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.