Pile Failure Determined 2

[haynewp]

After how much vertical movement do you consider a pile foundation as failed for a typical building? Does the movement happen all at onece?

The reason I am asking is that I have a large metal building, without any brittle finish, that is sitting on existing pile caps (caps between 6 and 9 piles). I need to add between 5 and 10% of additional load onto the caps, but I don't have existing pile capacities. I can't imagine 5 to 10% additional load failing this many piles.

 

[Qshake]

Remember that yielding the pile isn't all that load might do. Several other considerations that come to mind are displacement if the pile are supported by friction alone or overstressing the bearing strata, especially in a karstic topography. Other considerations may include cap design if a limits states approach wasn't used.

Just some thoughts.

Regards,
Qshake

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[72shortbox]

True failure is when pile deflection occurs with no increas in load. During load testing the Davison Method is often used stating failure is the point which pile deflection is (.15 +.008xdia) inches more than the elastic movement of the pile. Saying that I have called failure if the total deflection is greater than 3/4" depending on what the structure requires.

Piles are typicaly desinged with a FS of 2-3 so increasingthe load by 5-10% "should be" ok. Depends how comforatable you are on who designed the piles. The type of failure (plunging, or an increase in capacity with settlement) depends on the soil profile and whether it is controlled by endbearing or friction .

 

[BigH]

I'd consider a pile as "failed" if, under the applied load, the foundation moves sufficiently to cause distress in the structure - windows not closing, doors not hanging right, cracking of brickwork, etc. Basford has given a method of determining the failure of a pile - but based on the support of the pile - not of the consequences of movement for a structure. Some structures can tolerate a fair amount of movement (for shallow foundations, too) - others far far less. So, again, in my view, it is when the structure will not behave as intended.

 

[dik]

If it's an alteration to an existing structure, I've often put the owner on notice that any change in the loading regime can cause differential movement. Also have informed owner that overloading an existing pile 'may' cause problems and that if this happens then remedial work is called for... but, at the same time advised that failure won't likely be sudden.

Dik

 

[dmoler]

Metal buildings can sustain more angular distortion without problems, or with much less problems, than other structures.

If adding 5 to 10% additional loading to the piles causes problems, then the design was not very good to begin with.

 

[haynewp]

thanks,

I guess I was just feeling kind of negligent by putting new load on something I don't have existing information about.

But I also feel kind of the same way as the tone of most of the replies. I would think it might be (at most) a serviceability issue if a noticeable amount of settlement ocurred, due to the addition of this small percentage of load. (I also remembered the safety factor of 2 to 3 commonly associated with piles.)

 

[DRC1]

Unless you are dealing with crane runways, builings are typically designed for 1" differential settlement. I don't think 5% will cause anything to happen to the piles. Most piles are driven to a blow count that is in excess of required. It would be prudent to establish a benchmark on the pile before constrution, check it at least once prior to starting work, check a few times during the work anfd for a few after the work is done.