Cast-in-place Concrete Piles - Self Weight Included in Design

[JrStructuralEng]

This may sound like a stupid question, but hey, as they say, no such thing as a dumb question.....

Anyway,
Do I need to include the selfweight of the concrete pile in the design of the pile itself. I am designing on a basis of skin friction only.

 

[youngstructural]

Yes, you do. However you do not have to include the entire weight as a point load on the shaft; You can take it out over the length of the pile. I like to discount the capacity of the pile by the factored self weight while designing for length. That way after I do my primary calculations I know what my "true" carrying capacity is.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[JrStructuralEng]

thanks for the info

Does anyone know where I can find this in code/standards?

 

[frv]

Sometimes the geotech will specify this. I've seen where the geotech mentions that the self weight can be neglected.

 

[JrStructuralEng]

I just talked to a geotech. He figures the self-weight of a pile is generally insignificant for small diameter caissons.

His reasoning was that you are removing ~125lbs/cu.ft of soil and replacing it with ~150lbs/cu.ft of concrete.

Because of this, you are looking at less than 20% net increase in loading, which one could argue is insignificant based on their large safety factors. They base their skin friction capacities on safety factors of 2.5 to 3.5

Does everyone agree with this? I am guessing mixed opinions.

 

[UcfSE]

It sounds reasonable to me, similar to neglecting footing weight in footing design. You're missing 20% of the weight of the pile which is probably already small compared to the pile capacity.

 

[apsix]

I agree, most (all?) texts I've seen on the subject ignore pile self weight due to the reasons above.

 

[lkjh345]

I agree with others that ignore the self weight of the piles. I have never considered it.

I recently had a geotech engineer go so far as to tell me I could ignore the self weight of 6 foot diameter drilled piers that were up to 90 feet deep. I couldn't quite bring myself to that.

 

[BigH]

If the geotech says "Allowable bearing load" - then I would not consider the weight of the pile - as he "must have" figured it in.

MJ Tomlinson states (Foundation Design and Construction, 6th Ed): "At the limit state . . . the ultimate bearing capacity of a pile is given by the equation "Qp = Qs + Qt - Wp" . . . Usually Wp is small in relation to QP and is is often neglected because it is not much greater than the weight of the displaced soil. However, it must be taken into account for marine piling where a considerable proportion of the pile length extends above the sea-bed." He goes on to talk about Eurocode 7 where they talk about design bearing capacity rather than the ultimate value.