Elastomeric bearing pads are the unglamorous workhorses of bridge engineering. They absorb thermal expansion, accommodate rotation under live load, and transfer vertical reactions from girder to bent cap without ever being noticed — until one of them squirts out the side of a bearing seat ten years after construction. AASHTO LRFD §14.7.6, “Steel-Reinforced Elastomeric Bearings — Method A,” is the simplified procedure that most highway bridges in the United States are designed to. This walkthrough takes you from reaction loads to a finished pad in one example.
If you would rather drive the inputs interactively, the Elastomeric Bearing Pad Calculator implements every Method A check on this page.
Why Method A Exists
AASHTO offers two design methods for steel-reinforced elastomeric bearings:
- Method A (§14.7.6): simplified, conservative, based on shape-factor limits and unit shear strain checks. Adequate for ~80% of standard bridge bearings.
- Method B (§14.7.5): refined, performs explicit checks on combined compressive, rotational, and shear strains. Required when service loads or rotations exceed Method A limits.
Method B is more accurate. Method A is faster and lets you size a pad in five minutes with a calculator. For a routine prestressed concrete I-girder simple span over a county road, Method A is the right tool.
Worked Example: PSC I-Girder Bridge
Design a fixed bearing for an interior girder of a 90 ft simple span, supporting:
- Dead load reaction: DL = 95 kips
- Live load reaction (HL-93 + dynamic load allowance): DLL = 65 kips
- Total service reaction: P = 160 kips
- Total thermal movement (Service I): Δs = 0.55 in
- Girder rotation at service: θs = 0.012 rad
- Bottom flange width: 16 in
Specify a 60-durometer neoprene pad. Shear modulus G is 0.130 ksi (typical at 70°F per AASHTO Table 14.7.6.2-1).
Step 1: Trial Plan Dimensions
Start with the bottom flange as a width constraint. Use a 14″ wide pad. For length, Method A caps unit compressive stress at the lesser of 1.0 ksi or 1.25·G·S (we will check S in step 3). Try a trial length of 9″:
A = 14 × 9 = 126 in²
σs = 160 / 126 = 1.27 ksi
That is over 1.0 ksi — too aggressive. Try 14×12 = 168 in²:
σs = 160 / 168 = 0.952 ksi < 1.0 ksi ✓
Step 2: Total Elastomer Thickness from Shear Movement
Total elastomer thickness hrt must satisfy hrt ≥ 2·Δs:
hrt ≥ 2 × 0.55 = 1.10 in
Use hrt = 1.25 in. Add 1/8″ cover top and bottom — total pad height before steel shims will be 1.50 in.
Step 3: Choose Layer Thickness and Compute Shape Factor
Try 5 internal elastomer layers at hri = 0.25″ each (cover layers are 0.125″ but excluded from hrt). Shape factor for a rectangular pad:
Si = (L · W) / [2 · hri · (L + W)]
Si = (12 × 14) / [2 × 0.25 × (12 + 14)]
Si = 168 / 13.0 = 12.92
Now check the compressive stress limit including the shape factor:
1.25 · G · S = 1.25 × 0.130 × 12.92 = 2.10 ksi
The governing limit is min(1.0, 2.10) = 1.0 ksi. Our actual stress is 0.952 ksi < 1.0 ksi ✓.
Step 4: Stability Check
The total elastomer height plus cover cannot exceed hrt ≤ L/3 for rectangular pads in Method A:
L/3 = 12 / 3 = 4.0 in hrt = 1.25 in < 4.0 in ✓
Step 5: Rotation Check (Implicit in Method A)
Method A does not require an explicit rotation check at low rotations because the shape-factor and stress limits provide enough margin. The cutoff is θs ≤ (hrt/L)·(εc · n), where n is the number of internal layers. Our θs = 0.012 rad is small enough that this is satisfied. If you are above 0.020 rad, jump to Method B.
Step 6: Steel Reinforcement
Per AASHTO §14.7.5.3.5, internal steel shim thickness hs ≥ (3 · hmax · σs) / Fy:
hs ≥ (3 × 0.25 × 0.952) / 36 = 0.0198 in
Use 11-gauge (~0.120″) steel shims — vastly exceeds the requirement and is the practical minimum for fabrication.
Step 7: Final Pad
The finished bearing pad is:
- Plan: 14″ wide × 12″ long
- 5 internal elastomer layers at 0.25″ each = 1.25″
- 4 internal steel shims at 0.120″ each = 0.48″
- 2 cover layers at 0.125″ each = 0.25″
- Total pad height = 1.98 in
- 60-durometer neoprene, G = 0.130 ksi
Provide a bearing seat with sufficient anchorage to prevent walking-out under thermal cycles. A typical detail uses recessed bearing seats and a thin masonry plate.
Common Mistakes
- Ignoring temperature derating of G. AASHTO requires checking the bearing at the design low temperature. At 0°F, neoprene shear modulus increases significantly — your movement check tightens, and a pad that worked at room temperature may bind cold.
- Using durometer instead of measured G. Specify by shear modulus or by a Special Provision that requires test certification. Two pads at “60 duro” can have G values 30% apart.
- Forgetting the dead-load-only rotation. For prestressed girders with significant camber, the rotation at noon on Day 1 of erection can exceed the live-load rotation. Check both.
- Skipping the stability check. Tall thin pads roll out under shear. The L/3 rule is the cheap defense; ignore it and you will see pads everted in the first year.
When to Move to Method B
Switch to Method B when any of these apply:
- Service rotation exceeds approximately 0.020 rad
- You want to take credit for combined-strain capacity (Method A is conservative on this)
- Total movement exceeds about 4 inches (you may need a sliding bearing instead)
- The bridge is curved or skewed enough that biaxial movement is significant
Method B is implemented in the Elastomeric Pad Calculator Pro tier alongside Method A, so you can run both checks on the same input set and compare.
For more bridge engineering tools, see our Structural Engineering category — base plates, beam capacity, column buckling, and a growing set of bearing design tools.
Designed a tricky bearing recently? We are collecting Method A vs. Method B comparison cases for an upcoming article. Send us your inputs and the governing checks and we will feature it.