10.3 Bearings and expansion joints
4 min read•july 30, 2024
Bridge bearings and expansion joints are crucial components in superstructure design. They transfer loads and allow movement between bridge elements. Bearings come in various types, each suited for specific load and movement requirements. Proper selection is key for bridge performance.
Expansion joints accommodate thermal changes and provide smooth traffic transitions. Types include strip seal, modular, and . Selection depends on movement range and durability needs. Both bearings and joints require careful design and maintenance for optimal bridge function.
Bridge Bearings and Expansion Joints
Types and Functions of Bridge Bearings
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- Bridge bearings transfer loads between superstructure and substructure while accommodating movement and rotation
- Primary functions include load transfer, movement accommodation, and rotation allowance
- Common types
- (suitable for small to medium spans)
- (larger loads and rotations)
- (high vertical and horizontal load capacities)
- (complex geometries or significant rotational demands)
- Each bearing type offers specific load and movement capacities tailored to bridge requirements
Expansion Joint Systems
- Expansion joints allow and contraction of bridge elements while providing smooth traffic transition
- Types of expansion joints
- (movements up to 4 inches or 100 mm)
- Modular joints (movements up to 24 inches or 600 mm)
- Finger joints (very large movements exceeding 24 inches or 600 mm)
- Selection based on total movement range including thermal expansion, creep, shrinkage, and live load deflections
- Durability considerations involve resistance to fatigue, impact loads, and environmental factors (de-icing chemicals, debris accumulation)
Importance of Proper Selection
- Appropriate bearing and expansion joint selection critical for overall bridge performance and longevity
- Factors influencing selection
- Bridge geometry
- Anticipated loads
- Expected movements (thermal effects, creep, shrinkage)
- Maintenance requirements
- Cost-effectiveness
- Proper selection ensures optimal , movement accommodation, and long-term structural integrity
Bearing Selection for Bridges
Factors Influencing Bearing Selection
- Vertical and horizontal loads determine required load capacity
- Rotational demands influence bearing type and design
- Anticipated movements from thermal effects, creep, and shrinkage dictate movement capacity
- Bridge articulation scheme defines locations of fixed and expansion bearings
- Future maintenance and replacement requirements affect long-term performance and cost-effectiveness
- Environmental conditions (temperature extremes, marine environments) impact material selection
Bearing Types and Applications
- Elastomeric bearings
- Suitable for small to medium spans with moderate loads and movements
- Offer simplicity and low maintenance
- Examples: highway overpasses, short-span river crossings
- Pot bearings
- Used for larger loads and rotations
- Provide high load capacity with limited capabilities
- Examples: multi-span highway bridges, railway bridges
- Disk bearings
- Offer high vertical and horizontal load capacities
- Accommodate large rotations
- Suitable for long-span bridges (cable-stayed bridges, arch bridges)
- Spherical bearings
- Designed for bridges with complex geometries or significant rotational demands
- Allow for multi-directional rotation
- Examples: curved bridges, bridges with significant horizontal curvature
Bearing Design for Loads
Elastomeric Bearing Design
- Determine required plan dimensions, thickness, and number of layers based on load and movement demands
- Shape factor (ratio of loaded area to perimeter free to bulge) critical for load capacity and stiffness
- Design considerations as per Bridge Design Specifications
- Compression requirements
- Rotation limits
- Shear capacity
- Stability checks
- Design process involves iterative calculations to optimize bearing dimensions and performance
Mechanical Bearing Design
- Pot and disk bearings resist vertical loads through confined elastomers or polymers
- Horizontal loads managed by sliding surfaces or shear keys
- Design steps for mechanical bearings
- Size confined elastomer or polymer disk
- Design pot or retainer
- Select appropriate PTFE sliding surfaces for expansion bearings
- Consider fatigue, friction, and wear characteristics, particularly for sliding surfaces
- Seismic design may require additional checks and detailing (increased movement capacity, energy dissipation features)
Load and Movement Considerations
- Vertical loads include dead load, live load, and impact forces
- Horizontal loads encompass wind, braking forces, and seismic loads
- Movement calculations account for
- Thermal expansion and contraction
- Concrete creep and shrinkage
- Elastic shortening in prestressed structures
- Rotation demands arise from live load deflections and long-term deformations
- Load combinations as specified in design codes (AASHTO LRFD) must be considered
Expansion Joint Systems
Types and Selection Criteria
- Strip seal joints
