Geotech report- ultimate pile capacities

[haynewp]

I have a soils report from a geotech I haven't worked with before where H pile capacities are given as ultimate capacities. I remember previously getting reports that listed the allowable capacity or what F.S. to apply to the ultimate capacities given. I had to call the geotech to get the recommended F.S. for the piles and he suggested to use about 2. What is the point of giving a soils report with ultimate pile capacities when the allowable capacities are needed for the foundation design?

 

[JedClampett]

As I'm sure you know, there's a push in the building industry to use more of a strength approach in design. You get your loads, multiply them by a factor that accounts for their predictability and likelihood, and compare that to the absolute limit of the design element, times a reduction factor. As far as I know, it started in the concrete industry and has moved on the CMU, steel and finally foundations.
The advantage is, if you're designing a concrete structure, you don't have to carry two sets of loads(one factored for the concrete design, the other unfactored for the foundation design) in your calculations. You can also adjust your factor of safety based on the likelihood of loads occuring.
As long as it's clearly stated, it shouldn't be a problem.

 

[haynewp]

Can LRFD for loads on structural members translate to foundations like that? That is good if so, but I have always used the allowable loads per the geotechnical when sizing the foundations. I have seen some geotechs use a 2.5 as a S.F. which is higher than our structural ultimate/allowable ratios we use.

 

[JedClampett]

I hope they can. The code guidance for Geotechnical Work seems a lot less prescribed than what I'm used to, but that's what they've always had.

 

[ukengineer58]

well consider how arbitrary the factor of safety is why not use ultimate loads. in UK we are now using limit state approach with settlements expressly calculated not deemed to be ok by dividing the ultimate load by two to three

 

[FixedEarth]

It is more involved than simply stating F.S. value For a give pile, there are geotechnical capacities and structural capacities. For the geotechnical, we may have axial downward, axial upward, shear, moment and twist. Each one of those loads gives capacity with depth. So for example, you may get 35 ft embedment for axial downward, 24 ft axial upward, 19 ft for shear & moment and 30 ft for torque.

The geotechnical engineer may not know upfront if ASD or LRFD will be employed. So he may give the ultimate and then leave the F.S. to the designer. However, a range of appropriate F.S. should be given.

 

[BigH]

and, of course, the LRFD will determine the factors by calibrating to the old methods . . . so go figure . . .