AASHTO-LRFD Steel Reinforced Elastomeric Bearing:

AASHTO-LRFD Steel Reinforced Elastomeric Bearing tutorial is about design method A. It is needed to know shape factor,material properties, stress limits, shear deformations and also stability of bearings.

Firstly, provisions of Method A specified in Article 14.7.6 . It shall apply to steel-reinforced elastomeric bearings. But they must satisfies; Si2/n < 22 , Si is shape factor for an internal layer and n is number of internal layers . If the thickness of exterior layers greater than half of  the thickness of interior layer, number of interior layer may be increased by 0.5 for each exterior layer. And also for which primary rotation axis is about the axis parallel to the transverse axis of the bridge. Furtermore, the limiting value is taken as 20 if number of interior layers is greater than or equal to 3 and bearing is rectangular. Secondly, elastomer bearings designed according to Method A ;

  • Must have internal layers with same thickness.
  • Must have cover layers no thicker than seventy percent of the interior layer thickness.
  • Should not have a tapered layer.

Moreover, elastomers are flexible under shear and uniaxial deformation, but they are very stiff against volume changes. (Article The shear modulus of the elastomer at 23 C shall be used as the basis for design. Also, specified shear modulus of elastomer must be between 0.60 and 1.30 MPa or the nominal hardness scale must be between  50 and 60 on the Shore A scale.

Furthermore, the compressive deflection under instantaneous live load and initial dead load of an internal layer of a steel reinforced elastomeric bearing at service limit state without impact shall not exceed 0.09hri.( Article Deflection due to instantaneous live load must be smaller than 3.2 mm. ( Article In bearing design, AASHTO allows to ignore dynamic load allowance. However, some states uses dynamic allowance factor to be on the safe side.

Lastly, the minimum thickness of steel reinforcement shall be 1.6 mm.