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Section 5: Pretensioned Concrete U Beams

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Materials

Use Class H concrete with a minimum = 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.

Do not increase section properties of the beam to account for the transformed area of strands or mild steel.

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Geometric Constraints

The maximum skew angle for U-beam bridges is 45 degrees.

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Structural Analysis

Beam designs must meet the following requirements:

  • Distribute 2/3 of the rail dead load to the exterior beam and 1/3 of the rail dead load to the adjacent interior beam applied to the composite cross section.
  • Each U beam has two interior diaphragms at a maximum average thickness of 13 in. They are located as close as 10 ft. from midspan of the beam. Account for each diaphragm as a 2-kip load for U40 beams and as a 3-kip load for U54 beams applied to the non-composite cross section.
  • Use section properties given on the standard drawings.
  • Calculate composite section properties assuming the beam and slab to have the same modulus of elasticity (for beams with < 8.5 ksi). Do not include haunch concrete placed on top of the beam when determining section properties. 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 of comparable length.

  • For bridges with less than three girders in the cross section, assume the live load distribution factors for flexural moment and shear are equal to the number of lanes divided by the number of girders. Determine the number of lanes as required by Article 3.6.1.1.1.
  • 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.
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Design Criteria

Standard beam designs must meet the following requirements:

  • Stresses at the ends of the beam are controlled with the use of debonding. Draped strands are not permitted in U beams.
  • Debonded strands must conform to 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 is the lesser of:
    • The maximum debonding length is the lesser of: (a) one-half the span length minus the maximum development length; (b) 0.2 times the beam length; or (c) 15 ft.
    • 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).
  • Grouping of U-beam designs are at the discretion of the designer. However, no exterior U beam may have less carrying capacity than that of an interior U beam of equal length. If the designer chooses to group beams, a general rule is to group beams with no more than a four-strand difference.
  • See Section 4, Pretensioned I Girders for other design criteria.
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Detailing

Detail span sheets for a cast-in-place slab with prestressed concrete panels.


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