First, the characteristics of urban bridge reinforcement
The reinforcement of urban bridges is different from that of new bridges. It is an extremely complicated system engineering, and its objective conditions must be comprehensively analyzed on the original basis. In addition, the reinforced structures are mostly new and old combined structures, so their coordination and integrity need to be considered in design and calculation, and the influence of the original structure should be fully considered in reinforcement construction. Because the quality of the problem bridge contains uncertain factors, it is difficult to check its quality in depth and the construction is unsafe; Bridge reinforcement construction often lacks a broad site, and its construction site is always crowded and narrow, which also brings inconvenience to construction; In addition, we should also pay attention to the smooth flow of existing traffic, that is, in the process of the project, try not to interrupt traffic or interrupt traffic less.
Second, the basic requirements of urban bridge reinforcement
In view of the objective particularity of bridge reinforcement project, its implementation process should be paid attention to in many aspects and follow many requirements: economically, in order to make the direct and indirect economic benefits more obvious, the reinforcement cost is generally lower than the cost of new structures; Minimize the impact on urban traffic; It is also necessary to fully consider environmental factors and coordinate the old and new parts in appearance to meet certain landscape requirements; The most important thing is technology. Due to the fixed objective conditions, scheme design and construction organization, its technical requirements are very high. Therefore, it is necessary to make full use of the original structural members, minimize the damage to the original structure, meet various use requirements, and ensure the reliability, safety and durability of the project; Construction organization and scheme design to ensure convenient and fast construction; When dealing with it, the reduction of structural strength should also be fully considered to ensure the overall consistency of the old and new structures.
Third, the design principles of urban bridge reinforcement
If there are obvious diseases in the urban bridge structure, which affect traffic safety, it must be strengthened. Its reconstruction design should follow the following principles: first, carefully investigate the detailed situation of the original bridge, analyze its specific diseases and evaluate its structural state; Secondly, the reinforcement design should be closely combined with the construction method to ensure the reliable connection and coordinated development of the old and new structures; Strictly abide by the Code for Design of Highway Bridges and Culverts, so that the limit values of stiffness, strength and crack width of the new structure can meet the requirements of various codes when the bridge is reinforced; When checking the bearing capacity of the reinforced structure, the cross-sectional area of the structure is the actual effective cross-sectional area. Because the strain of the reinforced part is lagging behind, the actual stress of the reinforced structure should also be considered. If the weight of the reinforced structure increases, the reinforced structure and bridge foundation should be inspected; Structural calculation depends on the actual stress of the structure; If the bearing capacity of statically indeterminate structure is checked, additional internal forces caused by structural deformation, actual load eccentricity, temperature and other factors will be considered.
Four, urban bridge reinforcement work steps and matters needing attention
The steps of maintenance and reinforcement of old bridges mainly include: investigating the present situation of bridges and analyzing diseases; Consult the relevant historical data of bridges and analyze the existing traffic conditions; Put forward maintenance and reinforcement schemes, and make analysis and comparison; Finally, determine the plan and implement it.
In the process of reinforcement and maintenance of urban bridges, there are several problems that need attention:
(1) Accurately judge the actual bearing capacity of the original bridge, provide reliable basis, and better serve the technical transformation design; (2) Carefully analyze the causes of insufficient bearing capacity and bridge diseases, so as to take better measures; (3) Although some bridges only reinforce the superstructure, the bearing capacity and present situation of the substructure should be thoroughly investigated; (4) After reinforcement, it is necessary to evaluate its bearing capacity, check the reinforcement effect, observe it regularly, and record it carefully every time, so as to accumulate experience for better bridge reconstruction technology in the future; (5) The reinforcement design should be as convenient as possible to traffic without interruption, and the construction and traffic should be as smooth as possible.
Five, the means of urban bridge reinforcement
1. Crack repair before reinforcement
Most structural diseases occur in cracks. When cracks appear in the structure, its aesthetics and durability are seriously affected. Therefore, the repair of cracks is very important, and the commonly used methods are:
(1) surface treatment or surface spraying. Apply waterproof materials and fillers on the surface of microcracks, such as polymer cement slurry, elastic coating waterproof materials, cement fillers, etc. If the crack width changes, considering the environment in which the structure is located, materials with high ductility should be used; If the surface of the crack is roughened, a layer of high-strength and dense cement mortar can be sprayed, and the protective layer can seal the crack. Before spraying, it is necessary to remove the peeling part of the structure surface and rinse it with water before spraying.
(2) Filling method. Filling method is mostly suitable for wide cracks (above 0.5mm). First, a deep groove is cut along the crack, and then various bonding materials are embedded in the groove. If the reinforced concrete structure has rusted, the rust on the steel bar should be removed first, then the antirust primer should be applied, and finally the sealing material should be filled.
(3) Grouting method. When injecting cement or resin materials into cracks, pedal or manual infusion pump can be used. In order to better control the injection quantity, we often adopt the method of low pressure and low speed injection. In order to enhance the integrity of the crack, steel nail grouting can also be used.
(4) Paste steel plate sealing method. If cracks appear in the main tensile stress of reinforced concrete members, after the cracks are treated, steel plates can be pasted at the cracks and pressurized with expansion bolts, and the pasting direction of steel plates should be kept perpendicular to the direction of cracks.
