Building Frame Modeling for pile cap foundations
Building Frame Modeling for pile cap foundations
(OP)
I have a cast in place post tensioned building with frames spaced at 27 feet typical and 62 feet beam spans supporting 7 inch slab. Lateral system is ordinary moment frames. The foundation type is auger cast pile groups with individual pile caps. Other engineers and I have been debating which type fixity to assume for these foundations at the column base/foundation interface.
Your thoughts are appreciated?
Your thoughts are appreciated?






RE: Building Frame Modeling for pile cap foundations
RE: Building Frame Modeling for pile cap foundations
RE: Building Frame Modeling for pile cap foundations
RE: Building Frame Modeling for pile cap foundations
larsacious => two 62.42 ft spans.
hokie66 => Due to the nature of cast in place construction it seems that the connection between column and foundation tend to transfer moment. I would think it would take a special detailed connection (increased cost) between column and foundation to create a pinned connection and no moment transfer.
We have one concern with pile supported foundations (16 diameter auger cast piles), their lateral capacities controlled by the soil are very low. Under frame action the shears are enough to displace the piles 1/4 to 1/2 inch. This changes the support assumption and consequently some column moments and first level beam moments.
RE: Building Frame Modeling for pile cap foundations
I didn't say that I would detail a pinned connection, just that I would design the frames with the assumption that the base of the columns are pinned. That way, and assuming that you have no other lateral resisting elements, the column moments are maximum at the top and just act as cantilever elements. Lateral movement at the bottom doesn't affect the column and beam moments.
RE: Building Frame Modeling for pile cap foundations
Thanks
RE: Building Frame Modeling for pile cap foundations
RE: Building Frame Modeling for pile cap foundations
You are correct with the issue regarding displacement at the pile cap (which I where I am assuming you have modeled the restraint). Any rotation at this point (no matter how minor) will result in a redistribution of moment back to the top of the column and a large increase in frame sidesway. I have generally modeled a piled foundation with lateral spring restraints along the pile which allow up to 1/2" movement at the pile cap. I feel that this models the real behavior of the structure adequately.
What I have seen other engineers do in the past is to model full rotational fixity at a depth of 2*pile diameters below the pile cap. This will allow for some rotation and movement at the pile cap level. They then take the reactions from this model and design the substructure seperately.
RE: Building Frame Modeling for pile cap foundations