Geotechnical Resistance - from Allowable to Factored ULS Resistance

[J1D]

In all the geotechnical reports (in Canada) now Factored ULS (Ultimate Limit State) Resistances are given instead of traditional Allowable Resistance. When using factored ULS you find the capacity is lower than before.

For example, I have two geotech reports done by the same geotechnical consultant for the same site. One is done in 2004 with allowable values, another is done this year without new BH just changed the values to be factored ULS. As saying the new report is giving a bit higher capacity based on the piling records - this is what the client is expected. However the new "capacity" is not increased at all.

In the old report, the allowable steel pile shaft skin friction resistance is 16kPa, while in the new report the factored ULS resistance is 18kPa. However the factored load for the 18kPa increased more. The design loads to compare to these values are:

In allowable resistance design: Specified Load vs Allowable Resistance
In limit states design: Factored Load vs Factored ULS values

The Factored ULS value is about the same as the Allowable Resistance since it is 0.4 x ULS values (as per NBC). But the Factored Load is much higher (say 25% higher at least) than the specified load. This causes the lower design capacity.

This is what I experienced for a while. Thanks for your input.

j1d

ps. anothr question is that SLS values are given in some geo reports. If we consider the pile resistance (come with certain settlement, say 1/4") is actually a Serviceability Limit State issue then the design geotechnical resistance will be higher.

 

[jayrod12]

Get your geotech guy to give you a better resistance factor for ULS by doing proper testing.

Depending on the quality of the drilling, sampling and testing we sometimes are approved to use a factor of 0.55. That being said he gives us an unfactored ULS value of ~30 kPa when using 0.4 and an SLS value of ~15 kPa.

 

[motorbiketocrank]

I'm not familiar with the Canadian approach but I'd agree that you would talkto your geotech designer. If I'm understanding correctly and you have to have to use 0.4 uls, that's basically like the geotech applying a factor of safety of 2.5 to the ultimate capacity. Since that's about the same as applied in traditional design (with unfactored structural load) it inherently makes sense that you'd end up with a lower working capacity using a methodolgy that compares this against factored structural loads. Also, traditionally, the factor of safety is only applied to shear failure behavior, not settlement. So I wouldn't expect a uls capacity to change from traditional based on settlement criteria....again talk to your geotech.

 

[BigH]

. . . and all these factored values come from correlating to the old method . . . makes sense, eh?

 

[J1D]

Thanks.

I understand it is in a transitional period using limit states design in evaluating geotechnical capacities. But it should a smooth process to correlate to the traditional allowable capacities. What we encountered now, in many cases, is the real geotechnical design capacity is lower for the soil at the same spot. That means when you review an existing strucuture designed a few years ago using the new values you find the pile capacities are not enough. But the strucutre's been standing there for years without any issues.

I think the factors, either that for geotechnical resistance or for strucutral loading, should be adusted to make smooth change.

BTW, many old geotech reports used settlement (say 1/4") as well as the criteria determining the allowable resistances.

Thanks,
j1d