Elastomeric Bearing Pad Design per AASHTO Method A: A Worked Example

Elastomeric bearing pads sit between bridge girders and substructure to transfer vertical load and accommodate thermal movement, rotation, and (sometimes) seismic displacement. They are deceptively simple — a stack of neoprene or natural rubber laminated with thin steel shims — but the design checks are unforgiving. AASHTO LRFD Section 14 governs in the United States. This post walks through Method A, the simpler of the two design paths.

Run the calculation interactively with the Elastomeric Bearing Pad Calculator. For comparison with sliding bearings or pot bearings, see our broader Bearing Selector reference.

Method A vs Method B

AASHTO LRFD allows two design methods. Method A is conservative and limits compressive stress to about 1.0 ksi (6.9 MPa). Method B uses a more rigorous shear strain check and allows higher stress (up to 1.6 ksi / 11 MPa) but requires fabrication QA per AASHTO M251 and additional testing. Most state DOTs default to Method A for standard girder bridges; Method B shows up in long-span and high-load applications.

The Geometry: Shape Factor S

The shape factor controls almost everything. For a rectangular pad of plan dimensions L × W with internal layer thickness h_ri:

Higher shape factor means stiffer compression behavior. Method A typically requires S between 5.0 and 12. Below 5, the pad is too flexible and bulges excessively. Above 12, fabrication tolerances on the thin elastomer layers become impractical.

Worked Example: Setup

Consider a steel girder bearing pad with these inputs:

• Service dead + live load: P_SD = 180 kips total (assume 70 kips DL, 110 kips LL)
• Total thermal movement: 0.75 in (one-way)
• Live load rotation: 0.0125 rad about the transverse axis
• Elastomer: 50 durometer, G = 0.130 ksi (per AASHTO 14.7.6.2)
• Trial dimensions: L = 14 in (along girder), W = 18 in (across)
• Internal layers: 5 layers at h_ri = 0.50 in each, plus 0.25 in cover top and bottom

Total elastomer thickness h_rt = 5(0.50) + 2(0.25) = 3.0 in. Plus 4 internal steel shims at 0.075 in each = 0.30 in. Total pad height: 3.30 in.

Step 1: Shape Factor

S = 7.88 is comfortably in the Method A acceptable range.

Step 2: Compressive Stress Check (AASHTO 14.7.6.3.2)

For steel-reinforced pads under combined dead + live load:

Pad area = 14 × 18 = 252 in². Applied stress: σ_s = 180 / 252 = 0.714 ksi. Allowable: 1.25 × 0.130 × 7.88 = 1.281 ksi, capped at 1.25 ksi. Demand-to-capacity = 0.57. PASS with margin.

Step 3: Compressive Deflection

For 50 durometer elastomer at S = 7.88 and stress = 0.714 ksi, AASHTO Figure 14.7.6.3.3-1 gives an instantaneous strain ε_i ≈ 0.035 per layer. Total instantaneous deflection: 0.035 × 3.0 = 0.105 in. Long-term creep adds approximately 25% for natural rubber, 35% for neoprene: δ ≈ 0.142 in. The deflection between adjacent girders should not exceed 1/8 in to prevent deck cracking — borderline; consider matching pads on adjacent bearings.

Step 4: Shear Deformation

The pad must accommodate horizontal movement Δ_s with shear strain limit:

With Δ_s = 0.75 in, required h_rt ≥ 1.50 in. Provided: 3.0 in. PASS. The 2× factor caps the shear strain at 50%.

Step 5: Rotation Check (Method A)

Method A combines rotation and compression in a single uplift check:

This guards against pad uplift. With L = 14 in (along the rotation axis), h_ri = 0.50 in, θ_s = 0.0125 rad: required minimum stress = 0.5 × 0.130 × 7.88 × (14 / 0.50)² × 0.0125 / n where n = number of layers (5). Working through: 0.5 × 0.130 × 7.88 × 784 × 0.0125 / 5 = 1.005 ksi. The applied 0.714 ksi is < 1.005 ksi → FAIL by Method A. Either reduce rotation, redistribute load, increase shape factor, or move to Method B.

Step 6: Stability

To prevent buckling-like instability:

3.0 in ≤ 14/3 = 4.67 in. PASS.

Step 7: Steel Shim Reinforcement

Internal shim thickness from AASHTO 14.7.5.3.5:

For F_y = 36 ksi: h_s ≥ 3 × 0.50 × 0.714 / 36 = 0.0298 in. Provided 0.075 in (typical 14-gauge). PASS with large margin — most fabricators don’t go below 0.075 in regardless.

Conclusion of Example

Five of six checks pass; the rotation check fails by Method A. Realistic fixes: increase L to 16 in (gives more rotation capacity in the formula’s L² term) or increase shape factor with thinner internal layers. This is the bread-and-butter iteration loop for bearing pad design — change one dimension, recheck all six conditions.

What Method A Misses

Method A’s rotation check is empirical and tends to be conservative on wide pads and unconservative on narrow ones. It also doesn’t address combined shear strain explicitly. Method B’s shear strain check γ_total ≤ 5.0 (axial + rotation + shear, computed per 14.7.5.3.3) is more physically meaningful. For seismic applications, lead-rubber bearings (LRB) and friction pendulum systems (FPS) replace plain elastomeric pads entirely.

Related References

• Elastomeric Bearing Pad Calculator — AASHTO Method A automation
• Structural Engineering Tool Index — full structural calculator catalog
• Bearing Selector — comparison with mechanical bearing types

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