Net allowable capacity of concrete piles

[whiskypat]

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.

 

[InDepth]

Ask the geotech. usually you don't include the pile weight, because the geotech has already accounted for it in the recommendations.

 

[Mccoy]

There are different schools of thought on the topic, everyone with its own reasons wether to consider or not pile weight.

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.

 

[Riggly]

If I recall well, AASHTO subtracts weight of pile to get net net allowable.

 

[whiskypat]

thanks for the input. Yeah, especially with concrete piles (of significant weight themselves), it seems the self weight should be subtracted to get the net carrying capacity...will check AASHTO, thanks.

 

[JedClampett]

From the allowable loads I've seen (50 T for 12 inch, 65 T for 14 inch, etc.), I just figured that the level of accuracy took care of the pile self weight.

 

[hokie66]

Jed is right. Pile mass has very little effect on pile capacity, so it can be neglected.

 

[whiskypat]

hmmmmm, i disagree with the hilbilly and the Hokie, but perhaps i've gained insight as to it's (the pile self weight ) having been aparently neglected before.

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.

 

[hokie66]

Then be sure to deduct the mass of the soil the pile displaces. Just kidding. Seriously, you determine pile capacity by how it drives and the gross capacity of the shaft. Self weight has little to do with it.

 

[demayeng]

I was told that as the bearing capacity at the base of the pile is worked out based on in-situ soil parameters that have been obtained with undisturbed soil over, and the concrete is essentially just replacing the soil of similar density as the concrete, then you can neglect the weight of the pile in calculations.

 

[asixth]

We have had this discussion in our office many times before. When you mention, allowable capacity of a concrete pile, I immediately start thinking the structural capacity of a pile, this creates a problem in itself because an ultimate capacity is going to mean an ultimate axial force with a co-existant moment. I have never designed a pile which does not have a co-existant moment under some load case. To say the allowable capacity means that the actions are calculated using working loads and a factor of safety applied to the capacity (say 1.5 to 2.0 for 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.

 

[hokie66]

asixth,

Horizontal loads, if any, should be specified. As far as I know, listing horizontal forces is standard practice.

 

[vulcanhammer]

I never gave consideration to the pile weight (driven piles) until I started to teach Foundations and saw it in the textbooks. What I think has happened is that the practice of including pile weight has been picked up from drilled shaft practice (where it can be a big deal) into driven pile practice, and carried through for consistency of teaching the method. I know that the text I use (Coduto) tends to conflate the two.

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

 

[asixth]

hokie

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.

 

[hokie66]

asixth,

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.

 

[DRC1]

1. If you are worrying about the weight of the driven pile the a)your pile design is too close to capacity, b) the weight of the is far less than the the margin of error of your bearing capacity. (large drilled piles could be an issue)
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.

 

[BigH]

With respect to lateral capacity strickly a design function - yes, and NO. yes, if the piles were driven or bored as per specification with no deviation and NO if poor workmanship was used - gaps left in place for instance. The later is a construction problem.

 

[paddingtongreen]

The pile replaces the same volume of soil so the pile only adds the difference in density over that volume.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[DRC1]

Big H
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.