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Section 2: Steel-Reinforced Elastomeric Bearings for Pretensioned Concrete I-Girders

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Materials

Use 50-durometer neoprene for steel-reinforced elastomeric bearings.

Use a shear modulus range of 95 to 175 psi for design, using the least favorable value for the design check.

Make steel shims 0.105 in. thick.

Do not use adhesives between bearings and other components.

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Geometric Constraints

See standard drawings for standard pad details. Note the bearings shown on standard drawing IGEB may not be applicable for skews over 60 degrees.

Tapered bearings may be used if the taper does not exceed 0.050 ft./ft.

Use 1/4-in. exterior pad layers. If using 1/4-in. interior pad layers, disregard the requirements in Article 14.7.6.1, specifying exterior layers no thicker than 70% of internal layers.

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Structural Analysis

Assume a temperature change of 70 degrees Fahrenheit after erection when calculating thermal movement in one direction (not total). Take Tmin = 10 degrees F and Tmax = 80 degrees F. For the panhandle region use Tmin = 10 degrees F and Tmax = 115 degrees F, for a total temperature change of 105 degrees F. Optionally, the temperature ranges in Article 3.12.2.2 may be used if appropriate.

Do not include shrinkage, creep, and elastic shortening when determining maximum movement, which will be accommodated through infrequent slip.

Use appropriate shear live load distribution, modified for skew.

Use the critical DL condition (the lightest predicted DL) when checking against slip.

Use Load Combination Service I for all gravity loads.

Ignore limit on in Article 14.7.6.1.

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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 , where L is the pad length defined in AASHTO LRFD Bridge Design Specifications, and 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 , where is midspan LL deflection.
  • Check bearing pad slip as follows:

    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 degrees F, typical 0.175 ksi
    • A = plan arc of elastomer (sq. in.)
    • = 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.
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Detailing

Use standard drawing IGEB for guidance on detailing custom bearing pad designs.


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