| Pad Plan Size (mm) | Total Thickness (mm) | Internal Shims | Max Vertical Load (kN) | Max Horizontal Movement (mm) | Max Rotation (rad) |
| 300 × 400 | 39 | 3 | 720 | 13 | 0.010 |
| 350 × 450 | 44 | 4 | 1050 | 15 | 0.012 |
| 400 × 500 | 51 | 5 | 1350 | 18 | 0.014 |
| 450 × 550 | 57 | 6 | 1680 | 20 | 0.016 |
| 500 × 600 | 69 | 7 | 2100 | 24 | 0.018 |
Horizontal movement from thermal expansion and contraction places the pad in shear. The relationship is linear:
Shear strain γ = Δ / tr
Where Δ = horizontal displacement, tr = total rubber thickness (sum of all elastomer layers excluding shims)
Allowable γ per AASHTO M251: 0.5 for service load, 0.7 for ultimate
For a pad with total rubber thickness 37 mm (five 6 mm layers plus two 3.5 mm cover layers), the maximum allowable horizontal movement is:
Service: 37 × 0.5 = 18.5 mm
Ultimate: 37 × 0.7 = 25.9 mm
If the bridge design requires 30 mm of thermal movement, the pad must be thickened or a PTFE sliding surface added.
Rotation Capacity: The Often-Forgotten Limit
Beam-end rotation under live load and camber places the pad in non-uniform compression. The rotation capacity θ is given by:
θ = (2 × tr × σc) / (G × B²)
Where G = shear modulus (~1.0 MPa for 60 Shore A), B = pad width perpendicular to rotation axis
For a 400 mm wide pad with 37 mm rubber thickness and 5 MPa compressive stress:
θ = (2 × 37 × 5) / (1.0 × 400²) = 370 / 160,000 = 0.0023 rad
This is typically sufficient for simply supported spans up to 30 meters. Longer spans or continuous girders may require rotation-enhancing features such as tapered shims or pot bearings.
Case Study: Rotation Failure on a Curved Girder Bridge
A curved steel girder bridge in an urban interchange experienced bearing distress within three years of opening. The exterior girders rotated more than the interior due to torsional effects.
Investigation Findings:
Design rotation assumed 0.008 rad
Actual rotation measured 0.015 rad at exterior girders
Pad rotation capacity was 0.009 rad based on installed geometry
Damage:
Leading edge of pad crushed, trailing edge gapped 2 mm
Steel shim bent at leading edge
Girder sole plate showed fretting corrosion
Correction:
Replaced with thicker pad (total rubber increased from 41 mm to 59 mm)
Added tapered steel plate to match actual girder end slope
Rotation capacity increased to 0.017 rad
Follow-Up:
8-year inspection: no edge crushing, no gapping, uniform contact pattern
Quality Verification for International Shipments
When importing Bridge Bearing Pad units, request these verifications:
Full-scale compression test: Load to 1.5× design capacity. No delamination, no rupture, no steel shim exposure.
Shear modulus verification: G value within ±15% of specified target.
Bond peel test: Minimum 10 kN/m between rubber and steel shim.
Low-temperature stiffness test: If Grade 3 or Grade 5 specified, confirm shear modulus at -20°C does not exceed 2× room-temperature value.
Dimensional tolerance: Length/width ±2 mm, thickness ±1 mm, shim position within 2 mm of drawing.
What to Spec
A Bridge Bearing Pad is a load-rated structural component, not a commodity rubber part. Specify shape factor, rubber grade, shim count, and test verification in the same sentence as plan dimensions. The load capacity, movement allowance, and rotation tolerance are calculable from those inputs — and they determine whether the bridge sits safely for five decades or fails before the first deck overlay.
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