Section 4: Rectangular Reinforced Concrete Bent Caps
Anchor: #i1350886Materials
Use TxDOT Class C concrete (ƒ'c = 3.6 ksi) and Grade 60 reinforcing steel. Higher concrete strengths may be required in special cases.
Anchor: #i1350925Geometric Constraints
Cap depth should be in 3-in. increments and not less than cap width.
For caps supporting Type IV beams, Type VI beam, or U beams, do not use a cap smaller than 3 ft 3 in. by 3 ft. 3 in. For all other I-beam types, do not use a cap smaller than 2 ft. 9 in. by 2 ft. 9 in. For other structure types refer to the bridge standard drawings for recommended cap widths.
Anchor: #i1350956Structural 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.
Anchor: #i1351153Design Criteria
For cap design, check Strength I limit state and Service I limit state. Check distribution of reinforcement as required in Article 5.7.3.4 of the AASHTO LRFD Bridge Design Specifications using Class 1 exposure for moderate exposure conditions and Class 2 exposure for severe exposure conditions. Limit tensile stress in steel reinforcement, fss under Service I limit state to 0.6fy.
Check Service I with 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 wide columns, take design negative moments at the effective face of the column.
Do not use the simplified shear design procedures allowed by AASHTO LRFD 5.8.3.4.1 nor 5.8.3.4.3. Use the General Procedure as provided by AASHTO LRFD 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. Diagonal cracks have been observed in caps with large beam reactions and small pads designed using the traditional sectional model. Strut-and-tie analysis indicates that the diagonal compressive struts were overloaded in these instances.
When calculating the cracking moment of a member
in accordance with Article 5.7.3.3.2, take the
modulus of rupture, ƒr,
as 0.24
, for all normal weight concrete.
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, otherwise.
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 of the AASHTO LRFD Bridge Design Specifications in caps deeper than 3 ft. Caps 3 ft. and less should have two #5 bars equally spaced in each side face.