Detailed introduction of bridge bearings

Bridge bearings, as an important component of bridges, play a crucial role in the overall structure of the bridge. They make the bridge a cohesive whole, and bridge bearings are also an important supporting part of the bridge span structure. They are important components that connect the upper and lower structures of the bridge It can transmit the supporting reaction force of the bridge span structure to the bridge piers, and it also needs to ensure that the bridge span structure has some static conditions required by the design under the action of load and temperature changes, so as to adapt to the needs of beam rotation and free expansion and contraction And it should also have the function of easy installation, maintenance, and upkeep. The support must also be able to ensure sufficient fixation on the pier and abutment, and cannot slide off The quality of bridge bearings directly affects the entire structure of the bridge, so research on bridge bearings is very important

1.Classification of Bridge Bearings

According to the deformation of the support, it can be divided into fixed support, single movable support, and multi-directional movable support

According to the material of the support, it can be divided into steel support, concrete support, lead support, polytetrafluoroethylene support, and rubber support

According to the structural form of the support, it can be divided into arc-shaped support, plate type rubber support, rocker shaft support, pot type rubber support, spherical support, etc

Below is a brief introduction to several types of supports:

(1) Plate rubber bearing

Plate rubber bearings are made by embedding, bonding, and pressing several layers of thin rubber sheets and thin steel plates It needs to have sufficient vertical stiffness to withstand vertical loads, reliably transmit the reaction force of the upper structure to the pier, have good elasticity to adapt to the rotation of the beam end, and have significant shear deformation to meet the horizontal displacement of the upper structure Plate rubber bearings are suitable for medium and small span highway, urban, and railway bridges According to the specifications for highway bridges in China, beam and slab bridges with a standard span of less than 20 meters can generally use slab rubber bearings. However, in practical applications, they often exceed the span limits listed above. As long as the design principles are strictly considered, satisfactory results can be achieved There are two types of plate rubber bearings: rectangular and circular The bearing capacity range of domestically produced laminated rubber bearings can be between 150-7000KN

(2) Basin type rubber bearing

Basin type rubber bearings are a new type of bridge bearings composed of steel components and rubber It has the characteristics of high bearing capacity, large horizontal displacement, and flexible rotation, and is suitable for large-span bridges with bearing capacity of over 1000KN, as well as bridges in cities, forest areas, and mining areas Pot type rubber bearings can be divided into two types based on their performance: bi-directional movable bearings (also known as multi-directional movable bearings), which have rotational and longitudinal and transverse sliding properties; Unidirectional movable support, with rotational and unidirectional (longitudinal or transverse) sliding performance; Fixed supports only have rotational performance

(3) Large tonnage spherical steel bearings

With the development of large-span bridge structures, it is required that the bearing capacity of bridge bearings be large, while also meeting the requirements of adapting to large displacements and angles Spherical steel bearings have reliable force transmission and flexible rotation. They not only have the characteristics of large bearing capacity and allowable displacement of pot rubber bearings, but also better adapt to the needs of large bearing angles. Compared with pot rubber bearings, they have the following advantages

① Spherical steel bearings transmit force through the spherical surface without force necking, and the reaction force acting on the concrete is relatively uniform ② The bearing has consistent rotational performance in all directions and is suitable for wide bridges and curved bridges ③ The bearing does not require rubber compression, and there is no impact of rubber aging on the rotational performance of the bearing, making it particularly suitable for low-temperature areas (4) Tension bearings are used on continuous bridges, cantilever bridges, inclined bridges, wide cantilever flange box girder bridges, and small radius curved bridges. Due to the action of loads, tension is generated at certain points. In this case, bearings that can withstand tension and corresponding rotation and horizontal displacement must be installed Spherical steel bearings, pot type and plate type rubber bearings can all be functionally modified as tension bearings, which can be used for both fixed bearings and movable bearings Plate type rubber tension compression bearings can be used for bridges with low tension, while spherical tension steel bearings or pot type tension bearings are more suitable for bridges with high reaction forces However, when the support tension exceeds 1000KN, the above structure is not economical (5) Bridge bearings in earthquake prone areas should not only meet the support requirements, but also have various functions such as shock absorption and earthquake resistance Bearings designed according to seismic requirements must have the ability to resist seismic forces; The function of seismic isolation bearings is to separate the structure or components as much as possible from the seismic ground motion that may cause damage, in order to greatly reduce the seismic forces and energy transmitted to the upper structure At present, the main types of seismic and isolation bearings in China include new seismic rubber bearings, seismic spherical steel bearings, high damping rubber bearings, and lead rubber bearings

2.Design and Calculation of Two Bearings (Taking Plate Rubber Bearings as an Example)

