Spherical Bearings under AASHTO LRFD 6th Edition (2012)
Spherical bearings are categorized as HLMR (High‑Load Multi‑Rotational Bearings) in AASHTO LRFD Bridge Design Specifications, Section 14 "Joints and Bearings". It is critical to note that the extracted document only contains Sections 1‑3, and does not include the full text of Section 14. AASHTO LRFD serves as a design‑oriented code, while material requirements, manufacturing tolerances and prototype testing for spherical bearings are covered by the separate product standard ASTM D5977‑23.
Four limit‑state philosophies govern spherical bearing design under AASHTO LRFD: Service Limit State, Strength Limit State, Fatigue‑and‑Fracture Limit State, and Extreme‑Event Limit State. The fundamental LRFD formula ∑ηiγiQi≤ϕRn
applies to all bearing components. Service checks control PTFE contact pressure against creep and wear. Strength checks verify steel parts, anchorages and lateral stop lugs under factored horizontal loads such as braking force and seismic actions. Fatigue evaluates cyclic stress ranges for metallic components. Extreme‑Event Limit State covers earthquake, vessel‑vehicle collision and ice load scenarios.
Three functional types of spherical bearings are defined: fixed bearings (rotation only), unidirectional‑expansion bearings (rotation plus one‑direction translation with lateral stoppers), and multi‑directional expansion bearings (rotation plus free planar translation). The spherical‑on‑PTFE‑stainless‑steel sliding pair delivers large rotational capacity independent of rubber shear deformation, making them suitable for heavy‑load bridges. Designers shall account for both in‑service rotations and large construction‑stage rotations, and provide sufficient displacement capacity for temperature change, concrete shrinkage and creep.
PTFE‑stainless steel sliding interface requirements are specified in Article 14.9. Engineers shall define friction coefficients, PTFE thickness and embedment depth to prevent material extrusion, as well as surface finish for stainless steel mating plates. Anchor bolts or reinforcing connections need checks for tension, shear plus construction installation tolerances. Bearings shall allow jacking for future maintenance and replacement, together with protective measures against salt contamination and environmental corrosion.
There are clear gaps between AASHTO system and European standard EN 1337‑7. EN 1337‑7 is an integrated standard covering design, material, fabrication and prototype testing for both spherical and cylindrical PTFE bearings. AASHTO LRFD Section 14 only addresses spherical bearings; cylindrical PTFE sliding bearings have no dedicated AASHTO clauses and require project‑specific specifications. In AASHTO system, prototype tests are placed in ASTM D5977 rather than in the design code itself.
Important distinction exists between conventional spherical bearings and FPS friction‑pendulum seismic isolators. Ordinary bridge spherical bearings follow AASHTO LRFD Section 14 plus ASTM D5977. FPS seismic bearings are isolation devices and shall comply with AASHTO Guide
Specifications for LRFD Seismic Bridge Design, not Section 14, which is comparable to European standard EN 15129 for seismic devices.
For tender documents, typical specification statement reads: Spherical HLMR bearings designed in accordance with AASHTO LRFD Bridge Design Specifications, Section 14, manufactured and tested per ASTM D5977‑23.