- Suitable for movements up to 4 inches (100 mm)
- Commonly used due to simplicity and moderate cost
- Examples: short to medium-span highway bridges
- Modular joints
- Accommodate larger movements up to 24 inches (600 mm) or more
- Consist of multiple seals and support bars
- Examples: long-span bridges, bridges in extreme climates
- Finger joints
- Used for very large movements exceeding 24 inches (600 mm)
- Common in long-span bridges (suspension bridges, cable-stayed bridges)
- Selection based on total movement range and traffic conditions
Design and Detailing Considerations
- Specify appropriate materials for durability and performance
- Ensure water tightness to protect underlying structure
- Provide for ease of inspection and maintenance
- Account for skew effects causing non-uniform movement and additional stresses
- Design for fatigue loading from repeated traffic impact
- Consider noise reduction features for urban environments
- Incorporate debris management systems to prevent joint clogging
Installation and Maintenance
- Proper installation critical for joint performance
- Ensure correct positioning and alignment
- Use appropriate anchorage systems
- Regular maintenance required
- Clean debris accumulation
- Replace worn seals or components
- Monitor for water leakage or damage
- Develop inspection and maintenance schedules based on joint type and traffic volume
- Consider future replacement or rehabilitation in initial design and detailing
Key Terms to Review (21)
AASHTO LRFD: AASHTO LRFD stands for the American Association of State Highway and Transportation Officials Load and Resistance Factor Design. It is a design methodology that incorporates reliability-based principles into the structural design of bridges, ensuring safety and performance by applying factors to loads and resistances based on their statistical characteristics. This method connects directly to various aspects of bridge engineering, including design, analysis, and evaluation processes.
ASTM Standards: ASTM Standards are a set of guidelines established by ASTM International, which provide specifications, test methods, and practices to ensure the quality and safety of materials and products across various industries. These standards play a vital role in bridge engineering, particularly in defining the material properties of steel used in construction and ensuring the effectiveness of bearings and expansion joints, which are crucial for accommodating structural movements.
Deformation: Deformation refers to the change in shape or size of a material due to applied forces or moments. This process can occur in various materials, including metals and concrete, and is crucial for understanding how structures respond to loads and environmental changes. In bridge engineering, deformation is particularly relevant for ensuring that structures can accommodate movement and expansion without compromising safety or functionality.
Disk bearings: Disk bearings are specialized components used in bridge engineering to facilitate the movement and support of bridge structures. They are designed to allow for vertical and horizontal movements due to thermal expansion, contraction, and other forces while transferring loads between the superstructure and substructure effectively. Disk bearings help in minimizing stress concentrations and extending the lifespan of bridge elements.
Elastomeric Bearings: Elastomeric bearings are flexible support devices used in bridge engineering, designed to accommodate movements and rotations while providing load support. These bearings are made from layers of elastomeric materials, typically rubber, which allow for vertical and horizontal movements due to thermal expansion, vibrations, and other forces acting on the bridge structure. Their ability to reduce stress concentrations and dampen vibrations makes them essential components in bridge design.
Fatigue Failure: Fatigue failure refers to the gradual weakening and eventual failure of a material due to repeated cycles of stress or strain over time. This phenomenon is critical in structural design, especially for components that experience fluctuating loads, like those in bridges. Understanding fatigue failure is essential for predicting the lifespan of materials and ensuring the integrity of structures under dynamic conditions.
Finger Joints: Finger joints are specialized mechanical components used in bridge engineering, primarily designed to accommodate thermal expansion and contraction, while allowing for movement between structural elements. These joints play a critical role in maintaining the integrity of bridge structures by preventing damage due to temperature changes, and they help ensure that forces are evenly distributed across the bridge's components.
Flexibility: Flexibility refers to the ability of a structure or its components, like bearings and expansion joints, to accommodate movements without suffering damage. This includes adjustments to temperature changes, live loads, and seismic activities, allowing structures to maintain their integrity while adapting to various forces acting upon them.
