As the economy develops, the traffic volume increases and the load level changes. The guiding ideology of the early design focused on the saving of materials and low safety. Generally speaking, the section was thin and the safety reserve was low. The most typical one was a double-arch bridge; the bridge was poor in durability and aged, such as brick arch bridges. Due to improper design or poor construction quality, the built bridges also have problems with reinforcement.
Difficulty in construction
1. Bridges that have been opened to traffic have realistic traffic needs. Because they need to be reinforced without interrupting traffic, there will be traffic interference during reinforcement.
2. Restriction of structural form, the principle of reinforcement generally must use the original structure, so it is limited.
3. The combination of new and old structures is a difficult problem, including the changes and transitions of the new and old structural systems, as well as the joint surface of the new and old bridges.
4. The risk is high. All bridges to be reinforced are mostly dangerous bridges. The structure is already in an unfavorable state. Some old bridges lack original design materials and construction records. The internal structure of the structure is unknown and the existing stress conditions are different. , It is difficult to determine its force limit, which brings risks to the reinforcement of the old bridge.
Reinforcement principle
First, make an analysis and judgment on the necessity and feasibility of reinforcement according to the existing technical status of the bridge, the existence of diseases, the need for vehicle traffic and the trend of future traffic development, and then compare the technical and economic effects of various reinforcement schemes, and select reasonable Reinforcement scheme. Generally, the following requirements should be met: (usually)
1. It is feasible and meaningful to save more than 60-70% of the cost of rebuilding the new bridge. Including the economic loss caused by the reinforcement of the bridge and interrupted traffic.
2. After the bridge is strengthened, its structural performance, bearing capacity and durability can meet the requirements for use
3. The substructure of the bridge has sufficient potential to meet the requirements for the foundation of the reinforced bridge.
4. Conduct a comprehensive comprehensive evaluation of the advancement, economy and durability of the reinforcement technology, and strive to adopt reinforcement schemes with better indicators.
Common reinforcement methods
1. Reinforcement method of bridge deck reinforcement layer
All the original bridge deck paving is chipped or roughened, and then a certain thickness of reinforcement layer is added to increase the effective height of the main girder and improve the lateral load distribution capacity of the bridge, thereby increasing the single girder bearing capacity or the overall bearing capacity of the bridge structure .
2. Sticking steel plate reinforcement method
The sticking steel plate reinforcement method is a structural reinforcement method of resin-bonded steel plate and concrete. Its construction is fast, there is no wet work or only a small amount of wet work such as plastering, which has little impact on production and life, and it will affect the appearance of the original structure after reinforcement. The original headroom has no significant impact. However, the reinforcement effect depends to a large extent on the bonding process and the level of operation, and it is suitable for the reinforcement of bending or tension members that bear static forces and are in a normal humidity environment.
3. Enlarged section reinforcement method
The enlarged section method, also known as the outer-concrete addition method, is a method of strengthening the section area and reinforcement of concrete structures. This reinforcement method requires that the reinforced bridge substructure can withstand more self-weight and provide higher bearing capacity. Under normal circumstances, the main beam is to thicken the bridge and increase the width of the main beam.
4. Anchor shotcrete reinforcement method
The reinforcement method converted from tunnel construction is mainly used for the reinforcement and maintenance of concrete beams with insufficient shear strength due to the small size of the fulcrum section.
5. Pasting carbon fiber reinforced plastic reinforcement method
Carbon fiber has the advantages of high strength, light material and corrosion resistance. This method uses a special resin to paste carbon fiber on the tensile surface of the concrete structure. The carbon fiber and the original structure form a new force-bearing entity. The carbon fiber and the steel bar share the load, reducing the stress of the steel bar, so that the structure achieves the effect of reinforcement and reinforcement. Its main features are: almost no increase in structural weight and cross-sectional dimensions, no change in headroom height, convenient construction, almost no new damage to the original structure, good corrosion resistance, durability and fatigue resistance, according to the force The analysis can be reinforced by multi-layer pasting, and its directionality can also be flexibly grasped. The disadvantage of this method is that the ratio of elastic modulus to strength is too low; epoxy resin will soften when the temperature is higher than 60℃, and the bridge is generally exposed to direct sunlight, and the bridge deck temperature is higher than 60℃, which is not conducive to adoption. Resin glue is used as adhesive.
6. External prestress reinforcement method
The essential difference between the external prestressed structure and the internal prestressed structure is that the prestressed tendons of the external prestressed structure are arranged outside the main structure. Because the external prestressed cable is usually a concentrated steel cable composed of multiple steel strands, it is called It is an external prestressed cable. External prestressed reinforcement usually involves adding prestressed stiffening steel wire cables or prestressed thick steel reinforcement at the bottom of the beam or the lower part of the beam side and anchoring them to the two ends of the beam respectively, and the prestressed tie rods and the beam body are formed by setting certain joint structural members. A frame analysis system becomes a statically indeterminate structure. External prestress is applied to offset part of the dead load stress and play a role of unloading, thereby greatly improving the bearing capacity of the bridge structure.
7. Increase force members and change structural system
When the bearing capacity of the bridge is insufficient or the bridge is damaged due to various reasons, through certain technological and structural measures, new load-bearing members can be added to the original structure to increase the bearing capacity of the structure. Changing the structural system is mainly to use the continuous action of beams or the combined action of beams, slabs, and arch beams to change the stress system of the structure, improve the stress status of the structure, and improve the load bearing capacity of the bridge.
8. Add lateral connection reinforcement method
Increasing horizontal connections has also adopted more reinforcement methods in recent years. This method is to improve the lateral load distribution law of the superstructure by adding a bridge lateral connection, so as to achieve a reinforcement method that improves the overall bearing capacity of the structure. It is generally used for T-section and I-section beam bridges without internal beams or less internal beams. In engineering, cast-in-place concrete beams or steel beams are often added between adjacent main beams to improve lateral bending rigidity. The disadvantage of this technology is that it will cause a certain degree of damage to the original structure. For areas or components with more complex reinforcement, this method should not be used to reinforce the design, so as not to increase the potential safety hazards after reinforcement.
9. Composite material reinforcement technology (FRP)
The composite material FRP is composed of fiber and net-type resin. At present, there are three types of composite materials: E-glass fiber, carbon fiber, and aramid fiber. Among them, carbon fiber (CFRP) is the most widely used. The use of carbon fiber cloth to strengthen and repair bridges and building structures is a new type of structural reinforcement. Technology, which uses resin cement as the matrix, pastes and solidifies carbon fiber cloth on the surface of the concrete structure, and uses the high strength and high elastic modulus of carbon fiber to reinforce and reinforce the concrete structure, and to improve the stress status of the structure .
