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Section 8: Post-Tensioned Concrete Bent Caps

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Materials

Use TxDOT Class H concrete with a minimum = 5.0 ksi, and Grade 60 reinforcing steel. Higher steel grades may be used if needed in special cases.

Use 0.5-in. or 0.6-in. low-relaxation prestressing strand with a specified tensile strength, , of 270 ksi or high-strength steel bars meeting ASTM A722. All tendons and bars must be bonded.

Use duct as specified in Item 426, “Post-Tensioning,” of the Texas Standard Specifications.

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Geometric Constraints

Criteria in this section are not intended for C-shaped bents or through-girder bents.

Cap depth cannot be less than cap width unless the cap is widened for the purposes of:

  • meeting minimum support length per Article 4.7.4.4, or
  • accommodating a cap-to-column connection when one or both elements are precast, or
  • satisfying vertical clearance needs to a lower roadway.

See Section 6 for minimum cap widths, and column and drilled shaft dimensions based on beam or girder type.

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Structural Analysis

Column-to-cap connection details must reflect assumptions of fixity made for post-tensioned cap design. Column stiffness and secondary force effects due to post-tensioning must be considered.

Distribute the live load to the beams assuming the slab hinged at each beam except the outside beam.

Base live load reactions per lane on the combined effect of the truck loading added to the lane loading.

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Design Criteria

Round losses to 1 ksi. Determine prestress losses from elastic shortening, creep, shrinkage, and relaxation as prescribed for prestressed concrete in Chapter 3, Section 4, Pretensioned Concrete I Girders.

Use a minimum of 4 tendons in a cap.

Satisfy all stress limits for prestressing tendons and concrete as specified in Articles 5.9.3 and 5.9.4. Use limits for severe corrosive conditions in areas where de-icing agents are frequently used or where contact with salt water spray is possible.

Check limit states using the Strength I load combination and the Service I load combination for both tension and compression stress.

For multi-column bent caps, take design negative moments at the center line of the column. For hammerhead bents and multi-column bent caps with columns 4 ft. or wider, take design negative moments at the effective face of the column.

Do not use the simplified procedure for determining shear resistance allowed by Article 5.8.3.4.3. Use the General Procedure as provided by Article 5.8.3.4.2.

Minimize the number of stirrup spacing changes.

For typical multi-column bent caps supporting multiple beams, strut-and-tie modeling provisions of Articles 5.6.3 need not be considered. For bent caps supporting girders on high load multi rotational bearings or girders with large reaction forces that are defined as deep components according to Article 5.2, use the strut-and-tie design.

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Detailing

Use #5 stirrups, except as noted, with a 4-in. minimum and a 12-in. maximum spacing. Do not use stirrups larger than #6. Use double stirrups if required spacing is less than 4 in. If torsional resistance is explicitly addressed in the design, ensure the stirrup detailing is consistent with AASHTO requirements.

Provide a minimum 5ft. tangent length of tendon from the anchorage head before introducing any curvature. Determine minimum radius of curvature of duct based on published values from suppliers for individual duct sizes.

Use minimum duct spacing according to Article 5.10.3.3.

Provide elevation and plan views showing the profile of centerline of ducts.

Provide a separate elevation view, or table of dimensions, for the centroid of the tendon group. Show location and magnitude of final post-tensioning force at critical section.

Provide the stressing and erection sequence on the plans, including form removal and girder placement. Bottom cap forms must stay in place until after first tendons are stressed.

Include the following information on the cap detail sheets:

  • Assumed coefficient of friction and wobble coefficient for duct
  • Assumed anchor set
  • Assumed and maximum allowed eccentricity between duct and tendon
  • Assumed long term losses
  • Stressing and dead ends of tendon
  • Jacking force = 0.77 x x = xx kips

Provide alternate reinforcing steel details where a known conflict between duct and typical reinforcing steel will occur. Include notes indicating all other adjustments to reinforcing steel must be made as directed by the Engineer of Record.


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