Double dipping the safety factor?

[papa2ten]

When designing pile foundation for resisting up & down forces due to overturning moment from lateral forces, the code requires a safety factor of 1.5 against overturning (for stability.)

If the piles are being tested to 2.0X the design capacity, is applying the 1.5 F.O.S. to the down force side overly conservative or a minimum?

Thanks!

 

[Focht3]

In my view, No.

The FOS of 2 is intended for the soil alone, to try and achieve "allowable" behavior. The FOS of 1.5 addresses uncertainty in the loading, and the need for additional conservatism in the design. Yhe FOS of 1.5 should be applied to the allowable pile capacity, not the ultimate capacity.

The two FOS values address different issues -





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[PXC]

The 1.5 FOS applied to the allowable is what we want to achieve. The testing is to assure us that we have achieved this. Specifically, it is to prove the adequacy of the soil, not the steel/concrete of the pile.

If we tested all piles, we could test to only 1.5 times allowable and know that we had achieved the desired capacity. If we test only some piles, we need to test to a higher load to allow for the variability between piles. The magnitude of the increased factor is reported in various codes, and varies with the number of tests.

Note that if you are going to test a pile to 2.0 times the allowable load, then this is a design load that you should design the concrete/steel of the pile for. The length of the pile might have based on the soil strenght parameters with the 1.5 factor but the assumption is that the soil/rock has sufficient strength to resist this higher factor.

 

[Focht3]

I conditionally disagree with most of [blue]PXC[/blue]'s post. I do agree with the statement, The 1.5 FOS applied to the allowable is what we want to achieve. The remainder of the post implies that the FOS of 1.5 is applied to the ultimate pile capacity. The allowable pile capacity should be used with the FOS of 1.5 -

If I misunderstood [blue]PXC[/blue]'s post, I apologize.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[PXC]

Yes Focht3, I did mean that the 1.5 factor is applied to the allowable.

 

[VoyageofDiscovery]

Hi guys,

Now I am totally confused!

I have used my working loads and hope to God I have the capacity. Disregarding liquifaction and the like, for all intents and purposes the allowable capacity of the pile is the allowable capacity of the soil. This FOS is the business of the geotechs we pay.

Let's not talk about ultimate here. The philosophies of ultimate and FOS prohibit it.

Regards

VOD

 

[AlohaBob]

It seems to me these factors of safety refer to two different things.

The 1.5 FOS for stability of the structure overturning, typically means the geometries above the ground and interface statics at ground level are sufficient so that the structure would not require a net tension force to develop at the foundation to prevent the structure from tipping over. This FOS is required in most instances, regardless of foundation type. The OT FOS is sometimes very sensitive to small changes in geometry. Perhaps a relocation of 10% mass would provide a change FOS overturning 1.5 vs 1.0 however at the same time the actual pile stress may only change 10%. So FOS overturning is not necessarily directly related to FOS ultimate pile capacity.

If the structure however depends on pile tension capacity for stability, then that 1.5 FOS structure resulting forces in piles both up and down where they occur would not exceed the allowable load of the piles. Those allowable loads would relate to the ultimate capacity in some fashion, perhaps by 2.

 

[ishvaaag]

If we test a pile to twice the expected maximum loads, and the test results well, and if the limit load that the code mandates to consider is only 1.5 times that expected, the test informs us that for that particular pile, the pile would have behaved well and still have some (for this case) tensile strength margin.

The combinations to be used to ascertain what loads to use as the structural requirements for the foundations remain in the realm of the governing code.

 

[BigH]

If the test load was a compressive load, the tensile load may be different, of course. There is also many out there who believe that skin friction in tension is less than skin friction in compression - this is particularly so for tapered piles (wood, for example).

I look at the issue as follows: For the suitability of the pile to take a load, we test to 2x to ensure that, with a positive reserve of safety, those piles that have been driven or drilled in who are "less" than the test pile in their behaviour as within the limit to keep us "safe". (This is why in compaction testing, compaction these days are to a required level of relative compaction, but the average of the lot must be statistically larger than the specified to "accept" the compaction test results.).

Now the 1.5 factor of safety is a structure safety factor to overturning. This isn't the safety factor on the pile support.

If this were a retaining wall on spread footing, you take the allowable bearing capacity (which might be modified more for settlement) as a safety factor of 3 (or 2.5) on ultimate. Still, you use 1.5 for overturning FOS. Seems to me to be the same with your piles.

 

[J1D]

Good topic!
I've used the 1.5 overturning safty factor on top of the allowable overturning resistance for years, but never thought the thing deeper.

In light of your statements, it occurs to me that the structures with overturning resistance only by the weight of foundation and stuctural DL is less safer than the foundations with the 2x F.O.S. (The actural F.O.S. is even higher since, as far as I know, the F.O.S. usually determined on a strict displacement criterion rather than an ultimate strength). The total oveturning F.O.S. of the former foundation is however a deduction of dead load (say 0.85) + 1.5

What do you think?