Section 5: Inverted Tee Reinforced Concrete Bent Caps
Anchor: #i1351441Materials
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: #i1351466Geometric Constraints
Make inverted tee dimensions the same for all bents on the project.
Keep the top of stem at least 2.5 in. below the bottom of the slab; see standard drawing IBMS, available at http://www.dot.state.tx.us/insdtdot/ orgchart/cmd/cserve/standard/bridge-e.htm.
Anchor: #i1351497Structural Analysis
Analyze 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.
Anchor: #i1351712Design 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 areas where deicing chemicals are frequently used. Limit tensile stress in steel reinforcement, ƒss under Service I limit state to 0.6ƒy.
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.
Minimize the number of stirrup spacing changes.
When designing for beam ledge punching shear, replace AASHTO LRFD 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 1, shall not overlap. Therefore,
must be greater than 
to
prevent
longitudinal overlap. S must be greater than 
to prevent transverse overlap.
- Normal punching shear resistance,
, kip, shall be taken as:
- At interior pads:
- At exterior pads
, except
shall not be taken
greater than 
.
- At interior pads:
- Replace AASHTO LRFD Equation 5.13.2.5.5-1 with the following:
- Replace the following sentence in AASHTO LRFD 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.”
- Replace the following sentence in AASHTO LRFD Article 5.13.2.5.5: "The hanger reinforcement specified herein shall be provided in addition to the lesser shear reinforcement required on either side of the beam reaction being supported" with the following: "Do not superimpose loads on stirrups acting has hangers and loads on stirrups acting as shear reinforcement. Proportion the web reinforcement in the stem of an invert T-beam based on required hanger reinforcement or required shear reinforcement, whichever is greater."
When calculating the cracking moment of a member in accordance
with Article 5.7.3.3.2, take the
modulus of rupture, 
,
as 0.24
, for all normal weight concrete.
Detailing
Provide extra vertical reinforcing across the end surfaces of the stem to resist cracking. Do not weld bars together for development of ledge reinforcing. Use anchorage hooks to develop ledge reinforcing.
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.