Section 5: Pretensioned Concrete U Beams
Anchor: #i1372088Materials
Use Class H concrete with a minimum f_{ci} ́ = 4.0 ksi and f_{c} ́ = 5.0 ksi and a maximum f_{ci} ́= 6.0 ksi and f_{c} ́ = 8.5 ksi.
Use prestressing strand with a specified tensile strength, f_{pu} of 270 ksi.
Anchor: #i1372515Geometric Constraints
The maximum skew angle for Ubeam bridges is 45°.
Anchor: #i1372537Structural Analysis
Beam designs must meet the following requirements:
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 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. Anchor: #ONAJQJJI
 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 2kip load for U40 beams and as a 3kip load for U54 beams applied to the noncomposite cross section. Anchor: #DCSLMNEH
 Use section properties given on the standard drawings. Anchor: #XTQYRNAX
 Calculate composite section properties assuming the beam and slab to have the same modulus of elasticity (for beams with f_{c} ́ < 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. Anchor: #GQOIWXDT
 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. Anchor: #ITPYBRAM
 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:

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 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. Anchor: #MYOXTCQI
 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. Anchor: #GIJIBUMJ
 When the clear roadway width is less than 20.0 ft., design for one lane loaded with a multiple presence factor of 1.0.
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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.
Anchor: #QRFAATOA  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. Anchor: #XFSEBLXK
 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.
Design Criteria
Standard beam designs must meet the following requirements:
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 Stresses at the ends of the beam are controlled with the use of debonding. Draped strands are not permitted in U beams. Anchor: #NSVOYTFJ
 Debonded strands must conform to Article
5.9.4.3.3 except as noted below:
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 Debond no more than 75% of the total number of strands. Anchor: #XXFAQGYH
 Debond no more than 75% of the number of strands in that row. Anchor: #OGVYJFDA
 The maximum debonding length is the lesser of: (a) onehalf the span length minus the maximum development length; (b) 0.2 times the beam length; or (c) 15 ft. Anchor: #TVURFKLV
 Not more than 75% 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).
Anchor: #BIDDCBUM  Grouping of Ubeam 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 fourstrand difference. Anchor: #LJYWJAOU
 See Section 4, Pretensioned IGirders for other design criteria.
Detailing
Detail span sheets for a castinplace slab with prestressed concrete panels.