Design Criteria

Follow Design Method A in Article 14.7.6, with the following exceptions:
  • DL compressive stress limit is the lesser of 1.20 ksi and 1.2 GS.
  • Total compressive stress limit is the lesser of 1.50 ksi and 1.5 GS. This limit can be exceeded up to 15% at the engineer’s discretion.
  • For rotation check, disregard Article 14.7.6.3.5.
    Rotation is acceptable if the total compressive deflection equals or exceeds θ0.8L2,
    where L is the pad length defined in
    AASHTO LRFD Bridge Design Specifications
    , and θ is the total rotation. Estimate compressive deflection using Figure C14.7.6.3.3-1.
  • Calculate total rotation for dead and live load plus 0.005 radians for construction uncertainties as required by Article 14.4.2.1. Take maximum
    live load rotation as 4∆SpanLength
    , where Δ is midspan LL deflection.
  • Check bearing pad slip as follows:
    ∆sallow≤0.2-Gr x DL x hrtG x A
    where:
    • Gr = beam grade in ft./ft.
    • DL = lightest unfactored predicted dead load (kips)
    • hrt
      = total elastomer thickness (in.)
    • G = shear modulus of elastomer at 0° F, typical 0.175 ksi
    • A = plan area of elastomer (sq. in.)
    • Δs(allow) = maximum total allowable shear deformation (in.)
  • Use hrt
    , instead of total pad height when checking stability as required in Article 14.7.6.3.6.