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Section 4: Rectangular 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.

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Geometric Constraints

Cap depth should be in 3-in. increments and not less than cap width.

For bents supporting Tx62 or Tx70 girders, use a cap width of at least 4.00 ft. and 42 in. drilled shafts. For all other TxGirder types, use a cap width of 3.50 ft. and 36 in. drilled shafts.

For bents supporting Type IV beams, or U beams, use a cap width of at least 3.25 ft. and 36 in. drilled shafts. For bents supporting Type VI or VI(Mod) beams, use a cap width of at least 4.00 ft. and 42 in. drilled shafts. For all other I beam types, use a cap width of at least 2.75 ft. and 30 in. drilled shafts.

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

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Structural Analysis

In lieu of a more detailed analysis, 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 wide, 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 5.7.3.4 using Class 1 exposure for moderate exposure conditions and Class 2 exposure in areas where de-icing agents are frequently used. Limit tensile stress in steel reinforcement, fss under Service I load combination to 0.6 fy.

Check Article 5.7.3.3.2 for minimum reinforcement.

Check service dead load only, and limit reinforcement stress to 22 ksi to further minimize cracking.

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.

Do not use the simplified procedures for determining shear resistance allowed by Article 5.8.3.4.1 nor 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.

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. spacing.

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

For single-column bents, straddle bents, and other non-typical bents, provide minimum stirrups and longitudinal side face reinforcing in the region between each face of column and first girder such that the following are satisfied:

> 0.003 and,

> 0.003

Also, for longitudinal side face reinforcement in these regions, satisfy the requirements of Article 5.7.3.4 for Ask. The longitudinal side face demands of the two independent procedures need not be additive; use the larger value produced by Articles 5.6.3.6 and 5.7.3.4.

See the following illustration for further clarification of the terms Av and Ah:

Clarification of the terms Av and
Ah.. (click in image to see full-size image)

Figure 4-1. Clarification of the terms Av and Ah..

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Detailing

Use #5 stirrups 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.

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 5.7.3.4-2 in caps deeper than 3 ft. Caps 3 ft. and less should have two #5 bars equally spaced in each side face.


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