Section 10: Pretensioned Concrete Spread Box Beams (XBeams)
Anchor: #i1557238Materials
Use Class H concrete with a minimum of 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: #i1557263Geometric Constraints
The maximum skew angle for Xbeam bridges is 30° without modification to standard drawings.
Anchor: #i1557287Structural Analysis
Beam designs must meet the following requirements:
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 Distribute the weight of one railing to no more than three beams, applied to the composite cross section. Anchor: #BFWVDCDY
 Use section properties given on the Prestressed Concrete XBeams standard drawings. Anchor: #EVOHRCVP
 Composite section properties may be calculated assuming the beam and composite slab have the same modulus of elasticity (for beams with f_{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; however, this design assumption must be noted on the plans. Anchor: #WALRSADL
 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: #WFKCSKJR
 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: #DDPPKACQ
 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: #BLNIYFSK
 When the clear roadway width is less than 20.0 ft., design for one lane loaded with a multiple presence factor of 1.0.
Anchor: #MUQBVKNM  The live load used to design the exterior beam must never be less than the live load used to design an interior beam. Anchor: #BXYJIQKF
 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. Anchor: #TENQHPDE
 When prestressed concrete deck panels or stayinplace metal forms are allowed, design the beam using the basic slab thickness.
Design Criteria
Standard beam designs must meet the following requirements:
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 Add and debond strands in the order shown on the XBND standard drawings. Anchor: #JIXVMIED
 Debond strands in 3ft. increments at beam ends if necessary to control stresses at release. Anchor: #SDCQFRSJ
 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: #YWKEVCLM
 Debond no more than 75% of the number of strands in that row. Anchor: #LBRQAFKN
 The maximum debonding length is the lesser
of:
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 onehalf the span length minus the maximum development length; Anchor: #DASGDPGB
 0.2 times the beam length; Anchor: #LSVQSGCC
 15 feet.
Anchor: #KBDDVGBJ  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: #VTTLCLEY  Calculate required stirrup spacing for #4 Grade 60 bars according to the Article 5.7. Change stirrup spacing as shown on relevant standard drawings, only if analysis indicates the inadequacy of the standard design. Anchor: #HNYVSNTR
 TxDOT standard Xbeams satisfy Article 5.7.4 and Article 5.9.4.4. Anchor: #UTSOBAMM
 Compute deflections due to slab weight and composite dead loads assuming the beam and slab to have the same modulus of elasticity. Assume E_{c} = 5,000 ksi for beams with f_{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. Anchor: #CWCVPISY
 See Section 4, Pretensioned Concrete IGirders, for other design criteria.