Mixing LRFD and ASD for foundation design
Mixing LRFD and ASD for foundation design
(OP)
Hi, for pile design I am giving axial and lateral capacities calculated using some FOS (piles are end bearing piles), which may be considered as the ASD method from the structural perspective. However, the structural engineer is calculating the loads in the foundation using load combinations using a LRFD approach. My question is, if you are doing a LRFD based design, do you use the "allowable" capacities provided by the geotechnical (means the ultimate capacity affected by a FOS) or do you use the ultimate capacities (capacities non affected by FOS) since your loads are already factored? Thanks!






RE: Mixing LRFD and ASD for foundation design
RE: Mixing LRFD and ASD for foundation design
Dik
RE: Mixing LRFD and ASD for foundation design
RE: Mixing LRFD and ASD for foundation design
RE: Mixing LRFD and ASD for foundation design
RE: Mixing LRFD and ASD for foundation design
In theory, the Canadian Foundation Design Manual has phi factors for geotechnical resistances, so the Winnipeg thing should be do-able. It's the direction we're definitely going. I've run into the issue, though, that geotechnical engineers don't seem to trust it so they don't actually start with ultimate values. They put extra fudge factors in before they apply the phi, because they're afraid of using an ultimate value for design, and end up being more conservative than they would have been in ASD.
The above is more an issue with education and comfort than a problem with the system itself.
There's also, understandable, a lot of confusion over how to deal with serviceability based allowable.
RE: Mixing LRFD and ASD for foundation design
On a similar note, I thought the soil capacity is quite often governed by settlement (don't know if this is true for piles). More often than not, settlement is a serviceability issue and using service loads (ASD) makes more sense to me.
RE: Mixing LRFD and ASD for foundation design
No question, it's doable... just inflates the foundation costs, with little or no benefit.
Dik
RE: Mixing LRFD and ASD for foundation design
RE: Mixing LRFD and ASD for foundation design
Secondly, where does LRFD get the load reduction factors and the property factors? They "correlate" to the old methods - say assuming a SF = 3 for footings. So the question is why?
Thirdly, as far as I have been able to see, page after page is provided for shear capacity - of an isolated footing. I have never seen - and perhaps it is out there - of what happens when you have two footings close by whereby their potential shearing planes (aka prantdl) interact? Of course, for settlement computations we can take this into account by superposition of the vertical stresses of one to the other - but are there procedures for interacting footings? Again, it is the service limits that will govern.
Fourthly, I have never seen, actually in either method, what to do when one has slickensides or other zonal factors in the computations - pre-disposed sliding surfaces. I think this is a bigger issue with the LRFD where things are heavily "codified" and you are damned in you do and damned if you don't.
As one has stated and I have read this elsewhere as well - costs of foundations have gone up due to the LRFD approach . . . so is it really necessary? As Poulos has stated in his State of the Art paper to the Istanbul SSMEFE conference, the old methods work. . . . and, in any event, you need to critically look at your design. My fear is that, especially given the plethora of computer design these days, that the criticality of judgment may end up like the dodo.
My rambling take on the issue. No problem - criticize!
RE: Mixing LRFD and ASD for foundation design