Bearing stresses on pile caps

[Kramer]

I am designing a concrete pile cap/beam for a center bent of a 2 span bridge. The piles are HP 12x74. The concrete pile cap/beam is 28" wide and 30" high. The piles are to be embedded 12" into the bottom. Do I need to check the concrete bearing stress at the top of the piles? If so, do I use .85fc' with a reduction factor of 0.70? AASHTO allows for an increase of up to 2.0 if the bearing area is less than the surrounding area. Also, to evaluate shear on the beam, I was planning on checking beam shear using 2 x (fc')^1/2 on the full depth and checking punching shear using only the depth above the top of the pile.

 

[Dinosaur]

Forget about checking bearing stress over the pile. Place 2-#6 bars over the piles running the length of the pile cap. I wouldn't consider punching shear in a problem of this kind. I would make sure you have stirrups throughout the full length of the cap.

 

[Kramer]

Are you considering the load to be transferred only by bearing on the end of the pile or does bond between the concrete pile cap and the pile contribute to the load transfer? Are the #6 bars there to counteract the bursting (lateral) forces from the high bearing stresses? Do the #6 bars need to be set directly on top of the piles?

 

[mike80]

Kramer,

Typically in Ohio, we use 1'-6 over the tops of the piles and that is sufficient for punching shear. Reinforce the cap for bending moments and shear. As Dinosaur said, don't worry about bearing stress over the pile.


Mike