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Section 11: Pretensioned Concrete Spread Box Beams (X-Beams)

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Materials

Use Class H concrete with a minimum of = 4.0 ksi and = 5.0 ksi and a maximum = 6.0 ksi and = 8.5 ksi.

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

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Geometric Constraints

The maximum skew angle for X-beam bridges is 30 degrees without modification to standard drawings.

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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, applied to the composite cross section.
  • Use section properties given on the pretensioned concrete X-beam standard drawings.
  • Composite section properties may be calculated assuming the beam and composite slab have the same modulus of elasticity (for beams with < 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, however, this design assumption must be noted on the plans.
  • Live load distribution factors for interior beams must conform to Article 4.6.2.2.2 for flexural moment and Article 4.6.2.2.3 for shear.
  • Live load distribution factors for exterior beams must conform to Article 4.6.2.2.2 for flexural moment and Article 4.6.2.2.3 for shear, with the following exceptions:
    • When using the lever rule, multiply the result of the lever rule by 0.9 to account for better live load distribution arising from the beneficial torsional stiffness of the box girder system.
    • When the clear roadway width is greater than or equal to 20.0 ft., use a distribution factor for two or more design lanes loaded only. Do not design for one lane loaded.
    • When the clear roadway width is less than 20.0 ft., design for one lane loaded with a multiple presence factor of 1.0.
  • The live load used to design the exterior beam must never be less than the live load used to design an interior beam.
  • Do not take the live load distribution factor for moment or shear as less than the number of lanes divided by the number of girders, including the multiple presence factor per Article 3.6.1.1.2.
  • When prestressed concrete deck panels or stay-in-place metal forms are allowed, design the beam using the basic slab thickness.
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Design Criteria

Standard beam designs must meet the following requirements:

  • Add and debond strands in the order shown on the XBND standard drawings.
  • Debond strands in 3-ft. increments at beam ends if necessary to control stresses at release.
  • Debonded strands must conform to Article 5.11.4.3 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 is the lesser of:
    • one-half the span length minus the maximum development length;
    • 0.2 times the beam length;
    • 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 Article 5.8. Change stirrup spacing as shown on relevant standard drawings, only if analysis indicates the inadequacy of the standard design.
  • TxDOT standard X-beams satisfy Article 5.8.4 and Article 5.10.10.
  • 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 ksi for beams with < 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.
  • See Section 4, Pretensioned Concrete I Girders, for for other design criteria.

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