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Section 4: Pretensioned Concrete I Girders

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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 minimum number of I-girders in any roadway width is four if the span is over a lower roadway and the vertical clearance is less than 20 feet. Otherwise, a minimum of three I-girders per span may be used.

Intermediate diaphragms are not required for structural performance. Do not use intermediate diaphragms unless required for erection stability of girder sizes extended beyond their normal span limits.

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Structural Analysis

Girder designs must meet the following requirements:

  • Distribute the weight of one railing to no more than three girders, applied to the composite cross section.
  • Use section properties given on the standard drawings.
  • Composite section properties may be calculated assuming the girder and slab to have the same modulus of elasticity (for girders with < 8.5 ksi). Do not include haunch concrete placed on top of the girder when determining section properties. Section properties based on final girder and slab modulus of elasticity may also be used, however, this design assumption must be noted on the plans.
  • Live load distribution factors must conform to Article 4.6.2.2.2 for flexural moment and Article 4.6.2.2.3 for shear, except as noted below:
    • For exterior girder design with a slab cantilever length equal to or less than one-half of the adjacent interior girder spacing, treat the exterior girder as if it were an interior girder to determine the live load distribution factor for the interior girder. The slab cantilever length is defined as the distance from the center line of the exterior girder to the edge of the slab.
    • For exterior girder design with a slab cantilever length exceeding one-half of the adjacent interior girder spacing, use the lever rule with the multiple presence factor of 1.0 for single lane to determine the live load distribution.
  • 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.
  • Do not use the special analysis based on conventional approximation for loads on piles per Article C4.6.2.2.2d, unless the effectiveness of diaphragms on the lateral distribution of truck loads is investigated.
  • 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.
  • When prestressed concrete deck panels or stay-in-place metal forms are allowed, design the beam using the basic slab thickness.
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Design Criteria

Standard girder designs must meet the following requirements:

  • Add and drape strands in the order shown on the standard drawing IGND.
  • Straight strand designs with and without debonding are permitted provided stress and other limits noted below are satisfied.
  • Debonded strands must conform to Article 5.11.4.3 except as noted below:
    • 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).
  • Use hold-down points shown on the standard drawing IGD.
  • Strand stress after seating of chucks is limited to 0.75 for low-relaxation strands.
  • Initial tension stress up to 0.24 (ksi) is allowed for all standard TxDOT I-girder sections.
  • Initial compression stress up to 0.65 (ksi) is allowed.
  • Final stress at the bottom of girder ends need not be checked except when straight debonded strands are used or when the effect of the transfer length of the prestressing strand is considered in the analysis.
  • Final tension stress up to 0.19 (ksi) is allowed.
  • The required final concrete strength () is typically based on compressive stresses, which must not exceed the following limits:
    • 0.60 for stresses due to total load plus effective prestress.
    • 0.45 for stresses due to effective prestress plus permanent (dead) loads.
    • 0.40 for stresses due to Fatigue I live loads plus one-half of the sum of stresses due to prestress and permanent (dead) loads.
  • Tension stress up to 0.24 is allowed for checking concrete stresses during deck and diaphragm placement.
  • Use an effective strand stress after release of
  • Keep the end position of depressed strands as low as possible so that the position of the strands does not control the release strength. Release strength can be controlled by end conditions when the depressed strands have been raised to their highest possible position.
  • Do not use the simplified procedure for determining shear resistance as allowed by Article 5.8.3.4.3. Use the General Procedure as provided by Article 5.8.3.4.2. Do not use provisions of Appendix B5 of the AASHTO LRFD Bridge Design Specifications.
  • Calculate required stirrup spacing for #4 Grade 60 bars according to the Article 5.8. Change stirrup spacing as shown on standard drawing IGD for I girders only if analysis indicates the inadequacy of the standard design.
  • Replace Equation 5.8.4.2-1 with the following:

    where is the first moment of the area of the slab with respect to the neutral axis of the composite section.

    Take , width of the interface, equal to the beam top flange width. Do not reduce to account for prestressed concrete panel bedding strips.

  • Determine interface shear transfer in accordance with Article 5.8.4. Take cohesion and friction factors as provided in Article 5.8.4.3 as follows:
    • c = 0.28 ksi
    • = 1.0
    • = 0.3
    • = 1.8 ksi
  • Replace Equation 5.4.2.3.2-2 with the following:

    = 1.45 - 0.13 (V/S) > 0.0

  • Compute deflections due to slab weight and composite dead loads assuming the girder and slab to have the same modulus of elasticity. Assume = 5,000 ksi for girders with < 8.5 ksi. Show predicted slab deflections on the plans although 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.
  • TxDOT standard I girders reinforced as shown on the IGD standard drawings are adequate for the requirements of Article 5.10.10.
  • A calculated positive (upward) camber is required after application of all permanent (dead) loads.

Use the following equations to determine prestress losses:

  • Total prestress losses,
  • Elastic shortening,

  • Shrinkage loss,

  • Creep loss,, where

  • Relaxation loss,

Use of AASHTO LRFD Bridge Design Specifications 2004, 3rd E., Article 5.9.5, Loss of Prestress, is also allowed (available from the Bridge Division). Other methods to determine prestress losses are not allowed.


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