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Section 5: Inverted Tee Reinforced Concrete Bent Caps

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Materials

Use TxDOT Class C concrete ( = 3.6 ksi) and Grade 60 reinforcing steel. Higher concrete strengths may be required in special cases.

Higher reinforcing steel grades may be used provided their use satisfies requirements in AASHTO.

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Geometric Constraints

Make inverted tee dimensions the same for all bents supporting the same girder depth on the project.

Keep the top of stem at least 2.5 in. below the bottom of the slab; see standard drawing IGMS.

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Structural Analysis

In lieu of a more detailed analysis, it is permissible to analyze multiple-column caps as simply supported beams on knife-edge supports at the center of piling or columns. If the column is wider than 4 ft., consider a model that takes the stiffness of the column into consideration.

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

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Design Criteria

Check limit states using the Strength I and Service I load combinations. Check distribution of reinforcement as required in Article 5.7.3.4 using Class 1 exposure for moderate exposure conditions and Class 2 exposure for areas where de-icing agents are frequently used or where contact with salt water spray is possible.

Check Article 5.7.3.3.2 for minimum reinforcement.

For reinforced concrete straddle bents, the calculated shear, using the Service I Load Combination, shall be less than the web cracking load as given by the following equation:

, but not greater than or less than . Note that

in these equations is in units of psi, not ksi.

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. wide or wider, take design negative moments at the effective face of the column.

Minimize the number of stirrup spacing changes.

The punching shear resistance and hanger reinforcement provided at fascia girders must equal or exceed the factored punching shear demand and hanger reinforcement requirements of the adjacent interior girder.

When designing for beam ledge punching shear, replace Article 5.13.2.5.4 with the following:

  • Use df, not de, in all ledge punching shear calculations.
  • The truncated pyramids assumed as failure surfaces for punching shear, as illustrated in Figure 5.13.2.5.4-1, shall not overlap. Therefore, must be greater than to prevent longitudinal overlap. S must be greater than to prevent transverse overlap. If overlap does not occur with the minimum corbel depth, , needed to satisfy punching shear resistance requirements, the corbel depth can be increased without further concern with overlap.
  • Normal punching shear resistance, , kip, shall be taken as:
    • At interior pads:
    • At exterior pads , except shall not be taken greater than .
    • The distance between the bearing pad and corbel face may be included in the punching shear perimeter in the above equations.
    • Adjust the above equations as appropriate if round bearings are used.
  • Replace Equation 5.13.2.5.5-1 with the following:

    , with not taken larger than 60 ksi.

  • Replace the following sentence in Article 5.13.2.5.5: “The edge distance between the exterior bearing pad and the end of the inverted T-beam shall not be less than de" with the following: "The edge distance between the exterior bearing pad and the end of the inverted T-beam shall not be less than 12 inches.”
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Detailing

Provide extra vertical reinforcing across the end surfaces of the stem to resist cracking. Single #6 bars, anchored at each end with hooks and with spacing equivalent to the cap shear and hanger reinforcement, are considered adequate for this purpose for conventional inverted tee cap ends. Do not weld bars together for development of ledge reinforcing. Use anchorage hooks to develop ledge reinforcing.

Use stirrups with a 12-in. maximum spacing. If torsional resistance is explicitly addressed in the design, ensure the stirrup detailing is consistent with AASHTO requirements.

Use longitudinal skin reinforcement in accordance with Equation 5.7.3.4-2 in. caps deeper than 3 ft. Caps 3 ft. and less need at least two #5 bars, as a minimum, equally spaced in each side face.


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