Section 16: Spliced Precast Girders

Materials

Use Class H (HPC) concrete for girder elements:
  • Precast Elements:
    • Minimum fci'
      = 4.0 ksi, Maximum fci'
      = 6.0 ksi
    • Minimum fc'
      = 5.0 ksi, Maximum fc'
      = 10.0 ksi
  • Cast in Place Elements:
    • Maximum fc'
      = 6.0 ksi
Use Class S concrete for cast in place deck. Use Class S (HPC) if de-icing chemicals are routinely used at the site:
  • Maximum fc'
    = 4.0 ksi
Use prestressing strand with specified tensile strength,
fpu
 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
Texas Department of Transportation Standard Specifications for Construction and Maintenance of Highways, Streets, and Bridges, 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 permanent 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 should meet the following requirements:
  • Base the self-weight of the girder on a minimum of 160 pcf.
  • For I-Sections: Distribute the weight of one railing to no more than three girders, applied to the composite cross section.
  • For U-Sections: Distribute 2/3 of the rail dead load to the exterior beam and 1/3 of the rail dead load to the adjacent interior beam applied to the composite cross section.
  • 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.
  • Live load distribution can be determined from one of the following methods:
    • Must conform to 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; or
    • 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.
  • 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 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.
  • 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.
  • 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.
Provide a full depth diaphragm at all splice and anchorage locations. Diaphragms may be eliminated at these locations if all of the following are met:
  • CIP splice details do not promote honeycombing or constructability issues.
  • Lateral stability from a combination of permanent and/or temporary diaphragms is evaluated for the deck construction stage.
  • The superstructure system is demonstrated to successfully transmit lateral load to the substructure without global or local load effect issues.
  • Live load distribution for flexure and shear in the main girders considers the lack of these diaphragms.
Intermediate diaphragm use is not mandatory.
When providing pre-tensioning in addition to post-tensioning, debonded strands must conform to Article 5.9.4.3.3 except as noted below:
  • Debond no more than 75% of the total number of strands.
  • Debond no more than 75% of the number of strands in that row.
  • Replace Restriction B with, not more than 75% of the debonded strands, or 10 strands, whichever is greater, shall have the debonding terminated at any section.
  • Replace Restriction C with, longitudinal spacing of debonding termination locations shall be the larger of 36 inches or 60 db
    apart.
  • Do not design for Restriction E.
  • For I-Sections, replace Restriction I with:
    • Bond strands placed within the horizontal limits of the web, when strands are located in the web.
    • Uniformly distribute debonded strands.
    • Bond the outer-most strand in each row.
  • For U-Sections, replace Restriction J with:
    • Uniformly distribute debonded strands.
    • Bond the outer-most strand in each row.
Prestressed, precast sections must meet the following at release:
  • Use the concrete release strength fci'
    for the following stress limitations:
    • Tensile stress < 0.24λ f'ci
      (ksi)
    • Compressive stress < 0.65 fci'
      (ksi)
  • Do not drape pretensioning strands. Debond the strands as needed.
  • Strand stress after seating of chucks is limited to 0.75 fpu
    for low-relaxation strands.
  • Use an effective strand stress after release of 0.75 fpu- ΔfpES
The precast sections must meet the following requirements for transportation:
  • Prestressed Sections:
    • Factor the self-weight load by 1.33.
    • Use the concrete release strength fci'
      for the following stress limitations:
      • Tensile stress < 0.24λ f'ci
        (ksi)
      • Compressive stress < 0.65 fci'
        (ksi)
  • Non-Prestressed Sections:
    • Factor the self-weight load by 1.33.
    • Design the section as a reinforced concrete member, subject to the provisions in Article 5.6.3. Use the concrete release strength fc'
      in place of the concrete final strength fc'
    • 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).
  • Tendon stress before anchor set is limited to the lesser of 0.77fpu and the stress limits in Article 5.9.2.2 for low-relaxation strands.
  • Use the final concrete strength fc'
    for the following stress limitations:
    • Tensile stress < 0.24λ fc'
      (ksi)
    • Compressive stress < 0.6 fc'
The girder must meet the following requirements in the final (service) condition.
  • Use associated final concrete strengths fc'
    for the precast sections and cast in place splices.
  • Use effective prestress force after all short and long-term losses. Losses can be calculated by hand as outlined in Chapter 3 – Superstructure Design, Section 4 - Pretensioned Concrete 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 fc'
    • Effective Prestressing and Permanent (Dead) Loading < 0.45 fc'
    • Fatigue I live loads plus one-half of the sum of stresses due to prestress and permanent (dead) loads < 0.40 fc'
  • Tensile stress limitations:
    • Service III Loading
      • Non-Severe Corrosive Environment < 0.19λ fc'
        (ksi) ≤ 0.6ksi
      • Severe Corrosive Environment < 0.09λ fc'
        (ksi) ≤ 0.3ksi
    • Effective Prestressing and Permanent (Dead) Loading – No tension allowed
  • Principal Tensile stress limitations:
    • Service III Loading < 0.110λ fc'
      (ksi)
Evaluate principle tensile stresses using section properties that account for the presence of post-tensioning ducts in their ungrouted and grouted conditions.
All post tensioning must be done prior to placement of the deck. Post-tensioning after the deck is placed is permitted if a viable re-decking strategy is provided.
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)λ fc'
      (ksi), use No. 4 bars at 9 in. spacing.
    • Tensile stress in deck concrete is greater than (0.9)(0.24)λ fc'
      (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 5.7.3.3. Use the General Procedure as provided by Article 5.7.3.4.2. Do not use the provisions of Section 5, Appendix B. When the effective web width must be reduced, reduce it by 25% of the outer diameter of the splice coupler for grouted ducts. Apply a shear strength reduction factor for the presence of grouted post-tensioning ducts as outlined in Article 5.7.3.3.
Only apply the requirement in Article 5.7.3.5 from inside face of support to inside face of support. Do not calculate from the inside face of support to the end of the beam.
Design ultimate moment based upon Strength 1 Loading for the final condition.
Refer to Chapter 3 – Superstructure Design, Section 4 - Pretensioned Concrete I-Girders, for interface shear design of the deck to girder flange interface.
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 fc'
of the individual elements.
Include 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
  • Final girder elevation points
  • Post tensioning sequence
  • Jacking stresses for prestressing strand and post-tensioned tendons
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).
Provide anchorage zone details per Article 5.9.5.6 for the post-tensioning forces and Article 5.9.4.4 for the pretensioning forces. To determine the total required reinforcing for the anchorage zone, combine the required reinforcing for both the post-tensioned anchorage zone and the pretensioned anchorage zone. Provide anchor zone reinforcing at each debond section, for the strands terminated there.