Building Frame Modeling for pile cap foundations 1

[tbone73]

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?

 

[WillisV]

either. just design and detail for your assumption.

 

[larsacious]

How many spans are there in your frame?

 

[hokie66]

I would assume the columns are pinned at their base.

 

[tbone73]

Sorry for the late response.

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.

 

[hokie66]

thone73,
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.

 

[tbone73]

hokie66, ok I see your point of the pinned assumption at the base. However as far as lateral movement at the base of the column if the pile cap displaces 1/4 to 1/2 inch (similar to one spring)versus a true pinned connection how can the column moments remain unaffected?

Thanks

 

[hokie66]

I assume that if the piles caps moves that much, the whole building would be moving with the piles. With frame action alone, the base reaction equals the applied load, so the moment doesn't change with the deflection.

 

[rscassar]

I was late to jump on this thread. If (1) the CIP pile reinforcement extends through the pile cap and lapped to the column reinforcement and (2) the geotechnical engineer is prepared to certify the surrounding soil to resist the additional pressures resulting from moment resistance then 'yes', it would be a reasonable assumption to model the fixity as fixed against rotation.

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.

 

[tbone73]

kikflip thanks for your input. Your second paragraph is very similar to what we were thinking. Thanks to all for the input.