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Section 9: Prestressed Concrete Box Beams (Types B20, B28, B34, and B40)

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Materials

Use Class H concrete witha minimum of ƒ'ci = 4.0 ksi and ƒ'c = 5.0 ksi.

Use Class S concrete (ƒ'c = 4.0 ksi) for shear keys.

Design beams for 0.5-in low-relaxation strands.

Use prestressing strand with a specified tensile strength, ƒpu, of 270 ksi.

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Geometric Constraints

The maximum skew angle for box beam bridges is 30 degrees.

The maximum gap between adjacent box beams is 2 in. The minimum gap between adjacent box beams in 1 in.

A 5-in. minimum thickness composite concrete slab overlay or 2-in. minimum thickness asphaltic concrete pavement (ACP) overlay is required.

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Structural Analysis

Beam designs must meet the following requirements:

  • Distribute the weight of one railing to no more than three beams.
  • Use section properties given on the prestressed concrete box beam standard drawings.
  • Composite section properties may be calculated assuming the beam and composite concrete slab overlay have the same modulus of elasticity (for beams with ƒ'c < 8.5 ksi). When determining section properties, do not include haunch concrete placed on top of the beam. Section properties based on final beam and slab modulus of elasticity may also be used.
  • Live load distribution factors must conform to AASHTO LRFD Bridge Design Specifications, Article 4.6.2.2.2 and Article 4.6.2.2.3. Use:
    • Cross section (f) with bridges having a composite concrete slab
    • Cross section (g) with bridges having ACP applied directly to tops of beams, assuming beams are sufficiently connected to act as a unit.
  • Do not apply the skew correction factor for moment as suggested in Article 4.6.2.2.2e.
  • For interior as well as exterior girders, do not take the live load distribution factor, for moment or shear, as less than

  • Where,

    • m = multiple presence factor per AASHTO LRFD 3.6.1.1.2
    • = number of lanes
    • = number of beams or girders
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Design Criteria

Standard beam designs must meet the following requirements:

  • Strands should be added and debonded in the order shown on the BBND standard drawings, available at http://www.dot.state.tx.us/insdtdot/ orgchart/cmd/cserve/standard/bridge-e.htm.
  • Debond strands in 3-ft. increments at beam ends if necessary to control stresses at release.
  • Debonded strands must conform to AASHTO LRFD Bridge Design Specifications, Article 5.11.43 except as noted below:
    • Debond no more than 75 percent of the total number of strands.
    • Debond no more than 75 percent of the number of strands in that row.
    • The maximum debonding length must be the lesser of one-half the span length minus the maximum development length, 0.2 times the beam length, or 15 feet.
    • Not more than 75 percent of the debonded strands, or 10 strands, whichever is greater, shall have the debonding terminated at any section, where section is defined as an increment (e.g. 3 feet, 6 feet, 9 feet).
  • Calculate required stirrup spacing for #4 Grade 60 bars according to the AASHTO LRFD Bridge Design Specifications, Article 5.8. Change stirrup spacing as shown on relevant standard drawings, available at http://www.dot.state.tx.us/insdtdot/ orgchart/cmd/cserve/standard/bridge-e.htm, only if analysis indicates the inadequacy of the standard design.
  • TxDOT standard box beams satisfy Article 5.8.4 and Article 5.10.10 of the AASHTO LRFD Bridge Design Specifications.
  • For box beams with a composite concrete slab overlay, compute deflections due to slab weight and composite dead loads assuming the beam and slab to have the same modulus of elasticity. Assume Ec = 5,000,000 psi for beams with ƒ'c < 8.5 ksi. Show predicted slab deflections on the plans even though field experience indicates actual deflections are generally less than predicted. use the deflection due to slab weight only times 0.8 for calculating haunch depth.
  • Use shear keys for all box beam bridges. Do not consider composite action between beams and shear keys in computing live load distribution factors, nor for strength stress or deflection calculations.
  • Transverse post-tensioning is required for box beam bridges topped with an ACP overlay applied directly to the tops of beams. Space tendons at 10 ft. maximum with the first tendons set 10 ft. from bent centerlines. Post-tensioning details are provided on standard drawing BBCDO.
  • See Section 5, Prestressed Concrete I Beams and I Girders for other design criteria.
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