Section 9: Pretensioned Concrete Box Beams

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

Use Class S concrete ( fc ́ = 4.0 ksi) for shear keys.

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

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Geometric Constraints

The maximum skew angle for box beam bridges is 30° without modification to standard drawings.

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

A 5-in. minimum thickness composite concrete slab overlay is required. A 2-in. minimum thickness asphaltic concrete pavement (ACP) overlay can be used in lieu of the concrete deck for low-volume roadways at the discretion of the District.

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Structural Analysis

Beam designs must meet the following requirements:

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  • Distribute the weight of one railing to no more than three beams.
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  • Use section properties given on the Prestressed Box Beams standard drawings.
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  • Composite section properties may be calculated assuming the beam and composite concrete slab overlay have the same modulus of elasticity (for beams with fc ́ < 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.
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  • Live load distribution factors must conform to Article and Article Use:
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    • Cross section (f) with bridges having a composite concrete slab
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    • Cross section (g) with bridges having ACP applied directly to tops of beams, assuming beams are sufficiently connected to act as a unit.
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  • Do not apply the skew correction factor for moment as suggested in Article
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  • 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
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Design Criteria

Standard beam designs must meet the following requirements:

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  • Add and debond strands in the order shown on the BBND standard drawings.
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  • Debond strands in 3-ft. increments at beam ends if necessary to control stresses at release.
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  • Debonded strands must conform to Article except as noted below:
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    • Debond no more than 75% of the total number of strands.
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    • Debond no more than 75% of the number of strands in that row.
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    • 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.
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    • 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).
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  • Calculate required stirrup spacing for #4 Grade 60 bars according to Article 5.7. Change stirrup spacing as shown on relevant standard drawings only if analysis indicates the inadequacy of the standard design.
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  • TxDOT standard box beams satisfy Article 5.7.4 and Article
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  • 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 ksi for beams with fc ́ < 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.
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  • 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.
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  • 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, available from the Bridge Division on request.
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  • See Section 4, Pretensioned Concrete I-Girders, for other design criteria.
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