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(OP)
Hi, I have an existing building on piles which will be renovated and found that the piles will be overloaded about 10%~20% above the allowable pile capacity (ultimate capacity divided by a FOS of 3).

I saw also this:

I understand that Chapter 34 of the IBC allows for 5% of overstress when evaluating existing structures, but is this related to the ultimate or allowable resistance/capacities. As you know, geotechnical stuff is still using ASD method so just want to hear what you think.

(OP)
I am trying to answer myself: I think that the 5% allowance should be applied to the allowable capacities since if we exceed the ultimate capacities it will result in a failure.

How was the ultimate pile capacity determined? (May be a lot of conservatism there.)

Point bearing or friction piling? (IMHO, point bearing often have a lot of reserve capacity.)

How much would the building's design live load rating have to be reduced to bring the pile loading down to allowable?
(Not a joke, we did something like this on one project.)

May have another suggestion, if the above don't work out.

(OP)
Hi SRE,

The piles are end bearing in medium dense gravel. Friction contribution along the shaft is minimal. Ultimate capacities were calculated the upper bond of the available geotechnical parameters so it was not in the conservative side.

About reducing the loads, I will need to talk with the structural.

Appreciate the input !

Okiryu - If nothing else works, conduct a test pile program with new piling identical to existing. Not cheap... but may be cost effective if no other solution can be found. At least you will have an answer one way or the other instead of speculation.

Sounds like a lot of information on the original job is available. If the driving record is available for some, I'd apply different pile driving formulas to see where you stand. You might make some progress there.

Also,what sort of tolerance can the building handle as to differential settlement? Perhaps in the remodeling the added load caused cracks won't show up after finishing the walls, etc.

One precaution might also be adopted. That is a detailed record of floor elevations now and later what the new situation is like. You may be surprised-to see no change or at least minimal, considering the end bearing situation for the piles.

(OP)
SRE, yes, that is an option but costly.

Also, I noted that in the IBC 2015, Chapter 34 Existing structures was deleted...

(OP)
OG, thanks, yes, I asked for pile records to verify with dynamic formulas. Hope they can find it.

Chapter 34 indirectly allowed a 5% overstress. It allows a gravity load to be increased by 5% or the capacity to be reduced 5% from the time it was originally constructed. The only way you get to a 5% overstress is if the pile was originally loaded to 100% of its capacity. In your case, the original load on the pile can be increased 5% without justifying the piles capacity per the current code. If the pile originally had a 100 kip gravity load, it can be increased to 105 kips without further analysis. It doesn't matter if the load is calculated with ASD or LRFD. The load can be increased by 5% from the time it was originally constructed.

Chapter 34 from the IBC has been replaced with the International Existing Building Code. Free online read only access to I-codes at https://codes.iccsafe.org/public/collections/I-Cod...

Hi Okiryu
In my gut feeling with additional 20%load will give a FOS 2.5. In the existing building the piles had been no excess pore pressure anymore, it means the strength is higher (x%) than the strength during installment.

IBC 1810.1.2 requires load testing or redriving existing piles. This is for new construction, so I'm not sure how/if it applies to renovations.

A geotechnical engineer should be able to determine if "soil improvement" has occurred due to the loading over the duration of the structure lifespan. If so, you may be able to justify a lower Safety Fact or.

(OP)
Thanks all for your responses. We are looking at pile installations records for same type of piles installed close to the site and appears that they were installed to reach approx. 30% more than the designed pile capacity. We will take this into consideration if we allow overloading in the pile.

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