Section 5: Inverted Tee Reinforced Concrete Bent Caps

Materials

Use concrete with a minimum

f_{c}^{'}

of 3.6 ksi and Grade 60 reinforcing steel. Higher concrete strengths may be required in special cases. In highly congested bent caps due to reinforcement spacing, limit aggregate size of the concrete mix to a size that meets the minimum required reinforcement spacing in Article 5.10.3.1 based on the spacing provided.

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

Geometric Constraints

Minimize the variations in inverted tee cross section dimensions within a project.

Keep the top of stem below the bottom of the slab as shown on the Miscellaneous Slab Standard sheet for the girder type used; see standard drawings IGMS, SGMS, and UBMS.

Ledge width must meet the corbel width requirements shown on the Bearing Detail standard sheets; see standard drawings (i.e. IGEB and UBEB).

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., use 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.

Design Criteria

Check limit states using the Strength I and Service I load combinations. Check distribution of reinforcement as required in Article 5.6.7 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. Limit tensile stress in steel reinforcement,

f_{s s}

under Service I load combination to 0.6

f_{y}

For reinforced concrete straddle bents, check the calculated shear, using the Service I Load Combination, against the resistance from Equation C5.8.2.2-1.

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.

Limit

f_{y}

to 60 ksi for ledge and hanger resistance calculations in Articles 5.8.4.3 and 5.8.4.2.

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.

Replace Equation 5.8.4.3.5-1 with the following:

V_{n} = \frac{A_{a r} (\frac{2}{3} f_{y})}{s} (W + 3 a_{v})

with

f_{y}

not taken larger than 60 ksi.

The edge distance between the exterior bearing pad and the end of the inverted T-beam stem shall not be less than 12 inches.

Detailing

Provide extra vertical and horizontal reinforcing across the end surfaces of the stem to resist cracking. Single #5 bars, anchored at each end with hooks, 6-in. (+/-) spacing for vertical bars, and horizontal bars spaced with horizontal temperature and shrinkage bars 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.

Provide diagonal #7 bars at 6-in. spacing for 2’-6” at the end of ledge. Place the reinforcing such that it is developed at the point it intersects the crack at the ledge-stem interface. See the Bridge Detailing Guide
for more information on the bar detail.

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.