Section 2: Steel-Reinforced Elastomeric Bearings for Prestressed Concrete Beams
Anchor: #i1352007Materials
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.
Anchor: #i1352064Geometric Constraints
See standard drawings available at http://www.dot.state.tx.us/insdtdot/orgchart/cmd /cserve/standard/bridge-e.htm for standard pad details.
Tapered bearings may be used if the taper does not exceed 0.055 ft./ft. For beams on steeper grades, use a beveled steel sole plate field-welded (1/4-in. fillet) to a 1/2-in. steel plate embedded in and anchored to beams with headed stud anchors. Use a minimum of four 1/2-in.-by-3-in. stud anchors with studs located between strands and reinforcement. The minimum thickness of sole plate should be 1.5 in. of steel between weld and elastomer. The sole plate should extend at least 1 in. beyond the beam flange. Sole plates should not be vulcanized to the bearing to allow slip to occur at the beam/bearing interface.
Use 1/4-in. exterior pad layers. If using 1/4-in. interior pad layers, disregard the requirements in the AASHTO LRFD Bridge Design Specifications, Article 14.7.6.1, specifying exterior layers no thicker than 70% of internal layers.
Anchor: #i1352119Structural 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.
Do not include shrinkage, creep, and elastic shortening when determining maximum movement, which will be accommodated through infrequent slip.
Do not apply IM to live load when checking compressive stress (see AASHTO LRFD Bridge Design Specifications, Commentary C14.7.5.3.2).
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.
Anchor: #i1352223Design Criteria
Follow Design Method A in AASHTO LRFD Bridge Design Specifications, 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 AASHTO LRFD
Bridge Design Specifications, 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 θ is the total rotation. Estimate
compressive deflection using AASHTO LRFD Bridge Design
Specifications, Figure C14.7.5.3.3-1.
- Calculate total rotation for dead and live load plus 0.005
radians for construction uncertainties as required by AASHTO
LRFD Bridge Design Specifications, 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)
= total
elastomer thickness (M)
- 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.)
- You may use hn, instead of total pad height when checking stability as required in AASHTO LRFD Bridge Design Specifications, Article 14.7.6.3.6.
Detailing
Use standard drawing IBEB for guidance on detailing custom bearing pad designs.