COLUMN TENSION LOADS APPLIED TO PILE CAPS

The mechanical development of the tension resistance will typically be done with hooked or L-shaped bars. It is sometimes helpful, for visualization, to invert the drawing or design sketch. This shows the load/stress orientation in a direction you might be more acustomed to seeing. [sig][/sig]

What you are describing is typical for moderate to high seismic zones where uplift is expected.

In general, pile anchorages are designed to resist 10% of the ultimate strength (0.1xAxFy) in tension. Thus any anchorages (for steel piles) or reinforcing (for concrete piles) should meet this requirement. Also, for concrete piles, AASHTO, recommends that at least four dowels with a minimum reinforcing ratio of 0.01 be embedded in the pile cap.

As for the top mat of steel in the pile cap, it is generally a reflecion of the bottom mat for convienence in lower seismic zones. In moderate to high seismic zones one should check the top mat against the tension loads developed in the pile just as one would design the bottom mat based on compression in the pile. [sig][/sig]

Just to clarify what qshake wrote, you say that I should check the top steel with the tension and moment load applied to the pile cap, with the same method that I used to design the bottom mat of the pilecap??

thanks [sig][/sig]

Determine the tension/compression in the piles. Now using the forces determine the moment/shear at the face of the column. This method, for the tension loads, is exactly opposite what you did for the compression loads. This also agrees with Ron when he said to "invert the drawing or design sketch"

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YES. THE TENSION HAS TO BE CHECKED NOT ONLY THE INDIVIDUAL PILE BUT ALSO THE TOTAL SYSTEM (PILES AND PILE CAPS) TENSION GENERATE NOT ONLY BY REVERSIBLE LOAD CONDITIONS BUT ALSO FROM NEGATIVE SKIN FRICTION OF THE SOIL.SO BEFORE DESIGNING THE SOIL STRATIFICATION IS THE MOST VITAL. SO ACTION MUST BE BALANCED BY REACTION. I DO NOT AGREE THAT ONLY SOME TOP PORTIONED ADDITIONAL LONGITUDINAL AND CLOSELY SPACED STIRRUP TO BE PROVIDED. IT IS TO BE CALCULATED THE LENGTH REQUIRED TO COMPENSATE THE TENSION. CODES ARE THE GUIDING TOOLS. ONLY CODAL PROVISION IS NOT SUFFICIENT. ANY SOIL MECHANICS BOOKS CAN BE REFFERED.NO PILES AND ITS SYSTEM SHALL BE SUBJECTED TO ANY TENSION. [sig][/sig]

I disagree that "no pile and its system should be subjected to tension". Tension is a common consideration in designing deep foundations. It may come from a variety of sources including, seismic, wind, and dynamic lateral loads on structures transmitted to the foundation as uplift loads. A pile cap is no different than any other structural component in many respects. It is subjected to loads which must have a defined load path, which can then be resisted by appropriate reaction, including reinforcement. Qshake's diagram and explanation clearly shows this process. [sig][/sig]

I agree with Ron. Tension is commonly designed for in deep pile foundations. Sure, we would like the foundation to be in compression only but this is hardly feasible in some cases and is certainly not economical in areas where the loads are a result of a probability based design. It's hard to sell an oversized footing to your client that was designed to resist a load it may or may not see within its lifetime, especially when there is a practical alternative.

I hate to use that argument but we engineers have to find solutions that are both effective and economical. [sig][/sig]

I DID NOT MEAN THE PILE AND SYSTEM CAN NOT BE SUBJECTED TO TENSION. MY CONTENTION IS THAT WHATEVER TENSION COMES EITHER FROM DIFFERENT LOAD CONDITON OR FROM SOIL-PILE INTERACTION THAT ARE TO BE DESIGNED SO THAT ULTIMATELY NO TENSION IS TRANSFER TO SOIL.
WHEATHER QSHAKE HATE TO BELIEVE THE ARGUMENT THAT WE ENGINEERS HAVE TO FIND THE SOLUTION THAT ARE BOTH EFFECTIVE AND ECONOMICAL.BUT I BELEIVE THA MOST COST EFFECTIVE SOLUTION CAN BE EVOLVED SO THAT OUR CLIENT CAN HAVE TODAY'S TECHNOLOGY RESULTING COST EFFECTIVE ENGINEERING SOLUTION. [sig][/sig]

Is there any literature where I can find this method???

Tapan...the tension in a pile does not result in tension in the soil, it results in soil shear stress from the soil-pile interaction. This is common for both tension and compression in a deep foundation. The only difference is that the shear stress component in the soil behaves in a slightly different manner in the upward direction (uplift/tension) as compared to the downward direction (dead + live/compression). These are readily accountable in the physical and derived properties of the soil and soil-pile interaction.

Aldorankin...I'm not sure which "method" you are referencing, but there is plenty of literature on the interaction you have described. Is there a simple checklist and go-by for the exact condition you have described? I doubt it, but given the pieces and parts there is a method by which you can resolve the answer with support from many established and authoritative writings, including some of the tips in this thread (go back to Qshake's tip and diagram). It is called "Engineering"!