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Section 17: Spliced Precast Girders

Materials

Use Class H (HPC) concrete for girder elements:

  • Precast Elements:
  • Minimum = 4.0 ksi, Maximum = 6.0 ksi
  • Minimum = 5.0 ksi, Maximum = 10.0 ksi
  • Cast in Place Elements:
  • Maximum = 6.0 ksi

Use Class S concrete for cast in place deck (or Class S (HPC) if de-icing chemicals are routinely used at the site):

  • Maximum = 4.0 ksi

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

  • Use 0.6 in low-relaxation strands for pretensioning strands.
  • Use 0.6 in low-relaxation strands for post-tensioning tendons.

Provide post tension system in accordance with Item 426, “Post Tensioning” of the TxDOT Standard Specifications, with the following exceptions:

  • Non-Severe Corrosive Environments:
  • Galvanized or plastic duct can be used
  • Meet requirements for Protection Level 1B
  • Do not use tape-sealed connections
  • Severe Corrosive Environments:
  • Use plastic duct only
  • Meet requirements for Protection Level 2

All stressed tendons in the finished structure must be grouted. All tendons that are stressed at the precast yard must be grouted prior to transport.

Geometric Constraints

The minimum numbers of girders in any roadway width is as follows:

  • I-Section: 3 girders. If the span is over a lower roadway and the vertical clearance is less than 20 ft., a minimum of 4 girders are required.
  • U-Section: 2 girders.

Structural Analysis

Girder designs must meet the following requirements:

  • Use section properties given on the TxDOT website:
  • I-Section: http://ftp.dot.state.tx.us/pub/txdot-info/brg/long-span-i-girders.pdf
  • U-Section: http://ftp.dot.state.tx.us/pub/txdot-info/brg/long-span-u-girders.pdf Web width of these sections may be varied to optimize the sections in meeting design requirements.
  • Distribute the weight of one railing to no more than three girders.
  • Haunch concrete placed on top of the girder may be considered when determining composite section properties.
  • Composite section properties can be calculated assuming either constant modulus of elasticity for the girders and slab, or transforming the sections based upon their respective modulus. Determine respective modulus in accordance with AASHTO LRFD Bridge Design Specifications, Article 5.4.2.4, based upon unit weight of 0.150 kcf and .
  • Live load distribution can be determined from one of the following methods:
  • Must conform to AASHTO LRFD Bridge Design Specifications, Article 4.6.2.2.2 for flexure moment and Article 4.6.2.2.3 for shear when used in conjunction with a line girder analysis.
  • As determined by use of the lever rule when the span/girder arrangement is out of the applicable range of Articles 4.6.2.2.2 and 4.6.2.2.3 when used in conjunction with a line girder analysis.
  • As distributed by the model when used in conjunction with a grillage, finite element, or other refined model. The model must capture the effects of the complete unit and transfer loads in an acceptable fashion.
  • When prestressed concrete deck panels or stay-in-place metal forms are allowed, design the girder using the basic slab thickness.

Analysis must consider the effects of the following:

  • Staged construction
  • Addition and removal of temporary supports
  • Locked in forces
  • Staged post tensioning
  • Secondary forces due to post tensioning
  • Torsion due to horizontally curved alignments
  • Superstructure / Substructure interaction
  • Temperature variation

Design Criteria

Provide a minimum of two tendons per web.

Use diaphragms at all bearing locations.

Intermediate diaphragm use is not mandatory.

