Bridge Bearing Types

Why This Matters

Bridge bearings might seem like minor hardware, but they're the critical interface between a bridge's superstructure and its supports—and they determine how the entire structure responds to thermal expansion, traffic loads, seismic forces, and rotational demands. When you're analyzing bearing selection, you're really being tested on your understanding of load transfer mechanisms, degrees of freedom, and how engineers balance movement accommodation with structural stability.

Every bearing type represents a specific engineering solution to the fundamental challenge: how do you support massive vertical loads while still allowing necessary movements? Don't just memorize bearing names—know what movement each type permits, what loading conditions it handles best, and why an engineer would choose one over another. That conceptual understanding is what separates strong exam responses from simple recall.

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Elastomeric Bearings: Flexibility Through Material Properties

These bearings rely on the inherent deformation characteristics of rubber compounds to accommodate movement. The elastomer compresses under load and shears laterally, allowing the bearing to absorb movement without mechanical sliding surfaces.

Elastomeric Bearings

• Laminated rubber-steel construction—alternating layers of elastomer and steel plates provide vertical stiffness while allowing horizontal flexibility
• Accommodates rotation and limited translation through material deformation rather than sliding, reducing maintenance needs
• Cost-effective workhorse for highway and railway bridges where movement demands are moderate and predictable

Lead Rubber Bearings

• Lead core provides damping—the lead plug deforms plastically during seismic events, dissipating energy that would otherwise damage the structure
• Combines isolation and energy dissipation in a single unit, making them highly effective for seismic protection
• Standard choice for earthquake-prone regions where bridges need both flexibility and force reduction during ground motion

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Sliding Bearings: Low-Friction Movement Surfaces

Sliding bearings accommodate movement through relative motion between two surfaces, typically using PTFE (Teflon) or similar low-friction materials. The coefficient of friction determines both the ease of movement and the horizontal force transmitted to substructures.

Sliding Bearings

• PTFE-to-stainless steel interface—creates a low-friction sliding surface that accommodates thermal expansion and contraction
• Paired with other bearing types to separate load-carrying and movement-accommodating functions
• Requires careful detailing to prevent debris accumulation and ensure consistent friction behavior over the bearing's service life

Friction Pendulum Bearings

• Curved sliding surface creates restoring force—as the bridge displaces, gravity pulls it back toward center, providing self-centering behavior
• Exceptional displacement capacity for seismic isolation, accommodating movements that would exceed other bearing types' limits
• Period of isolation determined by geometry—the radius of curvature sets the isolated structure's natural period, allowing engineers to tune seismic response

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Confined Elastomer Bearings: High Capacity Through Containment

These bearings achieve high load capacity by confining elastomeric material within rigid containers. The confinement prevents lateral bulging under load, dramatically increasing the bearing's vertical capacity compared to unconfined elastomer.

Pot Bearings

• Elastomer confined in steel cylinder—the pot prevents lateral expansion, allowing the bearing to support very high vertical loads
• Rotation through elastomer deformation while translation requires a separate sliding surface on top
• Handles significant rotation demands making them suitable for continuous spans and curved bridges

Disk Bearings

• Polyether urethane disk provides rotation capacity through controlled deformation of the confined elastomer
• Simpler construction than pot bearings with fewer components and easier inspection requirements
• Combined with shear-restricted elements to create guided or fixed bearing configurations as needed

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Mechanical Bearings: Movement Through Geometry

These traditional bearing types use geometric mechanisms—rolling, rocking, or pivoting—to accommodate movement. The movement capacity is determined by the physical geometry rather than material deformation.

Roller Bearings

• Cylindrical rollers between plates—rolling motion provides very low resistance to horizontal movement in one direction
• Accommodates large longitudinal displacements from thermal expansion in long-span structures
• Requires maintenance attention to prevent corrosion and ensure rollers remain properly aligned and free to move

Rocker Bearings

• Curved surface rocks on flat plate—allows rotation about a single axis while transmitting vertical and horizontal loads
• Historically common in steel bridges before elastomeric alternatives became widely available
• Limited rotation capacity compared to modern alternatives, with potential for walking or displacement under repeated cycling

Pin Bearings

• Steel pin creates rotation axis—allows free rotation while providing fixed horizontal and vertical restraint
• Creates true pinned connection for structural analysis, matching idealized boundary conditions
• Used at fixed supports where rotation must be permitted but translation prevented

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Spherical Bearings: Multi-Directional Rotation

Spherical bearings use a ball-and-socket geometry to permit rotation about any axis. The spherical sliding surface maintains contact regardless of rotation direction, providing consistent load transfer under complex movements.

Spherical Bearings

• Convex-concave spherical interface—allows rotation in any direction while maintaining full contact area for load transfer
• High load capacity suitable for major bridges with large reaction forces and complex loading patterns
• Accommodates construction tolerances and differential settlement that would bind or overload bearings with limited rotation capacity

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Quick Reference Table

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Self-Check Questions

1. Which two bearing types specifically incorporate energy dissipation for seismic events, and what mechanism does each use to dissipate energy?

2. A bridge engineer needs a bearing that allows rotation about any axis while supporting very high loads. Compare spherical bearings and pot bearings—which is more appropriate and why?

3. Explain why friction pendulum bearings provide self-centering behavior while standard sliding bearings do not. What geometric feature creates this difference?

4. You're designing bearings for a long steel girder bridge in a non-seismic region where thermal movements are the primary concern. Compare elastomeric bearings versus roller bearings for this application—what are the tradeoffs?

5. Both pot bearings and disk bearings confine elastomeric material to achieve high load capacity. If an FRQ asked you to explain the principle behind this capacity increase, what would you describe about the confinement mechanism?