Section 4: Rectangular Reinforced Concrete Bent Caps

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Use TxDOT Class C concrete ( fc ́ = 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

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

For bents supporting Tx70 girders, use a cap width of at least 4.00 ft. and 42 in. columns and drilled shafts as a minimum.

For bents supporting U40 or U54 beams, use a cap width of at least 3.25 ft. and 36 in. columns and drilled shafts as a minimum.

For all other structure types refer to the bridge standard drawings for minimum cap widths.

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Structural Analysis

In lieu of a more detailed analysis, it is permissible to analyze trestle pile and 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.

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

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 in areas where de-icing agents are frequently used or where contact with salt spray is possible. Limit tensile stress in steel reinforcement, fss under Service I load combination to 0.6 fy.

Check Article for minimum reinforcement.

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.

Ignore requirements in Article requiring that concentrated loads located within dv from the face of support, the shear load and shear resistance shall be calculated at the face of the support. Loads close to the support are transferred directly to the support by compressive arching action without causing additional stresses in the stirrups.

Except for hammerhead bents, shear need not be considered in cantilever regions unless the distance from center of load to effective face of column exceeds 1.2d. Provide stirrups at 6-in. maximum spacing.

For typical multi-column bent caps supporting multiple beams, strut-and-tie modeling provisions of Articles 5.8.2 need not be considered. For bent caps supporting girders on high load multi rotational (HLMR) 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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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.

For flexural reinforcement, use #11 bars. Smaller bars can be used to satisfy development requirements. Do not mix bar sizes.

Use longitudinal skin reinforcement in accordance with Equation 5.6.7-3 in caps deeper than 3 ft. Caps 3 ft. and less should have two #5 bars, as a minimum, equally spaced in each side face.

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