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Section 10: Pretensioned Concrete Double-Tee Beams

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Materials

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

Use non-shrink cementitious grout for shear keys.

Design beams with 0.5-in. low-relaxation strands. Use 0.6-in. low-relaxation strands as necessary.

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

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Geometric Constraints

Do not use double-tee beams on skewed bridges.

A 5-in. minimum thickness composite concrete slab or 2-in. minimum thickness asphaltic concrete pavement (ACP) overlay is required.

Six-ft. wide beams are required for fascia positions when beams are not topped with a 5-in. slab.

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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.
  • Use section properties provided on the pretensioned concrete double-tee beam standard drawings.
  • Composite section properties may be calculated assuming the beam and composite concrete slab overlay have the same modulus of elasticity (for beams with < 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.
  • Regardless of topping, live load distribution factors for all beams, both interior as well as exterior, and both moment and shear, must conform to Table 4.6.2.2.2b-1, using cross section (i) if beams are connected only enough to prevent relative vertical displacement at their interfaces. Use K = 2.2 when determining the live load distribution factor. Use S/10 as maximum limit on live load 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.
  • For interior as well as exterior girders, do not take the live load distribution factor, for moment or shear, as less than ,

    Where,

    • m = multiple presence factor per Article 3.6.1.1.2
    • = number of lanes
    • = number of beams or girders
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Design Criteria

Standard beam designs must meet the following requirements:

  • Strands should be added on a 2-in. grid system by filling row “2” and working up. In HT beams, fill inner two positions then outer two positions of each web, in each row. Strands in row “2” are to remain straight, except inner position of HT beams which may be depressed.
  • Use hold-down points placed symmetrically 5 ft. from centerline of beam.
  • The end position of depressed strands should be as low as possible so that the position of the strands does not control the release strength. Release strength is occasionally controlled by end conditions when the depressed strands have been raised to their highest possible position.
  • Calculate required stirrup spacing for #3 Grade 60 bars according to the Article 5.8. TxDOT standard double-tee beams satisfy Article 5.8.4.
  • For double-tee 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 < 8.5 ksi. Show predicted slab deflections on the plans even though field experience indicates that actual deflections are generally less than predicted. Use the deflection due to slab weight only times 0.8 for calculating haunch depth.
  • TxDOT standard double-tee beams satisfy Article 5.10.10.
  • Connect adjacent beams with 1-in. diameter by 11 in. long, smooth lateral connector rods spaced at 5 ft. maximum, with the first lateral connectors set 5 ft. from bent centerlines.
  • See Section 6, Pretensioned Concrete I Beams and I Girders for other design criteria.

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