2. Strengthening technology of bridge structure and components
(1) Strengthening Technology of Bridge Superstructure
① Strengthening technology of bridge superstructure
According to the weakening degree of the bearing capacity of the bridge and the requirements for future use, the reinforcement of the superstructure of the beam bridge mainly has the following ways:
A. Expand the section of the original structural member. In order to improve the stiffness and strength of the structure, the cross-sectional area of the member can be increased to achieve the purpose of reinforcement.
B. external prestressing reinforcement method. Appropriate initial stress is applied to new members or original members to achieve the purpose of reinforcement. According to the different prestressed materials, it can be divided into external prestressed steel beam method and prestressed tie rod reinforcement method.
C. stress system method to change the original structure. In order to reduce the peak bending moment of the control section in the beam and improve its bearing capacity, the bridge structure system can be changed to achieve the purpose of reinforcement. The simply supported beam bridge can be changed into a continuous beam bridge, or bearings and piers can be added under the simply supported beam, or composite beams or stiffening beams such as steel analysis frames can be added under the beam.
D. external sticking method. In order to improve the bearing capacity of the structure, the steel plate, section steel, glass fiber reinforced plastic and other materials are bonded and strengthened outside the structural members with adhesives. However, it is necessary to ensure the reliable transmission of structural stress. The middle of steel plate, section steel or FRP shall not be broken, and both ends shall be anchored reliably. The surface is coated with antirust paint and sealed with epoxy resin.
E. method of adding main girder. If the pier foundation of the bridge is stable, but the bearing capacity of some main beams can not meet the requirements, or the geometric dimensions such as bridge width are not enough, the main beams with high stiffness and bearing capacity can be added to bear the same force as the old beams, so that the lateral distribution coefficient of vehicle load will be reduced.
(2) Reinforcement technology of arch bridge superstructure.
A. ferrule method. This method can be used if serious side wall bulging and longitudinal cracks appear in the main arch ring of arch bridge. That is to say, a steel hoop composed of steel tie rods and steel plates is added to the arch ring. The concrete implementation process is as follows: a steel plate with bolt holes is attached to the side of the hollow arch ring, and two or three steel tie rods with screws are added to the soffit and the back of the arch; In order to overcome the downward deflection of the lower tie rod due to its own weight, more U-shaped bolts can be set; For the mid-span solid web section, the upper tie rod can be arranged under the deck slab cut before. After the upper and lower tie rods are installed, the bolts can be tightened, and the circumferential force of the side wall and arch ring will increase.
B. bolting and shotcreting reinforcement method. This method can be used for solid-web arch bridges with thin vault filler and hollow arch bridges with brick, concrete and stone reinforced concrete. Before implementation, concrete and other floating layers must be removed, then chiseled and cleaned. After driving the steel nails, lay the steel mesh and spray a layer of concrete.
C. Reinforcement method of adding holes behind abutment to reduce load and anti-sliding horizontal support. This method can be used for double-curved arch bridges built on soft soil foundation. If the arch frame is settled, the arch axis of the superstructure will be deformed. This method can resist the horizontal thrust of the arch bridge and repair the damaged parts, thus improving the strength of the bridge. The concrete methods are: strengthening the damaged arch ring (or arch rib) at the arch foot; Build a supporting beam behind the abutment to resist the horizontal thrust of the arch bridge; Small span approach bridges are added at the rear ends of abutment.
(2) Strengthening technology of bridge substructure.
① Enlarged foundation reinforcement method. This method is more practical if the foundation is buried too shallow or the bearing capacity is insufficient, and the pier is a rigid solid body of masonry or concrete. In order to enlarge the bearing area of the foundation, stone concrete can be added around the rigid and solid foundation. It should be noted that the piling cofferdam must be carried out within the widening range first; When digging in the weir, it is necessary to ensure the safety of the old pier and abutment, and the combination of the old and new foundations should be firm. Anchorage reinforcement can be added during construction.
② Artificial foundation reinforcement method.
If the natural soil foundation under the bridge foundation is soft and unable to bear heavy load, or the foundation has settlement, manual grouting method or sand pile method can be used. Grouting method is to drive a pipe pile or drill a hole in the center of the pier, and inject various slurries or reinforcing agents into the soil layer through the pipe holes and holes. The solidification of slurry will bond the original loose soil into a whole with anti-seepage performance and block the rock cracks. Sand pile method is to drive wooden stakes or steel pipes into the soft soil around the foundation, pull them out, and then pour dry coarse sand into them to form sand piles.
③ Supplementary pile foundation reinforcement method. In order to increase the stability and bearing capacity of the foundation, reinforced concrete precast piles or bored piles can be driven around the pile foundation. This method is suitable for the occasion of pile foundation reinforcement. When strengthening single bent pile piers, if piles are added between piles, the original capping beam should be strengthened; If the spacing between the original piles is small, two rows of piles can be added on both sides of the original bent to form three rows of pier piles. If the bearing capacity of abutment is insufficient, a row of piles can be added to the abutment and the capping beam can be poured. The added capping beam can be stressed alone, or it can be connected with the old capping beam, the old pile and the new pile to share the load of the superstructure together.
Ending of intransitive verbs
In short, various diseases of urban bridges will not only threaten traffic safety, reduce bridge life, but also affect the city appearance and environment, so we must pay full attention to the reinforcement and transformation of urban bridges. In daily life, we should strengthen the inspection and maintenance of bridges, find problems in time, take economic and reasonable reinforcement measures, improve the quality of bridges, and ensure the safe and smooth urban traffic.
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