The selection of elastomeric bridge bearings is a critical step in bridge design, directly impacting the safety, serviceability, and durability of the bridge. The selection process requires comprehensive consideration of multi-dimensional parameters, including bridge structural characteristics, load conditions, and environmental factors. Specifically, it can be categorized into the following 8 core factor groups:
- Basic Parameters of Bridge Structure
The inherent characteristics of the structure lay the foundation for bearing selection, determining the load-bearing mode and deformation requirements of the bearings:
Bridge Type and Span
Different bridge types (e.g., simply supported beam bridges, continuous beam bridges, rigid frame bridges, cable-stayed bridges, suspension bridges) exhibit distinct force-bearing modes. For example, simply supported beams primarily bear vertical loads, while cable-stayed bridges need to withstand horizontal tension. This necessitates matching bearings with different functions: plain elastomeric bearings are commonly used for simply supported beam bridges, PTFE sliding elastomeric bearings for continuous beam bridges, and pot bearings for long-span bridges.
Smaller spans typically result in lower single-point loads on bearings, making plain bearings the preferred choice. For spans ≥ 30m or larger loads, bearings with higher load-bearing capacity (such as pot bearings) are required.
Superstructure Form
Bearings for precast beams (e.g., T-beams, box girders) experience concentrated forces, requiring the planar dimensions of the bearings to match the embedded steel plates at the beam bottom. Cast-in-place beams offer greater flexibility in bearing installation space and force distribution but require consideration of temporary support conversion during construction.
Beam Material and Dead Load
Concrete beams have a large dead load (e.g., a 30m precast T-beam weighs approximately 100-150 tons), demanding higher vertical load-bearing capacity from bearings. Steel box girders are lighter but may exhibit more significant thermal deformation, requiring bearings with superior horizontal displacement capacity.
- Load Conditions
Loads are the core indicators determining the load-bearing capacity and stability of bearings, requiring clarification of the following load parameters:
Vertical Load
This includes dead loads (beam weight, deck paving, guardrails, etc.) and live loads (vehicles, pedestrians, construction loads, etc.). The “maximum vertical reaction force” of the bearing must be calculated to ensure the design load-bearing capacity of the selected bearing ≥ calculated reaction force (typically with a safety factor of 1.2-1.5 reserved).
Example: For a simply supported T-beam bridge with a single bearing dead load reaction of 500kN and live load reaction of 200kN, a bearing with a vertical load-bearing capacity ≥ 700kN should be selected.
Horizontal Load
This includes horizontal forces from temperature stress, concrete shrinkage and creep, as well as vehicle braking force, centrifugal force, wind force, and seismic force.
For small horizontal forces (e.g., small-span highway bridges), the lateral stiffness of plain elastomeric bearings is sufficient. For large horizontal forces (e.g., long-span bridges, bridges in seismic zones), pot bearings with limit devices or separate seismic bearings are required.
Torque and Eccentric Force
Bearings for curved or skew bridges endure torque and lateral eccentric forces. Bearings with torsional resistance (e.g., pot bearings) or bearing arrangements (e.g., anti-uplift bearings) to counteract eccentric forces should be selected.
III. Displacement and Rotation Requirements
Bridges undergo horizontal displacement and rotation during operation, and bearings must possess corresponding deformation capabilities to prevent damage to the beam or the bearings themselves:
Horizontal Displacement
Primarily caused by temperature changes (e.g., beam elongation in summer, contraction in winter), concrete shrinkage and creep, and beam deflection under live loads.
For small displacements (≤ 50mm), plain elastomeric bearings can meet requirements through rubber shear deformation. For moderate displacements (50-200mm), PTFE sliding elastomeric bearings are needed (utilizing sliding between PTFE plates and stainless steel plates to reduce friction). For displacements > 200mm, pot bearings or coordinated design with expansion joints may be necessary.
Beam Rotation
Beams generate end rotation under load (e.g., the rotation angle at bearings of simply supported beams is typically 0.005-0.01rad). Elastomeric bearings must accommodate rotation through compressive deformation of the rubber layers.
The greater the “effective height” (total thickness of rubber layers) of the bearing, the stronger its ability to adapt to rotation. For large rotations, plain bearings with thicker rubber layers or pot bearings (adapting to large rotations through the interaction of steel pots and rubber blocks) are required.
- Environmental Factors
The environment directly affects the durability and material selection of bearings, requiring matching bearing types and protective measures for different environments:
Temperature Environment
In frigid regions (below -30°C), low-temperature resistant rubbers (e.g., chloroprene rubber, EPDM) should be used to prevent rubber embrittlement and cracking. In high-temperature regions (deck temperatures ≥ 60°C in summer), heat-resistant rubbers are required, and the high-temperature performance of PTFE slides (to avoid thermal deformation) must be considered.