Horizontal movement: Horizontal movement refers to the lateral displacement that can occur in structural elements, particularly in bridges, due to various factors such as thermal expansion, load changes, and seismic activity. This movement is essential to consider in the design of bearings and expansion joints to ensure the integrity and longevity of the structure. Properly accommodating horizontal movement helps to prevent structural damage and maintains the functionality of the bridge over time.
Load Distribution: Load distribution refers to the way in which loads are spread across a structure, impacting how forces are transferred throughout its components. Understanding load distribution is essential for assessing structural integrity and ensuring that all parts of a bridge can handle applied loads effectively, which is critical across various bridge designs and types.
Neoprene: Neoprene is a synthetic rubber material that is widely used in construction, particularly for bearings and expansion joints. It has excellent flexibility, durability, and resistance to temperature changes and various environmental factors, making it an ideal choice for applications that require reliable performance under dynamic loads. Neoprene’s properties help accommodate movement and stress in structures, ensuring longevity and safety.
Pot Bearings: Pot bearings are a type of structural bearing used in bridge engineering that allows for controlled movement and rotation while supporting loads. They consist of a cylindrical pot filled with a flexible material, such as elastomer or bearing pad, which accommodates vertical and horizontal movements caused by thermal expansion, wind forces, and seismic activity. The design ensures the bridge deck can expand and contract without transferring undue stress to the supporting structure.
Resilience: Resilience refers to the ability of a system, material, or structure to withstand and recover from external stresses or shocks without significant damage. In the context of engineering, particularly with structures like bridges, resilience encompasses how well these structures can handle factors like temperature changes, vibrations, and other environmental impacts while maintaining their integrity and functionality.
Rubber: Rubber is a versatile, elastic material derived from the latex sap of rubber trees, primarily used in the construction of bearings and expansion joints. Its unique properties, such as flexibility and resistance to wear, make it an ideal choice for accommodating movements in structures, ensuring stability and longevity while mitigating vibrations and stress.
Serviceability: Serviceability refers to the ability of a structure to perform its intended function without experiencing excessive deformation, discomfort, or damage under normal loading conditions. It focuses on user comfort and safety, ensuring that structures remain functional and visually acceptable throughout their lifespan. Key considerations for serviceability include deflection limits, vibration control, and durability, which directly relate to the performance of structures like composite steel-concrete beam bridges and their associated components.
Shear resistance: Shear resistance refers to the ability of a structural element to resist sliding forces that act parallel to its surface. This property is crucial in ensuring stability and integrity, particularly in bridge components where movements due to thermal expansion, traffic loads, and other dynamic forces occur. Shear resistance helps prevent structural failures such as sliding or buckling, maintaining the overall safety of the structure.
Spherical Bearings: Spherical bearings are specialized mechanical components designed to accommodate misalignments between connected parts, allowing for rotational movement in multiple directions. These bearings are crucial in bridge engineering as they support superstructures while enabling controlled movements due to temperature changes and structural settling, which are common in large constructions.
Steel: Steel is an alloy primarily made of iron and carbon, known for its high tensile strength and versatility in construction, particularly in bridge engineering. Its unique properties allow for the creation of robust structures that can withstand various loads and environmental conditions, making it a critical material in the design and construction of bridges.
Strip seal joints: Strip seal joints are flexible expansion joints used in bridge construction to accommodate movement between different components, allowing for thermal expansion, contraction, and other types of movement while preventing water and debris infiltration. These joints typically consist of a rubber or elastomeric strip that is set within a metal frame, ensuring durability and flexibility under various environmental conditions. They are crucial in maintaining the structural integrity of bridges by effectively managing movements caused by temperature changes and traffic loads.
Thermal expansion: Thermal expansion refers to the increase in size or volume of a material as it is heated. This phenomenon occurs because, at higher temperatures, the atoms or molecules within a material move more vigorously, causing them to occupy a larger space. Understanding thermal expansion is essential for designing structures that can accommodate these changes, especially in bridge construction where materials expand and contract due to temperature fluctuations.
Vertical movement: Vertical movement refers to the upward and downward displacements that occur in structural elements, particularly bridges, due to various factors such as thermal expansion, live loads, and settlement. This type of movement is crucial in understanding how bridges respond to environmental conditions and loading scenarios, impacting their overall performance and safety.