Design

While AASHTO Specifications allow two design approaches (Methods A and B, Section 14), many states have superimposed their own limitations on the design process. Local requirements should be checked before commencing design.

However, the following formulae offer simple predictions of the performance of bearings with fully bonded layers*:

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Where:

L is bearing length
B is bearing width
D is diameter of a circular bearing
K1 is the spring rate in compression
K2 is the spring rate in shear
A is plan area of the bearing
t is the thickness of the individual elastomer layers
n is the number of individual elastomer layers
G is the shear modulus of the elastomer (see Table 1)
EC is the compression modulus of the elastomer

Also note that:

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Where:

Eo is Young’s Modulus for the elastomer (see Table 1)
K is a numerical factor (see Table 1)
S is the shape factor, defined as the loaded area divided by the force area.

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In the interest of longevity, compression strain should be limited to 15% and shear strain to 50%.

To ensure stability, total bearing thickness should not exceed the smaller of L/3, B/3, or D/4 for Laminated Bearings.

*If either or both faces of an individual elastomer layer are not bonded to a steel plate, then compression will be modified by the b factor. The b factor is 1 for a fully bonded layer, 1.4 for a layer bonded on one face, and 1.8 for an unbonded layer. (Note that the b factor does not apply to shear calculations.)

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Elastomeric Bearings

Design

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