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Section 6: Pretensioned Concrete Slab 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.

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Geometric Constraints

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

The minimum gap between adjacent slab beams is 0.5 in. and the maximum gap is 3.31 in. A preferable gap range is 1 in. to 1.5 in.

A 5-in. minimum thickness composite concrete slab is required.

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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 slab beam standard drawings.
  • Composite section properties may be calculated assuming the beam and slab overlay 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 all beams, both moment and shear, must conform to Table 4.6.2.2.2b-1, using cross section (g), if the beams are connected only enough to prevent relative vertical displacement at their interfaces. This is called S/D distribution.
  • Do not apply the skew correction factors for moment as suggested in Article 4.6.2.2.2e nor for shear as suggested in Article 4.6.2.2.3c.
  • 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:

  • Add strands in the order shown on the PSBND standard drawing.
  • Debond strands in 3-ft. increments at beam ends if necessary to control stresses at release.
  • 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: (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).
  • Calculate required stirrup spacing for #4 Grade 60 bars according to Article 5.8. Change stirrup spacing as shown on relevant standard drawings only if analysis indicates inadequacy of the standard design.
  • TxDOT standard slab 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 other design criteria.

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