2.1 Determine the size of the support

The determination of support dimensions includes two parts of data: the support plane area and the support height

The planar area of the support can be calculated as follows: the total thickness of the rubber layer of the support ∑ σ 1: then ∑ σ 1 satisfies the requirement of (la/10) ≤ σ 1 ≤ (la/5), that is, 20 ≤ σ 1 ≤ 40 When the braking force is not considered, σ 1 ≥ 2 Δ L, where Nmax is the maximum fulcrum reaction force; A is the planar area of the rubber support; δ is the average allowable stress of the support Δ la is equal to - Δ t × α t × l × β. According to S=la × lb/2 × (la+lb) × δ 1, the height of the support is composed of two parts: the height of the rubber and the height of the steel plate. The height of the rubber is determined by the shear deformation provided by the support (which will determine the longitudinal displacement), and it must comply with the requirement in the specifications that the total height of the support is less than or equal to 1/5 of the longitudinal length of the support along the bridge The height of the steel plate is the sum of the thicknesses of the thin steel sheets that constrain the rubber sheet

2.2 Verification of Support Deflection and Compression Deformation

The bridge span structure will generate a turning angle at the support, and the support provides this rotational ability through uneven compression. At the same time, there should be no "gap" phenomenon between the support and the bridge span structure. The larger the average compression of the support, the stronger this rotational ability. This requires that under the condition of a certain turning angle, the support must ensure a minimum average compression Δ s. Δ s can be calculated as follows: Δ s is equal to - Δ t × α t × l × β, where E is the elastic modulus of the rubber support; ∑ σ 1 is the total height of the rubber layer In addition, the standard also stipulates that the maximum average compression of the support should not exceed 5% of the total rubber thickness

2.3 Checking the anti sliding performance of the support

In order for the support to function effectively, it must be ensured that it is in the designed position, and there must be no relative sliding on the contact surface between the support and the bridge span structure, as well as the contact surface between the support and the pier under horizontal force. This guarantee comes from the sufficient frictional force between the support and the concrete, and the magnitude of the frictional force can be determined by considering a certain empirical coefficient based on pressure and friction coefficient

3.Design analysis of 3 seismic isolation bearings

3.1 Design of shock absorption and isolation bearings

The purpose of designing seismic isolation bearings is to concentrate most of the energy under earthquake action on the seismic isolation bearings, while reducing the inertial force and ductility requirements borne by the lower structure The methods for seismic and seismic control of engineering structures can be divided into the following five categories based on whether there is external energy input: ① passive control (without external energy input); ② Active control (with external energy input); ③ Semi active control (with a small amount of energy input); ④ Hybrid control (with partial energy input); ⑤ Intelligent control (with a small amount of energy input) The vibration equation of a multi particle system under seismic action is: [M] {X ″}+[C] {X ′}+[K] {X} equals - [M] {I} {Xg ″}, where [M], [C], and [K] are the mass, damping, and stiffness matrices of the structure, respectively; {X} {X ′} and {X ″} are the displacement, velocity, and acceleration matrices of the structural response, respectively; {Xg ″} is the acceleration matrix of ground motion It can be seen that the seismic response of bridge structures is related to the acceleration of ground motion, the mass, damping, and stiffness of the structure Compared with the traditional "hard resistance" method that relies on increasing the cross-sectional size of structural components and increasing the reinforcement ratio to improve structural stiffness, using the above ideas and methods for seismic isolation design significantly saves engineering costs and has significant economic benefits

3.2 Construction of Seismic Bearings and Protective Facilities

In order to ensure the normal use of bearings and their protective measures, and to play a role in shock absorption and isolation, the construction quality of bearings requires high standards. Based on the construction experience of seismic bearings and their protective measures, the following construction precautions are summarized: (1) The top surface of bearings should be strictly kept horizontal First, use a grinding wheel to smooth the pad stones on the top surface of the pier and abutment, with a flatness error controlled within 1mm Then, evenly spread epoxy resin mortar of the same thickness, place the support, and tighten the bolts (2) After the support is installed, the displacement slot in the steel basin shall be filled with foam or cotton batting to prevent the concrete and sundries from falling into the support during the beam construction, which will eventually affect the free displacement of the support After the temporary support is removed, take out the foam or cotton wool in time (3) Temporary consolidation support construction and removal, do not damage permanent supports Especially for dismantling, if done manually, to prevent concrete debris and cutting steel bars from falling into the support sliding groove Micro expansion explosives can be used for dismantling, which is convenient and fast

Bridge bearings, as an important component of bridges, play a crucial role in the overall performance and lifespan of the bridge structure The quality hazards of bearings, as important components of bridges, in their application are worrying and have an undeniable impact on the safety and durability of the structure