Net allowable capacity of concrete piles
Net allowable capacity of concrete piles
(OP)
Hello,
Was asked by a colleague (another structural engineer) to determine the allowable capacity of a concrete pile.
Data from geotechnical report is used to calculate allowable capacity at base of pile. Shouldn't the net carrying capacity at top of pile subtract the weight of the concrete pile itself (which is significant in this case)from the allowable capacity at base?
Check of other piles at same site does NOT seem to indicate that weight of pile itself was subtracted from total capacity, thus the confusion.
Thanks for any input.
Was asked by a colleague (another structural engineer) to determine the allowable capacity of a concrete pile.
Data from geotechnical report is used to calculate allowable capacity at base of pile. Shouldn't the net carrying capacity at top of pile subtract the weight of the concrete pile itself (which is significant in this case)from the allowable capacity at base?
Check of other piles at same site does NOT seem to indicate that weight of pile itself was subtracted from total capacity, thus the confusion.
Thanks for any input.





RE: Net allowable capacity of concrete piles
RE: Net allowable capacity of concrete piles
Some regulations give explicit reference to what to do in this case though .The Eurocode 7, for example, require that the pile weight be considered as an action.
RE: Net allowable capacity of concrete piles
RE: Net allowable capacity of concrete piles
RE: Net allowable capacity of concrete piles
RE: Net allowable capacity of concrete piles
RE: Net allowable capacity of concrete piles
If the given pile capacity were only, say, 25 tons (dependant upon size and local substrate conditions, etc.) then a 5 ton self weight of a pile would NOT, in my humble opinion, be negligible.
RE: Net allowable capacity of concrete piles
RE: Net allowable capacity of concrete piles
RE: Net allowable capacity of concrete piles
I think the question is a geotechnical one, in which case allowable load would be right not to exceed the allowable bearing capacity. I ignore the self-weight of the pile because 1. it is generally much smaller compared to the loads that it is required to carry and 2. the weight of the pile is offset by the removal of the soil which it displaced.
What we do when we issue drawings is that we only give working vertical loads to the piling contractor. A dead load working, live load working and working wind loads if the pile is resisting overturning. This allows the piling contractor to install the piles knowing what is the capacity that is required.
I have never seem horizontal loads or moments ever called up on drawings and I believe they should, because I believe the lateral resistance of the pile is also a critical component of the design.
RE: Net allowable capacity of concrete piles
Horizontal loads, if any, should be specified. As far as I know, listing horizontal forces is standard practice.
RE: Net allowable capacity of concrete piles
Good, bad or indifferent practice, that's how I think it's seeped into driven pile design. Personally I'm inclined to agree with JedClampett.
http://www.pz27.net
RE: Net allowable capacity of concrete piles
On building structures or bridges. The only place that I have seen horizontal loads called up on drawings is hydraulic forces on bridge piers (usually distributed between 2 or 3 piles).
How are you calling up horizontal loads on your pile schedules? We basically give a pile mark, diameter, reinforcing, cut-off level, working dead load, working live load, and the +/- wind loads (if resisting overturning).
Interesting because I have not been specifying horizontal forces on piles.
RE: Net allowable capacity of concrete piles
That sounds like you are talking only about cast in place bored piles, where the design is typically by the structural engineer, using the geotech report for advice.
I meant the more general condition, where the piling design is accomplished on a performance basis by the piling contractor's engineer, using forces provided on the structural engineer's drawings. In this case, we include another line in the schedule for the horizontal forces, if in fact the piles have to be designed to resist these forces.
RE: Net allowable capacity of concrete piles
2. Piles are driven to a blow count for termination criteria, which only effects vertical capacity. Lateral capacity is controlled by soil and pile design, assuming driven depth is greater than 30 feet. Thus lateral capacity is strictly a design function.
RE: Net allowable capacity of concrete piles
RE: Net allowable capacity of concrete piles
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Net allowable capacity of concrete piles
I agree 100%, the point that I did not make very well inretrospect was that Axial capacity is often confirmed in the field via blow count or PDA, where as lateral load is not confirmed. Load tests can be done, but unless it is critical, it is often not done. Therefore the determination of lateral capacity is most likely by calculation. But as you point out if the contractor does a lousy job of installing the pile, the calulation is not worth much.