Bridge Bearing Types

Why This Matters

Bridge bearings are the critical interface between a bridge's superstructure and its supports. They determine how the entire structure responds to thermal expansion, traffic loads, seismic forces, and rotational demands.

Every bearing type represents a specific engineering solution to one 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.

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Elastomeric Bearings: Flexibility Through Material Properties

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

Elastomeric Bearings

• Laminated rubber-steel construction uses alternating layers of elastomer and thin steel shims. The steel plates provide vertical stiffness (preventing excessive bulging under compression) while the elastomer layers allow horizontal flexibility through shear deformation.
• Accommodates rotation and limited translation through material deformation rather than sliding, which significantly reduces maintenance needs since there are no mechanical parts to seize or wear.
• Cost-effective workhorse for highway and railway bridges where movement demands are moderate and predictable. These are by far the most common bearing type on shorter-span bridges.

Lead Rubber Bearings

• Lead core provides damping. A cylindrical lead plug is inserted through the center of a laminated elastomeric bearing. During seismic events, the lead deforms plastically, converting kinetic energy into heat and dissipating it.
• Combines isolation and energy dissipation in a single unit. The elastomer shifts the structure's natural period away from the dominant earthquake frequencies, while the lead core reduces displacement amplitudes.
• 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 (polytetrafluoroethylene, commonly known as Teflon) or similar low-friction materials. The coefficient of friction determines both the ease of movement and the magnitude of horizontal force transmitted to substructures.

Sliding Bearings

• PTFE-to-stainless steel interface creates a low-friction sliding surface that accommodates thermal expansion and contraction. Typical coefficients of friction range from about 0.02 to 0.08, depending on contact pressure and temperature.
• Often paired with other bearing types to separate the load-carrying function from the movement-accommodating function. For example, a pot bearing may include a PTFE sliding surface on top to allow translation.
• Requires careful detailing to prevent debris accumulation on the sliding surface and to ensure consistent friction behavior over the bearing's service life.

Friction Pendulum Bearings

• Curved sliding surface creates a restoring force. As the bridge displaces laterally, the slider moves up the concave dish. Gravity then pulls it back toward center, providing self-centering behavior without springs or elastomers.
• Exceptional displacement capacity for seismic isolation, accommodating movements that would exceed other bearing types' limits.
• Period of isolation is determined by geometry. The isolated period $T$ is governed by the radius of curvature $R$ of the concave surface: $T = 2\pi\sqrt{\frac{R}{g}}$, where $g$ is gravitational acceleration. This allows engineers to tune seismic response by selecting the appropriate dish radius.

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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. Think of it this way: unconfined rubber squishes outward when you press on it, limiting how much load it can take. Confine that same rubber in a steel pot, and it behaves almost like a fluid, supporting far greater compressive forces.

Pot Bearings

• Elastomer confined in a steel cylinder. The pot walls prevent lateral expansion, allowing the bearing to support very high vertical loads (often thousands of kN).
• Rotation occurs through elastomer deformation within the pot, while translation requires a separate PTFE sliding surface mounted on top of the assembly.
• Handles significant rotation demands, making pot bearings suitable for continuous spans and curved bridges where support rotations can be substantial.

Disk Bearings

• Polyether urethane disk provides rotation capacity through controlled deformation of the confined elastomer. The urethane material is stiffer than the natural rubber used in pot bearings.
• Simpler construction than pot bearings with fewer components, no internal seals to fail, and easier inspection requirements.
• Combined with shear-restricted elements (such as a central pin) 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 physical geometry rather than material deformation. Many of these designs date back to the early days of steel bridge construction, and while modern alternatives have largely replaced them in new designs, you'll still encounter them on existing structures.

Roller Bearings

• Cylindrical rollers between flat plates provide very low resistance to horizontal movement in one direction. The rolling contact means almost no friction opposing thermal movements.
• Accommodates large longitudinal displacements from thermal expansion in long-span structures, which is why they were historically favored for large steel bridges.
• Requires maintenance attention to prevent corrosion and ensure rollers remain properly aligned and free to move. A seized roller bearing can impose unexpected horizontal forces on substructures.

Rocker Bearings

• Curved surface rocks on a flat plate, allowing rotation about a single axis while transmitting vertical and horizontal loads to the support.
• Historically common in steel bridges before elastomeric alternatives became widely available. You'll see these on many mid-20th-century highway bridges.
• Limited rotation capacity compared to modern alternatives, with a known vulnerability to "walking" or incremental displacement under repeated load cycling. This is one reason they've fallen out of favor for new construction.

Pin Bearings

• A steel pin creates a defined rotation axis, allowing free rotation while providing fixed horizontal and vertical restraint.
• Creates a true pinned connection for structural analysis, closely matching the idealized boundary conditions used in design models.
• Used at fixed supports where rotation must be permitted but translation in all directions must be 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 full 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. The curved mating surfaces are typically finished with a PTFE or woven PTFE liner to minimize friction.
• 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 more limited rotation capacity. This makes them particularly valuable for skewed or curved bridge geometries where rotation demands are not confined to a single plane.

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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 flat 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. Explain the principle behind this capacity increase: what does confinement do to the elastomer's behavior under compressive load?