Section 9: Post-Tensioned Concrete Bent Caps
Materials
Use concrete with a minimum
= 5.0 ksi, and Grade 60 reinforcing steel. Higher concrete strengths and steel grades may be used if needed in special cases.
= 5.0 ksi, and Grade 60 reinforcing steel. Higher concrete strengths and steel grades may be used if needed in special cases.
Use 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.
, of 270 ksi or high-strength steel bars meeting ASTM A722. All tendons and bars must be bonded.
Provide post tensioning system in accordance with Item 426, “Post-Tensioning” of the
TxDOT Standard Specifications for Construction and Maintenance of Highways, Streets, and Bridges
.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
- accommodating a cap-to-column connection when one or both elements are precast
- satisfying vertical clearance needs to a lower roadway
See Chapter 4 – Substructure Design, Section 4 - Rectangular Reinforced Concrete Bent Caps, for minimum cap widths as well as column and drilled shaft dimensions based on beam or girder type.
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.
Design Criteria
Tendon stress before anchor set is limited to the lesser of 0.77
and the stress limits in Article 5.9.2.2 for low-relaxation strands.
and the stress limits in Article 5.9.2.2 for low-relaxation strands.
Round losses to 1 ksi. Determine prestress losses from elastic shortening, creep, shrinkage, and relaxation as prescribed for prestressed concrete in Chapter 3 – Superstructure Design, 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 5.9.2. 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.
Minimize the number of stirrup spacing changes.
For typical multi-column bent caps supporting multiple beams, strut-and-tie modeling provisions of Articles 5.8 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 in accordance with Article 5.8.
Follow the provisions in Article 5.9.5 for Post-Tensioning Details.
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 5 ft. 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 5.9.5.1.
Provide elevation and plan views showing the profile of centerline of ducts.
Show location of critical section(s) (i.e. the location of maximum flexural demand). Provide the magnitude of the initial stress in each tendon after anchor set at the critical section(s). Provide the assumed, post-tensioning stress, long-term loss of each tendon at the critical section(s).
Provide the stressing and erection sequence on the plans, including form removal and girder placement. Define when the bottom cap forms can be removed, based on the construction sequence designed.
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.XX x
x
= yy kips; replace “XX”, “
”, “
”, and “yy” with the values used in design. - Include stressing sequence; including constraints on partial stressing
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.
Include notes indicating that post-tensioning system/stressing sequence shop drawings must be submitted, reviewed, and approved by the Engineer of Record.