Corrosive Environment
Coastal bridges are exposed to seawater and salt spray corrosion. Bearings should undergo anti-corrosion treatment (e.g., galvanizing or anti-corrosion coating for steel components of pot bearings), and EPDM (salt corrosion resistant) is preferred for rubber materials.
Industrial area bridges face acid rain and dust corrosion, requiring enhanced bearing sealing and protection (e.g., dust covers for plain bearings, rubber seals for pot bearings).
Humidity and Geological Conditions
In humid areas or regions with high groundwater levels, rusting of bearing steel components must be prevented. For bridges on soft soil foundations (prone to uneven settlement), bearings adaptable to minor settlements (e.g., flexible plain elastomeric bearings) are suitable.
- Seismic Design Requirements
Bearing selection for bridges in seismic zones must prioritize seismic performance to prevent beam falling due to bearing failure during earthquakes:
Seismic Intensity
In areas with seismic intensity ≤ 6 degrees, plain or PTFE sliding elastomeric bearings can be used. In areas with intensity ≥ 7 degrees, seismic bearings (e.g., plain bearings with seismic stoppers, seismic pot bearings) or lead-rubber bearings (utilizing plastic deformation of lead cores to dissipate seismic energy) are required.
Seismic Load Type
For large horizontal seismic forces, bearings must have sufficient horizontal stiffness and anti-uplift capacity (e.g., anti-uplift devices on upper and lower base plates of pot bearings). In areas with significant vertical seismic forces (e.g., high-intensity mountainous regions), the vertical seismic load-bearing capacity of bearings must be verified.
- Construction and Maintenance Convenience
Selection must balance construction feasibility and long-term maintenance costs:
Construction Conditions
Plain bearings (small in size and light in weight) are preferred for narrow construction spaces (e.g., overpasses, tunnel portal bridges). Pot bearings can be used in areas accessible to large hoisting equipment, with attention to bearing protection during hoisting.
For precast beam bridges, bearings must precisely match the embedded steel plates at the beam bottom. Bearing planar dimensions and mounting holes should be clearly specified during selection.
Maintenance and Replacement
Plain elastomeric bearings are difficult to replace (requiring beam jacking) and are suitable for bridges with low maintenance needs within their design life. 易损件 (e.g., seals, PTFE slides) of pot bearings can be replaced individually, making them suitable for long-span or critical bridges.
For bridges in harsh environments, bearings facilitating inspection (e.g., pot bearings with observation holes) should be selected.
VII. Material Performance Requirements
The performance of core materials (rubber, steel plates, PTFE slides, etc.) directly determines bearing quality. Material specifications must be clarified during selection:
Rubber Materials
Common materials include chloroprene rubber (CR, aging-resistant and cost-effective, suitable for general environments), natural rubber (NR, good elasticity and high load-bearing capacity, suitable for heavy loads), and EPDM (weather-resistant and corrosion-resistant, suitable for coastal and high-temperature environments). The appropriate rubber type should be selected based on the environment and load, with rubber hardness (50-60 Shore A), tensile strength, and compression set meeting specifications.
Reinforcing Steel Plates
Steel plates in plain elastomeric bearings must have sufficient stiffness (typically 2-4mm thick) and rust resistance (galvanized or painted) to prevent bearing delamination due to steel corrosion.
Sliding Materials
PTFE slides must have a low friction coefficient (typically ≤ 0.03) and wear resistance. Stainless steel plates require surface polishing to ensure smooth sliding.
VIII. Code and Standard Requirements
Selection must comply with national or industry codes to ensure design compliance:
Highway bridges must adhere to JT/T 4-2019 Highway Bridge Plain Elastomeric Bearings and JT/T 391-2019 Highway Bridge Pot Bearings;
Railway bridges must follow TB/T 1893-2016 Railway Bridge Plain Elastomeric Bearings;
Municipal bridges should reference CJJ 2-2008 Code for Construction and Quality Acceptance of Urban Bridge Engineering.
Codes specify limits for bearing load-bearing capacity, deformation, and materials, which must be verified item by item during selection.
Summary
The selection of elastomeric bridge bearings is a comprehensive decision-making process integrating “structural requirements, load conditions, environmental adaptation, construction and maintenance, and code compliance”. In practical design, it is necessary to first clarify the three core parameters of bearings (load-bearing capacity, displacement, rotation) through structural calculations, then combine factors such as environment, seismic performance, and construction to ultimately select the most cost-effective bearing type (e.g., plain bearings for small-span ordinary bridges, pot or seismic bearings for long-span complex bridges).
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