or shallow, and the pier is a rigid entity or foundation of masonry or concrete. When the uneven settlement of the structural foundation is large and the foundation soil is 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 is relatively simple in construction. The disadvantage is that the new and old foundations must be combined to bear the upper load, so the reinforcement cost is high and the reinforcement effect is difficult to control.
2.2.2 High-pressure jet grouting reinforcement method High-pressure jet grouting, that is, first, grouting pipe with nozzle is drilled into the predetermined position of the soil layer with a drilling rig, and it is rotated and lifted at a certain speed. At the same time, slurry or water is ejected from the nozzle in the form of high-pressure flow, which impacts and destroys the soil layer, and the high-pressure flow cuts and smashes the soil layer, making it granular and dispersed, and one part is taken out of the hole by mud and water, and the other part is mixed with mud. With the solidification of the slurry, the composition has certain strength and resistance. The jet grouting reinforcement method is widely used, with good quality, low cost, obvious reinforcement effect and convenient construction. At present, it has gradually become one of the commonly used treatment methods for pier foundation in China.
2.2.3 Bonding steel plates and carbon fiber reinforcement can also be used to enhance the stiffness of the pier and improve the bending, compression and shear capacity of the pier and abutment. The reinforcement method and mechanism are consistent with 2. 1.4 and 2. 1.5, so they will not be described here.
3. Selection of reinforcement scheme
The reinforcement scheme is related to many factors, so it is very important to choose a reasonable reinforcement scheme, usually considering the following factors:
1, bridge structure type;
2. Topographic, hydrological and natural conditions of the bridge site;
3. The conclusion of the analysis and research on the present situation of bridges;
4. Construction technical level;
5. Can the traffic be closed?
6. Expected reinforcement effect;
7. Capital investment.
If defined:
Suggestion: β > 0.9, reinforce the bridge deck;
β=0.7-0.8. Stick the steel plate and change the system for reinforcement.
β=0.6-0.7, external prestressing, carbon fiber reinforcement.
When widening and strengthening at the same time, the widened part should be integrated with the original bridge to give full play to the unloading function of the new part.
The above commonly used reinforcement methods can be comprehensively applied and optimized, which can better reflect the reinforcement effect and economic benefits, but the following points should also be noted:
(1) Different reinforcement methods have corresponding design and calculation methods;
(2) The improvement of the bearing capacity of the strengthened bridge structure is influenced by the original structure.
Structural constraints, such as the original structure reinforcement ratio, section size, etc. , can not infinitely improve the bearing capacity;
(3) For the reinforcement calculation of long-span complex bridge structures, it is generally
Finite element method is an effective tool to analyze the whole structure, and nonlinear influence should be considered when necessary.
4. Conclusion
Bridge reinforcement and maintenance has become a common concern of all countries in the world. In order to standardize and guide the application of bridge reinforcement technology, the Ministry of Communications set up the project of "Research and Promotion of Detection, Evaluation and Reinforcement Technology of Old Highway Bridges" in the 200 1 Western Traffic Construction Science and Technology Project, aiming at putting forward a set of complete and practical detection, evaluation and reinforcement technology of highway bridges, providing technical support for the reconstruction of dangerous old highway bridges in China and ensuring the reconstruction of dangerous old bridges. The new code for inspection, evaluation and reinforcement of old bridges will be issued soon, when the bridge reinforcement will be "legal".
For more detailed information about "Common Methods and Mechanisms of Bridge Reinforcement", the link of Zhong Da Consulting Construction Design is simple, convenient and reliable.
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