The precast sections must meet the following requirements for transportation:

  • Prestressed Sections:
    • Factor the self-weight load by 1.33
    • Strand stress after seating of chucks is not greater than 0.75 for low-relaxation strands
    • Use the concrete release strength for the following stress limitations:

      Tensile stress < 0.24 (ksi)

      Compressive stress < 0.6 (ksi)

    • Do not drape pretensioning strands. Debond the strands as needed.
  • Non-Prestressed Sections:
    • Factor the self-weight load by 1.33
    • Design the section as a reinforced concrete member, subject to the provisions in AASHTO LRFD Bridge Design Specifications, Article 5.7.3.4. Use the concrete release strength in place of the concrete final strength
    • Limit the stress in the reinforcing steel to 36 ksi

The precast sections must meet the following requirements during construction stages:

  • Factor the self-weight load by 1.0
  • Include loads to represent weight of form work for splices and strong backs (if applicable)
  • Use the final concrete strength for the following stress limitations:

    Tensile stress < 0.24 (ksi)

    Compressive stress < 0.6 (ksi)

The girder must meet the following requirements in the final (service) condition.

  • Use associated final concrete strengths for the precast sections and cast in place splices
  • Use effective prestress force after all short and long term losses. Losses can be determined by hand as outlined in Section 4, Pretensioned Concrete I Beams and I Girders, or by analysis software that has concrete time dependent capabilities to capture the effect of creep and shrinkage.
  • Compressive stress limitations:
    • Service I Loading < 0.6
    • Effective Prestressing and Permanent (Dead) Loading < 0.45
  • Tensile stress limitations:
    • Service III Loading

      Non-Severe Corrosive Environment < 0.19 (ksi)

      Severe Corrosive Environment < 0.09 (ksi)

    • Effective Prestressing and Permanent (Dead) Loading – No tension allowed
  • Principal Tensile stress at Neutral Axis of Web

    Service III Loading < 0.110 (ksi)

All post tensioning must be done prior to placement of the deck.

The composite deck is not a prestressed element and is not held to the stress limitations listed above.

The deck must meet the following requirements:

  • Design Load includes effects due to the following:
    • Pouring sequence
    • Superimposed loads applied to composite section of Service III. Exclude the effects of creep and shrinkage of deck concrete.
  • Longitudinal steel must meet the following requirements:
    • Tensile stress in deck concrete is less than (0.9)(0.24) (ksi), use No. 4 bars at 9-in spacing
    • Tensile stress in deck concrete is greater than (0.9)(0.24) (ksi), deck reinforcement must equal or exceed 1% of the gross deck cross-sectional area (do not use bars larger than No. 6):

Design shear based upon Strength I Loading for the final condition and in accordance with Article 5.8.3.3 of the AASHTO LRFD Bridge Design Specifications. Use the General Procedure as provided by Article 5.8.3.4.2. Do not use provisions of Article 5.8.3.4.3 or Appendix B of the AASHTO LRFD Bridge Design Specifications. The effective web width must be reduced by 25% of the outer diameter of the splice coupler.

Design ultimate moment based upon Strength 1 Loading for the final condition.

Refer to Section 4, Pretensioned Concrete I Beams and I Girders for interface shear design of the deck to girder flange interface.

Show predicted slab deflections should be shown on the plans. Compute deflections using the same composite sections (constant modulus for girder and deck, or transformed sections) used in the analysis. Denote on plans the assumed modulus (if constant is used) or the assumed values of of the individual elements.

Included in plans the assumed construction sequence that includes the following:

  • Order of construction
  • Shore tower locations
  • Shore tower loads
  • Lifting / support points of precast members
  • Girder elevation points
  • Post tensioning sequence

Require contractor to provide a temporary bracing plan of the girders.

Require contractor to provide shoring and erection plan.

Detailing

Provide 2-in. clear cover to reinforcing steel for entire cross section. Also, increase top slab clear cover to 2.5 in. in areas of state where de-icing agents are frequently used.

Provide a minimum tangent length, dependent on duct size and type, of tendon from the anchorage head before introducing any curvature. Determine minimum radius of curvature for individual duct sizes based on published values from suppliers.

Reference Item 426 “Post Tensioning” in the General Notes for all post tensioning, grouting materials, and construction. Note exceptions if Protection Level 1B is used in the design (galvanized duct allowed)


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